430 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



For this paper four power plants having distinctly different per- 

 formance characteristics have been chosen : (1) a V-type, reciprocating, 

 liquid-cooled engine employing water injection for emergency power ; 

 (2) a turbojet engine having about 23 percent less sea-level static 

 thrust than the former; (3) a hypothetical subsonic ram-jet; and (4) 

 a bipropellant liquid-fuel rocket motor. For brevity, these power 

 plants will be referred to as a propeller, turbojet, ram-jet, and rocket, 

 respectively. 



Because these power plants differ so radically in performance char- 

 acteristics, it is difficult to determine a sound basis for comparison. 

 It has therefore been decided to design a single-engined, single-place 

 fighter or pursuit type airplane employing each type of power plant 

 and limiting the size to some reasonable weight, say less than 14,000 

 pounds. It is felt that the results submitted, together with the analy- 

 sis presented, offer sufficient justification for classifying these types of 

 power plants in specialized categories in which they excel. Thus, if a 

 particular type of engine shows outstanding merit in a high-altitude 

 and high-speed performance fpr a fighter, similar reasoning may be 

 Judiciously applied in considering other aircraft types. 



The text is divided into three parts. Part A compares the various 

 types of engines independently of airplane characteristics insofar as 

 possible. Part B briefly describes a logical and practical airplane 

 design configuration for each power plant. In part C, the results of 

 the airplane performance characteristics are presented from which 

 c.omparisons and conclusions may be advanced. 



PART A— POWER-PLANT CHARACTERISTICS 



1. Reci'procating engine and propeller. — A typical high-perform- 

 ance engine employing water injection is assumed, which delivers 2,000 

 b. hp. for take-off, the power varying linearly with density to 1,700 

 b. hp. at an engine critical altitude of 20,000 feet. Typical propulsive 

 efficiencies have been used in converting power to available thrust in 

 order to afford a direct comparison with the other types of power plants 

 in which thrust is the fundamental consideration. Exhaust jet thrust 

 has also been included. 



The gas turbine driving a propeller has not been considered because 

 the differences when compared to the remaining three power plants 

 are not radically different from those of the reciprocating engine. 

 Specific engine weight and size are improved, but fuel consumption 

 will be increased in comparison to reciprocating engines. Also, the 

 propeller itself imposes a definite limitation on maximum speed. Both 

 the gas turbine and compoimd engine may be considered to be alternate 

 developments in relation to propeller-driven airplanes. 



2. Turhojet engine. — The turbojet engine selected is typical of the 

 single-stage centrifugal blower type employed in a number of aircraft. 



