The term rocket refers both to any reaction motor that carries its own oxidant and to any vehicle that it propels. A reaction motor is a propulsion device that generates a forward push by expelling matter in a backward direction. Rocket fuels may be either solid or liquid.

In weapons terminology, a missile is an uncrewed rocket vehicle containing some form of guidance system and, generally carrying some type of explosive. A missile may also be guided by a ground-based command center. A rocket, in military parlance, is an unguided rocket-propelled weapon. A large part of the research and development on modern rocketry systems has been carried out by or under the supervision of various military organizations.

Rockets are used not only in weapons but to carry scientific instruments or human beings to locations not accessible by other kinds of transportation, such as the upper atmosphere, Earth orbit, or outer space. Rockets designed to carry instruments no farther than the upper levels of the atmosphere are known as sounding rockets. Those designed to lift spacecraft into orbit or into outer space are known as boosters or as launch vehicles.

History

The first rocket was almost certainly constructed in China, but the date of that invention is not known. There is evidence that the Chinese knew about black gunpowder at least two centuries before the birth of Christ, but the explosive was probably used exclusively for ceremonial purposes. The concept of using gunpowder to propel an object through space probably did not arise for more than a thousand years, perhaps during the thirteenth century. Records of the time indicate that gunpowder was attached to sticks for use as offensive weapons during battle. The birth of rocketry was, therefore, intimately associated with their first use as missiles.

For a short period of time, rockets were a reasonably effective weapon in warfare. For example, French

troops under Joan of Arc apparently used simple rockets to defend the city of Orleans in 1429. Military strategists of the time devised imaginative and sometimes bizarre variations on the rocket for use in battles, but such concepts were apparently seldom put into practice. The development of more efficient weapons of war, in any case, soon relegated the use of rockets to recreational occasions, such as those still popular in the United States at Fourth of July celebrations.

Scientific basis of rocketry

The scientific principle on which rocket propulsion is based was first described precisely in 1687 by the English physicist Isaac Newton (1642–1727). In his monumental work on force and motion, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), Newton laid out three laws of motion. The third of these stated that for every action, there is an equal and opposite reaction. For example, if you push against a wall, the wall pushes back at your hand with equal and opposite force.

The application of Newton’s third law to propulsion is used by a variety of marine animals as a means of movement. The body of the squid, for example, contains a sac that holds a dark, watery fluid. When the squid finds it necessary to move, it contracts the sac and expels some of the fluid from an opening in the back of its body. In this case, the expulsion of the watery fluid in a backward direction can be thought of as an “action.” The equal and opposite reaction that occurs to balance that action is the movement of the squid’s body in a forward direction.

Rocket propulsion

A rocket is propelled in a forward direction when, like the squid, a fluid is expelled from the back of its body. In the most common type of rocket, the expelled fluid is a mass of hot gas produced by a chemical reaction inside the rocket. In other types of rockets, the expelled fluid may be a stream of charged particles or plasma produced by an electrical, nuclear, or solar process.

Chemical rockets are of two primary types, those that use liquid fuels and those that use solid fuels. The most familiar type of liquid rocket is one in which liquid oxygen is used to oxidize liquid hydrogen. In this reaction, water vapor at very high temperatures (about 4,935°F [2,725°C]) is produced. The water vapor is expelled from the rear of the rocket, pushing the rocket itself forward.

The liquid oxygen/liquid hydrogen rocket requires an external source of energy, such as an electrical spark, in order for a chemical reaction to occur. Some combinations of fuel and oxidizer, however, will ignite as soon as they are brought into contact. Such combinations are known as hypergolic systems. An example of a hypergolic system is the liquid combination of nitrogen tetroxide and monomethylhydrazine. These two compounds react spontaneously with each other when brought into contact to produce a temperature of the order of 5,200°F (2,871°C).

The use of liquid fuels in rockets requires a number of special precautions. For example, with a liquid oxygen/liquid hydrogen system, both liquids must be kept at very low temperatures. Oxygen gas does not become a liquid until it is cooled below–297°F (–183°C) and hydrogen gas, not until it is cooled below–421°F (–252°C). The two liquids must, therefore, first be cooled to very low temperatures and then kept in heavily insulated containers until they are actually brought into combination in the rocket engine.

Hypergolic systems also require special care. Since the two liquids that make up the system react with each other spontaneously, they must be kept isolated from each other until combustion is actually needed.

