Thread: Mass-driver ending with a very long air balloon tube/cannon?

When I read the thread in which the "They modeled a 15-kilometer high tower," balloon,
I was wondering if a mass driver could ending up with a 15 km high tube-shaped balloon that could be pumped to create a very low pressure tunnel going up to 15 km in the atmosphere. I had read previously comments to the effect that a mass driver would have trouble because the high speed required would be problematic in the relatively dense air of the lower atmosphere. Is the atmosphere at 15km up sufficiently low pressure for a mass driver's projectile [cargo, building materials] to pass without problems, and can a cylindrical balloon have the required attributes to create a very low air pressure tunnel from a vacuum tube mass driver to the atmosphere 15km up?