Military researchers in the US are continuing efforts with industry to harness the most abundant element in the universe — hydrogen — as a means of reducing reliance on fossil fuels.
Hydrogen can be expensive to extract and it tends to be more difficult to store and transport than oil-based fuels, partly because it is highly flammable.
Also, while hydrogen energy is renewable and has a minimal environmental impact, fossil fuels are often used to extract it, thereby reducing or negating any emissions-reduction benefits.
There are potential advantages, though. In larger gas turbine engines, hydrogen can be burnt