It would fly at 4,000 mph and use hydrogen as fuel. Which theoretically means it could be both sustainable and clean. It’s just a concept now, but shows how seemingly impossible global warming conundrums can potentially be solved.
Imagine, a world with low emission, fast jets using renewable energy. It could happen.
I believe that there has been talk of its vapour trail damaging the ozone layer. I’m no expert but I suppose that has to be taken into account.