Original URL: http://www.theregister.co.uk/2011/01/13/usaf_scramjet_revived/

US air force has new scramjet hypersonic plane plans

Son of Blackswift, grandson of SR-71 Blackbird spy plane

By Lewis Page

Posted in Science, 13th January 2011 13:20 GMT

Who remembers the Blackswift – the planned hypersonic successor to the legendary SR-71 Blackbird spy plane of Cold War fame? The Blackswift was intended to take off and land from a runway like a normal plane and achieve speeds of Mach 6 on scramjet propulsion (comfortably eclipsing its illustrious turbo/ramjet predecessor's Mach 3.5) – and carry out a barrel roll while doing so.

Unfortunately for lovers of spiffy hyperplanes, the Blackswift (aka Falcon HTV-3X, as seen above) never got off the drawing board. Its funding was cut by sceptical politicoes back in 2008, and since then the US military has had to content itself with the missile-style WaveRider testbed. The WaveRider will not develop into a reusable aircraft: it is released at height from a B-52 bomber, accelerates to Mach 4.5 using a rocket booster, and then lights up its scramjet to accelerate – its designers hope – to Mach 6. In its only live test so far, however, the WaveRider topped out at Mach 5 in what was described as a partial success.

From a military point of view, the idea of hypersonic scramjet missiles is mildly interesting, though somewhat marginal: after all, if a warhead really needs to get somewhere in a hurry, one can simply fire it out of the atmosphere on a multistage space rocket (aka Intercontinental Ballistic Missile) and so avoid all the problems of high speed in an atmosphere.

But in civilian life we might like to move from having to use expensive, throwaway multistage rockets every time we want to put something into space. We might also like our orbital launch vehicles, as well as being much more reusable than a rocket stack or a Space Shuttle package, to use atmospheric oxygen in place of some or all of the huge amounts of oxidiser that rockets must carry to burn their fuel with. Also, we'd like to avoid the huge expense and trouble involved in pointing heavy vehicles vertically up into the sky for launch – we'd like runway takeoff as well as runway landing.

That's why people found the Blackswift exciting: while it was never intended to reach the Mach-25-equivalent speeds needed to achieve orbit, it was going to be a big step forward from the SR-71, previously the benchmark for sustained, practical-ish high-speed air breathing flight from a runway. Like its illustrious forebear, the Blackswift was to burn relatively normal JP-7 jet fuel rather than impractical, dangerous, bulky hydrogen; like the Blackbird, it was to take off and land on a runway and make no use of throwaway booster rockets to get up to ignition speed*.

And that's why today is an exciting day, because news has arrived that Blackswift is not dead after all. The US Air Force, we learn from Aviation Week, has revived the aspiration for a reusable, hydrocarbon-fuelled runway hyperplane under the new name "High-Speed Reusable Flight Research Vehicle" (HSRFRV). Lovers of hyperplanes will need to be patient, however, as the USAF intends to take a cautious path toward building the new Blackswift; it is not expected to fly until 2021, following years of trials and tests with WaveRider-derived missile-style weapon carriers.

These early tests – which might lead to a hypersonic scramjet missile that could fit into the weapons bay of a B-2 stealth bomber – would prove the main new technology required to beat the SR-71 Blackbird: that is, hydrocarbon-burning scramjets. The Blackbird was propelled down the runway and up to high speed by two hefty afterburning turbojets which were mounted inside cunning nacelles fitted with a retractable spike. At high supersonic speed, these nacelles functioned as ramjets and the turbojets nested within them were superseded, effectively acting as fuel injectors for the ramjet combustion chambers.

But a regular ramjet, even when travelling supersonically itself, slows down the flow of air through its combustion chamber to subsonic speed in order to avoid blowing out the flame. As speed climbs through the low Mach numbers this causes unacceptable levels of drag to build up, which is why the SR-71 couldn't beat Mach 3.5 or so.

Reagan's 'NASP' X-30 spaceplane failure is the skeleton at the scramjet feast

To go faster, it's necessary to let the intake air roar through the engine supersonically – hence the name, supersonic combustion ramjet – scramjet. Sustaining a flame in such an airstream is traditionally described as being like "lighting a match in a hurricane". It can be done relatively easily using explosive hydrogen as the fuel; rocket-boosted hydrogen scramjets have achieved Mach 10 in trials. But hydrogen fuel is cripplingly inconvenient, troublesome and expensive to use: also its low density means that any hydrogen-fuelled vehicle has to be mostly fuel tank**.