A third type of liquid propellant is known as a monopropellant. As the name suggests, a monopropellant consists of only a single compound. An example is hydrogen peroxide. When the proper catalyst is added to hydrogen peroxide, the compound decomposes, forming oxygen and water vapor, and producing heat sufficient to raise the temperature of the product gases to 1,370°F (743°C). The expulsion of these hot gases provides the thrust needed in a rocket.

Liquid fuel rockets have a number of advantages. For example, they can be turned on and off rather simply (at least in concept) by opening and closing the valves that feed the two components to each other. In general, they tend to provide more power than do solid rockets. Also, when problems develop in a liquid fuel rocket, they tend to be less serious than those in a solid-fuel rocket.

However, liquid-fuel rockets also have a number of serious disadvantages. One has been pointed out above, namely that the liquid components often require very special care. Also, liquid fuels must be added to a rocket just before its actual ignition since the components can not be stored in the rocket body for long periods of time. Finally, the mechanical demands needed for the proper operation of a liquid-fuel operation can be very complex and, therefore, subject to a number of possible failures.

Solid fuel rockets

Like liquid-fuel rockets, solid-fuel rockets have both advantages and disadvantages. With solid fuel, the rocket can be fueled a long time in advance of a launch without too much danger of the fuel’s deteriorating or damaging the rocket body. The construction of the rocket body needed to accommodate the solid fuel is also much simpler than that which is needed for a liquid-fuel rocket. Finally, the fuels themselves in a solid-fuel rocket tend to be safer and easier to work with than those in a liquid fuel rocket.

Still, solid-fuel rockets have their own drawbacks. Once the fuel in a solid-fuel rocket begins to burn, there is no way to slow it down or turn it off. That means that some of the most serious accidents that can occur with a rocket are those that involve solid-fuel combustion that gets out of control.

The solid fuels used in rockets tend to have a clay-like texture. The material, called the grain, contains the oxidizer, the fuel, a binder, and other components all mixed with each other. Ignition occurs when a spark sets off a chemical reaction between the oxidizer and the fuel. The chemical reaction that results produces large volumes of hot gases that escape from the rear of the rocket engine.

Many combinations of materials have been used for the grain in a solid-fuel rocket. One common mixture consists of powdered aluminum metal as the fuel and ammonium perchlorate or ammonium nitrate as the oxidizer. The flame produced by the reaction between these two substances has a temperature of at least 5,400°F (2,982°C). Nitroglycerine in combination with easily oxidizable organic compounds is also widely used. Such combinations have flame temperatures of about 4,100°F (2,260°C).

The shape into which the grain is formed is especially important in the operation of the solid-fuel rocket. The larger the surface area of grain exposed, the more rapidly the fuel will burn. One could construct a solid-fuel rocket by simply packing the rocket body with the fuel. However, simply boring a hole through the center of the fuel will change the rate at which the fuel will burn. One of the most common patterns now used is a star shape. In this pattern, the solid fuel is actually put together in a machine that has a somewhat complex cookie-cutter shape in its interior. When the fuel has been cured and removed from the machine, it looks like a cylinder of cookie dough with its center cut out in the shape of a seven-pointed star.

In some cases, a rocket engineer might want to slow down the rate at which a solid fuel burns. In that case, the surface area of fuel can be decreased or a slow-burning chemical can be added to the fuel, reducing the fuel’s tendency to undergo combustion. A grain that has been treated with an inhibitor of this kind is known as a restricted-burning grain.

Specific impulse

The effectiveness of a fuel in propelling a rocket can be measured in a number of ways. For example, the thrust of a rocket is the mass that can be lifted by a particular rocket fuel. The thrust of most rocket propulsion systems is in the range from 500,000 to 14,700,000 newtons (10,000 to 3,300,000 pounds).

The velocity of exhaust gases is also an indication of how effectively the rocket can lift its payload, the cargo being carried by the rocket. One of the most useful measures of a rocket’s efficiency, however, is specific impulse. Specific impulse (I_sp) is a measure of the mass that can be lifted by a given fuel system for each pound of fuel consumed per second of time. The unit in which I_sp is measured is seconds.

For example, suppose that a rocket burns up one pound of fuel for every 400 lb of weight (equivalent to 182 kg of mass at Earth’s surface) that it lifts from the ground per second. Then its specific impulse is said to be 400 seconds. A typical range of specific impulse values for rocket engines would be between 200 to 400 seconds. Solid rockets tend to have lower specific impulse values than do liquid rockets.

