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Reaching orbital velocity is not enough.
Eventually there should be a mass driver build that is big enough to launch people and delicate cargo. That makes it really long. The only way to have a large variance in the launch azimuth would be to put the whole thing on a huge turntable; unreasonably large for the size of machine we're talking about.
So we may see rotating machines for bulk cargo and very hardy payloads, while more delicate payloads settle for a long machine with very little choice of azimuth. That's not so bad. We really only have two targets for launch azimuth: the earth, and the plane of the ecliptic (the orbital plane in which most of the planets lie).
Earth wanders around in the sky inside a box 14 degrees on a side, but we can correct for the variation with course correction burns after launch.
In contrast, a lunar launcher aligned with the ecliptic will be spot on within 5 degrees of the desired orbital plane all the time; it just needs to wait for the moon's rotation to aim it correctly.
The concept of a circular mass driver is functionally the same as slinging a mass on a rope. The common bias toward mass drivers comes from the g-forces that result from trying to point the payload in the right direction once it's going at high velocity.
Why not just build a circular mass driver, or use tethers?
When you turn a corner at high speed, the acceleration is r * omega2, where r is the radius of your turn and omega is angular velocity in radians per second. Do some algebra and you'll get a = v2 / r, where v is your linear velocity, so you can see the influence of both high velocity and small radius of curvature. At orbital velocities, you can get some mind-boggling, payload-squishing, tether-breaking accelerations.
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