Base-Drag-Reduction Experiments on the X-33 Linear Aerospike SR-71 Flight Program

Abstract

Drag-reduction tests were conducted on the Linear Aerospike SR-71 experiment. The Linear Aerospike SR-71 experiment e ight 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 paper suggests a method for reducing base drag by adding surface roughness along the forebody. Calculations show a potential for basedrag reductions of 8 ‐14%. Flight results corroborate the base drag reduction, with actual reductions of 15% in the high-subsonic e ight regime. An unexpected result is that drag benee ts persist well into the supersonic e ight regime. Flight results show no overall net drag reduction. The applied roughness caused forebody pressures to rise and offset base-drag reductions. Apparently, the grit displaced streamlines outward, causing forebody e ow compression.Clearlythedrag optimizationmustbemodie edto includenotonly thebasepressuredragandviscous forebody drag coefe cients but must also include the forebody pressure distribution. Because of the mixed experimental results— there was no overall net drag reduction, the existence of an optimal forebody surface roughness must still be proven. Clearly, however, the forebody grit method has been proven as a viable drag-reduction tool.