Summary Session, 19 February 1957
Participants and Their Topics
(Panel Leader), President, Aerophysics Development Corp.
General topic: "A Comparison of Various Types of Propulsion Systems, Present and Future"
Robert A. Cornog, Ramo-Wooldridge Corp., Los Angeles
Gabriel M. Giannini, President, Giannini Research Corp., Los Angeles, Calif.
A.P. Graff, Vice-President and Executive Engineer, Propulsion Research Corp., Santa Monica, Calif.
Y.C. Lee, Head, Study and Applied Research Department, Liquid Engine Division, Aero-jet General Corporation, Azusa, California
Oliver E. Rogers, Chief Engineer, Utica-Bend Corp., Utica, Mich.
Ernst Stuhlinger, Chief, Research Section, Guidance Control Branch, Guided Missile Development Group, Redstone Arsenal, Huntsville, Ala.
George P. Sutton, Rocketdyne Division of North American Aviation, Canoga Park, California
Comdr. Robert C. Truax, Deputy Commander for Technical Operations, USAF Western Development Division, Inglewood, Calif.
Hans von Ohain, Supervisory Physicist, Fluid Dynamics Branch, Aeronautical Research Laboratory, Wright Air Development Center, Dayton, Ohio
The conclusions of the Propulsion Panel were that propulsion systems will probably pace the development of vehicles for space travel in very much the same way as power plant developments have paced the developments of aircraft and missiles over the past decade.
The propulsion systems for space travel may be divided into two types. First of all, the high acceleration propulsion systems which are needed in order to leave the surface of the earth or any other planet and to climb into a satellite orbit. These systems have to have an acceleration of greater than 1 g. It currently appears that chemical rocket propulsion systems will be required for this purpose. The second type of propulsion system which has been suggested is for travel from the earth satellite orbit to any other planetary satellite orbit. For this purpose, low acceleration propulsion systems may also be considered. For this second mission, therefore, either chemically powered rockets or some of the more unconventional propulsion systems may be considered.
Two such unconventional propulsion systems were described by Dr. Stuhlinger. The first of these is a propulsion system which utilizes a nuclear power plant for generating the necessary power for accelerating ions. This system would have an acceleration of the order of 10–4 g. It would be suitable for instance for such a mission as going from a satellite orbit around earth to a satellite orbit around Mars. Another system which has been suggested is the nuclear or solar power system heating a light gas. Such a system would have an acceleration of the order of 10–2 g. The technology of these two systems is known in principal; however, a great deal of research and development would have to be carried out to make them into practical propulsion systems.
Thus, at this time, the chemical rocket propulsion system may be considered as relatively proven and suitable for propulsion of spacecraft. The physical sizes of the space craft required would be very large if presently available chemical propulsion systems were used. There is some promise that over the coming decade the specific impulse which measures the efficiency of the chemical rockets may be increased approximately 50%. If this is accomplished, the size of the space craft could be appreciably reduced.
Photon propulsion systems were also discussed which convert the mass with a 100% efficiency into light in accordance with the Einstein energy equation: E= mc2. However, no one at this time can visualize any method for constructing a photon rocket with the necessary large thrust-weight ratios, and thus the trajectory calculations based on photon rockets must at this time be considered purely as an interesting mathematical exercise.
In conclusion it appears that the current state of the art of astronautics is somewhat comparable to the state of the art in aeronautics in the 1880's. Engineers can visualize how to construct large vehicles required for space travel; however, it is going to require a major engineering development effort to translate these concepts into practical devices.
Now going through the various papers in somewhat more detail, Dr. Stuhlinger presented a chart which showed a comparison of the four propulsion systems; namely, chemical, nuclear solar power, nuclear power plus ions, and photonic. The chemical system has exhaust velocities of the order of 3000 m/sec; the nuclear solar power system of the order of twice as much; the nuclear power system accelerating ions would have the order of thirty times as much; the photonic system would have of the order of one hundred thousand times as much; but, as we mentioned before, the technology on this latter one is completely unknown. Dr. Stuhlinger also reviewed briefly the trajectory of a photon rocket which is of somewhat academic interest.
