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4. HOW ROCKETS WORK

The vast and boundless space around the earth appears to hold nothing because we cannot see it or touch it. But as we know, the earth is surrounded for a few miles above its surface by atmosphere or air (Plate I) and this air is really quite solid. You can feel it if you hold a piece of cardboard on the palm of your hand and swing it broadside through the air. Planes make use of this air to fly. Sailboats are propelled through the water by making use of the air in the form of wind.

There is also an invisible force around the earth. This is the force of gravitation, which pulls everything toward the center of the earth (Plate 2). Like the air, the force of gravitation becomes less and less as the distance from the earth increases.

Drop a book and it will fall to the floor. Throw a stone into the air and it falls back to the ground. But if you could throw the book or the stone with enough force (speed), it could overcome the effects of gravita­tion and continue on into space.

Tie a small weight or stone to the end of a piece of string. Swing the stone around and it will pull on the string. The string acts like the force of gravitation, keeping the weight from flying away in a straight line. If you could swing the weight fast enough, however, it would break the string and fly of! in a straight line.

A rocket is able to escape from the earth's gravita­tional pull because it travels at tremendous speed. It has to travel at about 25,000 miles per hour to overcome the effects of gravity. In outer space, where there is little or no air and much less pull of gravitation, a small amount of power will propel a rocket or space ship at great speeds. Our scientists are now working on a rocket for use in outer space that will have a thrust (this word is explained below) of only one tenth of a pound! But at or near the surface of the earth immense power is needed to force the rocket through the dense atmos­phere and to overcome the enormous pull of gravity. The rocket must also achieve great speed in a very short space of time in order to escape from the earth; otherwise, the forces of gravity will overcome its mo­mentum and it will fall back to earth.

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 (Plate 3). 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 (Plate 4). 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 inter­continental 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 of! 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 (Plate 5).

 

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