ROCKET PROPULSION COOPER 311 



Table 4. — Vehicles for a lunar mission — 55,000 pounds landed on the lunar surface 



lant being used in upper stages. Solid propellant rockets will be used 

 for small final stages (e.g., retro rockets), low total cost, low payload 

 research rockets, and possibly as large, first-stage boosters. Nuclear 

 propulsion will be used in upper stages for difficult missions (lunar 

 and interplanetary) . The low-thrust electric propulsion systems will 

 be limited to orbital start interplanetary missions. 



AUXILIARY POWER SUPPLIES 



Up to the present, electrical power has been supplied to spacecraft 

 mainly by chemical batteries or solar cells. The former are relatively 

 heavy per unit of output, and have short lifetimes. The latter are 

 limited to low powers which will fluctuate with, the spacecraft's ori- 

 entation and position, and are affected by radiation. Some of these 

 difficulties are relieved by using the two in conjunction, allowing the 

 solar cells periodically to charge the batteries, w^hicli supply continu- 

 ous power. Nuclear energy represents a way to circumvent the life- 

 time and power limitations. One method, which has already been put 

 into practice, is to use the heat generated by radioactive isotopes to 

 supply energy to thermoelectric generators. These convert heat into 

 electricity in the same manner as do temperature-measuring thermo- 

 couples. Radioisotope sources are somewhat limited in power (sev- 

 eral hundred watts) but can have lifetimes ranging from 100 days to 

 100 years or more in practice, depending upon the isotope chosen. 



For high powers (kilowatts) and long times (years) nuclear re- 

 actors are the only practical source. A nuclear electric power supply 

 must include power conversion equipment, radiators to reject unusable 

 heat, and possibly some shielding, as well as the reactor. This leads 

 to system weights of the order of 1,000 pounds, useful only in large 

 payloads. At present, only rotating electric generators are sufficiently 

 developed to handle the high power. These will have a metal vapor 

 (such as mercury) heated by the reactor, to power a turbine which 

 drives a generator. The vapor is condensed and cooled in the radi- 

 ator to complete the cycle. E^•entually, high-power thermoelectric 

 conversion systems w-ill be developed with lower weight, higher effi- 

 ciency, fewer moving parts, and greater reliability than the turbo- 

 generator system. 



672-174—63 22 



