The physical law under which rockets operate was first set down almost three hundred years ago by Sir Isaac Newton, an English mathematician, without whose work our exciting space explorations of today would be impossible. This law states that to every action, there is an equal and opposite reaction. The recoil of a gun when it is fired is an example of the forces of action and reaction at work.

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 carries 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 indicates 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."

Unlike a bullet or shell, which is guided only while it is in the barrel of the gun, many of our missiles of today are guided while in flight. This is done in a number of ways. Sometimes electronic controls within the missile itself make adjustments in its flight path. Other missiles are guided by command signals radioed from the ground. However, our long-range missiles (the intermediate-range ballistic missiles and the inter-continental ballistic missiles) are guided only while their engines are burning. Once their fuel is exhausted no further changes can be made in their course and they follow a trajectory to their target, just like a bullet or shell.

Some rocket missiles and all the rockets used for space experiments are made up of two or more stages fastened together, each with its own engine or engines. The stages drop off! When their engines burn out, that is, when their fuel is consumed, and the next stage takes over to push the warhead to its target or the pay-load (or cargo) out into space.