The following are the various recommendations made to date for the practical solution of the space flight problem:
Professor Goddard uses a smokeless powder, a solid substance, as a propellant for his space rockets. He has not described any particular device, but recommends only in general packing the powder into cartridges and injecting it automatically into the combustion chamber, in a fashion similar to that of a machine gun. The entire rocket should be composed of individual subrockets that are jettisoned one after the other during the ascent, as soon as they are spent, with the exception of that subrocket containing the payload, and it alone reaches the destination. First of all, he intends to make unmanned devices climb to an altitude of several hundred kilometers. Subsequently, he also wants to try to send up an unmanned rocket to the Moon carrying only several kilograms of luminous powder. When landing on the Moon, the light flare is supposed to flash, so that it could then be detected with our large telescopes, thus verifying the success of the experiment. Reportedly, the American Navy is greatly interested in Goddard's devices.
The results of practical preliminary experiments conducted and published by Goddard to date are very valuable; the means for carrying out these experiments were provided to him in a very generous manner by the famous Smithsonian Institution in Washington. He was able to attain exhaust velocities up to 2,434 meters per second with certain types of smokeless powder when appropriately shaping and designing the nozzles. During these experiments, he was successful in using 64.5 percent of the energy chemically bonded in the powder, that is, to convert it into kinetic energy of the escaping gases of combustion. The result agrees approximately with the experiences of ballistics, according to which about 2/3 of the energy content of the powder can be used, while the remainder is carried as heat by the exhaust gases and, as a result, is lost. Perhaps, the efficiency of the combustion chamber and nozzle can be increased somewhat during further engineering improvements, to approximately 70 percent.
Therefore, an "internal efficiency" of approximately 60 percent could be expected for the entire propulsion systemthe rocket motorafter taking into consideration the additional losses caused by the various auxiliary equipment (such as pumps and similar devices) as well as by other conditions. This is a very favorable result considering that the efficiency is hardly more than 38 percent even for the best thermal engines known to date.
It is a good idea to distinguish the internal efficiency just considered from that addressed previously: the efficiency of the reactive force, which could also be designated as the "external efficiency" of the rocket motor to distinguish it from the internal efficiency. Both are completely independent from one another and must be considered at the same time in order to obtain the total efficiency of the vehicle (which is just the product of the internal and external efficiency). As an example, the values of the efficiency for benzene as the fuel are listed in Column 3 of Table 2, page 32.
Differing from Goddard, Professor Oberth suggests using liquid propellants, primarily liquid hydrogen and also alcohol, both with the amounts of liquid oxygen necessary for their combustion. The hydrogenoxygen mixturecalled "detonating gas"has the highest energy content (3,780 calories per kilogram compared to approximately 1,240 for the best smokeless powder) per unit of weight of all known substances. Accordingly, it yields by far the highest exhaust velocity. Oberth figured being able to attain approximately 3,8004,200 meters per second. If we were successful in using the energy chemically bonded in detonating gas up to the theoretically highest possible limit, then its exhaust velocity could even exceed 5,000 meters per second. The gas resulting from the combustion is water vapor.
Unfortunately, the difficulty of carrying and using the gas in a practical sense is a big disadvantage compared to the advantage of its significant energy content and therefore relatively high exhaust velocity, due to which the detonating gas would in theory appear to be by far the most suitable propellant for space rockets. Storing hydrogen as well as oxygen in the rocket is possible only in the liquefied state for reasons of volume.
However, the temperature of liquid oxygen is 183°, and that of the liquid hydrogen only 253° Celsius. It is obvious that this condition must considerably complicate the handling, even disregarding the unusual requirements being imposed on the material of the tanks. Additionally, the average density (specific weight) of detonating gas is very low even in a liquefied state so that relatively large tanks are necessary for storing a given amount of the weight of the gas.
In the case of alcohol, the other fuel recommended by Oberth, these adverse conditions are partially eliminated but cannot be completely avoided. In this case, the oxygen necessary for combustion must also be carried on board in the liquid state. According to Oberth, the exhaust velocity is approximately 1,5301,700 meters per second for alcohol, considerably lower than for hydrogen. It does have a greater density, however.
Due to these properties, Oberth uses alcohol together with liquid oxygen as propellants for the initial phase of the ascent, because the resistance of the dense layers of air near the Earth's surface must be overcome during the ascent. Oberth viewed a large crosssectional loading (i.e., the ratio of the total mass of a projectile to the air drag cross section of the projectile) as advantageous even for rockets and recommended, besides other points: "to increase the mass ratio at the expense of the exhaust velocity". This is, however, attained when alcohol and oxygen are used as propellants.
