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Pentagon wild-card warboffin agency DARPA has plunged back into the hypersonics fray with a vengeance, announcing plans to build a "recoverable" rocket plane capable of Mach 20 speeds within four years.

A bit like this, but with its own rocket

The new project appears under the banner "Integrated Hypersonics", acknowledging that several new technologies will be needed to get aeroplanes flying at twenty times the speed of sound.

Ramjet and scramjet fans will be disappointed, however. DARPA seems to have accepted that there's no realistic prospect of using air-breathing engines at these speeds, and instead the agency envisages a plane which will be powered up to flying speed by a disposable rocket stack or "launch vehicle" and then enter a hypersonic glide. However the 2016 "Hypersonic X-plane", or HX, will also have its own onboard rocket which can be used to give it a further push in order to extend its range.

DARPA specifies that the HX itself will be "recoverable" - that is, it won't simply crash into the Pacific or wherever as a total loss after flight. However it's not clear at this stage whether it will splash down for subsequent recovery - perhaps under a parachute - in the fashion of a space capsule, or actually come in for a runway landing like a space shuttle or the X-37B robot minishuttle.

Though the HX is expected to build on discoveries made during the recent DARPA HTV-2 boost-glide tests, in which one-shot hypersonic gliders were fired aloft on space launch rockets, the new plane is all about flight within the atmosphere. Its custom launch rocket will be "a single, integrated launch vehicle designed to precisely insert a hypersonic glide vehicle into its desired trajectory, rather than a booster designed for space missions".

That said, those keen to see some proper spaceplanes - ones able to actually reach orbit without throwing most of themselves away, already - needn't feel too downhearted. The new class of hyperspeed rocket planes and craft will be used for a range of missions:

The goal of the IH program is to develop, mature, and test next- generation technologies needed for global-range, maneuverable, hypersonic flight at Mach 20 and above for missions ranging from space access to survivable, time-critical transport to conventional prompt global strike.

"Space access" would seem to indicated that the spaceplane dream is not yet dead. "Survivable, time critical transport" is plainly a revisit to the long-touted idea of despatching not bombs or warheads but small groups of troops - most probably elite special forces ones - round the world extremely quickly, an idea long touted by enthusiasts in the US Marines.

"Conventional prompt global strike", however, is the near-term military hypersonics justification. The idea here is that you'd like to blow something up very fast - maybe a terrorist leader, rogue-nation head of state, a WMD, something of that sort - before it can move away or hide again and you lose your bead on it. It could take hours and hours to get a smartbomb or a cruise missile onto target: but a Mach-20 hypermissile could reach anywhere in the world in less than an hour.

Actually this can already be done, of course, by launching an ICBM - a space rocket which will despatch its warhead around the planet outside the atmosphere, so avoiding the vast technical challenges of hypersonics. However hypersonics advocates argue that an ICBM launch could trigger a nuclear war.

Naturally a hypersonic rocket could carry a nuclear warhead just as well as a conventional one, and frankly one big rocket lifting off would probably look much like another to a worried nuclear-armed enemy. (And indeed a subsonic bomber can take off, or a subsonic Tomahawk cruise missile can be launched, and either might be nuclear armed rather than conventional: yet Russia and China do not panic when these things happen.)

So the prompt-global-strike argument is rather unconvincing. But there are military reasons to want hypersonic aircraft all the same: Mach-20 missiles or bombers or special-forces insertion craft would all be very hard for an enemy to shoot down, for instance. This has led DARPA to claim that hypersonics could be "the new Stealth" for the US forces, a new tool making them more or less unstoppable in the skies (much though this has not turned out to make them invulnerable on the ground).

And then, space access and improved supersonic airliners would be nice to have even if one cares nothing for the Pentagon's problems. So we can probably all applaud the Integrated Hypersonics push for different reasons.

For now, all that's happening is that US arms'n'serospace behemoth Lockheed will be hired to modify a spare aeroshell remaining from the HTV-2 project and carry out another test flight. However once this has happened, the idea will be to release data from this trial and the earlier HTV-2 flights and have a competition to build the HX hyper-rocketplane for 2016.

As ever, the usual DARPA caveats apply. There's every chance that the new test will be a disaster (the HTV-2 flights were at best partial successes), or that sceptical politicians will refuse to fund the ambitious DARPA plans - as happened with the "Blackswift" plans to build a vastly enhanced version of the legendary SR-71 Blackbird ramjet spy plane.

It's good to see that the hypersonics fanciers of DARPA haven't given up, though. ®

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