Principles and theory of Spaceflight and jet aircraft

To demonstrate the basic applications of Newton’s laws of motion, the preceding examples have been purposely made simple and everyday. The most dramatic and up-to-date application of these laws is, of course, in the jet aircraft which span the seas and continents, and the satellites and rockets which are beginning to cross the frontiers of space.

Both jets and rockets and such speculative motors as the ion drive employ Newton’s third law: action and reaction are equal and opposite and always occur in pairs. When you blow up a balloon and let go of it without tying the end, it skitters about the room at a great rate before collapsing; this is about the simplest illustration of the third law as a motive force. In the balloon, as in jets and rockets, air or fuel is expelled at the rear of the motor, which then moves forward at a proportionate rate. A rocket depends on the fuel it carries for this effect, while most jets draw in air and compress, then expand and expel it. Rockets depend almost entirely for flight on this action-reaction effect, while jet planes, of course, use it only for forward motion, getting their lift from the difference in air pressure between the upper and lower surfaces of the wings.

Economical spaceflight is possible because of Newton’s first law: A body of itself is unable to change its condition of rest or motion. Since there is nothing in space except an infinitesimal quantity of dust, which can be for the moment discounted to create friction, there is no reason for an object such as a rocket or spaceship, ever to slow down. Therefore, once an initial velocity has been established, the ship can coast at the same speed indefinitely, without using up any more fuel until the time comes when it must slow down or manoeuvre. And this maneuvering even though there is nothing to push against is again possible because of the third law. Expelling any substance from the front of the ship in empty space will cause it to slow down; from the right side to the left; and so on.

The orbiting satellites that send back information on conditions in space and on Earth, or which have carried men around our planet, are not really “in space” but are subject to Earth’s gravitational force. They are constantly in “free fall” being drawn down to earth; but, as they start their orbit high enough above the planet’s surface, the surface itself curves or “falls” away at the same rate as does the satellite, which thus stays the same height above the surface until it is directed downward, or the “drag” of the fringes of atmosphere slows it enough for the angle of “fall” to become deeper. Thus the weightlessness that the astronauts and cosmonauts experienced is not a result of being out of reach of Earth’s gravity, but rather of constantly “falling”. Going down in an express elevator can give you mild idea of the effect.

In years to come, when longer trips into actual “deep space” will be undertaken, true “no-gravity” will be encountered; and the experience of the first orbiting spacemen will be useful. Contrary to some expectations, the weightlessness they encountered did not interfere with efficiency in bodily functioning; they were able to eat the drink comfortably and perform exacting tasks. Precautions had to be taken, of course when weightless, for instance, water does not fall, but hangs in a kind of ball. Drinking, therefore, had to be done from a plastic squeeze bottle, with the end held firmly in the mouth. In general, in space flight, it will be necessary for the travellers to learn not to take for granted that there is any such thing as up or down. It has been suggested that “partial gravity” may be supplied by imparting spin to the ship so that centrifugal force will give passengers a feeling that the outside of the ship is “down.”