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The United States Air Force is currently pursuing a national program to develop the technology necessary to build a single staged aircraft capable of flying from conventional runways into orbit. This vehicle, the National Aerospace Plane (NASP), will use airbreathing propulsion to accelerate through hypersonic speeds to orbital velocities. In order to achieve the aerodynamic and propulsive efficiencies necessary to accomplish this goal, the vehicle forebody must be used as a precompression surface for the engine flowpath. This results in a thick boundary layer which must be bled-off or ingested by the engine. A computational model for thickening boundary layers with mass addition is developed. The phenomena of uniform injection into a two-dimensional supersonic stream and subsequent boundary layer growth downstream is discussed. Analysis of the injection region provides the thickness of the boundary layer just aft of injection. An injection region velocity profile is then used to approximate the boundary layer profile just aft of injection and is input into a finite- difference boundary layer code. Downstream profiles and thicknesses are calculated and compared to experimental results. The computational model developed here provides a tool for the design of a boundary layer generation system for hypersonic engine inlet testing. This mass addition system is needed to simulate the boundary layer developed on the forebody of hypersonic vehicles. Theses. (aw)