Oberth's space rocket has, in general, the external shape of a German Sprojectile and is composed of individual subrockets that are powered either with hydrogen and oxygen (hydrogen rocket) or with alcohol and oxygen (alcohol rocket). Oberth also described in more detail two examples of his space vehicle. Of the two, one is a smaller, unmanned model, but equipped with the appropriate recording instruments and is supposed to ascend and perform research on the higher and highest layers of air. The other one is a large space ship designed for transporting people.
Figure 27. A longitudinal cross section through the main rocket of Oberth's small rocket model is shown schematically. The hydrogen rocket is inserted in the forward part of the alcohol rocket.
Key: 1. Parachute; 2. Tank; 3. Space for the recording instruments; 4. Propulsion system; 5. Control fins.
The smaller model (Figure 27) consists of a hydrogen rocket that is inserted into the forward part of a considerably larger alcohol rocket. Space for storing the recording instruments is located below the tank of the hydrogen rocket. At the end of the alcohol rocket, movable fins are arranged that are supposed to stabilize and to control the vehicle. The entire apparatus is 5 meters long, measures 56 cm in diameter and weighs 544 kg in the launchready state.
Furthermore, a socalled "booster rocket" (Figure 28)
is provided that is 2 meters high, 1 meter in diameter and weights 220 kg in the launchready state. Launching takes place from dirigibles at an altitude of 5,500 meters or more (Figure 29). Initially the booster rocket, which later will be jettisoned, lifts the main rocket to an altitude of 7,700 meters and accelerates it to a velocity of 500 meters per
Figure 28. The booster rocket of Oberth's small rocket model.
Figure 29. Launching the rocket from dirigibles, according to Oberth.
second (Figure 30). Now, the rocket is activated automatically: first the alcohol rocket and, after it is spent and decoupled, the hydrogen rocket. Fiftysix seconds after the launch, a highest climbing velocity of 5,140 meters per second is attained, which suffices for the remaining hydrogen rocket, now without propulsion, to reach a final altitude of approximately 2,000 km in a free ascent. The return to Earth takes place by means of a selfdeploying parachute stored in the tip of the hydrogen rocket.
Figure 30. The ascent of Oberth's small (unmanned) rocket model.
Key: 1. Free ascent up to an altitude of 2,000 km; 2. Powered ascent lasting 56 seconds; 3. The highest climbing velocity of 5,140 m/sec is attained; 4. Hydrogen rocket; 5. Alcohol rocket; 6. Complete rocket; 7. Altitude of 7,700 m, climbing velocity of 500 m/sec; 8. Booster rocket; 9. Altitude of 5,500 m, climbing velocity of 0; 10. Powered ascent by the hydrogen rocket; 11. The empty alcohol rocket is jettisoned. The hydrogen rocket starts to operate;
12. Power ascent by the alcohol rocket; 13. The empty booster rocket is jettisoned; the main rocket, beginning with its alcohol rocket, starts to operate; 14. Powered ascent by the booster rocket; 15. The launchready vehicle, suspended from dirigibles, as shown in Figure 29.
In the case of the second model, the large rocket space ship designed for transporting people (Figure 31), the total forward part of the vehicle consists of a hydrogen rocket set atop an alcohol rocket in the rear. The cabin designed for passengers, freight, etc. and equipped with all control devices, is located in the forward part of the hydrogen rocket. The parachute is stored above it. Towards the front, the vehicle has a metal cap shaped like a projectile, which later is jettisoned as unnecessary ballast along with the alcohol rocket (Figure 32), because the Earth's atmosphere is left behind at this point, i.e., no further air drag must be overcome. From here on, stabilization and controlling is no longer achieved by means of fins, but by control nozzles.
For this model, launching is performed over the ocean. In this case, the alcohol rocket operates first. It accelerates the vehicle to a climbing velocity of 3,0004,000 meters per second, whereupon it is decoupled and left behind (Figure 32); the hydrogen rocket then begins to work in order to impart to the vehicle the necessary maximum climbing velocity or, if necessary, also a horizontal orbital velocity. A space ship of this nature, designed for transporting an observer, would, according to Oberth's data, weigh 300 metric tons in the launchready state.
Figure 31. A longitudinal cross section of Oberth's large rocket for transporting people is shown schematically. The hydrogen rocket is set atop the alcohol rocket.
Key: 1. Parachute; 2. Cabin; 3. Hydrogen tank; 4. Oxygen tank; 5. Propulsion system; 6. Alcohol tank.
Figure 32. The ascent of Oberth's larger (manned) rocket model.
Key: 1. Horizontal velocity; 2. Parachute; 3. Hydrogen rocket; 4. Alcohol rocket; 5. Ocean; 6. Climbing velocity; 7. Cap; 8. Powered ascent by the hydrogen rocket. Depending on the purpose (vertical ascent or free orbiting), this rocket imparts either a climbing velocity or a horizontal velocity; 9. The empty alcohol rocket and the cap are jettisoned; the hydrogen rocket starts to operate. The climbing velocity attained up to this point is 3,000 to 4,000 meters per second; 10. Powered ascent by the alcohol rocket; 11. The launchready vehicle floating in the ocean.
