aerospace

Sputnik First manmade satellite and man's opening of a new quest

2008-06-02T00:40:00.000+05:30

Sputnik at 50: An improvised triumphBy VLADIMIR ISACHENKOV,

Associated Press Writer Sun Sep 30, 1:01 PM ETMOSCOW -

When Sputnik took off 50 years ago, the world gazed at the heavens in awe and apprehension, watching what seemed like the unveiling of a sustained Soviet effort to conquer space and score a stunning Cold War triumph.ADVERTISEMENTBut 50 years later, it emerges that the momentous launch was far from being part of a well-planned strategy to demonstrate communist superiority over the West. Instead, the first artificial satellite in space was a spur-of-the-moment gamble driven by the dream of one scientist, whose team scrounged a rocket, slapped together a satellite and persuaded a dubious Kremlin to open the space age.And that winking light that crowds around the globe gathered to watch in the night sky? Not Sputnik at all, as it turns out, but just the second stage of its booster rocket, according to Boris Chertok, one of the founders of the Soviet space program.In a series of interviews in recent days with The Associated Press, Chertok and other veterans told the little-known story of how Sputnik was launched, and what an unlikely achievement it turned out to be.Chertok couldn't whisper a word about the project through much of his lifetime.

His name, and that of Sergei Korolyov, the chief scientist, were a state secret. Today, at age 95 and talking to a small group of reporters in Moscow, Chertok can finally give full voice to his pride at the pivotal role he played in the history of space exploration."Each of these first rockets was like a beloved woman for us," he said. "We were in love with every rocket, we desperately wanted it to blast off successfully. We would give our hearts and souls to see it flying."This very rational exuberance, and Korolyov's determination, were the key to Sputnik's success.So was happenstance.As described by the former scientists, the world's first orbiter was born out of a very different Soviet program: the frantic development of a rocket capable of striking the United States with a hydrogen bomb.Because there was no telling how heavy the warhead would be, its R-7 ballistic missile was built with thrust to spare — "much more powerful than anything the Americans had," Georgy Grechko, a rocket engineer and cosmonaut, told AP.The towering R-7's high thrust and payload capacity, unmatched at the time, just happened to make it the perfect vehicle to launch an object into orbit — something never done before.Without the looming nuclear threat, Russian scientists say, Sputnik would probably have gotten off the ground much later."The key reason behind the emergence of Sputnik was the Cold War atmosphere and our race against the Americans," Chertok said. "The military missile was the main thing we were thinking of at the moment."When the warhead project hit a snag, Korolyov, the father of the Soviet space program, seized the opportunity.Korolyov, both visionary scientist and iron-willed manager, pressed the Kremlin to let him launch a satellite. The U.S. was already planning such a move in 1958, he pointed out, as part of the International Geophysical Year.But while the government gave approval in January 1956, the military brass wanted to keep the missile for the bomb program, Grechko, 76, said in an interview. "They treated the satellite as a toy, a silly fantasy of Korolyov."The U.S. had its own satellite program, Grechko said. "The Americans proudly called their project 'Vanguard,' but found themselves behind us."The Soviet Union already had a full-fledged scientific satellite in development, but it would take too long to complete, Korolyov knew. So he ordered his team to quickly sketch a primitive orbiter. It was called PS-1, for "Prosteishiy Sputnik" — the Simplest Satellite.Grechko, who calculated the trajectory for the first satellite's launch, said he and other young engineers tried to persuade Korolyov to pack Sputnik with some scientific instruments. Korolyov refused, saying there was no time."If Korolyov had listened to us and started putting more equipment on board, the Americans could have opened the space era," Grechko said.The satellite, weighing just 184 pounds, was built in less than three months. Soviet designers built a pressurized sphere of polished aluminum alloy with two radio transmitters and four antennas. An earlier satellite project envisaged a cone shape, but Korolyov preferred the sphere."The Earth is a sphere, and its first satellite also must have a spherical shape," Chertok, a longtime deputy of Korolyov, recalled him saying.Sputnik's surface was polished to perfection to better deflect the sun's rays and avoid overheating.The launch was first scheduled for Oct. 6. But Korolyov suspected that the U.S. might be planning a launch a day earlier. The KGB was asked to check, and reported turning up nothing.Korolyov was taking no chances. He immediately canceled some last-minute tests and moved up the launch by two days, to Oct. 4, 1957."Better than anyone else Korolyov understood how important it was to open the space era," Grechko said. "The Earth had just one moon for a billion years and suddenly it would have another, artificial moon!"Soon after blastoff from the arid steppes of the Soviet Republic of Kazakhstan, the satellite sent out what would be the world's most famous beep. But the engineers on the ground didn't immediately grasp its importance."At that moment we couldn't fully understand what we had done," Chertok recalled. "We felt ecstatic about it only later, when the entire world ran amok. Only four or five days later did we realize that it was a turning point in the history of civilization."Immediately after the launch, Korolyov called Soviet leader Nikita Khrushchev to report the success. Khrushchev's son, Sergei, who was alongside his father at the moment, recalled that they listened to the satellite's beep-beep and went to bed.Sergei Khrushchev said that at first they saw the Sputnik's launch as simply one in a series of Soviet technological achievements, like a new passenger jet or the first atomic power plant."All of us — Korolyov's men, people in the government, Khrushchev and myself — saw that as just yet another accomplishment showing that the Soviet economy and science were on the right track," the younger Khrushchev, now a senior fellow at Brown University's Watson Institute for International Studies, said in a telephone interview.The first official Soviet report of Sputnik's launch was brief and buried deep in Pravda, the Communist Party daily. Only two days later did it offer a banner headline, quoting the avalanche of foreign praise.Pravda also published a description of Sputnik's orbit to help people watch it pass. The article failed to mention that the light seen moving across the sky was the spent booster rocket's second stage, which was in roughly same orbit, Chertok said.The tiny orbiter was invisible to the naked eye.Excited by the global furor, Khrushchev ordered Korolyov immediately to launch a new satellite, this time, to mark the Nov. 7 anniversary of the 1917 Bolshevik Revolution."We didn't believe that you would outpace the Americans with your satellite, but you did it. Now you should launch something new by Nov. 7," Korolyov quoted Khrushchev telling him, according to Grechko.Working round-the-clock, Korolyov and his team built another spacecraft in less than a month. On Nov. 3, they launched Sputnik 2, which weighed 1,118 pounds. It carried the world's first living payload, a mongrel dog named Laika, in its tiny pressurized cabin.The dog died of the heat after a week, drawing protests from animal-lovers. But the flight proved that a living being could survive in space, paving the way for human flight.The first Sputnik beeped for three weeks and spent about three months in orbit before burning up in the atmosphere. It circled Earth more than 1,400 times, at just under 100 minutes an orbit.For Korolyov there was bitterness as well as triumph. He was never mentioned in any contemporary accounts of the launch, and his key role was known to only a few officials and space designers.Leonid Sedov, a member of the Soviet Academy of Sciences with no connection to space program, was erroneously touted in the West as the Father of Sputnik. Korolyov, meanwhile, was only allowed to publish his non-sensitive research under the pseudonym "Professor K. Sergeyev."Khrushchev rejected the Nobel committee's offer to nominate Korolyov for a prize, insisting that it was the achievement of "the entire Soviet people."Sergei Khrushchev said his father thought singling out Korolyov would anger other rocket designers and hamper the missile and space programs."These people were like actors; they would all have been madly jealous at Korolyov," he said. "I think my father's decision was psychologically correct. But, of course, Sergei Korolyov felt deeply hurt."Korolyov's daughter, Natalia, recalled in a book that the veil of secrecy vexed her father. "We are like miners — we work underground," she recalls him saying. "No one sees or hears us."The Soviet Union and the rest of the world learned Korolyov's name only after his death in 1966. Today his Moscow home, where Chertok met reporters, is a museum in the chief scientist's honor.Chertok was permitted to travel abroad only in the late 1980s, after Soviet leader Mikhail Gorbachev liberalized the Soviet Union.The surviving leaders of the space program are no longer anonymous or silent, and revel in the accolades so long denied them."The rivalry in space, even though it had military reasons, has pushed the mankind forward," said Valery Korzun, a cosmonaut who serves as a deputy chief of the Star City cosmonaut training center. "Our achievements today are rooted in that competition."In the end, it was the Americans who won the race to the moon, nearly 22 years later. Khrushchev wasn't interested in getting there, his son says, and the effort made under his successor, Leonid Brezhnev, was underfunded and badly hampered by rifts between Korolyov and other designers."We wouldn't have been the first on the moon anyway," Grechko said. "We lost the race because our electronics industry was inferior."Today, even as Sputnik recedes into the history books, its memory still exercises a powerful grip. In August, when a Russian flag was planted on the sea bed at the North Pole, the Kremlin compared it to Neil Armstrong stepping on the moon — an indication, perhaps, of how much Russians still treasure that first victory in space.

SolidFuel

2008-06-02T00:36:00.000+05:30

A solid propellant is a monopropellant fuel-a single mixture of several chemicals (the oxidizing agent and the reducing agent or fuel). This fuel, as implied, is in its solid state and has a preformed or molded shape. The propellant grain, this interior shape of the core is an important factor in determining a rocket's performance. The variables determining grain-relative performance are core surface area and specific impulse. The composite solid propellant for rocket motors of ISRO uses ammonium perchlorate (oxidiser), fine aluminium powder (fuel) and hydroxyl terminated polybutadiene (binder).

