434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



ing altitude specific engine weight increases, but not to the prohibitive 

 degree evidenced by the propeller type. Also, contrary to the propeller 

 type, specific weight improves at the higher air speeds, which empha- 

 sizes the practicability of higher air speeds. 



Considering the ram-jet, its design depends to a large extent on 

 both speed and altitude. Marked improvement obtains at higher air 

 speeds, and, when considering the reduction of thrust with altitude, 

 it tends to become bulky and heavy for high-powered, high-altitude 

 application. Apparently, the ram-jet is a medimn-low-altitude, high- 

 speed type of power plant. 



ALTITUDE • 1000 FEET 



FiGUBJE 4. — Power plant weight-thrust ratio. 



Simplest of all is the rocket, which again makes no compromise 

 for air speed or altitude variation. In terms of specific power-plant 

 weight it is unexcelled. 



7. SpecifiG fuel consumption {S. F. C.) at maximum^ continuous 

 thrust (fig. 5) . — The propeller-type plant offers a large saving in fuel, 

 especially at the lower air speeds, as evidenced by its specific fuel con- 

 sumption at maximum continuous thrust rating. At speeds exceeding 

 500 miles per hour this fuel consumption would become greater than 

 that of the turbojet. However, employing this thrust rating, speeds 



