Re: Launch rail and backplate issues
"As for the pressure differential, we are only concerned with ingress of air to launch tube..." umm... I think you may have got that the wrong way around. It'll be the _outside_ air pressure that's dropping, meaning that the pressure inside the tube will be increasing relative to the outside; the issue won't be _ingress_ of air in to the tube but air trying/wanting to leave the tube, to equalise with the outside pressure.
This presents another possible problem: when you get a a sudden pressure drop in a system containing any water vapour you're likely to get the formation of clouds i.e. water droplets, which could then freeze.
Another potential issue with using a long tube is that the plane will need to displace the air in the tube ahead of it as it traverses the tube, effectively acting like a piston, which will cost it launch energy. In an open air launch though, i.e. not in a tube, the air can also be displaced around the plane/rocket. To be sure, the expanding gas from the rocket will ameliorate against this factor and aid in pushing the air out of the tube but someone (a rocket scientist?) would have to work the numbers to find the trade-off balance.