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DARPA releases 'Blackswift' hyperplane details
Runway roboplane will do porkbarrel roll at Mach 6
DARPA, the Pentagon deathboffin office, has thrown its new "Blackswift" hypersonic-aircraft programme open to all (US) comers, and confirmed outline details of how the new unmanned hyperplane prototype will work.
In a government pdf released last week, the federal boffinry bureau (motto: "Making tomorrow's war-winning technology obsolete today") says that Blackswift will indeed, as speculation had it, be reusable and make takeoffs and landings from an ordinary runway. The hyperplane will be required to reach Mach 6 - well in excess of the best speeds yet reached by runway aircraft - and maintain that stably for at least a minute.
Wired magazine reported last year that DARPA would also require that Blackswift do a barrel roll, in order to demonstrate that it is a "real airplane", and this has been confirmed. In the new document, DARPA says that Blackswift "shall demonstrate testbed maneuverability at Mach 6+ including execution of an aileron [barrel*] roll".
Regarding propulsion, DARPA says that for subsonic and supersonic regimes, the new robobird will use relatively ordinary turbojets. For "high supersonic and hypersonic" flight it will use engines burning fuel in a supersonic air stream - supersonic combustion ramjets, or scramjets.
Most postulated scramjet designs assume the use of fast-burning hydrogen fuel, seen as the most practical to ignite in such a fast airstream. But hydrogen takes up huge amounts of volume - any hydrogen scramjet aircraft would be mainly fuel tank - and is the devil to store or transport.
DARPA have also been working on so-called "dual combustion ramjet" (DCR) kit which has two air flows running through the engine. In one, denser hydrocarbon fuel partially burns in a subsonic airflow; then the hot, light combustion products are further burned and release the rest of their energy in the other, supersonic stream. Since much of the air can move through the jet fast, drag doesn't rise to the prohibitive levels which develop above Mach 3 or 4 in a regular ramjet - but you don't have to carry hydrogen.
DCR has been trialled successfully in the wind tunnel and small-scale flying prototypes, but has lately suffered a failure in flight tests of the "HyFly" hypermissile demonstrator. Nonetheless, it seems to be DARPA's favoured option for use in Blackswift, as the description document says:
In high supersonic and hypersonic flight regimes, propulsion is provided by a scramjet engine (also referred to as a dual mode ramjet engine)... The Blackswift testbed shall use a hydrocarbon-fueled... propulsion system...
In order to make Blackswift a reality, DARPA will need not only to ensure that the troublesome DCR hyperjets work: ordinary turbojet mode will also need to be available, as ram and scram jets need to be airborne and moving fast before they'll kick in at all. Most present-day ramjets, used mainly in missiles, accomplish this by the use of rocket boosters, but DARPA want a fully reusable plane here.
There is precedent for turbojets integrated with ramjets, however. The ubercool cold-war era SR-71 "Blackbird" spyplane used monstrous afterburning turbojets, mounted in special nacelles whose intakes held retracting slotted spikes. These effectively transformed the engines and afterburners from turbojets into ramjets as the speed increased, allowing the SR-71 to fly at Mach 3.5-odd.
Given its name, Blackswift is fairly evidently intended as the new, enhanced Blackbird. Which is good news for all fans of super zoomy aeroplane tech, like us here on the Vulture zoom-tech desk - even if we can't really see the military need for it.
The original Blackbird was elbowed out of its job by spy satellites, and nothing has really changed there. Blackswift hypersonic-cruise planes might offer "prompt global reach" for "strike or other national need missions", but in fact ballistic exo-atmosphere rockets also offer this - rather more promptly and globally, in fact. (DARPA already has a wacky plan to lob robot spyplanes round the world on ICBMs.)
Fans of military hypersonics often contend that using ICBM-type rockets to deliver conventional warheads or spy payloads or whatever is foolish, as it will raise fears of a nuclear strike and so perhaps trigger an atomic war. But the US also has nukes on planes, both robot and manned; and nobody says you can't use planes for these jobs. Nobody says you can't launch satellites on rockets for fear that someone will think they're actually nukes waiting to de-orbit at some future point, which they perfectly well could be.
There again, reusable hypersonics could be cheaper in the long run than throwaway rocket stacks and satellites etc - if you had enough prompt-global-reach jobs to do, anyway.
In the end it doesn't matter. The US military may or may not genuinely need hypersonic planes: but the human race plainly needs cool new technology in general, and reusable ways of getting to orbit in particular. Mach 6 isn't anywhere near spaceplane speed (you need Mach 25 for orbit) but it's a lot better than Mach 3.5 - and that only in a barely-practical plane retired nearly 20 years ago. And if it's pollution or carbon burden that worries you, Blackswift hydrocarbon fuel isn't as clean in use as a hydrogen rocket or scramjet - but hydrogen manufacture is extremely energy intensive, hence carbon-intensive at present. Missile-style solid rockets, which are being used more and more in space launch, are far worse than either - outrageously filthy and noxious. So are most other liquid rocket fuels. Blackswift isn't at all dirty, as speedy flight tech goes.
So we all might wish Blackswift well. It may not be a boon to the Pentagon, but it might just be for everyone else.
If it works, that is - which not very many DARPA projects ever do. ®
*A barrel roll is so called because the aircraft spirals horizontally as it inverts and rights itself - as though stuck to the outside of a rotating barrel - rather than simply rotating round its own long axis. Barrel rolls in a normal aeroplane can be done using ailerons only - hence, aileron roll - but a straight-line or "slow" roll normally requires use of rudder as well.