Seems like a nifty idea
"An enormous ring of superconducting magnets similar to a particle accelerator could fling satellites into space, or perhaps weapons around the world, suggest the findings of a new study funded by the US air force."
http://www.newscientistspace.com/article/dn10180-huge-launch-ring-to-fling-satellites-into-orbit.html
The trajectory control problem is definitely soluble. The Excalibur guided artillery shell has a (fairly cheap) GPS/INS control package that is hardened to withstand >10,000 g and precise enough to hit a particular building 50 km away using small sideways-firing rockets. Using a rocket at the rear for guidance means that you have to somehow yaw the projectile - probably not a good idea at mach 25.
Constructing a vacuum seal that can be safely broached at that speed seems more difficult to me.
tty
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