"Time is critical. What we'll see with hypersonic air travel, and it's clearly a long way out there, is that it gives people that option of travelling anywhere in the world within four hours. That would be a fantastic offering and we believe there is a market for that.

"This is a really versatile propulsion system that we're developing. It is an air-breathing rocket engine that can go from zero to five times the speed of sound and for the space-access variant, 25 times the speed of sound, and has a huge range of operation. The other advantage of this engine is that it's highly scaleable."

The ability to up or down-size the concept is undoubtedly a trump card. Reaction has already studied a large "civil high-speed air transport vehicle". It seats 300 people, on a par with larger versions of Boeing's twin engine 787 Dreamliner.

"There would be a business case for a vehicle of that nature," Thomas contends. "It doesn't preclude you doing something that could be a step towards that. I think really the defining moment is going to be when we test the first engine. We're planning to do that by the end of this decade."

Thomas estimates that the world is 10-15 years away from commercial space flight. Returning to comparisons with SpaceX and other rocket engineers, he acknowledges Musk's ultimate goal of enabling people to live on other planets, but points to a key differentiator.