The advanced aircraft is expected to evolve into missiles able to hit mobile and time-critical targets.
The X-51A is wedge-shaped, with a scoop-like cowl on its underbelly, where air rushes into the inlet of the engine's combustor. It is critical that air entering the inlet be turbulent at hypersonic speeds or the engine could stop. Because of this, air must be converted to turbulent flow before entering the inlet. Engineers installed a raised strip of metal near the inlet to trip the air from smooth to turbulent.
Researchers at Purdue University, West Lafayette, Ind., are running tests on models of the aircraft using the only wind tunnel capable of running quietly at hypersonic speeds. They are collecting data showing how air flows over the vehicle's surface in flight. Specifically, engineers need more information on how airflow changes from laminar, or smooth, to turbulent as it speeds over the aircraft's surfaces.
The Purdue research, which focuses on the front portion of the craft, uses a foot-long model for wind tunnel testing. Key objectives are to maintain turbulent airflow into the engine's combustor to keep the scramjet running properly, and to increase the amount of smooth airflow over the vehicle's upper surface to reduce friction and heat generated in flight.
Researchers can switch the wind tunnel back and forth between quiet to high-noise, letting them compare data from each. For quiet flow, the throat of the Mach 6 nozzle is polished to a near-perfect mirror finish that turns airflow near the wall from laminar to turbulent. Then, for the wind tunnel to remain quiet, it must be entirely free of particles. Even a single speck can cause turbulence inside the tunnel, damaging the finish and disturbing the quiet.
Experiments under quiet conditions yielded more accurate findings, indicating that the smooth-to-turbulent strips should be raised about twice as high. At the same time, air flowing over the top of the vehicle should be as smooth as possible to reduce friction and heating. The researchers used temperature-sensitive paint to measure how hot the skin of the model got during testing. The paint was coated on a nylon strip inserted into the model. Shining a blue light onto the strip during testing generated a temperature-dependent red light from the paint. The intensity of the red light indicates how hot the surface is.
Scramjets, or supersonic combustion ramjets, could lead to space planes that are far less expensive to operate than the current space shuttles, making it more affordable to haul payloads into orbit. Because scramjets use air from the atmosphere as the oxidizer to burn fuel, they do not require liquid oxygen like rockets do. Vehicles with scramjets would carry less liquid oxygen, making the vehicle a lot lighter.
The X-51 project is lead by the Air Force Research Laboratory and the Defense Advanced Research Projects Agency, and the vehicle is being built by Pratt & Whitney and the Boeing Co. Purdue engineers are part of a national team of researchers handling different aspects of the vehicle.