The huge LEMV* surveillance airships now being built by British designers for the US Army may be able to carry substantial cargoes as an alternative to sky-spy equipment, according to reports.

Might not just be a spy-eye but a sky truck, too.

Aviation Week, in an update on the LEMV programme, quotes project chief Alan Metzger as saying that the vast airships are designed so that "mission and fuel modules can be removed and replaced with cargo modules".

In their standard configuration, the LEMV ships ("up to three" were ordered in June for $517m) would carry a 2,500lb payload of sensors and communications equipment on unmanned missions lasting up to 21 days. The 300-foot-long ships are "optionally manned" – they will be piloted where this is a legal requirement, as for example when transiting civil-controlled airspace, but for long surveillance missions they would normally be controlled from a ground station.

Swapping out the mission equipment would permit a LEMV to carry just 2,500lb of cargo to start with – not all that much. But three-week missions as standard indicates that the ships could spare a lot of fuel space for shorter flights of just a few days or less, suggesting that larger cargoes could be carried.

Then, an airship's lifting capacity decreases sharply as its maximum operating altitude increases. An airship's ceiling is set by its "pressure height" – the altitude at which its helium lifting gas has expanded to fill all available space inside the envelope. In order to have a higher pressure height, less gas must be put into the ship to start with, meaning that it will lift less stuff off the ground.