The problem with traditional scramjet planes (planes with the potential of hitting Mach 12-24) is they require supersonic airflow in order to function. So getting up to Mach 4 is a problem. That's why they've typically been piggybacked on other planes to get up to speed, and then released. Which isn't efficient or cost effective (since when did we start caring about this?).
Enter the DAPRA Vulcan. The Vulcan is a hybrid engine that can power a plane with a turbo jet until it's time to kick on the scramjet and feel your nuts climb up into your stomach. DARPA hopes to have a working prototype complete by 2012 and I'm all for it. Think about it -- Mach 24. That's like 250 trillion miles an hour. Which does comes with some inherent risk: I heard if you go that fast you may actually start aging in reverse and then crash the plane because you're seven and can't fly. I believe it.
Hit the jump for a conceptual video.
Thanks to Julian, who calls dibs on being the first person in the Mile High, Mach 24 club