Boeing X-51A programme officials acknowledge that they face long odds for completing a successful flight-test programme for the hypersonic vehicle with only four experiments funded.
Joseph Vogel, Boeing programme manager for X-51A, says he would prefer to have 10 flight tests, but is optimistic that the programme can be successful with funding for only four. Another two flight tests could be added if the programme can find a new source for funding, he adds.
Charlie Brink, X-51A programme manager for the US Air Force Research Laboratory, compares the hypersonic vehicle to cruise missile technology. But flight-test programmes for cruises missiles are notoriously problematic. Half of the first 20 flight tests in previous cruise missile programmes have failed.
The 1,815kg (4,000lb) test vehicle is designed with the ultra-high-speed cruise missile mission in mind. In contrast, the Lockheed Martin AGM-165 Joint Air to Surface Standoff Missile might take an hour to strike a target nearly 480km (260nm) away. A Waverider missile similar in size to the X-51A could travel 965km in only 10min.
The Waverider's angular nose is designed to ride atop the supersonic shockwaves that otherwise generate drag. The Pratt & Whitney Rocketdyne SJX61-2 scramjet, meanwhile, uses the speed of the air flow entering the inlet as a natural compressor stage.
Inside the engine, the air is initially mixed with ethylene, and later JP7, and burn at temperatures up to 1,870°C (3,400°F). To keep the engine from overheating, fuel is also used as a coolant and circulates around the engine structure.
The programme wants to rely on conventional fuels and materials as much as possible. That drove the decision to use JP7, which also powered the Lockheed SR-71 Blackbird. JP7 requires time to heat up, so the X-51A's combustion sequence starts with the injection of ethylene.
A storage tank with 2,7kg of ethylene fuel dominates the payload space, a requirement that is a minor irritant for the vehicle's designers. Brink derisively describes the ethylene storage unit as a cumbersome "scuba tank", albeit a necessary one for now.
If a follow-on programme is funded, Brink and Vogel believe a new Waverider vehicle design should rely solely on JP7 as fuel, thus removing the need for ethylene entirely.
The intense heat generated by hypersonic speeds creates thermal management requirements even outside the engine structure.
The X-51A is wrapped in special, spray-on treatments derived from the Space Shuttle called Boeing lightweight ablative coatings, which can withstand surface heat up to 1,260°C.
The coatings allowed the programme to use conventional alloys, rather than special composite materials, in the structure. For example, a nearly 31kg aluminium frame forms the majority of the engine structure, despite a material melting point of only about 150°C. The nose is formed from a 68kg tungsten block, rather than more exotic and risky composite materials.
Despite such extreme care, the experimental flight tests are highly risky and not only because nobody has tested a fuel-cooled hypersonic propulsion system in flight. The size and weight limitation of the X-51A test vehicle allowed engineers to build in only two redundant systems, both 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, Vogel says.
The 7.8m (25.5ft)-long X-51A is comprised of three stages: booster, interstage and cruiser, with the latter measuring 4.27m and weighing 680kg.
The booster stage is modified from the army tactical missile system with aerodynamic fairings and a lighter titanium nozzle. After the X-51A is released from a Boeing B-52H pylon, the booster will ignite for about 30s, lofting the vehicle to about 60,000ft and Mach 4.8.
After burn-out, both the booster and the interstage, which has a flow-through duct that helps pre-heat the JP7, will separate mechanically from the cruiser stage, as designers wanted to avoid explosive charges.
The cruiser stage will then coast unpowered for several seconds after separation before the hypersonic stage of the experiment begins. The cruiser's engine will burn for 300s, as the aircraft climbs to about 80,000ft and about M6. Upon burn-out, the vehicle will perform manoeuvres during a roughly 500s decent phase until splashdown in the ocean.