Rockets and Missiles

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 gases produced by a chemical reaction inside the body of 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.