The solid motors produced in ISRO include those for the first stage booster motor of the Polar Satellite Launch Vehicle (PSLV) - a five segmented motor of 2.8 m diameter and 22 m length, weighing 160 tons with a thrust level of 450 tons. Pitch and yaw control of the PSLV during the thrust phase of the solid motor is achieved by injection of an aqueous solution of strontium perchlorate in the nozzle to constitute Secondary Injection Thrust Vector Control System (SITVC). The shuttle's reusable solid rocket motor is the largest solid rocket motor ever flown, the only one rated for human flight and the first designed for reuse. Each shuttle launch requires the boost of two reusable solid rocket motors to lift the 4.5-million-pound shuttle vehicle.During space shuttle flights, solid rocket motors provide 80 percent of the thrust during the first two minutes of flight. Each motor generates an average thrust of 2.6 million pounds and is just over 126 feet long and 12 feet in diameter. It is the primary component of the shuttle's twin solid rocket boosters.During a shuttle launch, the solid propellant rockets take the shuttle to an altitude of 28 miles at a speed of 3,094 mph before they separate and fall into the ocean to be retrieved, then refurbished and prepared for another flight. The Ariane 5 launcher is fitted with two P230 boosters, each 30 m high and containing 237 t of propellant grain in three segments. Each booster has a thrust of 540 t, equal to that of the most powerful Ariane 4 (the Ariane 44l version), which makes it the most powerful booster ever built in Europe.It also uses a new binder For space shuttle solid rocket boosters or SRB sthe propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, ,16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star- shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure. Surface area is the amount of propellant exposed to interior combustion flames, existing in a direct relationship with thrust. An increase in surface area will increase thrust but will reduce burn-time since the propellant is being consumed at an accelerated rate. The optimal thrust is typically a constant one, which can be achieved by maintaining a constant surface area throughout the burn. Examples of constant surface area grain designs include: end burning, internal-core and outer-core burning, and internal star core burning (figure 3). Various shapes are used for the optimization of grain-thrust relationships since some rockets may require an initially high thrust component for takeoff while a lower thrust will suffice its post-launch regressive thrust requirements. Such a compromise has implications as seen, but it optimizes overall rocket performance. Complicated grain core patterns, in controlling the exposed surface area of the rocket's fuel, often have parts coated with a non-flammable plastic (such as cellulose acetate). This coat prevents internal combustion flames from igniting that portion of fuel, ignited only later when the burn reaches the fuel directly. Specific Impulse, the thrust per unit propellant burned each second, measures rocket performance and more specifically, internal thrust production a product of pressure and heat. Thrust in chemical rockets (inclusive of both solid and liquid fueled rockets, is a product of the hot and expanding gasses created in the combustion of an explosive fuel (a reduction-oxidation reaction). The degree of the fuel's explosive power coupled with the rate of combustion is the specific impulse. In designing the rocket's propellant grain specific impulse must be taken into account since it can be the difference betwixt a conflagration of failure (explosion), and a successfully optimized thrust producing rocket. If a propellant with a high specific impulse is used as the fuel for a rocket whose grain design offers a high surface area ratio, high amounts of thrust will ensue ignition. And if the engine grain casing cannot withstand the extreme pressure and temperature it will rupture and explode. Thus, the function involving the variables of both specific impulse and surface area must be considered in grain design. The departure from the use of gunpowder to more powerful fuels (higher specific impulses) marks the development of modern solid fueled rockets. Once the chemistry behind rocket fuels (fuels provide their own "air" to burn) was discovered, scientists sought the evermore-powerful fuel, constantly approaching new limits. A composite propellant is a mechanically mixed combination of the oxidizer and the fuel. Some common solid oxidizers are: ammonium perchlorate (NH4-ClO4) and ammonium nitrate (NH4-KNO3), chemicals providing far more oxygen than potassium nitrate (KNO3), the oxidizing agent in gunpowder. These oxidizers are often mixed, in making composite propellants, with synthetic rubbers such as: polystyrenes, polysulfides, and polyurethanes. Another type of propellant is homogeneous where the oxidizer and the fuel are combined as one molecule. Propellants of this type often use a double-base (combination of two propellants) of nitrocellulose and nitroglycerin (C3H5(ONO2)3). Advantages/Disadvantages: Solid fueled rockets are relatively simple rockets. This is their chief advantage, but it also has its drawbacks. Once a solid rocket is ignited it will consume the entirety of its fuel, without any option for shutoff or thrust adjustment. Another key disadvantage is the danger involved in the premixed fuels of monopropellant rockets. In the double-base homogeneous nitrocellulose-nitroglycerin propellant, for example the nitroglycerin is too unstable (sufficient shock will detonate) to use individually add thus a more stable propellant like nitrocellulose (a form of gunpowder) is added. Composite engines, having the fuel and oxidizer as separately mixed elements, are less sensitive to shock and therefore safer to use. A relatively low specific impulse limits the use of solid rockets when large amounts of thrust precondition. The Saturn V moon rocket used nearly 8 million pounds of thrust that would not have been feasible with the use of solid propellant, requiring a high specific impulse liquid propellant. The ease of storage of solid propellant rockets is another main advantage employing a high level use in the military. Some of these rockets are small missiles such as Honest John and Nike Hercules; others are large ballistic missiles such as Polaris, Sergeant, and Vanguard. Liquid propellants may offer better performance, but the difficulties in propellant storage and handling of liquids near absolute zero (0 degrees Kelvin) has limited their use unable to meet the stringent demands the military requires of its firepower.

Reasons of Agni Failure

2007-04-26T18:23:00.000+05:30

After the failed first test of Agni-III, about nine months earlier, there had been a test flight of Agni-III, the first ever. However, the maiden flight encountered problems a minute after lift off and was not entirely successful in meeting the mission objectives. From whatever data was available, we were able to determine the reason for failure. It turns out from the CFD simulation that I have carried out during thepost flight analysis that during the initial ascent phase of themissile, the external freestream air interacts with the hot rocket exhaust of the first stage rocket motor and creates a pocket of recirculating hot gases just behind the vehicle. Agni-III uses a flexible nozzle for thrust vector control. In order to allow the nozzle to deflect, an annular gap was left between the nozzle and the

cylindrical shell of the missile instead of closing it entirely.
It was through this gap that the recirculating hot gases entered the portion of the missile (called base shroud) and caused damage to the electronic components present there.

It took us nearly two months of extensive analysis, CFD simulation from my side and simulation of the flight electronics by other teams, to establish this phenomenon as the cause of failure. It took another six months to design a flexible barrier which would protect the base shroud from the hot gases while simultaneously allowing the nozzle to flex. Again from several CFD simulations, I was able to provide the thermal and pressure loads which the thermal barrier would experience and thus should be designed to withstand. Another month for launch preperations and we were ready for launch on 12th April.

The four minutes on 12th April, were undoubtedly among the most exciting moments of my career at DRDO. Four minutes during which our entire efforts are put to test. Four minutes because it corresponds to the powered phase of the flight during which all flight events including separation of the expended stages take place. Beyond that, good old gravity takes over Smile We were in a building about 1km from the launch pad from where we could feel the noise and vibration from a thrust of hundred tons. The flight video and the trajectory were projected in real time. The sky was fabulously clear and visibility was great. The cameras were able to track the missile more than a hundred kilometres. It was even possible to make out in the flight video when the first stage separation occurred.

Thirteen minutes from lift-off and it was all over. The round of applause that followed would probably have drowned the noise during lift-off Smile It was one of the most thrilling and professionally satisfying moments of my life and one which I would cherish for a long time to come.

Basics of Rocket and missiles

2007-04-26T17:52:00.000+05:30

n order for a satellite to go into orbit it must accomplish two major tasks. First, the satellite must rise above the atmosphere which surrounds the Earth's surface. The atmosphere contains enough particles which slow the spacecraft preventing it from orbiting the planet. A propulsion device must strain against gravity to rise above the atmosphere. Second, the satellite must also be provided with enough horizontal velocity above the atmosphere to at least equal the local circular speed upon orbital injection otherwise it will reenter the atmosphere and burn due to friction. Both of these jobs are done by rockets.

A simple rocket is usually a tall cylinder containing propellant. Propellant always contains two items: fuel and oxidizer. Fuel is the item which b urns to provide rocket thrust. In a simple liquid rocket it is stored in its own separate fuel supply tank. To support fuel combustion the rocket also contains a source of oxygen needed after the spacecraft passes above the atmosphere and cannot collect oxygen in any form. This oxygen is in the form of an oxidizer to aid in combustion; it is stored in a container which resembles the fuel supply tank.

The contents from fuel tank and the oxidizer tank flow from their respective tanks via plumbing; valves, pipes and pumps; into an area called the combustion chamber where the oxidizer joins with the fuel to burn. This combustion causes pressure to build up within the chamber's walls; the resultant pressure, called exhaust gas, is forced through a bell-shaped nozzle at the rocket's base. The nozzle is tapered in the middle in an area called the throat to allow the exhaust gas to build up even more pressure and to increase its flow rate out into the wider portion of the nozzle. The gas goes into the wide nozzle portion very fast and produces a force called thrust.

If the thrust is greater than the rocket's weight, the craft will lift off. This principle is called the thrust-to-weight ratio which must be greater than one or the vehicle will not lift off its pad. This thrust not only overcomes the payload's mass, but also its gravitational attraction to Earth. Any additional thrust above the thrust-to-weight ratio of one causes the rocket to accelerate. The greater excess thrust means greater rocket acceleration in a unit known as g's or numbers of times the norm al acceleration due to gravity at 9.8 meters/second2. In other words, one g equals 9.8 m/s2 , two g's equal an acceleration of 19.6 m/s2, three g's equal 29.4 m/s2, etc. Weight is also measured in g's because a natural part of weight is the acceleration due to gravity. Therefore at one g a 100 pound woman weighs 100 pounds; at 2 g's she weighs 200 pounds; 3 g's she weighs 300 pounds; etc. As more fuel and oxidizer are used the rocket's weight (mass) decreases and its thrust to weight ratio increases. To maintain the same thrust with a mass reduction, the spacecraft's acceleration must increase in order to obey Newton's second law, F = ma.

The first rockets needed wings to guide or steer them into space. Engineers soon found that these wings could be ripped off the rocket's body as it approached the sound barrier. The wings were then downsized into small little winglets called guide vanes . These guide vanes steered the rocket on to its appropriate trajectory to gain altitude and to increase the vehicle's horizontal velocity. As rockets matured, the engineers found that steering could be accomplished more efficiently by including small jets instead of vanes around the base of the craft. The original Atlas rocket employed this capability. As the engineers grew cleverer they found that the same steering could be done by moving the spacecraft engines and diverting the thrust into a different direction. This was called gimbaling and is used exclusively to launch today's modern rockets.

