ROCKET PROPULSION — COOPER 301 



The final mass includes the "dead" weight of the rocket (engines, 

 tankage, unused propellant), as well as the payload. For some value 

 of the ratio of velocity requirement to exhaust velocity, the dead 

 weight required for the propulsion system leaves nothing remaining 

 for the payload. This problem is circumvented by jettisoning used 

 portions of the propulsion system, resulting in a number of stages. 

 Usually the tankage and engines of a given stage are dropped when 

 it has exhausted its propellant. Occasionally, as with the Atlas, 

 which drops two of its engines, only portions are released. Staging 

 permits arbitrary mission velocities to be attained, although the pay- 

 loads may be small. One can find in general an optimum number 

 of stages for a given mission and propulsion system. A high exhaust 

 velocity allows one to use few stages, which results in a simpler, as 

 well as a lighter, vehicle. 



CHEMICAL PROPULSION: SOLID PROPELLANTS 



Solid propellant rockets are the simplest, and were first historically. 

 They were used in both China and Europe in the 13th century. Used 

 sporadically for centuries, they became very popular in warfare about 

 1800 ("the rockets' red glare") but were displaced by rifled artillery 

 which was much more accurate. They continued in use in a number 

 of minor applications as well as in warfare where much cheap, light- 

 weight, but inaccurate firepower was acceptable. These "powder" 

 rockets contained black powder and, later, smokeless powder grains 

 loosely packed in their cases, necessitating short burning periods to 

 keep the rockets from bursting because of the high temperature. 



During World War II, solid rockets using an asphalt base were 

 developed for assisting airplane takeoffs (JATO units). This pro- 

 pellant could be cast in a single piece but tended to crack or soften 

 with temperature changes. The development of rubber-based pro- 

 pellants after the war, combined with a design which kept the wall 

 cool, opened the way for large solid rocket engines. The propellant 

 is cast in place in a single mass with a central hole. The igniter in 

 this hole ignites the inside surface, and the burning surface moves 

 outward toward the case which remains cool until the propellant is 

 almost completely burned. During the entire burning period, the 

 case must be able to contain the high pressure (hundreds of pounds 

 per square inch) which originates in the hot gases and is transmitted 

 through the rubberlike propellant. Thus lightweight, high-strength 

 materials are an important requirement for solid rockets, which tend 

 to have high dead weights due to the case. The primary require- 

 ment for nozzle materials is ability to withstand extremely high tem- 

 peratures, and for this purpose, inserts of special refractory materials 

 (e.g., graphite or tungsten) are often placed in the nozzle throat. 



