Gainesy,
I'm trying hard not be a smartarse but have succumbed - surely semi-rigid airships don't inflate or shrink via Hydrogen ?
I'm reliably informed by someone who would know ( senior Rolls Royce aero-engines man ) that for example the Speys on the UK Phantom were cleared to run on coal dust.
Not sure what form the coal dust was in, as we had to break off our conversation.
If wearing a ' green ' badge, one must comment that all fossil fuels will run out sometime, the only question is whether coal or similar gives enough breathing space before a more intelligent power source comes along - for the medium term, " fuel you can grow " sounds about right to me.
As for Hydrogen, must have potential but you go first, and keep the ' no smoking ' sign on, also steer clear of static / lightning !