The advantages of hydrogen over helium are that it is less, cheaper to produce and less scarce (particularly on Earth, but also in the Universe at large). The downside is that it is flammable.
However, I think that good engineering could overcome the issues of flammability. Here are some ideas:
- Make the envelope out of flame retardant material
- Make the envelope into a cellular structure, such that one cells can be burst and catch fire without causing all of the other cells to do likewise
- Fill the outer cells of the aforementioned cellular structure with cheap inert gas (e.g. Nitrogen). This means that outer damage doesn't result in the release of flammable gas, and also means that if both an outer cell and an inner cells are burst, the burn rate is lowered as there's less oxygen in the immediate vicinity of the hydrogen
- Build in fire suppression systems
- Structure the gondola to a glider, such that it detaches from the balloon in the event of a fire and safely returns to Earth
- Add parachutes to both the gondola (as an alternative to the glider functionality) and the balloon. The latter is intended to prevent damage to people or property on the ground when, after a fire, the ballon falls to Earth
- Deploy such airships as AUVs / drone (at least until such time as a strong safety record could be established)
- Only use such airships in isolated areas / over the sea (at least until such time as a strong safety record could be established)
Would these safety features be effective? Would they be cost-effective compared with alternative lifting gases and alternative modes of transport?