Airships are attracting a lot of interest for persistent surveillance because of their ability to stay aloft at high altitudes for long periods - weeks, months, maybe even years. But lighter-than-air vehicles have well-known drawbacks. They can be hard to handle in winds because of their size, meaning they have to ascend and descend in calm weather. Managing buoyancy as fuel is consumed and the airship climbs is also a challenge.
Sanswire-TAO, a US-German joint venture, thinks it has the answers in its Stratellite segmented airship. Instead of presenting a single large surface to the wind, this non-rigid airship is divided into sections that make the envelope flexible and allow it to "wiggle" through gusts, making it easier to control, says Sanswire.
Bouyancy is provided by helium in the first segment only. The other sections are filled with what Sanswire calls "fuel gas", which has the same density as air. As the gas is compressed and consumed by the engine, air is pumped into the envelope to maintain the airship's mass and shape. The fuel gas is an unspecified mix of available gases that can burn in a range of engine types, the company says.
Stuggart-based TAO-Technologies has flown its latest version of the Stratellite, a 22m-long prototype. Unlike the previous version, which had propulsion units on each segment, this has a single vectorable engine mounted on a swivelling gimbal under the head segment. The powerplant is a modified Rotax piston engine, and Sanswire says it has looked at double- and triple-supercharging to enable the Stratellite to reach its stratorspheric surveillance altitude.
There is another aspect of the concept that is different. To increase the chances of a safe recovery, regardless of weather, the payload is designed to separate from the airship and return to earth via Sanswire's air-drop delivery system, a precision-guided parawing. Alternatively the whole airship could be deflated and parachuted to the ground, or just the payload recovered and relatively inexpensive envelope discarded, the company says.
Using capital from European investors, Sanswire-TAO plans to build and fly a 70m-long production version later in 2009. This will be designed to carry a 100lb payload at 60,000ft for 30 days. A heaver-payload, longer-duration 150-long version is also on the drawing board.