| The physical law under which rockets operate was | | | | momentum that carries the rocket forward. For |
| first set down almost three hundred years ago by Sir | | | | instance, on a forty-minute 4,000-mile flight, an |
| Isaac Newton, an English mathematician, without | | | | intercontinental ballistic missile is under power for only |
| whose work our exciting space explorations of today | | | | about 200 seconds. The speed it has gained while its |
| would be impossible. This law states that to every | | | | engines were burning then carries it along on course |
| action, there is an equal and opposite reaction. The | | | | until it eventually loses momentum and, like an artillery |
| recoil of a gun when it is fired is an example of the | | | | shell, arches over and falls to the earth. The course |
| forces of action and reaction at work. | | | | followed by an artillery shell or by a rocket-propelled |
| The action of the gases exhausting from a rocket's | | | | missile while it is in flight is called its "trajectory." |
| nozzles at great speed produces a reaction of equal | | | | Unlike a bullet or shell, which is guided only while it is |
| force against the inner walls of the rocket, and it is | | | | in the barrel of the gun, many of our missiles of |
| this force which propels it. This is why the rocket is | | | | today are guided while in flight. This is done in a |
| the only suitable device for space travel - it is | | | | number of ways. Sometimes electronic controls within |
| completely self-contained and independent of any | | | | the missile itself make adjustments in its flight path. |
| external force for its power. An airplane must have | | | | Other missiles are guided by command signals radioed |
| air to sustain it in flight and to provide the oxygen | | | | from the ground. However, our long-range missiles |
| needed to make its fuel burn. But the rocket needs | | | | (the intermediate-range ballistic missiles and the |
| nothing. It does not need air to support it, and it | | | | inter-continental ballistic missiles) are guided only while |
| carries its own oxidizer right on board. | | | | their engines are burning. Once their fuel is exhausted |
| The reaction force or push produced by a rocket | | | | no further changes can be made in their course and |
| engine is called thrust, and this power is expressed in | | | | they follow a trajectory to their target, just like a |
| terms of pounds. The thrust power of a rocket | | | | bullet or shell. |
| indicates how much weight it is capable of moving at | | | | Some rocket missiles and all the rockets used for |
| or near the surface of the earth. If a particular | | | | space experiments are made up of two or more |
| rocket engine has a thrust of 100,000 pounds, this | | | | stages fastened together, each with its own engine |
| means it can lift or propel that much weight. | | | | or engines. The stages drop off! When their engines |
| Our rocket-propelled missiles of today operate much | | | | burn out, that is, when their fuel is consumed, and |
| like a bullet or an artillery shell. The engines which | | | | the next stage takes over to push the warhead to |
| "fire" them burn for only seconds - after that it is | | | | its target or the pay-load (or cargo) out into space. |