Sustained
hypersonic flight above speeds of Mach 5 by vehicles using air-breathing,
jet-fuel-powered engines could become achievable within the next dozen years.
Successful recent
ground tests of jet-fueled, ramjet/scramjet demonstrator engines by Pratt &
Whitney Rocketdyne and Aerojet represent important progress toward
flight-testing of three separate hypersonic-vehicle programs.
In
September, Pratt & Whitney Rocketdyne (PWR) completed 10 months' testing of
a sub-scale combustor for a hydrocarbon-powered, dual-mode ramjet engine
designed to operate over a wide range of Mach-number speeds -- that is,
multiples of the speed of sound.
Using JP-7
jet fuel, PWR ran the combustor successfully at a variety of Mach numbers from
Mach 2.5 to Mach 6.0, demonstrating "desired operability and
performance" at each speed, the company said.
"No
engine, to our knowledge, has (previously) demonstrated a range from as low as
2.5 to a high of 6.0," said Michel McKeon, PWR's hypersonics and advanced
programs manager. "The FaCET (Falcon
Combined-Cycle Engine Technology) engine demonstrated a very wide Mach range,
with high performance. This really shows the technology lends itself to application
for a variety of different things."
A dual-mode
ramjet transitions between subsonic and supersonic modes of operation. Because
air entering the ramjet slows down as it goes through the engine, at lower Mach
numbers some of the air enters the combustor at subsonic speeds even though the
aircraft is traveling faster than the speed of sound, McKeon explained.
At higher
Mach numbers, all of the air goes through the engine at supersonic speeds.
"'Scramjet' basically means that all of the air is moving at supersonic velocity"
through the combustor, said McKeon.
Scramjet
challenges
Ramjets and
scramjets have few moving parts and there aren't any at all in their combustors.
But scramjet air is moving so fast that keeping the combustor lit is a major challenge.
Preventing the
engine from burning up from the heat of combustion, and making sure the JP-7 is
ignited in the fraction of a millisecond it takes for fuel to pass through the
combustor, are also crucial. "One of the key items in these systems is how
you manage the fuel," said McKeon.
PWR's approach
is to use a closed-loop or "heat sink" system, whereby the fuel is pumped
as a coolant throughout the engine casing to remove heat and pressure from the
combustor. This 3,000-degree heat also prepares the jet fuel for combustion by
cracking it into smaller molecules that burn very quickly when they enter the
combustor.
A full-sized
version of PWR's combustor will form the heart of the FaCET program, sponsored
by the U.S. Defense Advanced Research Projects Agency (DARPA) and the U.S. Air
Force. Lockheed Martin is FaCET prime contractor.
FaCET aims
to develop a hypersonic test vehicle -- which could fly in 2012 -- that would take
off and land by itself, use an advanced turbojet to get up to a speed of at
least Mach 4 and then use a liquid hydrogen-powered scramjet to get to Mach 10
and beyond. Jet fuel can't be used as a scramjet fuel at speeds as high as Mach
10.
"At
high Mach numbers, the engine would burn up using jet fuel," said Bob
Grabowski, PWR's FaCET program manager. "In that range, you're looking at
hydrogen or synthetic fuels."
The next
step for FaCET is to test a free-jet version of the engine on a rig at the U.S.
Air Force's Arnold Engineering Development Center (AEDC) at Tullahoma, Tenn., next summer. AEDC boasts the world's largest and most advanced complex of flight
simulation test facilities, including two hypersonic wind tunnels.
Cross-fertilization
between programs
FaCET isn't
linked to the DARPA/U.S. Air Force/NASA X-51A hypersonic aircraft that is due
to fly in 2009. But PWR, which is making the JP-7-powered X-1 scramjet engine
for the Boeing-built X-51A, uses what it learns from each program to improve
both engines.
"The
engines are not the same shape or configuration but, technology-wise, the FaCET
engine incorporates much of what we've learned through the X-51 engine,"
said McKeon. "The flip side is that we also have learned stuff with this
(FaCET) engine regarding different configurations that could also be used in
future X-51 activity."
PWR
successfully ground-tested its X-1 demonstrator engine in April, in simulated
Mach 5 flight conditions. The demonstrator, designated the SJX61-1, incorporated
a full-authority digital engine controller and a closed-loop thermal management
system.
In
September, Aerojet completed a full-duration ground test of an advanced
"flight-weight" combustor for the dual combustion ramjet (DCR) engine
it is developing for the Hypersonic Flight (HyFly) program sponsored by DARPA
and the U.S. Office of Naval Research (ONR). HyFly is a program to develop
hypersonic cruise-missile technology.
Aerojet's DCR
ramjet/scramjet, which is powered by JP-10 jet fuel, uses two air-inlet systems.
One feeds a subsonic gas generator in which a fuel-rich gas is created. This is
mixed with the supersonic gas from the second inlet system and then is passed
into a diverging combustor section that allows the combustor to function both at
subsonic and supersonic air speeds.
Aerojet's
different approach
The Aerojet
approach doesn't rely on using jet fuel as a coolant. Its DCR is made entirely
from advanced, lightweight ceramic materials, which Aerojet says makes the
engine less than half the weight of fuel-cooled metal engines.
Aerojet
conducted its September test in simulated Mach 6 flight conditions. "With
this test, HyFly took a big step forward," said Gil Graff, ONR's program
manager. "It proved the viability of an uncooled-structure concept to survive
extreme engine environments."
Together
with PyroDyne, Aerojet also is developing a turbine-based combined-cycle engine
it calls the PyroJet, which the companies hope will operate throughout a speed
range from zero to Mach 10. The aim is to provide an engine that will transition
smoothly from subsonic turbojet operation through to dual-mode ramjet/scramjet
operation.
Now, for
all manufacturers, the main challenge isn't demonstrating that their hypersonic
engines can fire. To sustain hypersonic
flight, the engines will need to fire for several minutes at a time, not
just the few seconds that the companies have achieved so far.
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