Channeled Inlet Means Better Supersonics
10:44 PM on Jan 03, 2012
NASA has flight tested an inlet that promises to improve the efficiency of supersonic and hypersonic engines. The Channeled Centerbody Inlet Experiment (CCIE) was flown at speeds up to Mach 1.74 mounted on a pylon under NASA Dryden's F-15B.
Photo: NASA Dryden Flight Research Center
As its name suggests, the CCIE has channels in the inlet centerbody. These slots increase the amount of air flowing into the engine, improving its performance at off-design Mach numbers. The CCIE is a fixed-geometry test version of the translating channel centerbody (TCCB) inlet patented by Ohio-based TechLand Research and originally designed to work with a NASA-designed rocket-based combined-cycle (RBCC) engine.
Photo and graphic: NASA
A typical supersonic inlet is designed to reduce the airflow to subsonic speed before it enters the engine. Supersonic fighters like the F-16 and F-18 have external-compression inlets - the terminal shock is located at the entrance to the inlet and all flow from there to the engine face is subsonic. But the SR-71 and advanced supersonic transports being designed by NASA and others have mixed-compression inlets - the terminal shock is inside the inlet and can be repositioned by moving the centerbody fore and aft, improving efficiency over a range of Mach numbers.
TechLand's TBCC was designed to address the problem of the changing mass-flow requirements for a supersonic inlet operating across different flight conditions. At the design cruise speed, inlet throat area and mass flow can be matched to maximize engine efficiency, but "off-design" at transonic and low supersonic speeds the inlet needs a much larger throat area to meet the engine's demand for air. This requires an inlet that can accomodate a wide variation in throat area, otherwise engine efficiency suffers.
TCCB closed (left) and open. (Graphic: TechLand Research)
In the TBCC, the centerbody not only translates fore and aft to reposition the terminal shock (as it did in the SR-71), but channels open and close to adjust throat area and vary mass flow into the engine depening on conditions. The CCIE test article has fixed open slots in the centerbody, and the F-15 flights are intended to collect data to compare both channeled and smooth centerbodies and to see how well the flight results compare with CFD calculations.
CCIE (left) vs smooth centerbody. Graphic: NASA
The advantage of the channels, NASA says, is that they allow the designer to select a mixed-compression inlet design with lower internal compression. A smooth centerbody would have a smaller throat area and would require higher internal compression to meet the engine's mass-flow demand. Lowering the internal compression reduces the risk the inlet will "unstart" - pushing the terminal shock forward, all the way out of the inlet, suddenly and dramatically increasing drag and potentially damaging the engine or even the aircraft.
awt, propulsion, NASA