Many such designs have a huge flaw: the fuel. We don't have a good way of producing hidrogen without high energy demand and it requires very low temperature for any significant amount to be stored. Basic 30-40% if the energy potential is wasted for production and storage. So it is a huge pain to work with it, this being the reason why it doesn't catch as a car fuel, it is to expensive just to have it and potentially dangerous. Now taking all of this in consideration and putting the system in a supersonic device that can suffer 10% thermic delation dosen't sounds a good idea, but who knows history favours the bold