Re: @AC Piggy back rocket?
It's better than that. Imagine an SR-71 type aircraft that can go from the ground, up to Mach 17 in orbit and back. That's basically what it's intended to do.
The precooler is needed only to make the jet mode operable to higher speeds / altitude, getting the aircraft closer to space before it has to switch to rocket mode and start using onboard O2 instead of atmosphere for combustion. By switching to rocket mode higher up, where there's a lot less drag, you get more delta V out of the amount of O2 carried.
In principal the pre-cooler is needed only between about Mach 3 and whenever they switch to rocket mode. Jets working up to Mach 3 are achievable without pre-cooling, using sane materials.
Biting the hand that feeds IT
