It is the purpose of this report to summarize certain considerations and proposals, some of which originated as long as ten years ago, and to discuss additional ideas concerning the attempt to attain velocities in the range of the missiles considered for intercontinental warfare and even more perhaps, for escape from the earth's gravitational field, for unmanned vehicles.
The methods most frequently proposed for obtaining such vehicles involve expulsion of material at high velocity froml rocket motors. This ejected material is heated in the rocket itself. either by a chemical reaction. or. in more recent schemes, by nuclear reactors.l In both cases there is a severe limitation on motor temperature and thus also on the velocity of material ejected. The well-known exponential rocket formula* then demlanlds impractical mass ratios for the attainment of final velocities Vf in the desired ranges, and multi-stage vehicles become necessary. The advantage of the nuclear rocket of this kind over the chemical type lies paradoxically not so much in its potentially enormous power source, which is limited by chamber temperature T to much the same range as chemical motors, but in its ability to use hydrogen as propellant. with molecular weight ,t lower than the average of chemical reaction products2 . thus permitting operation at higher specific impulse, which is a function of T-/.
The scheme proposed in the present report involves the use of a series of expendable reactors (fissioii bombs) ejected and detonated at a considerable distance from the vehicle, which liberate the required energy in an external "motor" consisting essentially of empty space. The critical question about such a method concerns its ability to draw on the real reserves of nuclear power liberated at bomb temperatues without smashing or melting thle vehicle.
General proposals of this sort were first made by S. Ulam in 1946. and some preliminary calculations were made by F. Reines and S. Ulam in a Los Alamos memorandum dated 1947. More recently, an additional idea was advanced. which consists in placing between each bomb and the rocket a "propellant" consisting of water or some plastic, which will be heated by the bomb. and which will propel the vehicle during its subsequent explosive expansion. Some of the advantages of this proposal will be mentioned in the final section.
In any such device. one of the principal difficulties is the heating of the rocket by the propellant. We seem to encounter a situation in which the base of the rocket will be. periodically, at one second intervals, in the proximity of a very hot gas for durations of about one millisecond each. Study of the effects of such a variable wall temperature on various materials will be made. and reported on subsequently.
The most recent idea is that the use of a sufficiently powerful magnetic field shielding the base of the rocket will have the effect of reflecting the (ionized) atoms of the hot propellant gas before they reach the rocket. thus avoiding heating of the base and incidentally gaining a factor on momentum transfer. It is hoped that this possibility
* AMo/Af = mass-ratio = exp(Vl/I). I = specific impulse.
also may be investigated at least schematically and reported on in Part II.* However, there appear to be many difficulties in such a study, involving the reaction of a plasma to the magnetic field. Whether the field strength required is impractically large remains to be seen. There is, it seems, the possibility of the formation of a powerful plasma current at the base of the rocket and a pinch effect, which may mean that the magnetic field becomes compressed to a smaller volume and the magnetic pressure considerably increased.