Multistage rockets

In some cases, rocket engineers combine solid and liquid rockets in the same vehicle in order to take advantage of the unique advantages each has to offer. A classical example is the National Aeronautics and Space Administration’s (NASA’s) space shuttles. The shuttles make use of 67 individual rockets in order to lift the vehicle off Earth’s surface, maneuver it through space, and control its re-entry to Earth’s surface. Forty-nine of those rockets are liquid engines and the other 18, solid motors.

The three largest of these rockets are liquid oxygen/liquid hydrogen engines that provide part of the thrust needed to lift the shuttle off the pad. Two more liquid rockets, powered by a nitrogen tetroxide/monomethylhydrazine mixture, are used to place the shuttle into orbit and to carry out a number of orbital maneuvers. Another 44 nitrogen tetroxide/monomethylhydrazine rockets are used for fine tuning the shuttle’s orientation in orbit.

Of the solid fuel rockets, two, the solid rocket booster motors, provide nearly 15,000 newtons (3,300,000 lb) of thrust at take-off. The remaining 16 rockets, composed of ammonium perchlorate, aluminum, and polybutadiene, are used to separate the solid rocket booster capsules from the main shuttle body for re-use.

Non-chemical rockets

Rockets that operate with solid and liquid chemicals are currently the only kinds of vehicles capable of lifting off Earth’s surface for scientific research or military applications. But both types of chemical rockets suffer from one serious drawback for use in vehicles traveling through outer space. The fuels they use are much too heavy for long distance travel above Earth’s atmosphere. In other words, their specific impulse is too small to be of value in outer space travel.

Rocket engineers have long recognized that other types of rockets would be more useful in travel outside Earth’s atmosphere. These rockets would operate with power systems that are very light in comparison to chemical rockets. As early as 1944, for example, engineers were exploring the possibility of using nuclear reactors to power rockets. The rocket would carry a small nuclear reactor, the heat from which would be used to vaporize hydrogen gas. The hydrogen gas would then be expelled from the rear of the rocket, providing its propulsive force. Calculations indicate that a nuclear rocket of this type would have a specific impulse of about 1,000 seconds, more than twice that of the traditional chemical rocket.

Other types of so-called low-thrust rockets have also been suggested. In some cases, the propulsive force comes from atoms and molecules that have been ionized within the rocket body and then accelerated by being placed within a magnetic or electrical field. In other cases, a gas such as hydrogen is first turned into a plasma, and then ionized and accelerated. As attractive as some of these ideas sound in theory, they have thus far found relatively few practical applications in the construction of rocket engines.

Missiles

The modern age of missile science began toward the end of World War II (1939–1945). During this period, German rocket scientists developed the ability to produce vehicles that could deliver warheads to targets hundreds of miles from their launch point. The missiles were effective only as terror weapons, however, and had little military significance; Germany’s war position was already hopeless and the V-2 was by modern military standards a mere rocket, stabilized by gyroscopes but without effective guidance.

The Cold War that followed the end of World War II provided a powerful incentive for the United States, the then Soviet Union, and a few other nations to spend huge amounts of money on the development of newer and more sophisticated missile systems. Missiles have the great advantage of being able to deliver a large destructive force at great distance from the launch site. The enemy can be damaged or destroyed with essentially no damage to the party launching the missile.

As the Cold War developed, however, it became obvious that the missile-development campaign was a never-ending battle. Each new development by one side was soon made obsolete by improvements in anti-missile defense mechanisms by the other side. As a result, there is now a staggering variety of missile types with many different functions and capabilities.

Missile classification

Missiles can be classified in a number of different ways. Some are said to be unguided because, once they are launched, there is no further control over their flight. The German V-2 rockets were unguided. Such devices can be directed at the launch site in the general vicinity of a target, but once they are on their way, there is no further way that their path can be adjusted or corrected.

Today, however, the weapons called “missiles” are guided. This means that the missile’s pathway can be monitored and changed either by instruments within the missile itself or by a guidance station.

Missiles can also be classified as aerodynamic or ballistic missiles. An aerodynamic missile is one equipped with wings, fins, or other structures that allow it to maneuver as it travels to its target. Aerodynamic missiles are also known as cruise missiles. Ballistic missiles are missiles that follow a free-fall path once they have reached a given altitude. In essence, a ballistic missile is fired into the air, the way a baseball player makes a throw from the outfield, and the missile (the ball) travels along a path determined by its own velocity and Earth’s gravitational attraction.

Finally, missiles can be classified according to the place from which they are launched and the location of their final target. V-2 rockets were surface-to-surface missiles since they were launched from a station on the ground in Germany and were designed to strike targets on the ground in Great Britain.

