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Hypersonic plane project confirmed by DARPA

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The US Air Force (USAF) has inked a deal with DARPA* - the famously luncheoning-elsewhere Pentagon boffinry outfit - to collaborate on a "combined cycle hypersonic vehicle that could take off and land like a plane", according to reports.

Rumours and whispers of a secret American hypersonic aircraft have, of course, been kicking about ever since the retirement of the famous SR-71 "Blackbird" spyplane in 1990.

The SR-71 was one of the more radical super-plane designs to date, driven by monster J-58 afterburning turbojets mounted behind an intake with a central retractable spike. The SR-71 engines effectively transformed into ramjets as they accelerated through low-supersonic speeds. The aircraft could make reasonably sustained Mach-3+ sprints at 85,000 feet.

Many have suggested that the SR-71 was so useful it makes no sense for the US never to have replaced it; this line of thought often leads to a belief in some ultra-secret successor aircraft. Such rumours are generally grouped under the heading "Aurora", owing to an interesting 1986 federal budget line item referring to such a project (it's now more widely thought that the disappearing Aurora money largely went into the Stealth bomber programme).

Aurora superplane speculation ranges from the low end, where it is a mild advancement on SR-71 type turbojet/ramjet kit, right up to a Mach-25+ orbital spaceship using exotic supersonic combustion ramjets (scramjets) or some refinement thereof. The higher figures are probably the result of high hopes initially held for the National Aerospace Plane (NASP, aka the X-30), which President Ronald Reagan mentioned in his 1986 State of the Union address.

He predicted "a new Orient Express that could, by the end of the next decade, take off from Dulles Airport and accelerate up to twenty-five times the speed of sound, attaining low earth orbit or flying to Tokyo within two hours".

Most aerospace engineers are less sanguine about the potential of scramjets nowadays, thinking it a good day if they can hit Mach 10. But it was originally hoped that by using a scramjet's fuel to cool the blazing hot front surfaces of the aircraft, heat energy could be moved into the engines and turned into thrust. That, it was thought, could let the NASP/X-30 hit the magical Mach 25, so flying itself up to orbital speed without the need for any rocket boost.

Quite apart from whether or not it actually offers much speed advantage over a ramjet, a scramjet is a nightmare to build because it runs on bulky, cryogenic liquid hydrogen rather than comparatively dense jet fuel.

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