More than just hot air

The US Air Force has set the tough task of designing a dirigible that can lift twice the payload of today’s largest transport aircraft. David Windle reports.

In previous columns, I’ve bemoaned the lack of ‘big ideas’ in aviation and sketched out the case for re-evaluating the practicability of large airships.

Well, I’m delighted to report that the Pentagon’s Defence Advanced Research Projects Agency (DARPA) has come up with an idea that’s not just big in every sense of the word, but also involves lighter-than-air craft. This could be my dream project.

DARPA’s Tactical Technology Office (TTO) has issued a revised draft solicitation for the development of what it terms an ‘objective air vehicle’ (OAV). The scaleable design should, it says, be capable of being developed into a craft that can move more than 500 tonnes over a distance of more than 6,000 nautical miles, all without refuelling.

In a departure from traditional airship designs, which use gas (generally helium) to provide most of its lift, the envisaged hybrid OAV would use aerodynamic lift to a much greater extent. As a result, its normal state would be heavier-than-air.

To give you some idea of the challenge this project represents, today’s heavyweight champion of the skies the Antonov-225, which was designed to transport space hardware (including the Buran, the Soviet’s answer to the Space Shuttle), can lift around 250 tonnes. And Lockheed Martin’s strategic transport aircraft, the enormous C-5 Galaxy, has a payload of around half that.

Both aircraft need considerable runways, whereas a goal of the Pentagon’s airship project, dubbed WALRUS, is to produce a vehicle that could operate without a runway, hangar or any specialised infrastructure.

The purpose of the OAV is to enable unprecedented amounts of equipment and personnel to be airlifted into a combat area in the shortest possible time. The intention is to be able to transport a new type of rapid-response force, comprising around 1,800 soldiers and their equipment.

This is exactly my kind of programme. It’s daring, it represents a major change in strategy and it’s going to cause a lot of sleepless nights for competing aerospace contractors, as it’s not clear how these tough objectives will be achieved.

But DARPA has never been afraid of going out on a technological limb – its Experimental Survivable Testbed programme led to the world’s first operational manned stealth aircraft, the Lockheed F-117a.

Oddly enough, the late Ben Rich, head of Lockheed Martin’s highly secretive Skunk Works at the time that the F-117a was designed and built, provided a rare glimpse into the organisation’s classified activities in his 1994 book Skunk Works, when he revealed: ‘We are looking into dirigibles as the ultimate heavy-lift cargo transports.’ When someone of his standing takes airships seriously, then you’d better take them seriously too.

His thinking was echoed in a 1996 study by the USAF’s Air University, which highlighted the desirability of a heavy-lift air vehicle similar to DARPA’s WALRUS, with ‘a 500-tonne lift capability, maximum airspeed of 250 knots, maximum range at maximum gross weight of 12,500 miles, and a defensive/stealth capability’.

The authors concluded: ‘The new vehicle would have three times the effective ability of the C-5B [currently the US’s biggest airship]. The cost of airship production is also low – the cost per unit produced could be around one-third that of the C-5B.’

The study shows a drawing of Aereon Corporation’s delta-shaped Dynairship hybrid; the company has been promoting the benefits of hybrid air vehicles since the late 1950s and it would be good to see Aereon get the recognition it deserves by becoming involved in the WALRUS programme.

No one knows what the end product of WALRUS will look like yet, as there is still much preparatory ground to cover, including initial system studies and the development of a detailed concept definition for the OAV.

The next stage will be the awarding of a contract to develop, build and flight test a demonstration vehicle with an airlift capability of around 18 tonnes. The first flight could be as early as 2007.

As if the WALRUS concept was not radical enough, DARPA has singled out two advanced technologies that it would like investigated in the programme’s first phase: vacuum/air buoyancy compensator tanks and electrostatic atmospheric ion propulsion.

Buoyancy control during and after cargo off-loading will be a major focus of research, especially as the air vehicle is intended to operate with little or no infrastructure. Whether ion propulsion provide sufficient thrust for such a giant aircraft will be a topic of hot debate – but that’s the idea.

When I said no one knows what the ATD could look like, I was tempted to say that it’s certain to look nothing like any aircraft we’ve ever seen before, but then again, perhaps not. The Bedfordshire-based Advanced Technologies Group first flew a sub-scale version of a heavy lift hybrid, called SkyCat, in July 2000. The one-sixth scale SkyKitten has demonstrated the stability and feasibility of a design that is said to be scaleable up to 1,000 tonnes.

David Windle is a freelance technology and aerospace writer.

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