An enormous airship built for the US forces has been bought back by its British designers and is to go into commercial service based in old Blighty.

Regular Reg readers will already be familiar with the ship, formerly designated as the first of the US Army's* planned fleet of Long Endurance Multi-intelligence Vehicle (LEMV) ships. It was expected that LEMVs, operating without crews aboard, would lurk for weeks on end high in the skies above Afghanistan, carrying spy podules to provide an unwinking watch over huge swathes of territory.

The current winding down of the Wars On Stuff, coupled with sweeping US government budget cuts following the recent economic troubles, saw the LEMV programme cancelled not long after the first ship flew at Lakehurst in New Jersey back in 2012 (famously the place where the mighty zeppelin Hindenburg burned and crashed in 1937, putting an end to the era of the great pre-war rigid ships).

The LEMV had actually been designed by a British firm, Hybrid Air Vehicles, latest in a long line of companies run by UK airship advocates. After the US Army cancellation, HAV negotiated successfully to buy the vessel. It was then taken apart and transported to Britain, and is now being restored to airworthy condition in the famous Number One shed at Cardington - the colossal building in which Blighty's answer to the Hindenburg, the likewise disastrous R101, was built.

The former LEMV - now to be known as the "Airlander" - was built to the HAV 304 design: the company has pulled the details of this off its website for some reason, but we had them noted. The ship is 91m long, 34m wide and 26m high - colossal, but even so it looks small inside the brobdingnagian Cardington hangar. The massive envelope maintains its shape by internal gas pressure, blimp-style, and is intended to generate extra dynamic lift over and above that from its helium filling as the ship flies along. The vessel will normally be heavier than air overall - it won't lift off the ground on its own. This is what is meant by a "hybrid airship".

The Airlander's engines are four 350hp, 4 litre supercharged V8 diesels, and the thrust they generate can be vectored upwards to get her off the ground. Once underway with forward speed generating dynamic lift, the bow engines are shut down in the cruise.

The LEMV was intended to operate mainly unmanned, but a single-pilot cockpit was provided for such occasions as transiting through normal civil airspace where a pilot is required for legal reasons. Apart from the cockpit, the Army version had a payload compartment for surveillance electronics and a "universal load beam". As a spy ship, the LEMV would have had a helium fill allowing operations up to 20,000 feet, and fuel to stay up for 21 days. It could carry 2,500lb (1134kg) of payload on such missions.

Pressure at 20,000 feet being about 0.45 of what it is at sea level, one would expect about 45 per cent of a LEMV's envelope volume to be helium at sea level, with the rest occupied by air in internal ballonets: as the ship climbed the air would gradually be expelled by the expanding helium until the "pressure height" ceiling was reached at 20,000 feet.

Total envelope volume of a HAV 304 is 38,000 cubic metres, so total gas lift of a LEMV at sea level would be about 18.8 tonnes. Given that we're told this might be supplemented by as much as two-thirds again by dynamic lift, maximum all-up weight as a LEMV would be in the region of 30 tonnes.

As the Airlander, we are told that the ship will carry a crew and "a host of celebrities" - plus a couple of competition winners - on its maiden British flight in 2016, so it would seem that the ship will be substantially reconfigured. As we discussed a while ago, it wouldn't be hard to put more helium in it than it would have carried as a LEMV: this would lower its ceiling, but allow the ship to lift more off the ground. Likewise, the Airlander will probably carry less fuel as it will no longer need 21 days of endurance, and this too will allow more payload.

So how many celebrities could you get on board one of these?

The Californian rival from Aeroscraft

It wouldn't be unreasonable to fill 80 per cent of the Airlander envelope with helium, cutting pressure ceiling down to 6,000 feet or so. This would gain an extra 14+ tonnes of lift.

It would also be possible to carry less fuel. A likely candidate for the engines would be the Thielert Centurion 4.0, said to consume an average of 45 litres of kerosene per hour when used in a light aircraft. A LEMV, though using two engines in normal flight, would probably not burn nearly as much fuel as two Centurion-fitted light aircraft when on station, however: in fact it couldn't lift the necessary 45,360 litres (35 tonnes) of kerosene to achieve 21 days' endurance if it did. But it's clear that it would have carried a lot of fuel to achieve such lengthy endurance. Bearing in mind the maximum all-up weight of 30 tonnes, the likely gas lift of 18-ish tonnes, and assuming the LEMV would always remain heavier-than-air in order to be able to land without using downward thrust, we might say it would carry as much as 10 tonnes of kerosene as a LEMV. As an Airlander, presumably at least 5-8 tonnes of this could be forfeited in favour of extra payload on shorter duration flights - structure permitting.

All in all then, it wouldn't seem unreasonable to suppose that the Airlander could manage a load of cargo or passengers (and supporting structures) of 20 tonnes or even more for short trips. The original US Army LEMV spec did say that the design was to allow for a payload area with generous deck space of 40 x 15 feet - perhaps such a large cargo bay or passenger deck can be attached to the "universal load beam".

So yes, it would seem that an inaugural flight with "hosts of celebrities" at several celebrities per tonne would be feasible. It would also seem that the existing converted LEMV ship could make round-the-world publicity flights: it seems that HAV/Airlander investor and frontman Bruce Dickinson - well known as a former Iron Maiden vocalist, perhaps less so as an occasional airline pilot and aviation businessman, though he is those things too - intends just that.