At the top of the rocket is its business end, a hollow cone containing the spacecraft's payload. The upper stage is shaped like a cone to minimize the rocket's cross section which has to penetrate the atmosphere. This reduces the amount of energy required to push the rocket through the atmosphere into space. The nose cone protects the payload against aerodynamic wind blast which is very prevalent when a vehicle speeds through the Earth's atmosphere.

As previously stated, rocket fuel is also called propellant. Propellant includes not only a fuel which is actually burned, but also an oxidizer which supplies oxygen for the combustion process. Propellant efficiencies are measured by a term called specific impulse, Isp. This measurement determines how much thrust a propellant produces; it is a similar gauge rating such as octane is for gasoline. Isp is measured in seconds; it is the amount of time one pound of propellant produces one pound of thrust. There are two classes of propellant mixtures: liquid and solid.

Liquid propellants develop the most efficient thrusts for rocket power. There are many liquid propellant combinations which are used for rocket flight such as kerosene/liquid oxygen (LOX), alcohol/LOX, gasoline/LOX, and liquid hydrogen (LH2)/LOX. Such a propellant with the highest Isp is LH2/LOX with a 400 second rating for operation in the atmosphere and a 453 second rating in a vacuum. Another liquid fuel with a medium efficiency is Aerozine 50 (kerosene) with Nitro Tetroxide (N2O4) for an oxidizer. This propellant has an Isp of 254 seconds at sea level and a 302 second rating in a vacuum. These highly efficient fuels and oxidizers still need an ignition spark to start the combustion process.

A special liquid propellant which does not need ignition to start combustion is a fuel which ignites spontaneously when it comes into contact with its oxidizer. This type of fuel is called hypergolic. A typical hypergolic propellant combination is hydrazine (N2H4) and nitrogen tetroxide (N2O4). Most spacecraft use a monopropellant for operation; the most popular of these fuels is hydrazine which is easily stored and used on orbit for many years.

There are two important advantages of using a liquid fuel. The first is the capability to throttle the thrust. A liquid engine can start, stop, restart, or be reduced in thrust as the rocket flies. The second big advantage of liquid fuel is its increased efficiency, Isp. LH2 has a typical Isp of about 496 seconds whereas the solid fuel has a typical Isp of about 250 seconds.

The disadvantages of using liquid fuel are included in three areas. The first is the cryogenic nature of the fuel meaning it is difficult to store because of the required cold temperatures. A second disadvantage is the handling of this fuel by workers and the special precautions such as wearing heavy, insulated gloves and eye protection must be taken. A third disadvantage is that liquid fuel rocket engines are extremely complex with many moving parts including pumps, valves, lines, and chambers. Every one of these parts must work perfectly or the engine fails.

Solid propellant consists of a flammable putty or rubber which contains both the fuel and the oxidizer within this mixture. For example, the mixture in the space shuttle's solid rocket booster is a typical solid rocket fuel. The ingredients include 16% atomized aluminum powder as the fuel, 70% ammonium per chlorate as the oxidizer, a 12% polybutadiene acrylic acid acrylonitril as a binding agent, 2% epoxy for curing and extremely small traces of iron oxide to control the burn rates during flight.

The solid rocket fuel fills the inside of the rocket from its top to the bottom. In the middle of the rocket is a shaped clearing that provides combustion area and allows the mixture to burn evenly. This shape may be a circle or a star depending upon the type of thrust desired for launch. The rocket's ignition commences by shooting flames down its entire length to initiate the combustion evenly throughout the grain, another term for solid rocket propellant. As the fuel burns and its waste parts are ejected out the nozzle the propellant area grows. As the propellant area grows there is more propellant to burn which means that the rocket's thrust increases.

The advantages of the solid rocket boosters include easy handling. Once the propellant is manufactured and shipped, the technicians need no extensive protective clothing or procedures. The solid rocket fuel can be stored indefinitely in its solid state with only random inspections to insure that its seals are still functioning. In a solid rocket motor there are no moving parts which can fail just by mechanical movement. Despite of the numerous advantages of solid rocket propellant there are a number of disadvantages as well.

The biggest disadvantage of a solid rocket booster is that once it starts it is not going to be stopped. Therefore, it has no control functions such as throttles to control the burn. If thrust is to be either reduced or increased it must be done in the design of the grain. For example, the space shuttle SRBs are designed so that the burn reduces during transonic region passage also known as the maximum dynamic pressure. A second disadvantage is the low Isp rating. Solid propellant is just not efficient because it burns so quickly. A third disadvantage is that the rocket emits solid particles not only polluting the atmosphere, but once the vehicle gets into space, these particles also become solid debris, a hazard for satellites and other launchers.

Which type of rocket propellant does one choose? It depends on the mission and the type of energy required for it. If the engineer needs fast and responsive energy, a solid would probably work best, but if the scientist needs a slow, but steady launch capability then perhaps a liquid would be better. The choice depends upon the mission.

Missile Grid Fins

2007-04-20T16:57:00.000+05:30

Grid fins, sometimes also called lattice fins, are a relatively recent development in guided missile technology. Most guided weapons use planar control fins that are mounted parallel to the body axis. These fins rotate back and forth to generate forces in the horizontal and/or vertical planes that cause the vehicle to yaw right and left, pitch up and down, or roll as the missile maneuvers towards a target. These moving control fins are located forward of the weapon's center of gravity (CG) in a canard control layout, at the CG in a wing control format, or aft of the CG in a tail control design.

Control fin options for guided weapons

These control options can also be subdivided further. Tail control fins, for example, include three different types that have been used to date. The first and most popular is the all-moving fin described above. Other missiles instead have fixed tail fins with movable flaps on their trailing edges. Examples of weapons employing this type of tail control find include the Hellfire missile as well as the Brimstone and Joint Common Missile based on the same basic design.

Diagram of the Brimstone missile showing its tail control flaps

A third form of tail control is the grid fin. A grid fin looks much like a rectangular box filled with a lattice structure similar to a waffle iron or garden trellis. The grid is formed by small intersecting planar surfaces that create individual cells shaped like cubes or triangles. This box structure is inherently strong allowing the lattice walls to be very thin, reducing weight and the cost of materials. Unlike conventional fins that are aligned parallel to the direction of the airflow, a grid fin is arranged perpendicular to the flow allowing the oncoming air to pass through the lattice grid cells. An example of a generic grid fin concept is illustrated below.

Example of grid fins on a missile body and a closeup of the lattice structure

Grid fins have appeared on a number of Soviet missile designs since the 1970s, particularly ballistic missiles like the SS-12 'Scaleboard,' SS-20 'Saber,' SS-21 'Scarab,' SS-23 'Spider,' and SS-25 'Sickle.' These fins have also been used on Russian spacecraft including the N1 lunar rocket and the Soyuz TM-22 capsule where they were used as emergency drag brakes. Perhaps the most recognized use of grid fins to date is on the Russian AA-12 'Adder' medium-range air-to-air missile.

All of these applications have been on vehicles traveling at high supersonic speeds. It is within this speed regime that grid fins are most attractive. The problem with planar fins is that they are relatively long surfaces. At high speeds, it becomes considerably harder to turn them because of the forces created by the mass of air rushing by. Imagine trying to open a door on a calm day versus trying to open it while a strong wind blows against it.

Missile fins are turned using servo mechanisms that rotate the fin to a new deflection angle. The sizes of these servos are dictated by the amount of torque it takes to rotate the fin about its axis. The large size of conventional planar fins causes their center of pressure to move over a relatively large region. This motion results in large hinge moments, which is a way of measuring of how much torque it takes to rotate the fin to a new position.

Grid fins on the Russian R-77 (AA-12) air-to-air missile

The primary advantage of grid fins is that they are so much shorter than conventional planar fins. As a result, they generate much smaller hinge moments and require considerably smaller servos to deflect them in a high-speed flow. The small chord length of grid fins also makes them less likely to stall at high angles of attack. This resistance to stall increases the control effectiveness of grid fins compared to conventional planar fins.

Another important aerodynamic characteristic of grid fins concerns drag, although it can be an advantage or a disadvantage depending on the speed of the airflow. In general, the thin shape of the lattice walls creates very little disturbance in the flow of air passing through, so drag is often no higher than a conventional fin. At low subsonic speeds, grid fins perform comparably to a planar fin. Both the drag and control effectiveness of the lattice fin are about the same as a conventional fin in this speed regime.

The same behavior does not hold true at high subsonic numbers near Mach 1. Drag rises considerably higher and the fins become much less effective in this transonic region because of the formation of shock waves. Even though the weapon may be flying below Mach 1, the airflow through the cells of the fin can be accelerated to supersonic speeds creating normal shock waves within the grid. This behavior is often referred to as a choked flow that blocks incoming air from passing through and increases drag. The drag grows even worse if the weapon flies slightly faster than Mach 1 since a detached normal shock wave, also called a bow shock, will form in front of the fin. This bow shock worsens the effect of the choked flow by causing additional air to spill around the fin further increasing drag.

Flow through a grid fin at transonic and supersonic speeds

This situation improves as the weapon accelerates beyond the transonic region of Mach 0.8 to 1.3. At higher speeds, the normal shock is "swallowed" and shock waves are instead formed off the leading edges of the lattice at an oblique angle. The angle is still fairly large at low Mach numbers causing the shock waves to reflect off the downstream lattice structure. These reflections can create a large number of shock waves within the grid fin resulting in high drag. The higher the Mach number becomes, however, the smaller the oblique shock angle becomes until the shock passes through the structure without intersecting it. This behavior causes drag to drop while control effectiveness increases considerably compared to a planar fin.

Compuational predictions showing the air flow through a grid fin at high speed and angle of attack

Because of their shape and small chord length, a further advantage of grid fins is that they can easily be folded down against the missile body. This folding makes the weapon more compact and easier to store or transport. This advantage is particularly attractive for weapons designed for internal weapons bays on stealth aircraft like the F-22 and F-35. Compact packaging is also important for submunitions dispersed by high-speed cruise missiles, and grid fins have been proposed as a means of controlling weapons ejected from supersonic munitions dispensers while also decelerating them to subsonic speeds.

