Rocket Power The action of the gases exhausting from a rocket's nozzles at great speed produces a reaction of equal force against the inner walls of the rocket, and it is this force which propels it. This is why the rocket is the only suitable device for space travel it is completely self-contained and independent of any external force for its power. An airplane must have air to sustain it in flight and to provide the oxygen needed to make its fuel burn. But the rocket needs nothing. It does not need air to support it, and it car-ries its own oxidizer right on board. The reaction force or push produced by a rocket engine is called thrust, and this power is expressed in terms of pounds. The thrust power of a rocket in-dicates how much weight it is capable of moving at or near the surface of the earth. If a particular rocket engine has a thrust of 100,000 pounds, this means it can lift or propel that much weight. Our rocket-propelled missiles of today operate much like a bullet or an artillery shell. The engines which "fire" them burn for only seconds—after that it is momentum that carries the rocket forward. For instance, on a forty-minute 4,000-mile flight, an intercontinental ballistic missile is under power for only about 200 seconds. The speed it has gained while its engines were burning then carries it along on course until it eventually loses momentum and, like an artillery shell, arches over and falls to the earth. The course followed by an artillery shell or by a rocket-propelled missile while it is in flight is called its "trajectory."