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Technology update
Arc jet testing reduces flight test risks
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At the U.S. Air Force's Arnold Engineering Development Center (AEDC) in
Tennessee, engineers are reducing the development costs and potential
flight-test risks of a new scramjet propulsion system by using a high
enthalpy arc heater. The heater tests materials at extreme temperatures
and pressures similar to those experienced during high-speed flight.
The USAF Research Laboratory's Hypersonic Technology Program Office
requested that the arc testing be done to support design and evaluation
of a liquid-hydrocarbon-fueled, dual-mode ramjet/scramjet propulsion
system for hypersonic missile applications.
A ramjet is an air-breathing jet engine without the mechanical compressor
or turbine of a turbojet. The engine relies on air flowing into it and
cannot be started from rest. A scramjet is a supersonic-combustion
ramjet in which the airflow through the combustor is supersonic.
The test team evaluated 24 2-in-wide, 10° wedge samples of refractory
materials that are being considered for the leading edge structure of
a scramjet inlet. Test conditions consisted of run times up to 10 min.,
hypersonic speeds up to Mach 4.8, 3500°F temperatures, and 17.6 psi
stagnation pressures. The purpose of the test was to screen the
performance and feasibility of passive, ceramic matrix composite materials
in a ground-test environment simulating Mach 8 missile operating
conditions.
According to Dwayne Carver, AEDC Contractor Project Manager, an
air-breathing missile traveling at Mach 8 experiences sustained component
heating temperatures in excess of 3000°F, considerably higher than what
conventional uncooled metallic structures can withstand. "This requires
the use of active cooling schemes or passive materials able to withstand
these extreme temperatures," he said. "Furthermore, sharp leading edges
are needed at the supersonic combustion ramjet (scramjet) inlet to
provide the highest overall engine efficiency, and these leading edges
must suffer only minimal recession from oxidation or erosion during
flight. The development of effective designs for the hypersonic inlet
leading edges and sidewalls is a critical component of the HyTech
scramjet missile development program."
"At hypersonic speeds, the leading edge heating rates exceed those that
can be sustained by state-of-the-art, uncooled metallic structures," said
Ray Dirling, Project Test Engineer of Science Applications International
Corp. "An alternate structural design is active cooling of the leading
edge, but this is difficult due to the small nose radius of typical cowl
leading edge designs."
Frank Bokulich
Aerospace Engineering August 2000
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