Folding grid fins used on the R-77 (AA-12) missile

It is this packing efficiency that helps explain why grid fins have been implemented on the enormous Massive Ordnance Air Blast (MOAB) bomb developed by the United States. This gigantic weapon, weighing over 20,000 lb (9,070 kg), is carried into combat aboard the C-130 Hercules. The bomb is fitted with four large grid fins that fold down against the bomb body. The weapon is launched by being pushed out of the C-130's cargo bay aboard a sled equipped with a stabilizing parachute. Once free of its sled, the four grid fins are deployed to steer the weapon as it maneuvers to a pre-programmed target using GPS signals for guidance. Grid fins have also been proposed for smaller bombs like JDAM and Small Diameter Bomb. Given the slow speeds these weapons fly at, the primary advantage of lattice fins would be for packaging rather than lower drag or increased control effectiveness.

Massive Ordnance Air Blast (MOAB) bomb equipped with grid fins

Nevertheless, an important disadvantage of grid fins that may limit their applications to future weapons is radar cross section. The cube-shaped cells formed by the lattice structure tend to create very strong radar reflections that would be very undesirable in any weapon designed for low observability. This limitation might not have any impact on short-range weapons like bombs but could be a significant concern for applications like long-range cruise missiles.

Despite these drawbacks, the improved maneuverability grid fins offer at high supersonic speeds and high angles of attack have resulted in several grid fin research efforts in recent years. Western nations like the US and Germany performed a great deal of experimental work on lattice fin concepts during the 1980s. More recent efforts have focused on using computational fluid dynamics (CFD) to better understand the characteristics of the flow passing through the fins in order to better optimize their performance. Thanks to this research and the advantages of the lattice design, grid fins could likely become a more common feature on guided weapons of the future.

More about grid fins and other missile design topics in the book Tactical Missile Design by Eugene Fleeman.

Missile Guidance part1

2007-04-20T16:18:00.000+05:30

Many questioners often seem "concerned" about how missiles are able to seek out and accurately navigate their way to the correct target without assistance from a human operator. However, there is no need for alarm because a weapon very rarely makes a mistake unless it is misprogrammed by the human who launches it in the first place. In fact, many of the methods used for missile guidance are the same as those used to navigate manned planes like the commercial airliners you and I fly aboard.

The first topic we need to address is establishing some basic definitions. Missiles can be categorized in many different ways, such as by their mission or lauch platform. The two conventions we will follow in this article concern the missile's type of guidance and type of sensor or seeker. These two concepts are often used interchangeably, but it is important to understand their differences.

Concept of missile guidance

Missile guidance concerns the method by which the missile receives its commands to move along a certain path to reach a target. On some missiles, these commands are generated internally by the missile computer autopilot. On others, the commands are transmitted to the missile by some external source. The missile sensor or seeker, on the other hand, is a component within a missile that generates data fed into the missile computer. This data is processed by the computer and used to generate guidance commands. Sensor types commonly used today include infrared, radar, and the global positioning system. Based on the relative position between the missile and the target at any given point in flight, the computer autopilot sends commands to the control surfaces to adjust the missile's course.

Phases of missile guidance

In many missiles, the guidance system is divided into three phases, as pictured above. The first is a launch or boost phase in which the guidance system is usually disabled to allow the missile to safely travel away from the launch platform. The majority of the flight is flown using midcourse guidance, during which the missile makes slight adjustments to its trajectory allowing it to reach the vicinity of the target. The final phase is terminal guidance when the missile uses a highly accurate tracking system to make rapid maneuvers for intercepting the target. Many missiles use a different type of guidance in the midcourse phase than in the terminal phase, as will be discussed later.

The primary forms of missile guidance are described below with examples of missiles and seekers used to accomplish that type of guidance.

Beam Rider Guidance

The beam rider concept relies on an external ground- or ship-based radar station that transmits a beam of radar energy towards the target. The surface radar tracks the target and also transmits a guidance beam that adjusts its angle as the target moves across the sky.

Beam rider guidance

The missile is launched into this guidance beam and uses it for direction. Scanning systems onboard the missile detect the presence of the beam and determine how close the missile is to the edges of it. This information is used to send command signals to control surfaces to keep the missile within the beam. In this way, the missile "rides" the external radar beam to the target.

Beam riding was often used on early surface-to-air missiles but was found to become inaccurate at long ranges. Limited improvement was possible using two different surface-based radar beams, but the beam rider method has been largely abandoned. The technique was used on the US Navy's Terrier ship-launched surface-to-air missile of the 1950s.

Command Guidance

Command guidance is similar to beam riding in that the target is tracked by an external radar. However, a second radar also tracks the missile itself. The tracking data from both radars are fed into a ground based computer that calculates the paths of the two vehicles.

Command guidance

This computer also determines what commands need to be sent to the missile control surfaces to steer the missile on an intercept course with the target. These commands are transmitted to a receiver on the missile allowing the missile to adjust its course. An example of command guidance is the Russian SA-2 surface-to-air missile used against US aircraft in North Vietnam.

Also note that command guidance is not limited just to radar. Another method that falls under command guidance is the use of wire guided systems. In this technique, commands are sent to the missile through a conventional wire or fiber optic cable that reels out from the missile back to its launcher. Wire guidance is often used on anti-tank missiles like TOW, which can be launched from both ground vehicles and helicopters. Many naval torpedoes fired from submarines also use wire guidance.

HOMING GUIDANCE

Homing guidance is the most common form of guidance used in anti-air missiles today. Three primary forms of guidance fall under the homing guidance umbrella--semi active, active, and passive. We will discuss each of these in turn, as well as a more unusual form called retransmission or track-via-missile homing.

Homing guidance

Semi-Active Homing Guidance

A semi-active system is similar to command guidance since the missile relies on an external source to illuminate the target. The energy reflected by this target is intercepted by a receiver on the missile. The difference between command guidance and semi-active homing is that the missile has an onboard computer in this case. The computer uses the energy collected by its radar receiver to determine the target's relative trajectory and send correcting commands to control surfaces so that the missile will intercept the target.

Semi-active homing guidance

The example shown above illustrates the guidance method used on an air-to-air missile like Sparrow. This missile relies on radar energy transmitted by the launch aircraft to track and home in on the target. This system is also sometimes referred to as bistatic meaning that the radar waves that intercept the target and those reflected back to the missile are at different angles to one another.

However, it should be noted that semi-active guidance is used by other types of seekers besides radar. Laser-guided weapons like the Paveway series can also be considered semi-active weapons because the laser energy these bombs track as they steer towards a target is supplied by an external source. The source could be a laser designation pod on the launch aircraft, on a second aircraft, or aimed by a soldier on the ground.

Active Homing Guidance

Active homing works just like semi-active except that the tracking energy is now both transmitted by and received by the missile itself. No external source is needed. It is for this reason that active homing missiles are often called "fire-and-forget" because the launch aircraft does not need to continue illuminating the target after the missile is launched.

Active homing guidance

Active homing missiles typically use radar seekers to track their target. These seekers are also sometimes called monostatic because, unlike semi-active guidance, the transmitted and reflected waves are at the same angle with respect to the line of sight between the missile and target. Examples of active homing missiles include the AMRAAM air-to-air and Exocet anti-ship missiles.

Passive Homing Guidance

A passive homing system is like active in that the missile is independent of any external guidance system and like semi-active in that it only receives signals and cannot transmit. Passive missiles instead rely on some form of energy that is transmitted by the target and can be tracked by the missile seeker.

Passive homing guidance

This energy could take many forms. For example, infrared seekers like those used on Sidewinder home in on the heat signature generated by a target. Anti-radiation missiles like HARM track the radio frequency energy transmitted by ground-based radar stations. Passive torpedoes use sonar, or sound waves, created by the engines of ships to attack their targets. Electro-optic sensors like those used on Maverick rely on visual images to guide towards a target.

Retransmission Homing Guidance

A more unusual example of homing guidance is the retransmission method. This technique is largely similar to command guidance but with a unique twist. The target is tracked via an external radar, but the reflected signal is intercepted by a receiver onboard the missile, as in semi-active homing. However, the missile has no onboard computer to process these signals. The signals are instead transmitted back to the launch platform for processing. The subsequent commands are then retransmitted back to the missile so that it can deflect control surfaces to adjust its trajectory.

Retransmission homing guidance

This method is also sometimes called "track via missile" (TVM) since the missile acts as a conduit of tracking information from the target back to the ground control station. The advantage of TVM homing is that most of the expensive tracking and processing hardware is located on the ground where it can be reused for future missile launches rather than be destroyed. Unfortunately, the method also requires excellent high-speed communication links between the missile and control station, limiting the system to rather short ranges. Retransmission homing guidance is used on the Patriot surface-to-air missile.

NAVIGATION GUIDANCE

Like homing guidance, navigation guidance includes several subcategories. In this section, we will describe inertial, ranging, celestial, and geophysical navigation techniques.

Inertial Navigation Guidance

Inertial navigation relies on devices onboard the missile that sense its motion and acceleration in different directions. These devices are called gyroscopes and accelerometers.

Mechanical, fiber optic, and ring laser gyroscopes

The purpose of a gyroscope is to measure angular rotation, and a number of different methods to do so have been devised. A classic mechanical gyroscope senses the stability of a mass rotating on gimbals. More recent ring laser gyros and fiber optic gyros are based on the interference between laser beams. Current advances in Micro-Electro-Mechanical Systems (MEMS) offer the potential to develop gyroscopes that are very small and inexpensive.

While gyroscopes measure angular motion, accelerometers measure linear motion. The accelerations from these devices are translated into electrical signals for processing by the missile computer autopilot. When a gyroscope and an accelerometer are combined into a single device along with a control mechanism, it is called an inertial measurement unit (IMU) or inertial navigation system (INS).

Inertial navigation concept

The INS uses these two devices to sense motion relative to a point of origin. Inertial navigation works by telling the missile where it is at the time of launch and how it should move in terms of both distance and rotation over the course of its flight. The missile computer uses signals from the INS to measure these motions and insure that the missile travels along its proper programmed path. Inertial navigation systems are widely used on all kinds of aerospace vehicles, including weapons, military aircraft, commercial airliners, and spacecraft. Many missiles use inertial methods for midcourse guidance, including AMRAAM, Storm Shadow, Meteor, and Tomahawk.

