[r9etb]

Would there be some way to get around this by keeping some of the propulsion system earthbound? Like a humongous slingshot or catapult?

The biggest hurdles for that approach have to do with the fact that any ground-based boost method will induce high accelerations; and after that, the payload ends up having to go very fast in thick atmosphere.

The only way to handle the first problem is by adding length to your booster system. You'd have to handle the second by adding sheilding (heavy!), or somehow keep the atmosphere in the launch system at low density.

There is potentially a lot of merit in developing an air launch capability, although that's got to deal with other issues.

[The Red Zone]

You'd have to handle the second by adding sheilding (heavy!), or somehow keep the atmosphere in the launch system at low density.

An ablative shield possibly? Cast a shield from a material that carries away heat when vaporized? (Ice?)

[SampleMan]

Again, see my post #10. A rail gun, with a mountain top terminus solves both problems. Shooting a vehicle into space while keeping acceleration structural loads below 7 G. Atmospherics are greatly helped by not entering the atmosphere until the vehicle is already above the thickest part of the atmosphere (higher is better). And high energy beams may be able to split the flight path, thus decreasing resistance even more.