The late, great SR-71 doing its thing

Note the cunning turbo/ramjet engines with adjustable intake spike.

So a practical scramjet needs to burn hydrocarbon, not hydrogen. This is the technology the WaveRider and its possible hypermissile successor are intended to prove. Once that's done, the next challenge will be to dispense with disposable rocket boosters in order to reach operating speed: it will be necessary to somehow add a normal turbojet to the scramjet, as the SR-71 incorporated them into its ramjets.

There are those who consider that this is actually a step too far, even on a 2021 timetable: and who raise the dread spectre of the overly ambitious Reagan-era plan for a National Aerospace Plane, aka NASP or X-30. The NASP was to be a superpowered, ultra-hypersonic turbo/scram/rocket combo – able to take off from a standing runway start and achieve full orbit unassisted. It was well beyond the realms of possibility, and ended in ignominious failure.

Former USAF Chief Scientist Mark Lewis, for instance, tells Av Week:

"You don’t have to have a turbine ... Be bold, but be realistic in that reach. If not you risk going back to a NASP-like failure."

Lewis considers that trying to get a turbojet into the mix too soon would be fatal, and argues instead for more work at the early WaveRider stage aimed at pushing for longer scramjet endurance. He suggests that scramjet weapons can be practical and useful a lot sooner than runway hyperplanes can.

That's one point of view, of course. One might alternatively suggest that if the only thing that comes out of scramjets is Mach-6 airbreather missiles, one might as well not bother – for strike missions of the sort the proposed B-2 scram-bomb might carry out, alternative rocket-propelled approaches such as ICBMs or Nazi-style "Boost Glide" space-bomber systems would seem easier to achieve and much quicker over the ground. There are other competitors for this rather niche application, too – for instance the proposed US Navy bombardment railgun, intended to fling its projectiles hundreds of miles at speeds in excess of Mach 5.

Then, one should note that the proposed HSRFRV is not nearly as ambitious as NASP: nobody is suggesting a runway-to-orbit spaceplane as yet, merely a runway-to-Mach-4+ hypersonic aeroplane. If HSRFRVs were ever used as orbital launchers, they would act merely to give useful initial height and speed to piggybacked rockets, rather in the style of the existing airliner-based "Stargazer" – but much more powerful.

Quite frankly, we here on the Reg hyperplane desk want our stepping-stone towards a proper NASP style spaceplane and we applaud the new HSRFRV plan. We'd also like it if the British government would get on already and cough up some serious cash to see if Blighty's Reaction Engines team can make good on their alternative "Skylon" runway spaceplane plan, which would eschew scramjets in favour of SABRE engines - described as a "rocket engine with an additional precooled turbo-compressor".

The Skylon suffers from being hydrogen-fuelled, of course, meaning that many would doubt it can ever be economically useful: certainly it is basically a huge hydrogen tank with wings and a small payload bay amidships. Meanwhile the USAF scramjet plans, even if they are successful, will still be far short of orbital capability.

It's possible to contend, along with famous internet-tycoon-turned-space-kingpin Elon Musk, that reusable runway spaceplanes will never work and that the answer is instead to make normal rocket stacks much cheaper. But even this simpler plan may well founder during the next decade, as it menaces a lot of jobs at NASA and in the huge, well-established US rocket industry.

All in all, it may be a while before access to space seriously improves. But at least some people are still working on it. ®

Bootnotes

*A ramjet, employing no compressor fans at the intake, needs to be going quite fast before it will fire up – the speed is necessary to ram air into the scoop with sufficient force, hence the name.

**A look at a Shuttle launch stack bears this out: and the Shuttle gets a lot of poke from its strap-on solid rockets burning denser fuel. If it used only hydrogen, the fuel tank would have to be even more enormous. Also, the expense of using leaky, explosive, cryogenic hydrogen is vast – this is why those seeking to cut the cost of rocket launch tend nowadays to favour hydrocarbon fuels such as rocketgrade kerosene.