When the National Aerospace Plane program was canceled in 1993, it was realized that a greatly shrunken test vehicle could contain costs. With no pilot, the costs of man-rating such a vehicle could be avoided, and test data would still be highly valuable for the eventual development of a manned vehicle. Thus was born the Hyper-X program and the tiny X-43 hypersonic lifting body.
NASA's unmanned X-43 supersonic combustion ramjet research vehicles were 12 ft long, weighed 3,000 lb, and carried just 2 lb of hydrogen fuel. Because they breathed air, they did not require any onboard oxidizer. They were launched with the help of two prestages, a B-52 mothership and an air-launched rocket to bring the test vehicle up to operating altitude and speed. Three were built, tested, and flown at a cost of $230 million. The first flight, in June 2001, failed because the scramjet's booster rocket, a modified lower stage of a Pegasus rocket, lost control in the transonic region and had to be destroyed, obliterating the research vehicle as well. The second vehicle was flown in March 2004 and was accelerated to operating speed by Pegasus, whereupon it demonstrated scramjet flight at Mach 6.83, or about 5,000 mph for some 10 s. The vehicle then became a hypersonic glider, continuing to collect aerodynamic data as it gradually decelerated and descended.
Its flight path led it to an oceanic ending in the Pacific, where it sank. The third flight took place in November 2004, using a modified X-43 with a goal of flying at about Mach 10. This flight also succeeded, with the vehicle reaching Mach 9.68 or nearly 7,000 mph at an altitude of 109,000 ft. This vehicle also ditched in the Pacific. In both flights, it was the Pegasus rocket that boosted the Hyper-X vehicle to its operational speed and altitude. The tests merely validated the operation of scramjet engines at those speeds, without any appreciable self-induced acceleration. As for the Pacific ditchings, this was part of the planned program. None of the X-43 craft were equipped with landing gear or flotation devices, and so they could not be recovered. The tests were performed over the open ocean in order to ensure the safety of the dry-land population. And of course, the vehicle was unpiloted because of its small size; so there was no one aboard to land it.
The Mach 6.83 flight heated the X-43 to 2,600°F, so carbon-carbon was used on the twin vertical tailplanes. The second flight, at nearly Mach 10, was expected to heat the nose to 3,600°F. Because both the successful test flight vehicles ditched in the ocean, they could not be recovered. Therefore, they could not be examined for the thermodynamic effects at the high speeds they endured. In other words, we do not know how much they melted. Nevertheless, the X-43 test program was considered a success, proving that scramjets work at hypersonic speeds in the upper atmosphere.5,6
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