X-51 hypersonic scramjet test bird ready for second flight

Mach 6 success could indicate spaceplane potential

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The X-51A hypersonic scramjet project is back on track, according to its chiefs, with problems that occurred on the inaugural test flight now ironed out and a second flight pencilled in for 22 March.

The first X-51A and its booster rocket mounted ready for release from B-52 mothership. Credit: USAF

All right, very nice, but where's our spaceplane already?

"We are proud of the first flight results, and at the same time we understand the inherent risk in a high-technology demonstrator like the X-51A. We can't wait to get this second vehicle in the air and show what we can do," says Curtis Berger of Pratt & Whitney Rocketdyne, the firm which built the X-51's radical engine.

The initial flight took place last May, with an X-51 dropped from a modified B-52 bomber mothership above the Pacific. A booster rocket accelerated the unmanned hypersonic craft to Mach 4.5 before being jettisoned.

It had been hoped that the X-51's engine would then burn for 300 seconds and boost it up to speeds of Mach 6. In the event, according to programme officials, it didn't accelerate as fast as it was intended to - and a design flaw led to hot exhaust gas leaking from the engine into electronics bays.

At 140 seconds into the flight, telemetry was lost and firing-range controllers ordered the destruction of the craft while it was still moving at only Mach 5.

Now, however, project chiefs believe that the remaining three test craft have been modified such that the exhaust leak problem won't occur in them. This should let them fly for as long as they have fuel and accelerate faster.

"In a demonstrator you learn things," says Charlie Brink, supervising the X-51 for the US air force research laboratory.

The X-51 is intended to deliver scramjet technology which can run on relatively ordinary JP-7 hydrocarbon fuel to achieve Mach 6. Previous test scramjets have touched Mach 10, but they required the use of hydrogen fuel - generally seen as impractical for military use in such applications. Any future hypersonic missile or aircraft powered by a hydrogen scramjet would be mainly fuel tank and would require troublesome logistic support.

However, a scramjet has to let intake air blow through its combustion chamber at supersonic speeds - hence the name, supersonic combustion ramjet, scramjet. Keeping fuel burning in a supersonic airstream is traditionally compared to "lighting a match in a hurricane", and so far has only been reliably achieved using highly volatile hydrogen.

The X-51's engine starts up burning a mix of ethylene and JP-7 to get going, then switches to JP-7 only once properly fired up. It promises to offer a means of reaching hypersonic speeds without the use of rockets, which have to carry oxidiser as well as fuel and thus have shorter endurance and/or lesser payloads.

Possible applications for scramjet tech in the near term include hypersonic cruise missiles, which might let US bombers strike targets possessing very strong air defences. There is also talk of use in so-called "prompt global strike" to deliver a conventional warhead round the world in a matter of hours just as an ICBM can do with a nuke. The idea here is that launching a conventionally-armed ICBM would cause other nations to panic and assume that a nuclear war was underway, whereas they would be un-bothered by a hypersonic.

In the longer term there are perhaps more exciting applications in prospect. Brink himself has previously speculated that the X-51A's engine could be persuaded to ignite at lower supersonic speeds achievable by ordinary turbojets. This would open up the prospect of a successor to the legendary SR-71 "Blackbird" spy plane of Cold War fame, which took off from a runway on jet power.

It could then transform the nacelles within which its jets were housed into ordinary old-school ramjets as soon as it was going fast enough to cram adequate amounts of air into their intakes. At this point the internal turbojets and their afterburners became essentially fuel injectors for the ramjets, and the Blackbird could accelerate to speeds of Mach 3.5.

There were formerly ambitious Pentagon plans for a turbo-scramjet Mach 6 "Blackswift" runway hyperplane to follow the Blackbird, but these were scrapped in 2008 by sceptical Washington politicos. The US air force has since stated that if the X-51 effort and possible successor hyper-missiles are a success, it would like to proceed with a "High-Speed Reusable Flight Research Vehicle" (HSRFRV) to fly in 2021.

If that happens, it would still be a long way from being a true, Mach-25 space plane able to take off from a runway and fly itself into orbit. But it would perhaps be a bigger step down that route than seems to be the case at first sight.

Consider the "Skylon" spaceplane design, long promoted by British firm Reaction Engines though seemingly as far from being built as ever. It is intended to use radical hydrogen-burning, air breathing SABRE engines rather than scramjets for takeoff and initial acceleration: but the key point is that once it is flying at Mach 5.5 it converts to rocket mode, as the air has become so thin as to not be worth scooping.

An HSRFRV derivative, then, might add rocket mode and oxidiser tanks to achieve a Skylon-style trajectory into orbit from a runway takeoff. This could open up the prospect of affordable access to space at long last. Not only would such a turbo/scram/rocketplane not have to throw large bits of itself away to reach orbit, not only would it be able to use comparatively compact and easy-to-handle hydrocarbon fuel instead of hydrogen, it would also avoid the expense and difficulty of being pointed vertically into the sky for liftoff like almost all current launch systems*.

So there are reasons for space enthusiasts as well as weapons buffs to look forward to the next X-51 test later in the month (provided there are no holdups) and hope that it goes well. ®


*One exception is the Pegasus, which takes off from a runway carried by an airliner mothership/first-stage and is released to boost onward to orbit at 40,000 feet.

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