This method is, for all practical purposes, not useable. The energy that the space vehicle requires for overcoming gravity and air drag, as well as for moving in empty space, must be supplied to it in another manner, that is, by way of example, bound in the propellants carried on board the vehicle during the trip. Furthermore, a propulsion motor must also be available that allows the propulsion force during the flight to change or even shut off, to alter the direction of flight, and to work its way up gradually to those high, almost cosmic velocities necessary for space flight without endangering passengers or the payload.
But how do we achieve all of this? How is movement supposed to be possible in the first place since in empty space neither air nor other objects are available on which the vehicle can support itself (or push off from, in a manner of speaking) in order to continue its movement in accordance with one of the methods used to date? (Movement by foot for animals and human beings, flapping of wings by birds, driving wheels for rolling trucks, screws of ships, propellers, etc.)
Figure 11. The "reactive force" or recoil when firing a rifle
Key: 1. Powder gases; 2. Recoil; 3. Pressure of the powder gases.
A generally known physical phenomenon offers the means for this. Whoever has fired a powerful rifle (and in the present generation, these people ought not to be in short supply) has, no doubt, clearly felt the socalled "recoil" (maybe the experience was not altogether a pleasant one). This is a powerful action that the rifle transfers to the shooter against the direction of discharge when firing. As a result, the powder gases also press back onto the rifle with the same force at which they drive the projectile forward and, therefore, attempt to move the rifle backwards (Figure 11).
Figure 12. Even when a person quickly shoves an easily movable, bulkier object (e.g., a freely suspended iron ball) away from himself, he receives a noticeable "reactive force" automatically.
1. Action; 2. Reaction.
However, even in daily life, the reaction phenomenon can be observed again and again, although generally not in such a total sense: thus, for example, when a movable object is pushed away with the hand (Figure 12), exactly the same thrust then imparted to the object is, as is well known, also received by us at the same time in an opposite direction as a matter of course. Stated more precisely: this "reaction" is that much stronger, and we will as a result be pushed back that much further, the harder we pushed. However, the "velocity of repulsion," which the affected object being pushed away attains as a result, is also that much greater. On the other hand, we will be able to impart a velocity that much greater to the object being pushed away with one and the same force, the less weight the object has (i.e., the smaller the mass). And likewise we will also fall back that much further, the lighter we are (and the less we will fall back, the heavier we are).
The physical law that applies to this phenomenon is called the "law maintaining the center of gravity." It states that the common center of gravity of a system of objects always remains at rest if they are set in motion only by internal forces, i.e., only by forces acting among these objects.
In our first example, the pressure of powder gases is the internal force acting between the two objects: projectile and rifle. While under its influence the very small projectile receives a velocity of many hundreds of meters per second, the velocity, on the other hand, that the much heavier rifle attains in an opposite direction is so small that the resulting recoil can be absorbed by the
Figure 13. If the "reaction" of the rifle is not absorbed, it continually moves backwards (after firing), and more specifically, in such a manner that the common center of gravity of rifle and projectile remains at rest.
Key: 1. Prior to firing; 2. Common center of gravity of the rifle and projectile; 3. After firing
shooter with his shoulder. If the person firing the rifle did not absorb the recoil and permitted the rifle to move backwards unrestrictedly (Figure 13), then the common center of gravity of the projectile and rifle would actually remain at rest (at the point where it was before firing), and the rifle would now be moving backwards.