The Return to Earth


The previous explanations indicate that obstacles stand in the way of the ascent into outer space which, although significant, are nonetheless not insurmountable. Based solely on this conclusion and before we address any further considerations, the following question is of interest: Whether and how it would be possible to return to Earth after a successful ascent and to land there without experiencing any injuries. It would arouse a terrible horror even in the most daring astronaut if he imagined, seeing the Earth as a distant sphere ahead of him, that he will land on it with a velocity of no less than approximately 12 times the velocity of an artillery projectile as soon as he, under the action of gravity, travels towards it or more correctly stated, crashes onto it.

The rocket designer must provide for proper braking. What difficult problem is intrinsic in this requirement is realized when we visualize that a kinetic energy, which about equals that of an entire express train moving at a velocity of 70 km/hour, is carried by each single kilogram of the space ship arriving on Earth! For, as described in the beginning, an object always falls onto the Earth with the velocity of approximately 11,000 meters per second when it is pulled from outer space towards the Earth by the Earth's gravitational force. The object has then a kinetic energy of around 6,000 metric ton meters per kilogram of its weight. This enormous amount of energy must be removed in its entirety from the vehicle during braking.

Only two possibilities are considered in this regard: either counteracting the force by means of reaction propulsion (similar to the "reverse force" of the machine when stopping a ship), or braking by using the Earth's atmosphere. When landing according to the first method, the propulsion system would have to be used again, but in an opposite direction to that of flight (Figure 37). In this regard, the vehicle's descent energy would be removed from it by virtue of the fact that this energy is offset by the application of an equally large, opposite energy. This requires, however, that the same energy for braking and, therefore, the same amount of fuel necessary for the ascent would have to be consumed. Then, since the initial velocity for the ascent (highest climbing velocity) and the final velocity during the return (descent velocity) are of similar magnitudes, the kinetic energies, which must be imparted to the vehicle in the former case and removed in the latter case, differ only slightly from one another.

Figure 37. Landing with reaction braking. The descending vehicle is supposed to be "cushioned" by the propulsion system, with the latter functioning "away from the Earth" opposite to the direction of flight, exactly similar to the ascent.

Key: 1. The space ship descending to the Earth; 2. Direction of effect of the propulsion system; 3. Earth

For the time being, this entire amount of fuel necessary for braking must still and this is critical be lifted to the final altitude, something that means an enormous increase of the climbing load. As a result, however, the amount of fuel required in total for the ascent becomes now so large that this type of braking appears in any case extremely inefficient, even nonfeasible with the performance levels of currently available fuels. However, even only a partial usage of the reaction for braking must be avoided if at all possible for the same reasons.

Another point concerning reaction braking in the region of the atmosphere must additionally be considered at least for as long as the travel velocity is still of a cosmic magnitude. The exhaust gases, which the vehicle drives ahead of it, would be decelerated more by air drag than the heavier vehicle itself and, therefore, the vehicle would have to travel in the heat of its own gases of combustion.

Figure 38. Landing during a vertical descent of the vehicle using air drag braking.

Key: 1. Descent velocity of 11,000 m/sec; 2. Parachute; 3. The space ship descending to Earth; 4. Braking distance, i.e., the altitude of the layers of the atmosphere (approx. 100 km) probably suitable for braking; 5. Earth.

The second type of landing, the one using air drag, is brought about by braking the vehicle during its travel through the Earth's atmosphere by means of a parachute or other device (Figure 38). It is critical in this regard that the kinetic energy, which must be removed from the vehicle during this process, is only converted partially into air movement (eddying) and partially into heat. If now the braking distance is not sufficiently long and consequently the braking period is too short, then the resulting braking heat cannot transition to the environment through conduction and radiation to a sufficient degree, causing the temperature of the braking means (parachute, etc.) to increase continuously.

Now in our case, the vehicle at its entry into the atmosphere has a velocity of around 11,000 meters per second, while that part of the atmosphere having sufficient density for possible braking purposes can hardly be more than 100 km in altitude. According to what was stated earlier, it is fairly clear that an attempt to brake the vehicle by air drag at such high velocities would simply lead to combustion in a relatively very short distance. It would appear, therefore, that the problem of space flight would come to nought if not on the question of the ascent then for sure on the impossibility of a successful return to Earth.


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