Ranging Navigation Guidance

Unlike inertial navigation, which is contained entirely onboard the vehicle, ranging navigation depends on external signals for guidance. The earliest form of such navigation was the use of radio beacons developed primarily for commercial air service. These beacons transmit radio signals received by an aircraft in flight. Based on the direction and strength of the signals, the plane can calculate its location relative to the beacons and navigate its way through the signals.

Global Positioning System used in ranging navigation guidance

The advent of the global positions system (GPS) has largely replaced radio beacons in both military and civilian use. GPS consists of a constellation of 24 satellites in geosynchronous orbit around the Earth. If a GPS receiver on the surface of the Earth can receive signals from at least four of these satellites, it can calculate an exact three-dimensional position with great accuracy. Missiles like JSOW and the JDAM series of guided bombs make use of GPS signals to determine where they are with respect to the locations of their targets. Over the course of its flight, the weapon uses this information to send commands to control surfaces and adjust its trajectory.

Celestial Navigation Guidance

Celestial navigation is one of the earliest forms of navigation devised by humans. and it saw its greatest application in the voyages of the great maritime explorers like Christopher Columbus. Celestial navigation uses the positions of the stars to determine location, especially latitude, on the surface of the Earth. This form of navigation requires good visibility of the stars, so it is only useful at night or at very high altitude. As a result, celestial navigation is seldom applied to missiles, though it has been used on many ballistic missiles like Poseidon. The missile compares the positions of the stars to an image stored in memory to determine its flight path.

Geophysical Navigation Guidance

Perhaps even older than celestial navigation is geophysical navigation, which relies on measurements of the Earth for navigation information. Methods that fall under this category include the use of compasses and magnetometers to measure the Earth's magnetic field as well as gravitometers to measure the Earth's gravitational field.

While these methods have not found much application in missiles, a more useful technique is terrain matching. This method typically requires a radar altimeter that uses radar waves to determine height above the ground. By comparing the contours of the terrain against data stored aboard the missile, the autopilot can navigate its way to a particular location.

A related but more accurate technique is called digital scene matching. In concept, digital scene matching is little different than looking out the window of your car and using landmarks to navigate your way to a specific location. Missiles make use of this technique by comparing the image seen below the weapon to satellite or aerial photos stored in the missile computer. If the scenes do not match, the computer sends commands to control surfaces to adjust the missile's course until the images agree. Digital scene matching is used on the Tomahwak cruise missile.

Summary

As discussed earlier, a missile may use one particular form of guidance throughout its flight or it may depend on different types of guidance at different times. The following tables compare the midcourse and terminal guidance methods used on a variety of weapons.

Note that many weapons also make use of a combination of methods simultaneously. In particular, a common technique is a combined GPS/INS system that takes advantage of both inertial and ranging guidance to improve accuracy.

Missile Control Systems, part1

2007-04-20T16:14:00.000+05:30

You are correct in pointing out that most missiles do not have conventional rudders, ailerons, or elevators like those used on typical airplanes. Nonetheless, missiles do employ similar aerodynamic control surfaces in order to maneuver the vehicle during flight. To begin our discussion, we need to introduce some terminology used to describe the major components of a missile.

The heart of a missile is the body, equivalent to the fuselage of an aircraft. The missile body contains the guidance and control system, warhead, and propulsion system. Some missiles may consist of only the body alone, but most have additional surfaces to generate lift and provide maneuverability. Depending on what source you look at, these surfaces can go by many names. In particular, many use the generic term "fin" to refer to any aerodynamic surface on a missile. Missile designers, however, are more precise in their naming methodology and generally consider these surfaces to fall into three major categories: canards, wings, and tail fins.

Major components of a missile

The example shown above illustrates a generic missile configuration equipped with all three surfaces. Often times, the terms canard, wing, and fin are used interchangeably, which can get rather confusing. These surfaces behave in fundamentally different ways, however, based upon where they are located with respect to the missile center of gravity. In general, a wing is a relatively large surface that is located near the center of gravity while a canard is a surface near the missile nose and a tail fin is a surface near the aft end of the missile.

Most missiles are equipped with at least one set of aerodynamic surfaces, especially tail fins since these surfaces provide stability in flight. The majority of missiles are also equipped with a second set of surfaces to provide additional lift or improved control. Very few designs are equipped with all three sets of surfaces.

We have discussed how aircraft use control surfaces to turn the plane in different directions in a number of previous questions (see parts of an aircraft, origins of control surfaces, and adverse yaw). Whereas most aircraft have fixed horizontal and vertical tails with smaller movable rudder and elevator surfaces, missiles typically use all-moving surfaces, like those illustrated below, to accomplish the same purpose.

Deflection of a control surface on a missile

In order to turn the missile during flight, at least one set of aerodynamic surfaces is designed to rotate about a center pivot point. In so doing, the angle of attack of the fin is changed so that the lift force acting on it changes. The changes in the direction and magnitude of the forces acting on the missile cause it to move in a different direction and allow the vehicle to maneuver along its path and guide itself towards its intended target. An example of a control surface deflection on an AIM-9M Sidewinder model is illustrated below.

Canard deflections on an AIM-9M Sidewinder

Canards, wings, and tails are often lumped together and referred to as aerodynamic controls. A more recent development in missile maneuvering systems is called unconventional control. Most unconventional control systems involve some form of thrust vector control (TVC) or jet interaction (JI).

We have now introduced four major categories of missile flight control systems--tail control, canard control, wing control, and unconventional control--so let's briefly take a closer look at each type.

Four main categories of missile flight controls

Tail Control:

Tail control is probably the most commonly used form of missile control, particularly for longer range air-to-air missiles like AMRAAM and surface-to-air missiles like Patriot and Roland. The primary reason for this application is because tail control provides excellent maneuverability at the high angles of attack often needed to intercept a highly maneuverable aircraft. Missiles using tail control are also often fitted with a non-movable wing to provide additional lift and improve range. Some good examples of such missiles are air-to-ground weapons like Maverick and AS.30 as well as surface-to-surface missiles like Harpoon and Exocet. Tail control missiles rarely have canards, although one such example is AIM-9X Sidewinder. A selection of 23 representative missiles using tail control is pictured below.

Missiles with tail control

In addition to missiles, some bombs also use tail control. An example is the JDAM series of GPS-guided bombs.

Canard Control:

Canard control is also quite commonly used, especially on short-range air-to-air missiles like AIM-9M Sidewinder. The primary advantage of canard control is better maneuverability at low angles of attack, but canards tend to become ineffective at high angles of attack because of flow separation that causes the surfaces to stall. Since canards are ahead of the center of gravity, they cause a destabilizing effect and require large fixed tails to keep the missile stable. These two sets of fins usually provide sufficient lift to make wings unnecessary. Shown below are twelve examples of canard control missiles.

Missiles with canard control

A further subset of canard control missiles is the split canard. Split canards are a relatively new development that has found application on the latest generation of short-range air-to-air missiles like Python 4 and the Russian AA-11. The term split canard refers to the fact that the missile has two sets of canards in close proximity, usually one immediately behind the other. The first canard is fixed while the second set is movable. The advantage of this arrangement is that the first set of canards generates strong, energetic vortices that increase the speed of the airflow over the second set of canards making them more effective. In addition, the vortices delay flow separation and allow the canards to reach higher angles of attack before stalling. This high angle of attack performance gives the missile much greater maneuverability compared to a missile with single canard control. Six examples of split canard missiles are shown below.

Missiles with split canard control

Many smart bombs also use canard control systems. Most notable of these are laser guided bombs such as the Paveway series.

Wing Control:

Wing control was one of the earliest forms of missile control developed, but it is becoming less commonly used on today's designs. Most missiles using wing control are longer-range missiles like Sparrow, Sea Skua, and HARM. The primary advantage of wing control is that the deflections of the wings produce a very fast response with little motion of the body. This feature results in small seeker tracking error and allows the missile to remain locked on target even during large maneuvers. The major disadvantage is that the wings must usually be quite large in order to generate both sufficient lift and control effectiveness, which makes the missiles rather large overall. In addition, the wings generate strong vortices that may adversely interact with the tails causing the missile to roll. This behavior is known as induced roll, and if the effect is strong enough, the control system may not be able to compensate. A few examples of wing control missiles are shown below.

Missiles with wing control

Unconventional Control:

Unconventional control systems is a broad category that includes a number of advanced technologies. Most techniques involve some kind of thrust vectoring. Thrust vectoring is defined as a method of deflecting the missile exhaust to generate a component of thrust in a vertical and/or horizontal direction. This additional force points the nose in a new direction causing the missile to turn. Another technique that is just starting to be introduced is called reaction jets. Reaction jets are usually small ports in the surface of a missile that create a jet exhaust perpendicular to the vehicle surface and produce an effect similar to thrust vectoring.

Unconventional control technologies

These techniques are most often applied to high off-boresight air-to-air missiles like AIM-9X Sidewinder and IRIS-T to provide exceptional maneuverability. The greatest advantage of such controls is that they can function at very low speeds or in a vacuum where there is little or no airflow to act on conventional fins. The primary drawback, however, is that they will not function once the fuel supply is exhausted.

Missiles with unconventional controls

Examples of missiles employing unconventional controls are shown above. Note that most missiles equipped with unconventional controls do not rely on these controls alone for maneuverability, but only as a supplement to aerodynamic surfaces like canards and tail fins.

