Original URL: http://www.theregister.co.uk/2007/09/19/blackswift_hypersonic_confirmed_uprated_sr71_blackbird/

Hypersonic plane project confirmed by DARPA

Retro 60s tech unlikely to melt much tinfoil

By Lewis Page

Posted in Science, 19th September 2007 15:08 GMT

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.

In the end - unless you're at the more tinfoil end of the Aurora spectrum - NASP was shelved. DARPA didn't have the money, the USAF was more interested in Stealth, and NASA had all it could do to fund the space shuttle. Since then, researchers have been puttering about with relatively limited efforts such as DARPA's Falcon programme, often ambitious in scope but limited in funding.

One aspect of Falcon is the so-called HTV-3X hypersonic demonstrator. This is a fighter-sized unmanned job, intended to burn regular jet fuel in turbojet/ramjet combo engines much like those of the SR-71.

Veteran aerospace analyst Bill Sweetman reports that the famous Lockheed "Skunk Works" reckon they could design such a modernised Blackbird for speeds of up to Mach 6.5 or so.

Now Wired magazine reports confirmation from DARPA that this relatively limited design has the go-ahead from the Air Force under the name "Blackswift", with funding initially estimated at $800m for two planes.

There are still many questions hanging over the Blackswift, not least what it's supposed to do. Being very fast, very hot, very high and made of exotic refractory material, it won't be at all stealthy - which could limit its usefulness as a spy plane.

Fans of the old SR-71 always said spy birds are more responsive than satellites, but according to some accounts SR-71 operations were actually highly involved, requiring a two-day lead in and multiple prearranged air-to-air refuellings. The crippling expense and complexity of SR-71 missions is one of many official reasons why the aircraft was binned.

As an alternative to spyplane work, a Blackswift hypersonic bomber could offer the ability to strike anywhere in the world at fairly short notice, but then so does an existing ICBM.

All in all, at $0.4bn or more per aircraft, the ability to go at a relatively uninteresting Mach 6 doesn't seem worth the money. Blackswift's true value is perhaps more as a stepping stone to something better.

Unless, of course, you're of the opinion that the something better already exists, and this is merely a billion-dollar coverup. ®

*The Defence Advanced Research Projects Agency