a = v^2 / r = 15,000 x 15,000 / 500 = 450,000 m/sec^2 or 45,000g. It's on the same order of magnitude as the previous calcs.

That's obviously also a big engineering challenge because it's freaking HUGE, especially for the precision required.

The really great thing about going super-super fast is that at 15km/sec you're in space in ~10 seconds. Obviously you're going to start scrubbing speed really fast since drag is proportional to velocity squared. But you can evacuate the whole launch assembly and put some kind of an explosively opened door at the exit point and maintain your speed up until the last second.