446 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



afforded by the addition of some 10 percent in fuel. Some compromise 

 wing loadings must be chosen. In this case 77 pounds per square foot 

 for take-off represents a relatively low wing loading for this type 

 of airplane but is assumed to allow for auxiliary means of take-off 

 and flight up to 350 miles per hour, at which point the ram-jet power 

 plant is started for the first time, take-off obviously not being possible 

 with the ram- jet alone. The resultant landing wing loading of 42 

 pounds per square foot is quite reasonable. 



High performance dictates reduction in wing thickness to 10 per- 

 cent. 



As is obvious from the phenomenal fuel consumption, the rocket- 

 motor-powered airplane must carry an abnormal amount of fuel, in 

 this case 8,200 pounds, or 61 percent of the take-off gross weight. 

 Wing loading is dictated primarily by the landing condition and has 

 been held at 40 pounds per square foot, resulting in 104 pounds per 

 square foot for take-off, wliich seems entirely feasible. Since super- 

 performance is obvious, aerodynamic considerations dictate the entire 

 arrangement. A practicable wing thickness of 10 percent for the wing 

 and 8 percent for the tail are chosen. As in the case of the ram-jet, 

 700 pounds of armament are installed. Tanks are not self -sealing 

 owing to the nature of the fuels, the extremely short duration, and 

 the exceptional performance. All fuel is carried in the fuselage. 



PAET C— PERFORMANCE COMPARISONS 



1. Calculation of airplane drag. — Drag estimates for all four air- 

 planes are based upon an identical analysis wherein the wetted areas 

 of the component parts are assigned drag coefficients depending upon 

 fineness ratios, Reynolds Numbers, and Mach Numbers. In this man- 

 ner, the drag coefficients of the component parts are obtained and 

 adjusted for lift coefficient or angle of attack. The cleanliness factors, 

 6'^=the parasite drag coefficient based upon total wetted area at 6'l=0 

 and /?i\^=8XlO% and the parasite drag coefficients, based upon wing 

 area, are given in the following table : 



Cleanliness factor and drag coefficients 



Propeller 

 Turbojet. 

 Ram-jet-. 

 Rocket... 



Wetted area 



931 

 881 

 672 



Cf 



0,0038 

 .0034 

 .0035 

 .0034 



250 

 240 

 130 

 130 



Cd 



0.014 

 .013 

 .019 

 .017 



Both the turbojet and the rocket exhibit extremely good aerodynamic 

 cleanliness, being about an 11-percent improvement over the propeller 

 airplane. Drag coefficients based on wing area are shown in which 

 high wing loadings, because of the relatively large fuselages, result 



