Experimental aircraft sets speed record for air-breathing vehicle and raises hopes NASA may continue programme
A successful Mach 7 flight by NASA's X-43A has rekindled hopes that the agency can be persuaded to reverse its recent cancellation of future hypersonic research. NASA has agreed to fund an M10 flight attempt towards the end of the year, and programme officials hope to win the agency's approval for a new five-year hypersonics initiative, beginning in fiscal year 2006.
The 27 March flight of the unmanned X-43A set a speed record for an air-breathing vehicle and marked the first free flight of an aircraft powered by a supersonic-combustion ramjet (scramjet). Alliant TechSystems built both the air vehicle and the airframe-integrated, hydrogen-fuelled scramjet, while Boeing Phantom Works designed the thermal protection and onboard systems.
"Everything worked as planned" when the second X-43A hypersonic research vehicle, mounted on a modified Orbital Sciences Pegasus booster, was launched from NASA's Boeing B-52, says project manager Joel Sitz. The first X-43A flight attempt in June 2001 ended when the booster's fins failed during transonic acceleration. "This time we got through transonic clean as a whistle," he says.
The Pegasus was redesigned for the second flight and launched from a higher altitude - 40,000ft (12,000m) compared with 20,000ft - to reduce dynamic pressure on the booster. After release from the B-52, the Pegasus accelerated to M3 then pulled over into negative angle-of-attack flight to reach M7, where the X-453A separated from the booster. "The vehicle came off very smoothly at M7, and was stable from the beginning," says Sitz.
"Some felt separation was high risk, as there is a lot of non-linear flow. They were surprised how smooth it was," says Sitz. "It was quick, about half a second." The scramjet inlet then opened and started. Combustion was achieved using silane to ignite the hydrogen in the supersonic flow through the engine. The scramjet burned for 11s, until its fuel was exhausted, accelerating the X-43A. NASA would have been happy to have achieved supersonic combustion: "Acceleration was gravy," says Sitz.
The biggest surprise was the fact that the X-43A stayed intact all the way to splashdown in the Pacific, Sitz says. The vehicle was expected to disintegrate as the scramjet, a copper heat-sink engine with no active cooling, melted after burning out. Instead NASA was able to collect data from vehicle manoeuvres at M6, M5 and so on down to impact with the ocean at around M1. "We even got the water temperature," says Sitz.
NASA subsequently released around $20 million for the third and final planned flight of the X-43A, an M10 attempt scheduled for late in the fourth quarter. Sitz says hardware for the flight is essentially complete and integration will begin in May. Whereas booster propellant was offloaded for the second flight, and a steel aft skirt added for balance, a full propellant load will be required for the M10 flight, he says. The steel ballast will be removed and the adaptor changed from steel to aluminium to balance the booster. Carbon-carbon thermal protection, used on the leading edges of the X-43A's horizontal stabilisers for the M7 flight, will be added to the vertical fins for the M10 flight.
Funds for the M10 flight had to be transferred back to the agency's Aeronautics enterprise from the new Exploration Systems office, which took over hypersonics research under the Next Generation Launch Technologies programme. Last month NASA decided to cancel all hypersonic work beyond the M10 flight because it did not support the new space exploration vision.
This included the X-43C hydrocarbon-fuelled scramjet demonstrator, work on an M4-plus turbine accelerator for a planned reusable combine-cycle flight demonstrator, and a proposed X-43D follow-on to the hydrogen-fuelled X-43A capable of M15.
The only US hypersonics programmes now planned after the M10 flight are the US Air Force Research Laboratory's Scramjet Engine Demonstration (SED) and the Defense Advanced Research Project Agency/Office of Naval Research HyFly hypersonic strike missile demonstrator. Whereas NASA's X-43 series was aimed at reusable hypersonics for future space access vehicles, the military programmes are targeted at expendable missile applications.
The SED will use the same Pratt & Whitney-developed HyTech hydrocarbon-fuelled scramjet planned for the cancelled X-43C, but in a single-engine configuration. The X-43C was to use three of the fuel-cooled engines. The programme plans to test a Boeing-designed waverider vehicle at speeds ranging from M4 to M7-plus, with the first of several flights planned for fiscal year 2008. Flights of the Boeing/Aerojet dual combustion ramjet-powered HyFly are planned for this year, initially at M4, with M6 flights planned for next year.
GRAHAM WARWICK / WASHINGTON DC
Source: Flight International