Base-drag-reduction experiments on the X-33 Linear Aerospike SR-71 flight program

Drag-reduction tests were conducted on the Linear Aerospike SR-71 experimen t. The Linear Aerospike SR-71 experiment flight tested a 20% scale model of an X-33 forebody with a linear-aerospike engine mounted at the rear of the body. The entire apparatus was mounted on top of an SR-71 aircraft. This p aper suggests a method for reducing base drag by adding surface roughness a long the forebody. Calculations show a potential for base-drag reductions o f 8-14%. Flight results corroborate the base drag reduction, with actual re ductions of 15% in the high-subsonic flight regime. An unexpected result is that drag benefits persist well into the supersonic flight regime. Flight results show no overall net drag reduction. The applied roughness caused fo rebody pressures to rise and offset base-drag reductions. Apparently, the g rit displaced streamlines outward, causing forebody flow compression. Clear ly the drag optimization must be modified to include not only the base pres sure drag and viscous forebody drag coefficients but must also include the forebody pressure distribution. Because of the mixed experimental results-t here was no overall net drag reduction, the existence of an optimal forebod y surface roughness must still be proven. Clearly, however, the forebody gr it method has been proven as a viable drag-reduction tool.