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The US military will deploy an "optionally manned" 250-foot surveillance airship to Afghanistan by the middle of 2011, according to reports. The dirigible spy-ship will be able to lurk high above Afghan battlefields for up to three weeks at a time, relaying information to ground commanders.

Aviation Week reports that the US Army Space and Missile Defence Command is putting together a group of organisations aimed at building an enlarged version of the well-known prototype P-791 airship test-flown in 2006 (see the vid above). The new, bigger ship will be termed the Long Endurance Multi-intelligence Vehicle (LEMV).

The P-791 and the LEMV which will follow it are termed "hybrid" airship designs as they are not supported entirely by the buoyancy of their lifting gas like a conventional sky-ship. Rather, 80 per cent of their weight is supported by the gas cells and the remainder by aerodynamic lift when the ship is underway.

To move about on the ground, the missing 20 per cent lift is generated by a hovercraft-style aircushion system (the round openings underneath), allowing the craft to be taxied about. The aircushion can also be switched into reverse, allowing the ship to suck itself down firmly in place and potentially avoid the need for mooring masts and/or large ground-handling teams.

In order to lift off before gathering speed, such a ship uses vertical thrust from its swivelling propulsors, in this case driven by powerful turbodiesels. Once in the cruise, these fuel-hungry engines are shut down and the props are driven electrically by a central generator.

The P-791 was actually built with a view to heavy military airlift. The much, much bigger "Walrus" transports intended to develop from the prototype were intended to haul hundreds of tons to a landing far from base deep in rough terrain - perhaps dropping off a large, well-equipped ground combat unit.

There's nothing too hard about designing an airship able to lift such a cargo, but dropping it off quickly at the other end is a huge headache. With hundreds of tons of weight suddenly gone, the ship is liable to become uncontrollably buoyant and soar wildly up past "pressure height", where expanding helium must vent off so as not to burst the ship.

Solutions to this could include the taking on of water ballast (if any were available at the landing site), use of the reverse-aircushion to hold the ship down, making a landing in a very heavy condition supported mainly by vectored thrust, and possibly some kind of miraculous "static heaviness control" gas-compression method - or several of the above.

But the "Walrus" plans never progressed beyond some initial trials like the P-791. This may not have meant that the landing issues were insoluble: there are other reasons why a Walrus heavy-lifter wouldn't be practical. It was only spec'd for 10,000 foot cruising height, for instance, meaning that it could be attacked by handheld missiles all along its route - and a bigger, more high-value target would be hard to imagine.

In any event, the airship-lift plan has remained firmly on the back burner - or even off the stove altogether - for the last few years.

But now it appears that the technology will be going to war soon in the surveillance role. This means a much lighter payload - just 2,500 lb according to Av Week - though the cargo pay will remain large at 40' x 15' deck space. Less load means that the ship can work at a much more survivable 20,000 feet and still carry enough fuel for a long mission, even with the central generator powering the payload as well as the propulsion. The ship would be piloted for legal and safety reasons when in transit to and from warzones or in civil airspace, but most probably unmanned on the job.

The US Army Space & Missile Defence Command issued a request for LEMV proposals in April: now Av Week reports that a contract will be issued by the end of the year for a ship ready to deploy within 18 months from that point. ®

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