Will the supersonic-combustion ramjet ever be a usable means of propulsion? Boeing, Pratt & Whitney Rocketdyne and the US Air Force are convinced it can be
Hypersonic windtunnel tests have cleared the way for Pratt & Whitney Rocketdyne to begin building engines for the US Air Force Research Laboratory's X-51A scramjet demonstrator programme.
The Boeing-designed X-51 will demonstrate the first "practical scramjet" packaged to fit in a free-flying vehicle that starts reliably, operates continuously on jet fuel and accelerates through multiple Mach numbers.
Recently completed in the 8ft (2.4m) High Temperature Tunnel (HTT) at NASA Langley Research Center, Virginia, tests of the first prototype of PWC's SXJ61 scramjet for the X-51A, engine X-1, met or exceeded expectations.
X-1 has demonstrated the X-51 will reach thermal balance, jet fuel flowing through the scramjet sidewalls cooling the engine while heating and cracking the JP-7 fuel, making it easier to burn in the supersonic flow. Once thermal balance is achieved, the scramjet becomes a practical means of propulsion.
Flight International was shown X-1 during a break in testing, the welded-Inconel scramjet looking small atop its copper-clad plinth in the HTT's cave-like test section. The engine had completed 34 cycles, heating up to beyond 1,000ºC (1,832ºF) then cooling down - in flight it will endure just half a cycle.
The cycles were accumulated in 34 tunnel runs at Mach 4.6 and 5 - picked because M4.6 is the speed at which the scramjet is to start after the cruiser separates from its booster, and M5 is when fuel will begin to be staged - injected at different locations inside the engine - as it accelerates and more of the flow through the combustor goes supersonic.
"Thirty-four cycles is huge," says PWR programme manager Curtis Berger. "That's 15min of combustion on an engine where all the hardware is in the basic configuration that will fly." Except the flight fuel pump, which will be on X-2, the flight clearance engine to be built by October for testing in the HTT.
In June, a series of runs at M6.5, close to the X-51's expected top speed, completed testing of the X-1 engine for a total of 40 cycles and 17min combustion. This is more time than on any scramjet back to the X-30 National Aero-Space Plane (NASP) programme.
After the X-30 was cancelled in 1992 hypersonic work continued, but at a much lower level, and P&W's NASP engine design was the basis for the hydrogen-fuelled scramjet that powered NASA's X-43A Hyper-X demonstrator to Mach 9.68 in 2004. AFRL, meanwhile, initiated the HyTech programme to maintain a capability in hypersonic propulsion and in 1996 awarded P&W a contract to demonstrate a hydrocarbon-fuelled scramjet.
This led to ground tests in 2001 of the performance test engine (PTE), followed by two ground demonstrator engines (GDE). While the uncooled PTE was a heavy copper heat-sink engine, both the GDEs were flight-weight, fuel-cooled engines. The PTE had a 150mm (6in)-wide flowpath, GDE-1 230mm and GDE-2 270mm. These led to the SJX61, which is a smaller engine with a 230mm flowpath.
Original efforts were directed towards the X-43C, a derivative of NASA's Hyper-X with three hydrocarbon scramjets, but this was cancelled. "We developed an engine, but there was no programme," says AFRL programme manager Charlie Brink. Work was redirected to AFRL's Scramjet Engine Demonstrator - WaveRider programme, which started late in 2003 and became the X-51A.
Run in 2003, GDE-1 used two sets of fuel, one for cooling and one for combustion, but by matching the thermodynamics of the flows it was proved the loop could be closed and the engine thermally balanced, says Berger. GDE-2, run in 2005, had a closed-loop fuel system, variable inlet flap and full-authority digital engine control. Brink says the GDE-2, which was run 24 times in the HTT at M5, "provided early risk reduction for the X-51, and looked a lot like the SJX61".
Four X-51A flights are planned over the Pacific, beginning in August 2009. After take-off from Edwards AFB, the Boeing B-52 will climb to 49,500ft (15,000m), "to get as much potential energy as possible", then release the "stack" - the scramjet-powered cruiser atop its modified missile booster.
The booster will light 4-5s later and burn for 26-28s, climbing and accelerating to M4.7-4.8. At M3.5, while still attached to the booster, air will start flowing through the cruiser's inlet to establish supersonic flow. "When the inlet starts on the booster, we will begin flowing fuel to start heating the JP-7," says Berger.
At burnout, four separation bolts will fire, pushing the cruiser away as the booster drops off. The X-51 will coast for a half a second, then ethylene will be injected into the combustion chamber and ignited to act as blowtorch, heating the sidewalls and bringing the JP-7 up to temperature. Jet fuel will then be introduced, the engine initially burning the JP-7 along with ethylene to boost acceleration then transitioning to jet fuel only.
The scramjet is expected to reach thermal balance within seconds, all the heat absorbed by the circulating fuel being dumped overboard through its combustion. The X-51 will fly under power for 300s, accelerating from M4.7 to M6.7. Initially the scramjet will be dual-mode, a significant portion of the airflow in the combustor subsonic. Beginning at M5, fuel will be staged as the vehicle accelerates and combustion becomes fully supersonic.
After 5min, its fuel exhausted, the scramjet will shut down and the X-51 will begin a glide to an impact with the ocean. During its unpowered descent to a watery end, the cruiser will manoeuvre to collect aerodynamic data. From scramjet ignition to splashdown will be less than 15min. A picket line of US Navy P-3s will receive telemetry throughout the flight, recording it and relaying it back to Edwards.
The X-51 is a true flying vehicle, says Boeing programme manager Joseph Vogel, with all the systems of an aircraft. It is also autonomous and unstable in all directions, and will be stabilised by digital flight controls. It is a small vehicle, and packaging as much fuel as possible along with the ethylene bottle and batteries has proved problematic. The primary structure is aluminium diverter, nozzle and fins are titanium and the nosecone is tungsten and inlet cowl carbon-carbon because of the high temperatures.
The X-51 team is sensitive about direct connections being made between the missile-sized vehicle and any hypersonic weapon that could result from the research. "The X-51 is a technology demonstrator. You would not design a missile this way," says Brink. But a high-speed cruise missile is the nearer-term of two targets for the technology. The other, longer term, is air-breathing access to space, but this would require the scramjet to be scaled up. "The [X-51] design is for a technology demonstrator, but we have a quick off-ramp to a missile," he says.
The Woracle: a quick guide to hypersonic programmes