ROCKET PROPULSION — COOPER 309 



principles applied in "atom smashers" and television tubes. There 

 are two major problems associated with these engines. First, 

 although their exhaust velocity is very higli, they are limited to very 

 small total thrust compared to the weight of the engine and power 

 supply. Thus they can be used only in space where the vehicle is 

 already in orbit and a very small acceleration (less than one-thou- 

 sandth that of the earth's gravity) acting over a long period (montlis 

 or years) is acceptable. Secondly, the long period of operation has 

 several unfortimate consequences. Electrical power sources must be 

 available which are very light in weight and capable of operating 

 many months to several years with little or no attention. This leads 

 to a small nuclear reactor as the only energy source with either a tur- 

 bine-generator or a thermoelectric system to convert the reactor's heat 

 to electricity. The latter, with no moving parts, would be preferable 

 for a long-lived operation. For electrical propulsion, the power/ 

 weight of the energy source is the measure of performance i-ather than 

 the exhaust velocity. The long period of thrust and low acceleration 

 also implies long periods spent in the radiation belts and in transit to 

 planets, which is undesirable for manned flight. Nevertheless, the 

 high exhaust velocity makes this form of propulsion attractive for 

 very high velocity missions such as exploration of the solar system 

 with probes or manned vehicles. Botli the engines and the power 

 sources (which are the crucial components) are being developed. 



There are other low-thrust propulsion systems which may be of in- 

 terest for scientific payloads. One type would use solar energy to 

 heat hydrogen with a plastic mirror system replacing the reactor as 

 the energy source. The weight of aluminized plastic film and the 

 relatively low energy rate in solar radiation limit this to low thrust/ 

 weight ratios. A very intriguing propulsion technique, based on the 

 momentum carried by solar radiation, has been called "solar sailing." 

 Here a veiy large aluminized plastic sail is used to catch and reflect 

 sunshine much as ship sails catch the wind. The accelerations are 

 small, but significant as has been shown by the effects of the sun upon 

 the large Echo balloon satellite, which also demonstrated that present 

 materials could withstand the space environment for long periods. 

 Note also that the "propellant" is composed of particles of light 

 ("photons") supplied by the sun and thus the solar sail need not carry- 

 any propellant or ever need refueling. 



PERFORMANCE COMPARISON 



There is as much variety in space propulsion as in surface trans- 

 portation, and consequently there are varied performance levels and 

 areas of application. The method of propulsion used in practice is 

 dependent upon availability, cost, and other criteria, as well as per- 

 formance simply on a gross weight basis. Table 2 gives approximate 



