see my comment below: radius of an 8g turn at mach 20 (330 m/s * 20) is 555 km. I choose 8 g because that's roughly what we think a sunburn (ss-n-22) can do, and that thing is built like brick shit-house.

now, to add to that: the radius of a 1 g turn at mach 20 is 4444 km. The radius of the earth is 6378 km. They've got to at least consider orbital mechanics just to hold altitude constant. Able to maneuver and highly maneuverable are two different things. The Titantic was able to maneuver. Just couldn't move fast enough.

Now, let's think about some other fun stuff, everybody join in.

Here's one: at mach 20, you need something along the lines of the shuttle's thermal protection system. We're talking plasma hot. Wrap this thing up in 3 inches of glass and I'll bet you've lost significant payload volume.