Saturday, March 27, 2010

X-51A WaveRider gets first ride aboard B-52

PALMDALE, Calif -- As the first Mach 6.0+, air-breathing, fuel-cooled, hypersonic test vehicle, the Boeing/Pratt & Whitney Rocketdyne X-51A Waverider is the aerospace community's best hope for pushing forward the boundaries of hypersonic science. [Read related news story.]

So why, on close inspection, does it seem so very ... jury-rigged?

For example, the X-51A's booster stage comes from the Army Tactical Missile Systems (ATACMS). The FADEC is re-gifted from the F-35 program, which had inherited a surplus digital control system from the F119. Its super-combusion ramjet engine, the most sophisticated of its kind, boasts an igniter pilfered from -- of all things -- a rotting TP-33 TF33 turbojet that once powered a Lockheed C-141 Starlifter.

Charlie Brink, the X-51A's Waverider programme manager, explains.

The $250 million Waverider experiment is designed to test the capabilities of supersonic combustion propulsion at hypersonic (Mach 5.0+) speeds. If anything fails during any of the four planned flight tests, it better have something to do with the engine, Brink says.

That philosophy drives a rule for Brink's contractors: take no unnecessary risks by integrating all-new and untested systems or components outside the critical path of the engine. The idea also carries over into structures.

"I didn't want to have a scramjet experiment and spend all the money fixing [an unrelated] structural problem," Brink says.

Rather than develop all-new, exotic structures to withstand the intense heat created by hypersonic flight, Brink ordered contractors to strictly rely on conventional materials aided by liberal amounts of thermal-ablative coatings. That's why aluminum frame, with a melting point of 300-degrees F, surrounds the engine bay inside the X-51A cruiser stage. Titanium (900-degree melting point) and nickel-based incanel (1,500-degree melting point) are required closer to the nose. The nose itself is made out of tungsten, the only relatively lightweight, non-composite material capable of surviving temperatures up to 2,700-degrees without melting.

All of these decisions were made to increase the chances that the X-51A will have four successful test flights. The history of the early stages of flights for missile-like vehicles like the X-51A is not promising. The track record indicates that half of such flight tests will fail, Brink says, meaning that the X-51A experiment may only have two test flights for collecting data.

Such long odds explain the program's ultra-conservative design decisions. A good example is the size of the storage tank for ethylene, which is required to kick-start the hypersonic ignition sequence as the JP7 fuel heats up. Brink jokingly calls the ethylene unit the "scuba tank". It is sized to hold 6lbs of ethylene, even though the designers estimate the engine will need only 2-4lbs. But the designers are playing it safe, building in extra margin in case more something fails and more ethylene is needed.

Despite such extreme care, the experimental flight tests remain highly risky. This is not only because nobody has tested a fuel-cooled hypersonic propulsion system in flight. The size and weight limitations of the 4,000lb X-51A test vehicle allowed engineers to build in only two redundant systems, and both of those are in case of failure. If the X-51A veers off-course, the flight test team wants to be sure there is a second option for destroying the vehicle in case the primary self-destruct sequence fails, says Joseph Vogel, Boeing's X-51A programme manager.

There is an upside to the X-51A's conservative design approach. The vehicle is designed to a safety factor of two. Remove all equipment not required for an operational system -- for starters, the FADEC, flight data instruments and ethylene tank -- and there is suddenly ample round for more fuel or other payload.


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