High hopes are pinned on the first flight of NASA's Hyper-X dedicated hypersonic research aircraft
Graham Warwick/WASHINGTON DC
It is more than 30 years since NASA last conducted a dedicated hypersonic research flight. That was in 1969 when the X-15 raised the speed bar to Mach 6.7. Now the USA plans to lift the bar to M10 and pave the way for a new generation of hypersonic vehicles.
NASA will return to hypersonic flight testing on 2 June, when it launches the first X-43A. If successful, the mission will mark the first free flight of an air-breathing hypersonic vehicle. It will also be a key step on the road to operational hypersonic craft, including a third-generation reusable launch vehicle (RLV).
Whereas the X-15 was manned and rocket-propelled, the X-43 is unmanned and powered by a supersonic-combustion ramjet (scramjet). Just 3.6m (12ft) long and weighing 1,270kg (2,800lb), the X-43 is little more than a flying engine, the airframe acting as both inlet and nozzle for the scramjet.
The first two X-43s are planned to fly at M7, the highest speed that can be achieved in ground test facilities, says deputy programme manager Paul Reukauf. The third will break new ground. It will fly at M10 and provide the first real data from which to benchmark computational fluid dynamics models.
Scramjet key to development
The successful operation of an airframe-integrated scramjet at M10 is key to developing operational hypersonic vehicles. Whereas it was once thought speeds as high as M25 would be desirable, the likely operational range looks to be M7-12. "Real vehicles are likely to operate at M12. Any faster and there is a huge drag rise through the engine," says Reukauf.
In a typical air-breathing RLV concept, the vehicle is accelerated by rocket or turbine engines to M4 where it switches to ramjet propulsion. At M6, the powerplant transitions to scramjet mode, accelerating the vehicle to M12 where rocket propulsion takes over for orbital insertion. Using atmospheric oxygen for at least some of the flight saves weight, increases payload and reduces cost.
NASA cleared the way for the first launch with a successful captive-carry flight on 28 April from Dryden Flight Research Center at Edwards AFB, California. The X-43, attached to its Orbital Sciences Pegasus booster and carried beneath the wing of NASA's Boeing B-52 mother ship, went through two practice launch sequences over the Pacific to check out the systems and procedures.
One key test was to confirm that sealing and purging of the hydrogen-fuelled X-43 is sufficient to ensure there is no more than 1% atmospheric oxygen intrusion into the vehicle. Any more could result in an explosive mixture that would ignite when the scramjet is started. To prevent this, the inlet cowl door is closed and the vehicle is purged with nitrogen up to the moment of separation from its booster.
While the captive-carry flight confirmed the X-43A vehicle and its Pegasus booster - a combination dubbed Hyper-X by NASA - are ready to fly, it left the most important questions unanswered. Will the vehicle separate safely from its booster? And will the scramjet work?
Safe separation
NASA believes the second question has been answered by ground tests in which the X-43's scramjet has started and operated successfully at M7. The first question is the real unknown and safe separation of two autonomously controlled vehicles travelling at M7 is the real objective of the first flight. Successful operation of the scramjet will be a bonus. "There is enough risk in the separation event that the rest is just gravy," says Reukauf.
Even if the first flight is unsuccessful, NASA has a second M7 mission scheduled for December before it embarks on the M10 flight, currently planned for October 2002. The first mission will begin 36h before launch with fuelling of the vehicle. The X-43 has tanks for hydrogen fuel, stored at 590bar (8,500lb/in²); for silane to ignite the hydrogen, stored at 310bar; and for water to cool the vehicle's leading edges and engine during its brief free flight. Nitrogen for purging, and water/glycol coolant, which will be pumped through the leading edges at speeds above M3, are stored in the adaptor between the vehicle and its booster.
After take-off from Edwards, the B-52 will head for the coast, following the corridor once used for cruise missile testing, says Reukauf. Over the Pacific, off Pt Mugu, the mother ship will enter a racetrack pattern. On the westward leg, flying towards San Nicholas Island at 25,000ft (7,600m) and M0.5, the B-52 will release the booster, which will drop for 2.5s then ignite and climb for 88s to 100,000ft, accelerating its payload to M7 and a dynamic pressure (Q) of 75bar.
"When it burns out, the booster will sense the negative acceleration and tell the X-43 to get ready to separate," says Reukauf. Three seconds later, the booster will initiate the pyrotechnics which will push the vehicle away with a velocity differential of just over 2.7m/s (530ft/min). "It will take less than two tenths of a second for them to separate," says Reukauf.
Free at last, the X-43 will have 2s to stabilise itself at the correct angle of attack before the inlet cowl door opens to allow air to flow through the engine. The vehicle will then execute the first of several "parameter identification" (PID) manoeuvres, to collect aero- dynamic data with the cowl open.
A hypergolic mix of silane and hydrogen will be injected into the scramjet to start supersonic combustion. As it begins to burn, the hydrogen flow will be increased and silane flow reduced. "The engine will ramp up to a preset fuel mix then burn for 5-7s, until it runs out of hydrogen," says Reukauf.
Tested to destruction
Fuel exhausted, the cowl door will close and the X-43 will fly a constant-Q descent to its destruction in the Pacific, the vehicle performing PID manoeuvres at every Mach number until it hits the water.
There was no point in making the X-43 recoverable, says Reukauf, because even after it has run out of fuel, the scramjet will continue to heat up, with cowl door closed and coolant water exhausted, and the copper-alloy engine will "melt away" during the descent. Instead, NASA will extract as much data as possible from the X-43 during its brief life. Telemetry antennas on the vehicle will transmit 600 parameters back to Edwards: 400 from the vehicle's databus and 200 "real data" variables, ranging from engine pressures and temperatures to vehicle accelerations and aerodynamic derivatives.
As a back-up, the X-43 will store the 30 most important parameters measured during engine burn, then rebroadcast them every 20s "all the way to the water", Reukauf says, in the hope the data will be captured by range-control aircraft positioned over the Pacific to monitor the entire Hyper-X mission.
If successful, the X-43 will take over the mantle of world's fastest air-breathing aircraft from the Lockheed M3-plus SR-71. More importantly, it will mark a renaissance in US hypersonics research. NASA is already planning X-43B and C derivatives, says Reukauf.
The X-43C would be powered by a hydrocarbon-fuelled scramjet under development by the US Air Force. This would use JP8 jet fuel, instead of hydrogen, and burn for 200s after being accelerated to M6 by the B-52 mothership and Pegasus booster. The X-43C is scheduled to fly in 2006.
The X-43B is "in very early planning", says Reukauf, but would be a much larger, fully recoverable vehicle for use as a propulsion system demonstrator in support of third-generation RLV development. The 13- to 15m-long vehicle would be accelerated to M4 by a rocket or turbine engine then to M7 by a ram/scramjet before coming back to land.
The X-43A may be small, but with cancellation of the X-33 and X-34 RLV technology demonstrators, and industry saying a breakthrough in propulsion technology is required before the Space Shuttle can be affordably replaced, NASA's first hypersonic research aircraft for 30 years is taking on new significance.
Source: Flight International
