Today`s rocket engines use chemical propellants, which may be either liquid or solid. Notice that we say “propellants” and not “fuel”. This is because a rocket engine usually has to have two propellants. One is called fuel and the other is called the oxidizer. An automobile engine or an airplane engine needs to carry only fuel, because it uses the oxygen from the air to burn with the fuel. But most of the time a rocket has to operate where there is no air, so it must carry its own source of oxygen, and that is called the oxidizer.
Most liquid rocket engines have separate tanks for the fuel and the oxidizer, which are combined and burned in the combustion chamber of the engine pretty much the way gas and air are combined and burned in the burner of an ordinary gas stove. The difference is that in the rocket engine both fuel and oxidizer are stored in liquid form in the tanks and pumped or gravity-fed to the combustion chamber, where they are sprayed in and burned.
Different rocket engines use different combinations of fuel and oxidizer, but the most common oxidizer is just plain liquid oxygen. In the Atlas and in the Saturn V first stage, the fuel used is kerosene, very much like jet airplane fuel. For upper stages like the Centaur or the third stage of the Saturn V, liquid hydrogen is used as the fuel. Both oxygen and hydrogen have to be very cold to become liquid (-297 degrees F for oxygen and -423 degrees F for hydrogen), and are hard to store and handle. These propellants are called cryogenic, from the Greek word kryos, meaning icy cold.
The Titan II rocket and some others used propellants that did not have to be cold, such as hydrazine and nitrogen tetroxide or nitric acid. Some liquid propellant combinations are called hypergolic, which means that they ignite just by coming together and therefore do not need any ignition system. The TRW-built Descent Engine for the Apollo Lunar Module used hypergolic propellants.
A solid propellant engine also has a fuel and an oxidizer, but they are mixed together before-hand and poured into the rocket casing. The rocket case is placed nose down and a core is placed in the middle of it. The mixture is then poured in around the core and cured by heating until it becomes a solid rubbery mass. The core is then taken out, leaving a hole in the middle of the propellant. When it is time to fire the engine, an igniter shoots a tongue of flame down this hole and the propellant burns all along this inner surface, shooting the hot gases out the nozzle.
Solid-propellant rockets have no tanks or pumps or valves and are very simple and reliable. The Minuteman missile, for example, has three solid propellant stages and a ready to take off ant time at less than a minute`s notice. One trouble with a solid rocket, however, is that once you have started it you can`t shut it off as easily and safely as you can a liquid engine. Also, we haven’t perfected a way yet to fire it, shut it off, and then start it again later as we sometimes need to do for space missions.
Another type of rocket that will be used to power the deep-space missions of the future is the nuclear rocket. The first such engine was designed by NASA and the Atomic Energy Commission. It was called NERVA (Nuclear Engine for Rocket Vehicle Application). It was supposed to use the heat from a nuclear reactor to heat liquid hydrogen to a high-temperature gas, which was then expelled out the nozzle like the gases from a chemical rocket engine. The Isp from such an engine is much higher than that of chemical propulsion; it would be about 1000 seconds, compared to 300-500 seconds for chemical propellant rockets.
Still more advanced is the electrical propulsion engine, which has much higher Isp`s. This engine uses electricity to heat hydrogen to a high-temperature gas that is expelled from the nozzle. So far, only relatively small electrical propulsion systems have been built, because it is hard to generate in space the enormous amounts of electrical power that would be needed to produce a thrust comparable to that of a chemical or nuclear rocket engine. The ion engine and the plasma engine are other types of electrical propulsion system that are also very efficient, but so far they too are limited to very small thrusts of a pound or less.
Notice that electricity by itself could not propel a rocket at all. It can make the rocket move only by accelerating some kind of particle and expelling it.
And remember that even the most efficient rocket engine works on the same principle as the toy balloon, that is, it must expel something from the nozzle in order to create thrust. Photo: © Megan Jorgensen