From UAVs to novel lighter-than-air craft

My friend and business colleague Richard Wishart came back from the Farnborough Airshow 2010 enthused by the future of UAVs (unmanned aerial vehicles).

We moved on to discussing the advances in solar powered flight that would allow these UAV's to stay airborne for weeks at a time, demonstration vehicles such as the Helios shown above and the manned Solar Impulse are already in existence.

Back home my mind was churning over other developments that would assist in this area.

Ultralight materials would be a definite benefit, and there are currently solids such as silica aerogels of ultra light density. Amazingly an evacuated version, the silica nanofoam, apparently has a density equal to or even lighter than that of air. I suspect there are structural problems with it at the moment!

We could of course also look to lighter-than-air aircraft, something I would love to see commercially done again. Currently either Helium airships or hot air balloons are available options.

The ideal would be a vacuum buoyed airship, as conceived by Francesco Lana de Terzi in 1670 - but we do not have materials light yet strong enough to hold large volumes of vacuum without collapsing under air pressure.

There is another option, replacing hot air with steam - this can give twice as much lift as hot air at the same temperature; the  Flying Kettle project and the German Heidas have done feasibility testing, with the latter achieving a 30 minute flight with a 5kg payload!
What thought experiments or ideas have you played with recently?