[ChuckMcM]

That was the interesting takeaway from the Elon Musk talk about how cheap the fuel was compared to everything else. Having the Skylon be reusable then shifts some of the operational costs back to fuel.

Of course a plane that can fly a Mach 5 and 100,000' has other uses, the most obvious one being the SR-71 replacement. Nothing like a bit of high speed aerial surveillance "right now" to help folks on the ground make better decisions. I'm sure someone would pony up the $400M they need.

[stcredzero]

Yes, but having anything be reusable does that. If you can make up what you would have gained with Skylon by making the fuel tank 5% larger, then the opportunity cost of developing a radically new engine isn't worth it. Mach 5 does have other uses, though.