Ballistic missile

2007-04-18T18:28:00.000+05:30

A ballistic missile works by burning propellant and ejecting the hot gases through a nozzle, typically at a velocity of around 2500 meters per second (m/sec). The thrust from the exhaust causes the missile to accelerate. A given thrust will cause progressively higher accelerations as the missile lightens due to the consumption of its propellant. All of the propellant is consumed in the first few minutes of flight, following which the missile coasts above the atmosphere at a speed of several kilometers per second to its target. In an idealized case, the burnout velocity would be equal to the exhaust velocity times the natural logarithm of the ratio of gross missile weight to the payload. In real life, the missile will need additional impulse to reach a given velocity. Account must be taken of: the structural weight of the missile (typically discarded in several stages during boost phase); air resistance during boost; and gravity during boost. Still, the idealized relationship is useful: it provides an optimistic upper bound on what can be achieved when parameters are varied. At short ranges, the range of the missile will go as the square of its burnout velocity. Due to the curvature of the earth, at longer ranges the range will increase more rapidly. Table 2 shows the burnout velocity needed to reach various ranges, together with the payload fractions associated with missiles that attain any given velocity.

For example, a 6000-km range missile would need a 6200 m/sec burnout velocity and could achieve this while devoting 2-5% of its gross weight to payload; a 10,000-km range missile would need 7200 m/sec and could devote 1.3-3.5% of its weight to payload, about two-thirds as much. Thus, the table can be used to scale payload fractions as the missile range is varied. The scaling indicated in Table 2 is probably a bit optimistic from the missile designer's perspective. The original missile design is optimized to produce the best possible distribution of propellant and structural weight among the stages. Adding a new upper stage (adapted from a different missile) or offloading payload will not necessarily yield an optimal mix. Thus, the payload penalty for increasing the range could be greater than the table indicates.

The concept of payload merits discussion because definitions can vary widely. Consider the weight remaining when the missile reaches burnout velocity. It includes the empty weight of the burned-out final stage. If this is deducted, what remains is the throw-weight or payload (shown in Table 2 as a fraction of gross weight). This includes the MIRV bus (if any), the decoys (if any), the guidance system, and one or more re-entry vehicles (RVs). Typically, a single RV will account for two-thirds or more of the payload; but because of the need for a bus, multiple RVs will add up to only about half the payload. The RV consists of a nuclear warhead, a fuze, and a heat shield. The heat shield may account for about one-third of this weight. Thus, less than half the payload will commonly be available for the weight of a nuclear warhead.

Table 2. Burnout Velocity and Payload Fraction vs. Missile Range

Range (km)	Burnout Velocity (m/sec)	Payload Gross Weight
300	1700	.150 - .200
600	2400	.080 - .120
1000	3000	.070 - .110
1500	3600	.055 - .095
2000	4100	.040 - .080
3000	4800	.030 - .065
4000	5400	.025 - .060
6000	6200	.020 - .050
8000	6800	.016 - .042
10,000	7200	.013 - .035
12,000	7400	.011 - .030

The first US nuclear warheads weighed 4100-4500 kg. The likely weight of a first warhead produced by a proliferating country has been variously estimated at 450-1000 kg. The lower estimate was for the first effort of an advanced, industrialized country and the higher estimate for a third world country. The United States and the Soviet Union each needed six to eight years to reduce their warhead weights to 1000 kg. Existing North Korean ballistic missiles could carry a 500 kg nuclear warhead. ICBM-range derivatives of these missiles could carry only 200-300 kg. Thus, even assuming that North Korea's first generation nuclear warhead is at the low end of the estimated range (450 kg), an ICBM derivative of the existing missiles could not lift the warhead.

A guidance system is needed to hit a predictable target. This functions only during the boost phase, correcting the flight path to adjust for various deviations. After burnout, the missile is unguided and any further deviations from course, such as those caused by winds during the re-entry phase, are not corrected by the guidance system. The ballistic missiles now in use by North Korea, Iraq, and Iran achieve CEPs (Circular Error, Probable, the radius of a circle within which one-half of the warheads can be expected to fall) of several kilometers. This corresponds to velocity errors on the order of 0.1%, which in turn could lead to CEPs as great as 40 kilometers at ICBM range. With such accuracy, a missile aimed at Los Angeles would run a significant risk of missing the entire metropolitan area. This would seem to preclude any very near-term threat. However, on the extended timetables suggested above (2010 for North Korea, 2015 for Iran and Iraq), guidance should not be a problem. There will be time to develop a new system. If adapted to an ICBM, strapdown systems coming into use in civil aviation could yield CEPs of a few kilometers at full range. Global Positioning System updates (even on the clear channel) could reduce guidance system errors to a level that is small compared to the re-entry error discussed below. Thus, while North Korea, Iraq, and Iran are not remotely prepared for ICBM guidance now, they should not be expected to have difficulty hitting large cities at ICBM range after 2010.

A final problem is re-entry. As the RV re-enters the atmosphere at a velocity of more than 7000 m/sec, it encounters tremendous drag and slows down, eventually striking the ground at somewhere between 200-3000 m/sec. While slowing down, the RV generates tremendous heat that must be removed or else the RV will burn up. The ICBMs that could be developed by North Korea, Iraq, or Iran would use a blunt, high-drag heat sink-essentially a dome of copper armor. As the RV decelerates, the heat sink warms up and transfers most of the heat to the air rushing past. Most of the deceleration occurs at high altitude where the air is thin. By the time the RV reaches the denser lower atmosphere, it is no longer traveling at tremendous speed. The heat-sink approach was used on all of the early US and Soviet ICBMs. It has three drawbacks: it is heavy; its low final velocity gives the wind a longer time to alter the RV's course, degrading its accuracy; and its low final velocity makes it vulnerable to terminal interceptors. One consequence of relying on heat sinks is that no matter how good the guidance system is, the ICBM will be limited to a CEP of at least 500 meters at full range. Shorter range missiles often use pointed, low-drag heat sinks, but this approach is not available at ICBM re-entry velocities: the RV would burn up.

http://www.bharat-rakshak.com/MISSILES/Temp/Images/ImagesA3_family_contrast%20A2_K15_tube_r11A-1500x1400pix.jpg

Smallest aircraft in the World

2007-04-18T18:21:00.000+05:30

We addressed the largest plane in the world in a previous article. The subject of the smallest plane is a bit more complex since one must ask what class of aircraft to consider. For example, the remote controlled planes that you and I can buy in a store are obviously smaller than anything a human could fly aboard. A paper airplane flown by a child is even smaller than that, and many researchers are currently working on Micro Air Vehicles (MAVs) that rival insects in size.
However, we will assume that this question is asking about the smallest manned plane in the world, or the smallest plane flown with a human pilot aboard. Even this category can be confusing since it could possibly include vehicles like hang gliders or ultralights, but we will limit our discussion to more conventional airplane types. This definition leaves us with three primary sets of aircraft designers who have competed against each other to build and fly the "world's smallest plane" since the end of World War II.

The first plane to be so titled was known as the Wee Bee. The diminutive plane was designed, built, and flown by Ken Coward, William Chana, and Karl Montijo in San Diego, California, during the late 1940s. Just over 14 ft (4.25 m) long and with a wingspan of only 18 ft (5.5 m), the Wee Bee was powered by a 30-hp piston engine and could carry a maximum of 200 lb (90 kg), including the pilot and fuel. Weight was kept to a minimum by having the pilot lay in a prone position atop the fuselage.

Several views of the Wee Bee
Only one example of the unusual Wee Bee was built, making its first flight in 1948. The Wee Bee was later placed on display at the San Diego Aerospace Museum but was lost in a fire that swept through the museum in 1978. A replica has now taken the place of the destroyed original. The creators of the Wee Bee also formed a small company called Bee Aviation, or Beecraft, that went on to build a larger version called the Honey Bee. With a wingspan of 28 ft (9.5 m), the Honey Bee could carry a single pilot in a more comfortable and conventional seated position.
Following in the footsteps of the Wee Bee was an even smaller plane called the Stits Junior. Designed by Ray Stits and Martin Youngs, the Junior was rebuilt from the components of a surplus World War II Taylorcraft L-2. The Junior was around 11 ft (3.4 m) in length and had a wingspan that varied between 8.8 and 9.3 ft (2.7 to 2.8 m) over the course of three different modifications. Power was provided by a single piston engine ranging from 36 to 75-hp that gave the Junior a top speed up to 150 mph (240 km/h).

Stits Junior compared to a Taylorcraft L-2
Also competing against Stits to build the world's smallest plane during the 1940s was Wilbur Staib, an engineer living near Los Angeles. Believing he could build a plane even smaller than the Junior, Staib set out to design and build a tiny monoplane called the Little Bit. Only 11 ft (3.4 m) long and with a wingspan of just 7.5 ft (2.3 m), the Little Bit weighed a mere 390 lb (177 kg) empty. Unfortunately, the plane was never successfully flown and was dismantled as a "hazard to the life and limb of Wilbur Staib." The parts of the Little Bit were later reused in a biplane with a 13 ft (4 m) wingspan.
However, Staib's challenge only encouraged Stits and co-builder Robert Starr to build a new plane even smaller than the Junior or the Little Bit. Known as the Sky Baby, this new plane differed from earlier entries into the arena by adopting a biplane design instead of a monoplane. This change allowed the wingspan to be reduced even further to just over 7 ft (2.1 m). With a maximum length under 10 ft (3 m), the Sky Baby weighed just 452 lb (205 kg) empty and 666 lb (302 kg) fully loaded. Its 65-hp piston engine gave the Sky Baby a maximum speed of 185 mph (300 km/h). Pilot Robert Starr flew the Sky Baby at air shows from April to November of 1952, and the plane was donated to the National Air and Space Museum in 1972. The Sky Baby is currently exhibited at the Experimental Aviation Association AirVenture Museum in Oshkosh, Wisconsin.

Stits Sky Baby
The Sky Baby remained unchallenged as the world's smallest plane until the 1980s when its former pilot, Robert Starr of Phoenix, Arizona, built the Bumble Bee. The Bumble Bee biplane was heavier than the Sky Baby at 547 lb (248 kg) empty and 725 lb (329 kg) loaded. However, its dimensions were otherwise smaller with a length under 9.5 ft (2.9 m) and a wingspan of just 6.5 ft (2 m). As a result, the Guinness Book of World Records considered the Bumble Bee to be the smallest plane in the world following its first successful flight on 28 January 1984. Powered by an 85-hp piston engine, the Bumble Bee was capable of 180 mph (290 km/h).

