A Comparison of Active and Passive Methods for Control of Hypersonic Boundary Layers on Airbreathing Configurations

Active and passive methods for control of hypersonic boundary layers have been experimentally examined in NASA Langley Research Center wind tunnels on a Hyper-X model. Several configurations for forcing transition using passive discrete roughness elements and active mass addition, or blowing, methods were compared in two hypersonic facilities, the 20-Inch Mach 6 Air and the 31-Inch Mach 10 Air tunnels. Heat transfer distributions, obtained via phosphor thermography, shock system details, and surface streamline patterns were measured on a 0.333-scale model of the Hyper-X forebody. The comparisons between the active and passive methods for boundary layer control were conducted at test conditions that nearly match the nominal Mach 7 flight trajectory of an angle-of-attack of 2-deg and length Reynolds number of 5.6 million. For the passive roughness examination, the primary parametric variation was a range of trip heights within the calculated boundary layer thickness for several trip concepts. The prior passive roughness study resulted in a swept ramp configuration being selected for the Mach 7 flight vehicle that was scaled to be roughly 0.6 of the calculated boundary layer thickness. For the active jet blowing study, the blowing manifold pressure was systematically varied for each configuration, while monitoring the mass flow, to determine the jet penetration height with schlieren and transition movement with the phosphor system for comparison to the passive results. All the blowing concepts tested were adequate for providing transition onset near the trip location with manifold stagnation pressures on the order of 40 times the model static pressure or higher. * Approved for public release; distribution is unlimited. Introduction Recently NASA refocused space access research into the Space Launch Initiative (SLI), which established the Next Generation Launch Technology (NGLT) program. The objective of NGLT is to advance the state-ofthe-art in space transportation systems (STS) technologies for affordable and reliable transportation to and from earth orbit through development of innovative approaches and concepts for future missions of human and robotic exploration of space. A major component of NGLT is the advancement of rocket and air-breathing propulsion technologies: to lower the cost of proven rocket-powered systems, while developing and demonstrating the revolutionary air-breathing technologies of the scramjet engine. Since the advent of the National Aerospace Plane (NASP) program in the mid-80’s, a scramjet powered vehicle has been a vision of affordable and rapid access to space. Scramjet powered vehicles scoop the oxygen required for fuel combustion from the atmosphere, reducing the tanking requirements, which can lead to improved payload capability. However the promise of scramjet as a propulsion system will be only an illusion until a flight program can conclusively demonstrate the technology. The Hyper-X program, a subset of NGLT, endeavors to provide a demonstration with a Mach 7 flight. Conceptually, a scramjet-powered system appears very simple, with no moving parts required to provide flow compression and only fuel addition and ignition needed to provide thrust. However, the feasibility of just such a system has always been tied to the details of trying to integrate the engine to the airframe using innovative materials and structures to control the highly complex flow field while handling the high heat loads. For airframe-integrated scramjet engines, the forebody ahead of the inlet is designed to process and precondition the flow that will be ingested. As shown in Fig. 1, a full-scale air-breathing vehicle will likely have competing transition mechanisms in the forebody boundary layer that will naturally force turbulent flow ahead of the inlet, which is desirable as susceptibility to flow separations within the engine will be diminished. However, on a sub-scale vehicle such as the Hyper-X, the shortened forebody is likely to provide laminar flow to the engine, based on the Mach 7 trajectory. The Hyper-X program decided early in the design to utilize