[bluehawk]

But where is the 200km of track? You also have to deal with the atmosphere. If the track is on the ground or low in the atmosphere, then when you come out of the "cannon" you would be torn apart. So that means the track (or at least the exit) needs to be above the atmosphere, which is quite a challenge in itself.

[jcoffland]

You could use a 200km evacuated tube. There is of course the issue of maintaining the vacuum as the vehicle exits. This could be managed using a pair of diaphragm shutters. Then only the space between the shutters would need to be reevacuated after a launch. Shutter failure would be disastrous.