Starr Bumble Bee
Not to be outdone, Ray Stits' son Donald set out to recapture the title by building an even smaller monoplane called the Baby Bird. At 11 ft (3.4 m) in length, the Baby Bird was longer than the Bumble Bee but had a smaller wingspan of 6.25 ft (1.9 m) and weighed less at only 250 lb (115 kg) empty. Its 55-hp piston engine gave the Baby Bird a maximum speed of 110 mph (175 km/h) when it first flew at Camarillo, California, on 4 August 1984. Pilot Harold Nemer completed 35 flights aboard the Baby Bird by the time it was retired in 1989.

Donald Stits posing with the Baby Bird
However, Robert Starr believed he could go even smaller and completed a new Bumble Bee II in 1988. Though still heavier than the Baby Bird with an empty weight of 396 lb (180 kg), the overall dimensions of the Bumble Bee shrank even further to 8.8 ft (2.7 m) in length and a mere 5.5 ft (1.7 m) wingspan. Its 85-hp piston engine made possible a maximum speed of 190 mph (305 km/h).

Starr Bumble Bee II
Unfortunately, the first flights of the Bumble Bee II on 8 May 1988 proved a mixed success. Starr was able to make several passes over the airfield demonstrating the plane could fly and earning his place in the record books, but the attempt nearly cost Starr his life. While at an altitude of about 400 ft (120 m), the Bumble Bee II suffered an engine failure and fell from the sky. The plane was completely destroyed in a crash that left Starr seriously injured, though he later made a full recovery.

Comparison of Several of the World's Smallest Planes
Builder	Plane	Length	Wingspan	Empty Wt	MTOW	Propulsion	Max Speed
Bee Aviation	Wee Bee	14'2"	18'0"	?	?	30-hp Kiekhaefer	82 mph
Stits	Junior	10'10" to 11'4"	8'10" to 9'4"	?	?	75-hp Continental C-75	150 mph
Staib	Little Bit	11'0"	7'6"	390 lb	?	85-hp Continental C-65	?
Stits	Sky Baby	9'10"	7'2"	452 lb	666 lb	65-hp Continental C-65	185 mph
Starr	Bumble Bee	9'4"	6'6"	547 lb	725 lb	85-hp Continental C-85	180 mph
Stits	Baby Bird	11'0"	6'3"	252 lb	?	55-hp Hirth	110 mph
Starr	Bumble Bee II	8'10"	5'6"	396 lb	?	85-hp Continental C-85	190 mph

Perhaps hoping to prevent further life-threatening crashes in this battle of the small, the Guinness Book decided to credit both Stits and Starr with world records. The Stits Baby Bird is considered the world's smallest monoplane while Starr's Bumble Bee II is the world's smallest biplane. Although the Bumble Bee II was tragically lost, the original Bumble Bee is on display at the Pima Air & Space Museum in Tucson, Arizona. Like his father's Sky Baby before it, Donald Stits' Baby Bird is also on display at the EAA AirVenture Museum.
As tiny as these planes were, they were all built of conventional construction materials like aluminum. Recent advances in composite materials that provide the same strength as metals yet weigh far less may make it possible for a future innovator to build an even lighter and smaller plane.

Largest Helicopters in the World

2007-04-18T18:19:00.000+05:30

The article you mention discusses several of the largest planes in the world. It also describes how aircraft are typically ranked by weight rather than by physical dimensions like length or wingspan. The same holds true for helicopters. Although the diameter of the rotor is often a useful measure of a helicopter's size, these vehicles are most often ranked by maximum takeoff weight. As was also true of the largest planes, the ranks of largest helicopters are largely dominated by craft originally built in the Soviet Union. Perhaps that should be no surprise given the vast size of that nation and the need to transport large cargoes to distant and remote locations.

1. Mil V-12 : 231,485 lb (105,000 kg)
The largest helicopter ever built was a massive aircraft developed in the Soviet Union during the 1960s called the Mil V-12. The V-12 was a rather unusual test vehicle that featured two rotors mounted side by side at the ends of a large wing. Each rotor had a diameter of nearly 115 ft (35 m). The helicopter was so enormous that the distance from the edge of one rotor disk to the other was almost 220 ft (67 m), even wider across than the wingspan of a Boeing 747!

Mil V-12, largest helicopter in the world
Even more remarkable was the lifting capacity of this powerful craft. The V-12 had a maximum takeoff weight of 231,485 lb (105,000 kg), nearly twice that of the second largest helicopter, and set a record in 1969 for carrying a payload of 88,635 lb (40,205 kg). So massive was the V-12 that this payload weight alone is greater than the maximum takeoff weight of the third biggest helicopter in the world! These impressive figures have earned the V-12 the record as the world's largest helicopter according to the FAI and the Guinness Book of World Records.
Only two V-12 prototypes were built, yet despite its amazing size and lifting power, the design was considered a failure by its manufacturer and Soviet authorities. The V-12 was simply too big and difficult to maneuver to be a practical machine. A production model would have been called the Mi-12, but these plans were cancelled following evaluation of the two test vehicles. One of the V-12 prototypes was retired to the Russian Air Force Museum in Monino while the second is reportedly housed at the Mil factory near Moscow.
2. Mil Mi-26 : 123,455 lb (56,000 kg)
Because the V-12 had been judged unsatisfactory, Mil was again tasked with designing a large heavy-lift helicopter for military and civil use. The resulting Mi-26 was far more compact than the V-12 due to advancements in propulsion technology, making the Mi-26 much less cumbersome to operate. With a maximum takeoff weight of 123,455 lb (56,000 kg), the Mi-26 is the world's second largest helicopter overall and the largest ever to go into production as well as the largest flying today.

Mil Mi-26 recovering an American CH-47 Chinook helicopter
Though substantially smaller than the V-12, the Mi-26 carries a considerable payload up to 44,000 lb (20,000 kg). The Mi-26 has proven much more powerful and efficient than its predecessors thanks in part to its eight-bladed main rotor. This rotor is only 106 ft (32 m) in diameter compared to the 115 ft (35 m) diameter rotors used aboard other giant Russian helicopters but can lift greater loads due to improvements in engines and blade design. Over 200 examples of the Mi-26 were built and the type remains in use throughout Russia. Several have also been sold to civilian firms and foreign nations.
3. Mil Mi-6 : 97,000 lb (44,000 kg)
The third and fourth largest helicopters are also Mil designs that helped prepare the way for the V-12 and Mi-26. The Soviet's first attempt to create an enormous heavy-lift helicopter came during the mid-1950s with the introduction of the Mi-6. Designed for many of the same duties as the improved Mi-26 some 25 years later, the Mi-6 had a maximum takeoff weight of 97,000 lb (44,000 kg) and could carry up to 24,250 lb (11,000 kg) of payload. The rotor used on the Mi-6 was the same 115 ft (35 m) diameter rotor system that would also be used on the V-12.

Mil Mi-6 lifting a large payload
The Mi-6 was the world's largest helicopter when it was first built and retained the title for many years until the appearance of the V-12. This Mi-6 proved quite successful for its time as demonstrated by the large number built. Some 860 were manufactured between 1960 and 1981 primarily for the Soviet Air Force and the civil airline Aeroflot. Additional models were also exported to several Soviet allies. These helicopters remained in use for over 40 years but are finally being phased out. In spite of its age, the Mi-6 still remains a remarkable vehicle and one of the largest helicopters ever conceived.
4. Mil Mi-10 : 83,775 lb (38,000 kg)
Yet another member of this related family of giant helicopters was the Mi-10. The Mi-10 was a specialized variant of the Mi-6 called a flying crane. The Mi-10 borrowed the engine and dynamic systems of the Mi-6 but these were mounted within a new fuselage. Unlike the wide Mi-6 fuselage that featured hinging doors for loading cargo internally, the Mi-10 fuselage was instead configured for passengers. This fuselage stood atop four widely spaced, extended-length landing gear legs designed to straddle large, bulky payloads carried below the helicopter.

Mil Mi-10 showing its long landing gear legs
Both the Mi-6 and Mi-10 shared the same 115 ft (35 m) diameter rotor that was also used aboard the V-12. Compared to the Mi-6, however, the Mi-10 had a reduced maximum takeoff weight of 83,775 lb (38,000 kg) ranking it as the fourth largest helicopter ever built. Only about 55 Mi-10 helicopters were made and the type saw relatively little use. Both its flying crane mission and the transport duties of the Mi-6 have since been taken over by the far superior Mi-26.
5. Sikorsky CH-53E : 73,500 lb (33,340 kg)
Coming in as the fifth largest helicopter in the world is the biggest ever built in the United States, the Sikorsky CH-53E Super Stallion. The CH-53E is an enlarged model of the earlier CH-53 Sea Stallion. Both vehicles were originally developed as heavy-lift helicopters for the US Marine Corps, and they have also been used by the Navy, Air Force, and allied countries. The largest variant of the CH-53 family developed so far is the CH-53E with a maximum takeoff weight of 73,500 lb (33,340 kg). Though comparable in physical dimensions to other H-53 models with a main rotor 79 ft (24.1 m) in diameter, the CH-53E and related MH-53E are equipped with an extra engine of increased power to lift a significantly greater payload.

CH-53E Super Stallion lifting an F-15 Eagle fighter
Approximately 115 examples of the CH-53E and MH-53E Sea Dragon variant were built, and these remain the largest helicopters in military service in the West. However, a new version of the venerable H-53 design is currently under development. Known as the CH-53K, this improved model will carry more powerful engines and new rotor blades to increase maximum takeoff weight to 84,700 lb (38,420 kg). Once this enhanced variant becomes available, the CH-53K should narrowly beat the Mi-10 to become the world's fourth largest helicopter.
6. Boeing Vertol MH-47E/G : 54,000 lb (24,495 kg)
The next largest helicopter is the Boeing Vertol CH-47 Chinook also developed in the United States. The H-47 family began development during the late 1950s as an enlarged variant of the CH-46 Sea Knight and has proven to be one of the most successful series of military heavy-lift helicopters. The design features a long, spacious fuselage powered by two engines driving a pair of tandem rotors 60 ft (18.3 m) in diameter. The type is widely used for transporting troops, carrying heavy military equipment like artillery slung underneath the fuselage, and resupply missions. Though the primary users are the US Army and British Royal Air Force, the CH-47 has been sold to over 20 nations as well as civilian operators.

