Re: Another mighty initial thrust idea
I think a suspended test would be more useful.
If you can't scrounge a crane to suspend the launch platform from its tether(s) then use a tethered balloon to suspend it i.e. three tethers to hold the balloon in place, which then suspends the launch platform at a convenient height above the ground. You'll have to wait for a windless day to try it because in the actual launch the balloon will be moving with the surrounding air and not stationary wrt the ground.
Hmm... you could also use this rig to try various balloon-platform tethers too. IIRC, the Vulture 1 vid seemed to show that it was spinning quite a lot and I mentioned in an earlier post that a single long tether from the balloon to two short attachment tethers joined just above the launch platform would be subject to wind-up i.e. if the launch platform acquired any spin during its ascent then a single long tether would offer little resistance to that spin but would store that spin energy like a spring; when the balloon reached smoother air this 'spring' would start to unwind, transferring its energy into the launch platform in the form of rotational momentum, which then causes the tether to wind up again (sure, much of the energy will go into spinning the balloon too, but reducing the total amount of energy stored in the tether as much as possible is a Good Thing). Most people will have already seen this phenomenon for themselves, one way or another, but it can be demonstrated by dangling a weight on a bit of string, manually winding it up and then releasing the weight; the string won't just unwind and then stop but, after transferring its stored energy to the weight, will wind itself up again (in the opposite direction, of course).
Dunno how many attachment points you have to the balloon but I doubt it's just, in effect, a bit of string tied around the neck (is there not a clamp?). Consider attaching a (relatively) short (say ~30 cm) spreader bar just beneath the balloon on two short tethers, such that the spreader bar forms the hypotenuse of a right-angled triangle [with the two short tethers], and then use two long tethers, attached at each end of the spreader bar to the ends of the launch platform. Should reduce the horizontal spin of the launch platform quite a lot.