If the rifle was now attached to a lightweight cart (Figure 14) and fired, it would be set in motion by the force of the recoil. If the rifle was fired continually and rapidly, approximately similar to a machine gun, then the cart would be accelerated, and could also climb, etc. This would be a vehicle with reaction propulsion, not the most perfect, however. The continual movement of a vehicle of this type takes place as a result of the fact that it continually accelerates parts of its own mass (the projectiles in the previous example) opposite to the direction of motion and is repelled by these accelerated parts of mass.
Figure 14. A primitive vehicle with reaction propulsion: The cart is moved by continuous firing of a rifle, as a result of the "reaction" generated thereby.
Key: 1. The masses flung away (the projectiles in this case); 2. Recoil; 3. Direction of travel
It is clear as a result that this type of propulsion will then be useful when the vehicle is in empty space and its environment has neither air nor something else available by which a repulsion would be possible. Indeed, the propulsion by recoil will only then be able to develop its greatest efficiency because all external resistances disappear.
For the engineering design of a vehicle of this type, we must now strive to ensure that for generating a specific
propulsive force, on the one hand, as little mass as possible must be expelled and, on the other hand, that its expulsion proceeds in as simple and operationally safe way, as possible.
To satisfy the first requirement, it is basically necessary that the velocity of expulsion be as large as possible. In accordance with what has already been stated, this can be easily understood even without mathematical substantiation, solely through intuition: for the greater the velocity with which I push an object away from me, the greater the force I have to apply against it; in accordance with what has already been stated, then the greater the opposite force will be that reacts on me as a result; this is the reaction produced by the expulsion of precisely this mass.
Furthermore, it is not necessary that larger parts of mass are expelled over longer time intervals, but rather that masses as small as possible are expelled in an uninterrupted sequence. Why this also contributes to keeping the masses to be expelled as low as possible, follows from mathematical studies that will not be used here, however. As can be easily understood, the latter must be required in the interest of operational safety, because the propulsive thrust would otherwise occur in jolts, something that would be damaging to the vehicle and its contents. Only a propulsive force acting as smoothly as possible is useful from a practical standpoint.