Re: Another mighty initial thrust idea
As with any launch method discussed here there is essentially no fixed point. Any forces on the launch mechanism can only be counteracted by the inertia from the mass of the rest of the payload (which is deliberately kept to a minimum) and the balloon itself (little mass, negative weight and little air resistance at altitude). Even it doesn't freeze it will have to be remarkably friction free to prevent the rocket simply pulling the whole girder/balloon combo along with it as there is so little 'pulling the other way'. Without sufficient mass there is nothing to keep a rail pointing in the same direction all the time either.
Without a fixed point to launch from the craft must simply be released and be able to set its own course.