Then the discussion proceeded to the more nearly standard chemical propulsion systems. Commander Truax pointed out that for propulsion in outer space the chemical rocket should be optimized in a different manner than the manner in which I chemical rockets are currently built; namely, because of practically zero back pressure, it is possible to reduce the combustion chamber pressure to perhaps 2 arm. instead of 300–800 psi, which corresponds to 20–40 atm. He pointed out that such a reduction in chamber pressure would result in a saving of tank weight but might involve some penalties in engine thrust-to-weight ratio. However, the penalty is off-set by the small flight path angle which reduces gravity losses and by low stress and heat flux density which permit lighter engine design. New problems will arise in heat transfer injection because there is some possibility of fuels boiling before they get into the rocket combustion chamber. His conclusion was that the time is right for an intensive development program in this area.
Dr. Lee of Aerojet covered some of the same territory mentioned by other speakers. He concentrated in addition on a discussion of the areas of research which are needed to obtain the goal of astronomical flight. Some of the major points which he mentioned in connection with chemical rockets are: improvement of component reliability, weight reduction, and optimum integration of the rocket propulsion system and the overall missile. He also pointed out that for the astronautical flight there is a very major payoff in using the high energy propellants which currently appear on the horizon. There are however many problems in combustion dynamics which need to be overcome.
It was his estimate that nuclear rockets utilizing fission reaction may appear feasible provided little or no biological shielding is required. The problem of high temperature for such a nuclear rocket is, however, a major one because in the nuclear rocket the wall temperature will be higher than that of the combustion gases in order to be able to affect any heat transfer. Consequently, the materials problem is a much more difficult one for a nuclear rocket than for a conventional rocket. The fusion rocket, based on what he has been able to learn from the unclassified literature, cannot be achieved except in the very distant future. Some of the more promising areas for future research, which he points out, are the use of free radicals for a propellant and the ion rocket which was already mentioned before by Dr. Stuhlinger. Dr. Lee recommends a continued research on the ion rocket, particularly in the problems of obtaining a high yield of ions, methods of separating the positive and negative ions, and practical methods of acceleration.
Mr. George Sutton of North American Aviation covered the various chemical propulsion systems. His overall conclusions are that presently known devices should make flight possible to the Moon, Mercury, Venus, and Mars; that multiple stage devices with reasonable flight times make it also theoretically possible to fly missions to Jupiter and Saturn; that there is no known power plant today which would permit trips to Neptune or Pluto or which would permit escape from the solar system in any reasonable length of time.
Mr. Rogers of the Utica Bend Corporation commented briefly on the possibilities of using light weight turbo jet engines for the initial phase of boost of multiple stage rocket systems, and on the possibility of utilizing turbo jet propulsion systems after re-entry into the earth's atmosphere for extending the gliding range for the landing craft. He commented upon the fact that when landing on planets other than the earth, it might become necessary to consider the different chemicals as fuels for reaction with the atmosphere in these planets.
Mr. Cornog presented some charts in which he pointed out the methods of oxidizing the ion rocket, and his conclusions confirmed pretty much the analysis which has also been carried out by Dr. Stuhlinger.
Dr. von Ohain gave a comprehensive survey of the various non-a& breathing propulsion systems and the corresponding vehicles. His conclusions were that chemical propulsion systems will require elaborate staging for planetary or moon operations and return missions. Application of high energy fuels will considerably reduce required fuel weights; however basic principles of staging as described by Dr. von Braun in "The Mars Project" will still be required. The realization of free radical recombination or nuclear heat transfer rockets will have a great influence on space missions. He also concludes that ion rockets are restricted to missions where thrust-to-weight ratio below unity can be applied, such as missions from the earth's orbit to planetary orbit.
Dr. Giannini reported on recent experiments on highly ionized gas jets with energy content greater than chemically accelerated jets. This work is an extension of some methods developed in Germany for the direct transfer of electrical energy into kinetic energy by means of discharge of gas. He points out that continuous operation at temperatures in excess of 1,000 degrees K has been obtained, and powers in excess of 50 kw/mm2 of nozzle area have been absorbed. This plasma jet will be a flexible and powerful means for study of gas dynamics. The question of whether this type of a jet will be usable for propulsion purposes is still open because of power requirements: the problem of generating sufficient electrical power on board the vehicle to create these high velocity jets.