ROCKET PROPULSION — COOPER 305 



a portion of the boost phase of flight. Since only the fuel need be 

 carried in the vehicle, much greater efficiency is possible in the region 

 up to 10,000 feet per second (7,000 miles per hour) which is one-third 

 of orbital velocity. If sufficiently large, high-speed aircraft engines 

 and airframes could be built, they could be used as flyable, recoverable 

 boosters. In combination with rocket propulsion such "planes" 

 might even be powered into orbit (the so-called "aerospace plane"), 

 although this seems a very formidable task. 



NUCLEAR PROPULSION 



Nuclear energy can be a very compact type of almost limitless 

 energy, and it is natural to seek some way of utilizing it for space 

 propulsion. Any form of rocket will require some form of propellant 

 to provide the thrust by being expelled from the vehicle, but with 

 a separate energy supply, this could be used much more effectively 

 (i.e., with higher exhaust velocity). There are many methods of 

 nuclear propulsion, with efficiency and complexity generally increas- 

 ing together. Emphasis here is on those which are closest to becom- 

 ing a practical reality. 



The simplest and most straightforward way of using nuclear power 

 in a rocket is to replace the liquid rocket combustion chamber with a 

 nuclear reactor to supply heat to the propellant. The reactor is an 

 array of solid nuclear fuel elements containing a fissionable fuel. 

 Wlien the reactor is brought to power, the heat generated in its fuel 

 is transferred directly to the liquid propellant which is pumped 

 through the reactor. The liquid is vaporized, heated to a very high 

 temperature, and expelled through a nozzle to provide the thrust. 

 Since the heat energy is supplied by a source independent of the pro- 

 pellant (rather than by the propellant's chemical energy), one has 

 a freer choice of propellant. 



By choosing hydrogen, which has the lowest molecular weight, one 

 can readily achieve exhaust velocities of 25,000 to 30,000 feet per 

 second, about twice those of the best chemical propulsion system. 

 This high performance is partially offset by the heavier dead weight 

 (10 to 15 percent necessitated by the reactor and H2 tankage) and by 

 the complications arising from the nuclear radiation emanating from 

 the reactor. Nevertheless, the high performance is invaluable for 

 missions in the interplanetary class and useful for less ambitious ones. 



The nuclear reactor is basically a simple device and its chain reac- 

 tion can be easily controlled through the movement of neutron- 

 absorbing materials in the core (the fuel-bearing region) or the 

 reflector (an outer layer of material which helps to keep the neutrons 

 from escaping). Since there is no combustion, explosions are un- 

 likely, and the nuclear engine should prove to be quite reliable from 