An air-to-air missile is one fired from the air (usually from an aircraft) with the objective of destroying another aircraft. One of the best known air-to-air missiles is the United States’ Sidewinder missile, first put into operation in 1956. The first Sidewinders were 9.31 ft (2.84 m) long and 5.00 in (12.7 cm) in diameter, with a weight of 165 lb (5 kg) and a range of 0.68 mi (1.1 km).

A surface-to-air missile is one fired from a ground station with the goal of destroying aircraft. The first surface-to-air missile used by the United States military was the Nike Ajax, a rocket with a weight of 2,295 lb (1,042 kg), a length of 34.8 ft (10.6 m), a diameter of 12.0 in (30.5 cm), and a range of 30 mi (48 km).

Some other types of missiles of importance to the military are anti-ship and anti-submarine missiles, both of which can be launched from ground stations, from aircraft, or from other ships. Military leaders were at one time also very enthusiastic about another type of missile, the anti-ballistic missile (ABM). The ABM program was conceived of as a large number of solid rockets that could be aimed at incoming missiles. U.S. engineers developed two forms of the ABM: the Spartan, designed for long-distance defensive uses, and the Spring, designed for short-range interception. The former Soviet Union, in the meanwhile, placed its reliance on an ABM given the code name of Galosh. The ABM program came to a halt in the mid-1970s when the cost of implementing a truly effective defensive system became apparent.

Structure of the missile

Any missile consists essentially of four parts: a body, known as the airframe; the propulsive system; the weapon; and the guidance system. Specifications for the airframes of some typical rockets were given above. The propulsive systems used in missiles are essentially the same as those described for rockets above. That is, they consist of one or more liquid rockets, one or more solid rockets, or some combination of these.

In theory, missiles can carry almost any kind of chemical, biological, or nuclear weapon. Anti-tank missiles, as an example, carry very high powered chemical explosives that allow them to penetrate a 24-in (60 cm) thick piece of metal. Nuclear weapons have, however, become especially popular for use in missiles. One reason, of course, is the destructiveness of such weapons. But another reason is that anti-missile jamming programs are often good enough today to make it difficult for even the most sophisticated guided missile to reach its target without interference. Nuclear weapons cause destruction over such a wide area, however, that defensive jamming is less important than it is with more conventional explosive warheads.

Guidance systems

At one time, the methods used to guide a missile to its target were relatively simple. One of the most primitive of these systems was the use of a conducting wire trailed behind the missile and attached to a ground monitoring station. The person controlling the missile’s flight could make adjustments in its path simply by sending electrical signals along the trailing wire. This system could be used, of course, only at a distance equal to the length of wire that could be carried by the missile, a distance of about 984 ft (300 m).

The next step up from the trailing wire guidance system is one in which a signal is sent by radio from the guidance center to the missile. Although this system is effective at much longer ranges than the trailing wire system, it is also much more susceptible to interference

KEY TERMS

Ballistic missile— A missile that travels at a velocity less than that needed to place it in orbit and which, therefore, follows a trajectory back to Earth’s surface.

Grain— The fuel in a solid propellant.

Hypergolic system— A propellant system in which the components ignite spontaneously upon coming into contact.

Monopropellant— A system in which fuel and oxidizer are combined into a single component.

Specific impulse— The thrust provided to a rocket by a fuel as measured in pounds of payload lifted per pound of fuel per second.

(jamming) by enemy observers. Much of the essence of the missile battles that took place on paper during the Cold War was between finding new and more secure ways to send messages to a missile, and new and more sophisticated ways to interrupt and “jam” those signals.

Some missile systems carry their own guidance systems within their bodies. One approach is for the missile to send out radio waves aimed at its target and then to monitor and analyze the waves that are reflected back to it from the target. With this system, the missile can constantly make adjustments that keep it on its path to the target. As with ground-directed controls, however, a system such as this one is also subject to jamming by enemy signals.

Another guidance system makes use of a TV camera mounted in the nose of the missile. The camera is pre-programmed to lock in on the missile’s target. Electronic and computer systems on board the missile can then keep the rocket on its correct path.

Resources

BOOKS

Chun, Clayton K.S. Thunder over the Horizon: From V-2 Rockets to Ballistic Missiles. Westport, CT: Praeger, 2006.

Harland, David M. and Ralph D. Lorenz. Space Systems Failures: Disasters and Rescues of Satellites, Rockets and Space Probes. Chichester, UK: Praxis Publishing, 2005.

Vab Riper, A. Bowdoin. Rockets and Missiles: The Life Story of a Technology. San Francisco: Greenwood Press, 2004.

David E. Newton

The Gale Encyclopedia of Science