Regarding these various proposals, the following is added as supplementary information: as far as can be seen from today's perspective, the near future belongs in all probability to the space rocket with liquid propellants. Fully developed designs of such rockets will be achieved when the necessary technical conditions have been created through practical solutions (obtained in experiments) of the questions fundamental to their design: 1. methods of carrying the propellants on board, 2. methods of injecting propellants into the combustion chamber, and 3. protection of the chamber and nozzle from the heat of combustion.
For this reason, we intentionally avoided outlining our own design recommendations here. Without a doubt, we consider it advisable and necessary, even timely, at least as far as it is possible using currently available experiences, to clarify the fundamentals of the vehicle's structure; the question of propellant is predominantly in this context. As stated earlier, hydrogen and oxygen, on the one hand, and alcohol and oxygen, on the other, are suggested as propellants.
In the opinion of the author, the pure hydrocarbon compounds (together with the oxygen necessary for combustion) should be better suited than the ones mentioned in the previous paragraph as propellants for space rockets. This becomes understandable when the energy content is expressed as related to the volume instead of to the weight, the author maintaining this as being the most advantageous method in order to be able to evaluate the value of a rocket fuel in a simple fashion. Not only does it matter what amount of fuel by weight is necessary for a specific performance; still more important for storing the fuel, and as a result for designing the vehicle, is what amount of fuel by volume must be carried on board. Therefore, the energy content (thermal units per liter) of the fuel related to the volume provides the clearest information.
This energy content is the more significant the greater the specific weight as well as the net calorific value of the fuel under consideration are, and the less oxygen it requires for its combustion. In general, the carbon rich compounds are shown to be superior to the hydrogen rich ones, even though the calorific value per kilogram of the latter is higher. Consequently, benzene would appear very suitable, for example. Pure carbon would be the best. Because the latter, however, is not found in the fluid state, attempts should be made to ascertain whether by mechanical mixing of a liquid hydrocarbon (perhaps benzene, heptane, among others) with an energy content per liter as high as possible with finely dispersed carbon as pure as possible (for instance carbon black, the finest coal dust or similar products), the energy content per liter could be increased still further and as a result particularly high quality rocket fuel could be obtained, which may perhaps be overall the best possible in accordance with our current knowledge of substances.
Of course, an obvious condition for the validity of the above considerations is that all fuels work with the same efficiency. Under this assumption by way of example, a space rocket that is supposed to attain the final velocity of 4,000 meters per second would turn out to be smaller by about one half and have a tank surface area smaller by one third when it is powered with benzene and liquid oxygen than when powered by liquid hydrogen and oxygen (Figure 35).
Figure 35. Size relationship between a hydrogen rocket and a benzene rocket of the same performance, when each one is supposed to be capable of attaining a velocity of 4,000 meters per second.
Key: 1. Hydrogen rocket; 2. Benzene rocket.
Therefore, the benzene rocket would not only be realized sooner from an engineering point of view, but also constructed more cheaply than the hydrogen rocket of the same efficiency, even though the weight of the necessary amount of fuel is somewhat higher in the former case and, therefore, a larger propulsion force and, consequently, stronger, heavier propulsion equipment would be required. Instead, the fuel tanks are smaller for benzene rockets and, furthermore, as far as they serve the purposes of benzene at least, can be manufactured from any lightweight metal because benzene is normally liquid. When considering its abnormally low temperature (253° Celsius) according to Oberth, a point made previously, rockets for liquid hydrogen would have to be made of lead (!). This discussion ignores completely the many other difficulties caused by this low temperature in handling liquid hydrogen and the method of using this fuel; all of these difficulties disappear when using benzene.
Figure 36. Size relationship between a hydrogen rocket and a benzene rocket of the same performance, when each one is supposed to be capable of attaining a velocity of 12,500 meters per second (complete separation from the Earth!).
Key: 1. Hydrogen rocket; 2. Benzene rocket
However, this superiority of liquid hydrocarbons compared to pure hydrogen diminishes more and more at higher final velocities. Nevertheless, a benzene rocket would still turn out to be smaller by one third than a hydrogen rocket, even for attaining a velocity of 12,500 meters per second as is ideally necessary for complete separation from the Earth (Figure 36). Only for the final velocity of 22,000 meters per second would the volumes of propellants for the benzene rocket be as large as for the hydrogen rockets. Besides these energy efficient advantages and other ones, liquid hydrocarbons are also considerably cheaper than pure liquid hydrogen.