304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 62 



are burned, and a nozzle to expel the gases efficiently. The fluid pro- 

 pellant can be used before combustion to cool the nozzle and combus- 

 tion chamber, allowing longer periods of operation than uncooled 

 solid rocket motors. The propellants may be forced into the engine 

 under the pressure of gas in the tanks, but since this requires heavier 

 tankage, for large rockets the propellant is pumped into the combus- 

 tion chamber. Propellants enter through an "injector" which is simi- 

 lar to a showerhead and serves to disperse and mix the propellants for 

 efficient combustion. The pump, which requires considerable power 

 in large engines, is usually powered by a gas turbine. The turbine 

 may have its own gas generating system or utilize the propellant 

 combustion products to supply its working fluid. The nozzle or en- 

 tire engine can be swiveled to provide flight control for the vehicle. 

 The dead weight of a large liquid rocket propulsion system is 5 to 

 10 percent of the stage gross weight. The most commonly used pro- 

 pellant combinations (for example, "EP," a kerosenelike hydrocarbon, 

 and oxygen) yield exhaust velocities of about 10,000 feet per second, 

 while the use of liquid hydrogen as a fuel yields 1-4,000 feet per second, 

 which is close to the maximum possible with chemical propulsion. 

 Hydrogen has a veiy low boiling point and very low density, but its 

 high performance has led to its choice as the fuel for future U.S. 

 spacecraft. For liquid chemical propulsion, two or three stages are 

 optimum for the earth orbit mission, and only a high energy fueled 

 rocket with a light structure can place itself in orbit with only a 

 single stage. The liquid propellants are less expensive than solid 

 fuel, but the engines are more complicated and therefore more expen- 

 sive to develop and build. At this time, it is not clear whether it will 

 be economically feasible to reduce costs by recovering spent boosters 

 for reuse. 



OTHER CHEMICAL SYSTEMS 



Naturally many proposals have been made for improving the per- 

 formance of chemical systems by increasing their exhaust velocity, 

 reducing dead weights or complexity. Specialized systems have been 

 or will be developed for particular purposes, including monopropel- 

 lants (single chemical liquids which decompose to give hot gas), 

 hybrid solid-liquid rockets, engines with controllable thrust levels for 

 landing. Relatively little improvement can be expected in the exhaust 

 velocity, even with quite exotic propellant combinations, and it is this 

 which primarily determines the performance. Many advances in 

 simplicity, reliability, and structural weight can be expected, but a 

 "breakthrough" in performance of chemical propulsion seems 

 unlikely. 



One area where great improvement is possible is in "aerospace" 

 vehicles, which use air-breathing engines (turbojets or ramjets) for 



