302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



cast propellant 



igniter 



case 



nozzle 



Figure 1. — a, Typical modern solid propellant rocket; b, recession of internal surface with 



time. 



The propellant itself must contain both a fuel and an oxidizer, 

 which supplies the oxygen for the combustion process. Both can be 

 contained in the same molecule (such as nitroglycerine-nitrocellulose) 

 which forms a homogeneous "double base" propellant. For large 

 motors, a rubberlike fuel with small, discrete particles of oxidizer 

 dispersed throughout ("composite" propellant) is more appropriate, 

 since large pieces can be cast in place with little danger of cracking 

 or softening. The exhaust velocity for such materials is in the range 

 of 7,000 to 8,000 feet per second, which is not as good as many liquid 

 propellant combinations. To place a payload in orbit (at a mission 

 velocity requirement of 30,000 feet per second), four solid propellant 

 stages are required, and such a vehicle (Scout) is being developed 

 by NASA. Although it puts less than 1 percent of its initial weight 

 in orbit, it has many of the favorable characteristics of solids — sim- 

 plicity, ease of handling and launching, relatively low cost, use of 

 various stages for diflerent missions — which commend it to scientific 

 researcli work with small pajdoads (about 100 pounds). The ease of 

 scaling up or clustering solid rocket motors has led to their considera- 

 tion as large boosters. When used as first stages only, their lower 

 performance and higher dead weight are less significant. In very 

 large sizes, the propellant cost (about $1 per pound) becomes signifi- 

 cant, as does the difficulty of handling the large quantities of poten- 

 tially explosive material. 



