Science Forums 2

[CraigD]

Originally Posted by Pyrotex

Trans-oceanic commercial travel -- Airships -- Hot Air….

Very cool

On the subject of solar-heated hot air balloons, actual ones like Dominic Michaelis's 1981 English Channel crosser shown at this webpage are, I think, a must-see/read.

I’d have to run a lot of numbers and sketches to have any idea how much one could scale these things up. From Michaelis’s balloons, we can get a rough volume to lifting capacity ratio of about , about 7 times the required volume of a gas burner-heated balloon. This gives Pyro’s mile-wide sphere a maximum mass of about 54,000,000 kg, about the same as the 250 m WWII battleship Bismarck.

A major design challenge in making a purely solar-heated balloon (Michaelis’s had gas burners for use in initial inflation and landing) is to designing something that doesn’t crash. - especially in real outdoor weather. Once a hot air balloon is cooled, and begins descending, you need to have a ready, controllable supply of heat to heat the gas and make it stop. Michaelis’s balloons handled by using their gas burners during landing – otherwise, their landings would have been abandon-ship-with-parachute-type crash landings. A pure solar balloon would need a better system. Over-design it so that you constantly vent hot air in level flight, and/or actively control its transparency and/or opacity are a couple of approaches that come quickly to mind.

Staying up at night is another. At night, obviously, you won’t get significant solar energy input (moonlight really doesn’t count). You’d need to either have super insulation, dump-able ballast, some sort of gas pressure creation and control system, etc. The controls in the previous paragraph would be useless (other than for descending, which is exactly what you want to avoid occurring at night), and all of the solutions mentioned have big drawbacks.

The worst case – and not an unlikely – scenario I can imagine is floating into a region of warmer air at night in the rain. Throw in a strong downdraft, and its even worse.

Of course, these are mere technical challenges – but they’re pretty daunting ones.

No one appears to have mentioned so far what I consider a very straight-forward and proven alternative approach for heavier-than-air craft: battery electric with or without solar photovoltaic. Thousands of sizable model airplanes are airborne as I write using LiPo battery powered electric motor-driven propellers, while some fairly ordinary-looking small manned airplanes have flown respectable distances on photovoltaic power alone. There don’t appear to me to be any major show-stopper barriers to, in the next few years, building a “plug-in solar” electric airliner in the 30-seat, 500 km/h, 1500 km range “regional” class (eg: the BAe Jetstream 41). Though cost will likely be high, major increases in fuel price – which, according to peak oil predictions, is inevitable – you just might be able to get rich selling such a plane.

Though I’ve not worked out even rough engineering details yet, I’ve got a pretty strong sense that we won’t be seeing the last of fast, heavy air travel anytime soon.