In both models, the subrockets are independent; each has, therefore, its own propulsion system as well as its own tanks. To save weight, the latter are very thinwalled and obtain the necessary stiffness through inflation, that is, by the existence of an internal overpressure, similar to nonrigid dirigibles. When the contents are being drained, this overpressure is maintained by evaporating the remaining liquid. The oxygen tank is made of copper and the hydrogen tank of lead, both soft metals, in order to prevent the danger of embrittlement caused by the extreme low temperatures discussed previously.
Figure 33. The propulsion system of Oberth's rocket:
Right: the small model. The combustion chamber discharges into only one nozzle.
Left: the large model. A common combustion chamber discharges into many nozzles arranged in a honeycombed fashion.
Key: 1. Sectional view; 2. Pumps; 3. Injectors; 4. Combustion chamber; 5. Nozzles; 6. View from the rear; 7. Nozzle.
The propulsion equipment is located in the rear part of each rocket (Figure 33). For the most part, that equipment consists of the combustion chamber and one or more thin sheet metal exhaust nozzles connected to it, as well as various pieces of auxiliary equipment necessary for propulsion, such as injectors and other devices. Oberth uses unique pumps of his own design to produce the propellant overpressure necessary for injection into the combustion chamber. Shortly before combustion, the oxygen is gasified, heated to 700° and then blown into the chamber, while the fuel is sprayed into the hot oxygen stream in a finely dispersed state. Arrangements are made for appropriately cooling the chamber, nozzles, etc.
It should be noted how small the compartment for the payload is in comparison to the entire vehicle, which consists principally of the tanks. This becomes understandable, however, considering the fact that the amounts of propellants previously calculated with the rocket equation and necessary for the ascent constitute as much as 20 to 80 percent of the total weight of the vehicle, propellant residuals, and payload!
However, the cause for this enormous propellant requirement lies not in an insufficient use of the propellants, caused perhaps by the deficiency of the reaction principle used for the ascent, as is frequently and incorrectly thought to be the case. Naturally, energy is lost during the ascent, as has previously been established, due to the circumstance that the travel velocity during the propulsion phase increases only gradually and, therefore, is not of an equal magnitude (namely, in the beginning smaller, later larger) with the exhaust (repulsion) velocity (Figure 17). Nevertheless, the average efficiency of the reaction would be 27 percent at a constant exhaust velocity of 3,000 meters per second and 45 percent at a constant exhaust velocity of 4,000 meters per second, if, for example, the vehicle is supposed to be accelerated to the velocity of 12,500 meters per second, ideally necessary for complete separation from the Earth. According to our previous considerations, the efficiency would even attain a value of 65 percent in the best case, i.e., for a propulsion phase in which the final velocity imparted to the vehicle is 1.59 times the exhaust velocity.
Since the internal efficiency of the propulsion equipment can be estimated at approximately 60 percent on the basis of the previously discussed Goddard experiments and on the experiences of ballistics, it follows that an average total efficiency of the vehicle of approximately 16 to 27 percent (even to 39 percent in the best case) may be expected during the ascent, a value that, in fact, is no worse than for our present day automobiles! Only the enormous work necessary for overcoming such vast altitudes requires such huge amounts of propellants.
If, by way of example, a road would lead from the Earth into outer space up to the practical gravitational boundary, and if an automobile were supposed to drive up that road, then an approximately equal supply of propellants, including the oxygen necessary for combustion, would have to be carried on the automobile, as would be necessary for the propellants of a space ship with the same payload and altitude.
It is also of interest to see how Oberth evaluated the question of costs. According to his data, the previously described smaller model including the preliminary experiments would cost 10,000 to 20,000 marks. The construction costs of a space ship, suitable for transporting one observer, would be over 1 million marks. Under favorable conditions, the space ship would be capable of carrying out approximately 100 flights. In the case of a larger space ship, which transports, besides the pilot together with the equipment, 2 tons of payload, an ascent to the stable state of suspension (transition into a free orbit) would require approximately 50,000 to 60,000 marks.
The study published by Hohmann about the problem of space flight does not address the construction of space rockets in more detail, but rather thoroughly addresses all fundamental questions of space flight and provides very valuable recommendations related to this subject. As far as questions relating to the landing process and distant travel through outer space are concerned, they will be addressed later.
