Hypersonic Jets Prepare to Soar
Under a DARPA/U.S. Air Force program, Boeing is making the X-51A hypersonic demonstrator aircraft, with the aim of making the first test flight in 2009. Pratt & Whitney Rocketdyne is making the X-1 JP-7 jet-fuel scramjet engine for the X-51A, which is also known as the WaveRider, because effectively it will surf the shock wave of compressed air that the aircraft creates in front of it as it flies at speeds well in excess of Mach 5.
Credit: Pratt & Whitney

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.