Officials at the U.S. Air Force's Arnold Engineering Development Center (AEDC) are heralding a successful first freejet test on a dual mode, combined ram/scramjet hypersonic engine in the center's Aerodynamic and Propulsion Test Unit (APTU), a major milestone on two fronts.
"Not only did we complete a successful first hot-flow test on DAPRA's (Defense Advanced Research Projects Agency) Falcon Combined Cycle Engine Test (FaCET) article, but this test also represents our first entry for a customer since APTU underwent an extensive multi-year facility upgrade," said Matthew Bond, the Air Force manager for APTU.
"The main purpose of this program is to test the common combined cycle flow path at Mach 4, he explained. "What is unique about this test article is that it combines two propulsion systems into a single package which could be used to power a flight vehicle from zero airspeed out to hypersonic flight and back."
"The test is to verify that the flow paths are designed correctly," Bond said. "The most critical issue in a turbine engine, or a ramjet is making sure you can manage the airflow. Each propulsion mode has different airflow requirements. For example, with a ramjet, even though the vehicle is going Mach 3 or Mach 4, when the air comes in the inlet, it actually gets choked down to subsonic flow."
He said the challenge to getting a combined cycle propulsion system to function properly is to manage the flow transition between supersonic and subsonic.
"APTU is a fixed Mach facility, you install a nozzle and you run one Mach number," Bond said. "You can't vary the Mach number during the run. So, you try to run a set of test points that capture the different modes of the test article.
"In the original test plan, we were going to run several Mach 3 conditions, which has the bypass door to the turbine partially open so it will test being able to split those flow paths. Next we would run the test article under Mach 4 conditions, which is a ramjet mode and then at Mach 6 conditions which gets into scramjet mode, distinguished by supersonic combustion.
"We would test at the different modes - and it basically allows our customer to verify that they've got their airflow calculations right and that they get the right flow through the engine to actually light the fuel/air mixture in the combustor and maintain that combustion."
Regarding APTU's Initial Operational Capability (IOC) milestone, Bond said that process has been performed in progressive steps over the years with a major objective being realized two years ago.
"APTU went IOC with the new Combustion Air Heater (CAH) burner in September 2007," he said. "We went through a big modernization program to put that new heater into the facility, and it took time to get it to operate properly at all the different Mach numbers."
He said the facility's current configuration still only allows the facility's operators to put in one nozzle at a time for a test run.
"The ultimate objective for APTU is to provide a real-time variable Mach capability. The current design is based on having a flexible walled nozzle, similar to what's in our 16-foot supersonic and 16-foot transonic wind tunnels," Bond continued. "Then we can vary the Mach conditions as we're running the facility and this would allow us to actually simulate the vehicle accelerating and test those transitions from turbine to ram or from ram to scram.
"That capability is still in the future," Bond admitted. "Our primary objective right now is to deliver as much data as we can with the FaCET test article to support future combined cycle engine development."
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