Tandem rotor design of the MH-47E Chinook
The single largest versions of the H-47 developed to date are the MH-47E and MH-47G special operations models. While most CH-47 variants have a maximum takeoff weight around 50,000 lb (22,680 kg), the MH-47E/G have been upgraded to 54,000 lb (24,495 kg). Over 1,000 Chinooks have been built to date and production of newer CH-47F, MH-47G, and HH-47 models continues today.
7. Hughes XH-17 : 50,000 lb (22,680 kg)
Coming in as the seventh largest helicopter is a unique prototype called the XH-17 Sky Crane. The Sky Crane was the first helicopter built by Hughes Aircraft, the same company that had also built the behemoth Spruce Goose flying boat during the 1940s. Shortly thereafter, Howard Hughes became interested in helicopter technology and purchased a design for a giant transport helicopter from the Kellett company. Similar to the Russian Mi-10, the Sky Crane and was equipped with four long landing gear legs to straddle large oversized payloads carried underneath the fuselage. The H-17 was lifted by a massive two-blade rotor system 134 ft (40.8 m) across, still the record for the largest rotor ever flown.

Massive rotor of the Hughes XH-17 Sky Crane
Construction of a single XH-17 prototype began during the late 1940s for use as a research vehicle. The craft was completed by 1952 when it began a three-year testing period. The maximum takeoff weight ever attempted aboard the Sky Crane was over 50,000 lb (22,680 kg) during a flight in 1953. However, the research program concluded that the design was too large and cumbersome to be useful and no further models were built. Hughes also designed an enlarged variant called the H-28 that would have doubled the maximum weight to nearly 104,000 lb (47,000 kg), but further development was cancelled following experience with the XH-17.
8. Sikorsky CH-54 : 47,000 lb (21,320 kg)
Rounding out this list of large helicopters is another American design called the Sikorsky CH-54 Tarhe. Like the Mi-10 and XH-17, the CH-54 was a flying crane helicopter designed to haul large payloads slung below the fuselage. The unusual looking helicopter featured an upper spine containing the engines driving the 72 ft (21.9 m) diameter main rotor. At the front of this spine was the cockpit with windows facing both forward and aft allowing crew to monitor the payload. This minimal fuselage structure made the CH-54 a very efficient design compatible with various cargo modules that could be hoisted into position behind the cockpit for low drag.

CH-54 Tarhe recovering a damaged F-4 Phantom II
With a maximum takeoff weight of 47,000 lb (21,320 kg), the CH-54 was designed primarily for transporting military cargoes like artillery, ground vehicles, and other bulky supplies. Its lifting capabilities were frequently used during the Vietnam War to move aircraft or recover downed planes and helicopters. A civil model called the S-64 Skycrane was also developed and became popular with the logging industry and for firefighting. About 105 of these helicopters were built, and though retired from military service, several remain in civilian use.
Summary
Further illustrating just how large the helicopters described above are is the fact that no other helicopter exceeds a maximum weight of 35,000 lb (15,875 kg). The majority of helicopters are indeed quite small next to these giants since there is relatively little demand for such enormous vehicles compared to more moderately sized designs. Yet even among these giants, the size differential between them is hard to comprehend. The following diagram compares the Russian V-12 and Mi-26 to the American CH-53E. Despite its enormous size relative to any other Western helicopter, the Super Stallion almost seems a toy next to the two largest helicopters ever built.

Relative size comparison of the V-12, Mi-26, and CH-53E

you may enjoy the books Helicopters: Military, Civilian, and Rescue Rotorcraft by Robert Jackson or Steve Crawford's Twenty-First Century Military Helicopters.

Largest Aircraft In the World

2007-04-18T18:17:00.000+05:30

Aircraft records can indeed be confusing. If you need any reminder of that observation, try looking through the records on the FAI website. FAI, short for Fédération Aéronautique Internationale, is the international body that officially recognizes aviation and space records. However, most of the information on that site concerns aircraft performance, such as speed and altitude, rather than size.
Your question really comes down to how you decide to compare one plane to another. One of the measurements you mention is wingspan. While this dimension is often used to compare overall size, it is generally not the most accepted measure. Aircraft are usually ranked by weight, the maximum takeoff weight in particular. By this measure, the world's largest plane is the Antonov An-225 built in Ukraine when it was part of the Soviet Union.

An-225, largest plane in the world

The An-225 is quoted as having a maximum takeoff weight of over 1.3 million pounds (600,000 kg). The FAI gives credit for a slightly lower weight of 1.12 million pounds (508,200 kg), the record set by an An-225 in 1989 for the largest mass ever lifted by an airplane to an altitude of 6,500 ft (2,000 m). The An-225 was originally built to ferry the Soviet Buran space shuttle but it is now used to transport various outsize cargos. Only one example was ever completed, and it is currently available for charter flights through the British company Air Foyle. The An-225 is a cargo transport only and has never been used as an airliner.
If we continue to measure by maximum takeoff weight, the next largest plane in the world would be either the related An-124 transport or the Boeing 747 airliner. Both aircraft are quoted with a maximum weight around 900,000 lb (405,000 kg). The An-124 was also built by the Soviet Union as a large cargo plane for both military and civil use. The basic design was later enlarged and adapted to produce the An-225.

An-124
Over 50 examples of the An-124 were ultimately built, and many are now operated on charter flights through Air Foyle or other commercial firms. The American EP-3 reconnaissance plane that was forced to land after colliding with a Chinese fighter was actually transported back to the US aboard an An-124 chartered by the American government.

Boeing 747
The Boeing 747, meanwhile, is currently the world's largest commercial airliner and typically carries about 400 passengers on intercontinental flights. The 747 will not retain its title for long, however, since the new Airbus A380 will carry over 550 passengers and have a takeoff weight over 1.23 million pounds (560,000 kg).

Airbus A380
Once it enters service, the A380 will become the world's largest airliner and at least the second largest plane. Additional stretched models are also planned, and one of these will almost surely supersede the An-225 to take the official title as world's largest plane.
Your question also mentions two other classic aircraft that were certainly enormous by any measure. Unfortunately, both proved to be disastrous follies that were great embarrassments to their proponents. The Spruce Goose, officially known as the Hughes H-4 Hercules, was the brainchild of flamboyant billionaire Howard Hughes. The aircraft had originally been ordered by the US government during World War II as a giant cargo plane ferrying up to 750 armed troops or two 30-ton tanks. As the war concluded, Hughes believed he could adapt the massive plane to revolutionize long-distance transportation in the fledgling airline and air cargo industires. His creation was the world's largest plane at the time and is still the largest flying boat ever built with a maximum takeoff weight of 400,000 lb (180,000 kg). The enormous craft also holds records for the largest wingspan at 319 ft 11 in (97.5 m), tallest airplane at 79 ft 3 3/8 in (24.2 m), and the largest aircraft ever made from wood.

Spruce Goose during its only flight
After pouring $7 million of his own money and another $18 million of government funds into the behemoth, Hughes only succeeded in making a single mile-long flight in 1947. The record-setting plane never flew again and is today a tourist attraction at the Evergreen Aviation Museum in Oregon.
Like the Spruce Goose, the Bristol Brabazon was another a government-sponsored effort intended to advance the state of the art in long-distance travel. The Brabazon was funded by the UK during the 1940s to develop a large airliner capable of non-stop flights between London and New York. Though smaller than the Spruce Goose, the Brabazon would have easily been the largest airliner of the day with a takeoff weight of 250,000 lb (115,000 kg) and a wingspan of 230 ft (70 m).

Bristol Brabazon compared to other planes of the day
If successful, the Brabazon offered the opportunity to challenge American manufacturers for dominance in the commercial aviation market. Unfortunately, the ambitious design failed to receive government certification following the discovery of fatigue cracks in the propeller mountings. While the problem was certainly correctable, the UK decided to cancel further funding in the early 1950s even though £3 million had already been invested. Only one prototype had been built, and it was broken up for scrap in late 1953 after flying only 400 hours. An improved prototype with structural enhancements and turboprop engines was also under construction, but it too was scrapped after the project was cancelled.

Relative size comparison of the Spruce Goose, An-225, A380, and 747
In summary, aircraft sizes are typically compared by weight rather than length or wingspan. The largest plane in the world at the current time is therefore the An-225, though the A380 will likely overtake it in the near future. The largest wingspan, however, still belongs to the Spruce Goose at nearly 320 feet. By this measure, the An-225 would be a distant second place at 290 ft, and the A380 falls short at 262 ft.
- answer by Molly Swanson, 8 August 2004
Additional Rankings:
Since this article was posted, we have received several messages from readers who believe the C-5 Galaxy transport operated by the US Air Force ranks second or third place and should be included. The C-5 was the world's largest plane when it was introduced in the late 1960s, but it has since been overtaken by the jet-powered aircraft discussed above.

C-5 Galaxy
The C-5 remained the world's largest plane until the introduction of the An-124, and the C-5 has since been surpassed by the An-225, enlarged models of the 747, and the new A380 as well. The current ranking of the world's largest aircraft by maximum takeoff weight (MTOW) is shown in the following table.

Rank	Aircraft	MTOW [lb]	Notes
1.	An-225 Mriya	1,300,000	-
2.	Airbus A380	1,230,000	-
3.	Boeing 747-400ER	910,000	the 747-8 under development will reach 960,000 lb
4.	An-124 Ruslan	892,870	-
5.	C-5 Galaxy	840,000	wartime max load, peacetime load is limited to 769,000 lb
6.	Airbus A340-600 HGW	837,755	high-gross weight variant
7.	Boeing 777-300ER	775,000	highest available weight, loading restrictions apply

With a wingspan of 223 ft, the C-5 currently ranks sixth after the Spruce Goose (320 ft), An-225 (290 ft), A380 (262 ft), An-124 (240 ft), and a tie between the Brabazon and B-36 (230 ft). The C-5 has a larger span than the 747-400 (211 ft) but will soon be narrowly beaten by the new 747-8 variant with a wingspan of nearly 225 ft.