That's all good clean fun, but nothing we've heard so far would lead sensible investors to pour cash into HAV/Airlander. The British government has coughed up £2.5m on the grounds that the Airlander is "quieter, more energy efficient and environmentally friendly", according to Vince Cable. The plan is to move on and build proper 50-tonne-capacity Airlanders for commercial sale or lease. These would most probably be something along the lines of the former HAV 366 design, also proposed to the US military as a "LEMV Heavy" airlifter.

But will anybody want such ships?

Seventy per cent greener?

It's true, vessels along Airlander lines are quieter than jets when you're standing next to them. But when you're standing on the ground, ducted props driven by Centurion V8s at a few thousand feet wouldn't be as quiet as a cargo jet at stratospheric cruising altitude.

It's true in the same way that a cargo airship could move a given load using less fuel than a jet: but Dickinson's claim that it would be "70 per cent greener" is a bit suspect. As we've established on these pages before, an airship would burn perhaps 40 per cent less fuel to move a given load compared to a plane. The other thirty per cent green-ness comes from the not terribly well-supported idea that exhaust CO₂ emitted at higher altitudes is much more ecologically damaging than it would be if released down near the surface: hence low-flying airships are greener than high-flying jets.

Still, at least Dickinson and HAV/Airlander have eased down their predecessor firm SkyCat's claim to be ninety per cent greener than planes. And a 40 per cent fuel saving is definitely good.

Unfortunately an airship takes an awful lot longer to move its load than a jet. The Airlander will probably cruise, like the LEMV, at 80 knots (around 90 mph), as opposed to a jet's 550-odd. This will tend to rule it out of most of the passenger trade, fuel savings or no.

There's always air cargo, but generally things are sent by air because they need to arrive in a hurry: if there's no hurry then ships, trucks and railways will beat airships on cost, energy efficiency and green-ness. Most air cargo travels on passenger flights anyway. There might be a small niche in the business of moving cargo between airports for Airlanders, but probably not.

So Airlander will need to do something else: something that aeroplanes and helicopters can't do.

There is such a thing, of course.

"It doesn't need a runway," Dickinson tells the BBC. "And it can plonk 50 tonnes anywhere in the world you like, which is 50 times more than a helicopter."

There are a lot of rival plonker ideas out there

Actually there are helicopters which can plonk 14 tonnes or even more - but he's right, there aren't any that can do 50. And he's right again, there are plenty of places that helicopters can't get to due to their limited range. This is why airship enthusiasts tend to envision future ships being used to deliver heavy loads to inaccessible, undeveloped places, perhaps supporting oil or gas drilling or for disaster relief.

Plonk, suck, squeeze ... or blow

Unfortunately there's a snag. An Airlander/HAV 366 which plonks down 50 tonnes will unavoidably become seriously positively buoyant as it does so, and will surge uncontrollably upwards. If the captain doesn't vent off a lot of helium himself, automatic dump valves will do it for him when the ship soars up through its pressure ceiling.

A ship which has to throw away large amounts of its expensive helium every time it delivers a cargo isn't much use. So in fact, as it stands, an Airlander can only plonk things in locations where it can easily take on a similar weight of ballast. This probably means a large quantity of water - and if the water is found in the form of a large river or the sea, this in turn probably means it would be simpler to get there by boat. If the water comes out of a tap, it would be simpler to get there by road - taps are not usually found away from other infrastructure.

There are some possible technical solutions to the buoyancy issue. A hover-skirt undercarriage, as seen on the well-known Lockheed P-791 demonstrator, could be switched to suck instead of blow to help hold the ship down - or it could on a suitable surface, anyway. The HAV 366 design includes this feature. Unfortunately, once the ship took off after using that strategy, it might struggle ever to land again.

Other possibilities include the use of condensers to harvest water ballast from the engine exhaust, or the plan of compressing the ship's helium to the point where it becomes heavier than air - as the Aeroscraft "Pelican" ship is supposed to do. But the engineers of HAV/Airlander don't appear to favour these plans.

All in all, then, there's reason to doubt that future 366-style Airlanders will really be able to plonk as well Dickinson suggests. Even if they can, it remains unproven that the wilderness plonking market really exists.

Perhaps the existing ship, already paid for courtesy of the US Army, can prove the concept. But it doesn't even have hover-skirts, not having been designed for cargo operations: it sets down on a pair of inflated bumpers.

We here on the Reg airship desk are extremely chuffed at the thought that a big, serious airship may soon be flying from the UK. We hope that HAV has enough cash to actually get this done, or that it can raise that money: we note that the company is seeking funds both from the markets and from ordinary enthusiasts.

We hope that the plonk market exists, or some other market such as slow luxury air travel. We personally would far rather cough up for a berth on an Airlander liner than one on a cruise ship or the Orient Express.

But after years covering the airship beat and pretty much nothing but disappointments to show for it, we'd have to say we aren't totally convinced any of those things will happen. ®

*Yes, Army, not Air Force or Navy. All three main US services have had a variety of airship projects on the go recently, in much the same way that all three have aeroplanes, helicopters and space forces.