IIRC, hydrogen gas will basically detonate at any mixture ratio in oxygen or in air. Quick search says 4% to 75% to burn in air, and 18% to 60% to fully detonat

IIRC, hydrogen gas will basically detonate at any mixture ratio in oxygen or in air. Quick search says 4% to 75% to burn in air, and 18% to 60% to fully detonate. By contrast it looks like kerosene will only burn at between 0.7% to 5% in air, so that's a much narrower range that's dangerous. And of course that's just burning, lots of heat and light, but no brisant supersonic shockwave to shred everything around like you get with hydrogen. Plus apparently if you end up with atomic hydrogen somehow, it will embrittle and structurally ruin a lot of common metals on contact. It also seems generally just not very practical even ignoring the tendency to leak and explode, needing either cryo or high pressure to be stored at any reasonable density, and even in that case taking up over three times as much space as hydrocarbon fuels by volume, despite also packing three times as much energy by mass. The British designs for Skylon are basically just one massive hydrogen fuel tank with wings and engines.