Simple inlet concept could help pave way for combined-cycle space access
Fifty years after researchers tested a unique engine inlet design for Lockheed's Mach 3.2-capable A-12, a novel inlet concept is being evaluated that could lead to a new generation of simpler and lighter supersonic and hypersonic propulsion systems.
Detailed analysis of results from tests of the Channeled Centerbody Inlet Experiment, or CCIE, will begin this month following the final evaluation flight onDryden Flight Research Center's testbed aircraft on Jan. 5. The CCIE was flown on a pylon mounted under the F-15B over a series of eight flights, starting with initial tests in late August 2011.
The CCIE concept is being explored to address the problem of the changing mass-flow requirements for a supersonic inlet operating across different flight conditions. This is vital, because future supersonic and hypersonic vehicles, including air-breathing combined-cycle space launchers, must be capable of operating at low supersonic and transonic speeds as well as higher Mach numbers. The inlet will therefore be required to cope just as efficiently with a throat area and mass flow matched to cruise requirements as one configured for “off-design” transonic and low supersonic conditions.
Unlike typical supersonic, external-compression inlets designed to reduce the airflow to subsonic speed before it enters the engine, higher-speed supersonic and hypersonic combined-cycle designs are configured with mixed-compression inlets. These contain the terminal shock associated with the airflow inside the inlet instead of by the entrance to the inlet. In previous mixed-compression inlets like those used in the A-12 and its better-known SR-71 Blackbird-family stable mate, the position of the shock is controlled by moving the centerbody fore and aft.
However, the mechanisms for moving and controlling the centerbody are relatively complex and heavy, andhopes the simpler, lighter CCIE can produce the same effect more efficiently. The CCIE has channels, or slots, in the inlet centerbody that increase the amount of air flowing into the engine, improving its performance over a wide range of Mach numbers. Although the CCIE is a fixed-geometry test unit, a fully operational inlet would have movable channels that would open and close to adjust throat area and vary mass flow into the engine, depending on conditions.
The CCIE is a version of the translating channel centerbody (TCCB) inlet patented by Ohio-based TechLand Research and originally intended to work with a now-shelved NASA-designed rocket-based combined-cycle (RBCC) engine. “The proof-of-concept test unit is relatively simple in configuration” and is being tested to “determine if a new design is worth pursuing,” says CCIE principal investigator Clint St. John.
The main research objective was to define the airflow through the experimental jet engine inlet, then compare it to the flow through a standard inlet. Inside, airflow around two interchangeable centerbodies installed in an inlet tube was measured. One centerbody was channeled while the other had a conventional, smooth shape (see drawing).
First flights were undertaken with the channeled shape attached to a series of three nozzles to vary mass flow rates. Following these, the smooth centerbody was tested with the same three nozzles. Data from the smooth centerbody—including inlet mass airflow, internal surface pressure distribution and airflow distortion—were used to benchmark performance data for the channeled unit. The results will also be compared with computational fluid dynamics predictions.
Flights were aimed at capturing equivalent inlet performance at Mach 1.3 and 1.5 at 40,000 ft., which required the F-15B to fly at speeds up to Mach 1.74. The last flights, conducted in early January, repeated an earlier flight with a smooth centerbody and medium-flow-rate nozzle to gather additional data and perform further evaluation of the local flow field under the F-15B, says St. John. To avoid the worst of this, and to position the CCIE correctly for testing, “we angled it to a zero angle of attack when flying at Mach 1.5, which is the primary case,” he adds.