What is interesting at this point is designing a space vehicle for transporting two people including all necessary equipment and supplies. Hohmann conceives a vehicle structured in broad outlines as follows: the actual vehicle should consist only of the cabin. In the latter, everything is storedwith the exception of the propellant. A solid, explosivelike substance serving as the propellant would be arranged below the cabin in the shape of a spire tapering upward in such a way that the cabin forms its peak (Figure 34). As a result of a gradual burning of this propellant spire, thrust will be generated similar to that of a fireworks rocket. A prerequisite for this is that explosive experts find a substance that, on the one hand, has sufficient strength to keep itself in the desired shape and that, on the other hand, also has that energy of combustion necessary for generating a relatively large exhaust velocity.
Figure 34. The space rocket according to Hohmann.
Key: 1. Cabin cell; 2. Propellant tower; 3. Exhaust gases of combustion.
Assuming that this velocity is 2,000 meters per second, a space vehicle of this nature would weigh, according to Hohmann, a total of 2,800 tons in the launchready state, if it is to be capable of attaining an altitude of 800,000 km (i.e., twice the distance to the Moon). This corresponds approximately to the weight of a small ocean liner. A round trip of this nature would last 30.5 days.
Recent publications by von Hoefft are especially noteworthy. His original thought was to activate the propulsion system of space ships using the space ether. For this purpose, a unidirectional ether flow is supposed to be forced through the vehicle by means of an electrical field. Under Hoefft's assumption, the reaction effect of the ether would then supply the propulsive force of the vehicle, a concept that assumes ether has mass. Hoefft, however, maintained that was assured if the opinion held by Nernst and other researchers proved to be correct. According to this view, the space ether should possess a very significant internal energy (zero point energy of the ether); this was believed to be substantiated by the fact that energy is also associated with mass in accordance with Einstein's Law.
He intends initially to launch an unmanned recording rocket to an altitude of approximately 100 km for the purpose of exploring the upper layers of the atmosphere. This rocket has one stage, is powered by alcohol and liquid oxygen, and is controlled by means of a gyroscope like a torpedo. The height of the rocket is 1.2 meters, its diameter is 20 cm, its initial (launch) weight is 30 kg and its final weight is 8 kg, of which 7 kg are allocated to empty weight and 1 kg to the payload. The latter is composed of a meteorograph stored in the top of the rocket and separated automatically from the rocket as soon as the final altitude is attained, similar to what happens in recording balloons. The meteorograph then falls alone slowly to Earth on a selfopening parachute, recording the pressure, temperature and humidity of the air. The ascent is supposed to take place at an altitude of 10,000 meters from an unmanned rubber balloon (pilot balloon) to keep the rocket from having to penetrate the lower, dense layers of air.
As the next step, von Hoefft plans to build a larger rocket with an initial weight of 3,000 kg and a final weight of 450 kg, of which approximately 370 kg are allocated to empty weight and 80 kg to the payload. Similar to a projectile, the rocket is supposed to cover vast distances of the Earth's surface (starting at approximately 1,500 km) in the shortest time on a ballistic trajectory (Keplerian ellipses) and either transport mail or similar articles or photograph the regions flown over (for example, the unexplored territories) with automatic camera equipment. Landing is envisaged in such a manner that the payload is separated automatically from the top before the descent, similar to the previously described recording rocket, descending by itself on a parachute.
This singlestage rocket could also be built as a twostage rocket and as a result be made appropriate for a Moon mission. For this purpose, it is equipped, in place of the previous payload of approximately 80 kg, with a second rocket of the same weight; this rocket will now carry the actual, considerably smaller payload of approximately 5 to 10 kg. Because the final velocities of both subrockets in a twostage rocket of this type are additive in accordance with the previously explained staging principle, a maximum climbing velocity would be attained that is sufficiently large to take the payload, consisting of a load of flash powder, to the Moon. When landing on the Moon, this load is supposed to ignite, thus demonstrating the success of the experiment by a light signal, as also proposed by Goddard. Both this and the aforementioned mail rocket are launched at an altitude of 6,000 meters from a pilot balloon, a booster rocket, or a mountain top.
In contrast to these unmanned rockets, the large space vehicles designed for transporting people, which Hoefft then plans to build in a followon effort, are supposed to be launched principally from a suitable body of water, like a seaplane, and at the descent, land on water, similar to a plane of that type. The rockets will be given a special external shape (somewhat similar to a kite) in order to make them suitable for their maneuvers.
The first model of a space vehicle of this type would have a launch weight of 30 tons and a final weight of 3 tons. Its purpose is the following: on the one hand, to be employed similarly to the mail rocket yet occupied by people who are to be transported and to cover great distances of the Earth's surface on ballistic trajectories (Keplerian ellipses) in the shortest time; and, on the other hand, it would later have to serve as an upper stage of larger, multistage space ships designed for reaching distant celestial bodies. Their launch weights would be fairly significant: several hundred metric tons, and even up to 12,000 tons for the largest designs.