Rocket engines are the only means now available that can furnish enough force to'lift vehicles from the earth. In one way a rocket engine is similar to an automobile en-gine. Both require a mixture of fuel and oxidizer to be burn-ed.Hot gases are produced which expand and create the force necessary to move the vehicle.A basic difference between a rocket and an automobile engine is the speed with which the fuel is burned.
In a rocket engine, gine, hot gases are produced by very rapid burning of fuel. As the hot gases expand, tremendous pres-sures push against the walls of the engine.If the engine had no opening to the outside, the gas pressure would be very nearly equal in all directions and the engine would quickly explode.However, a rocket engine is provided with an exhaust nozzle to permit the hot gases to escape. As the gases flow out of the exhaust, the force on the front of the engine is no longer balanced by a force to the rear.Unbalanced gas pressure pushing on the front of the engine thrusts the whole body of the rocket forward. This force is called its thrust. A rocket engine does not have to push against anything but itself.It operates even in the near vacuum of space.
As part of its load, a rocket vehicle must carry along both the fuel and oxidizer it needs. The fuel can be almost anything that is quickly and smoothly combustible.The most commonly used oxidizer is oxygen itself. But the oxygen used in rockets is in the liquid form because it oc-cupies the least amount of space.At least 75 percent of the weight of a rocket vehicle at take-off is taken up by its fuel and oxidizer. Rockets that arc intended to trave great distances must be very large in order to carry suffi-cient fuel and oxidizer. Such rockets must also be very powerful to get the entire load off the earth.
