ROCKET PROPULSION — COOPER 307 



The liquid hydrogen presents several difficulties. First, it is only 

 one-fourteenth as dense as water, or most chemical fuels) and thus 

 requires relatively larger and heavier tankage. Secondly, it boils at 

 — 423° F., wliich is close to absolute zero ( — 459° F.) , and thus requires 

 special insulating and handling techniques. It is cold enough not only 

 to liquefy air, but to freeze it solid. Nevertheless, hydrogen is being 

 used as a liquid fuel in the chemical rocket program, and techniques 

 are being developed which should make its use routine. 



The nuclear radiation presents a number of problems, but these 

 can be met with straightforward solutions in most cases. It has been 

 shown that even with regular launchings in the atmosphere, the world- 

 wide contamination would be negligible. Hazards for manned oper- 

 ations must be minimized, but the problems may not be very 

 different from those associated with chemical propulsion. ]\Ianned 

 space flight may require extensive shielding against space radiation, 

 which will be effective against the nuclear engine as well. 



The exhaust velocity for nuclear propulsion is about equal to the 

 velocity increment needed to achieve earth orbit, and therefore a single 

 nuclear stage is capable of going into orbit with considerable (about 

 20 percent) payload. For difficult missions, only about half as many 

 nuclear stages as chemical stages need be used, increasing reliability 

 and decreasing launch costs. Finally, operation in space reduces 

 many of the radiation problems and weight penalties associated with 

 nuclear propulsion (lower thrust, lower weight engines can be used, 

 lighter structured H2 tanks employed) . 



ADVANCED PROPULSION SYSTEMS 



Since the nuclear heat exchanger engine uses less than 0.1 percent 

 of the fission energy available, we can see that only the beginnings 

 of nuclear propulsion have been touched upon. The problem lies 

 not in obtaining the energy as much as in dealing with the higher 

 temperatures involved when this energy is transferred to the pro- 

 pellant. An additional incentive for seeking temperatures above 

 3,000° C. is the disassociation of the Ho molecules into H atoms; 

 disassociation occurs over a range of temperatures, wliich allows much 

 greater storage of energy in the propellant at these temperatures. 

 This could lead to exhaust velocities of up to 50,000 feet per second. 

 There has been hope of making gaseous core reactors, operating at up 

 to 10,000° C, but the problem of separating the gaseous fuel from 

 the propellant appears insurmountable if reasonable thrust is desired. 

 One possibility for circumventing the material temperature problem 

 is the use of small nuclear explosions in what might be called an 

 "external combustion engine." The nuclear explosive heats the pro- 

 pellant behind the vehicle. The propellant impinges on a large. 



