Of those considering a rockoon approach has anyone considered solar heating the lifting gas in the balloon. a clear drop cloth envelope can be sollarized with tempra powder ( not dust free ) or powerd charcoal. could at least decease the gas volume needed solar hot air balloons can reach 70000 feet without any lifting gas except air. the romanians are attemping to use a solar hot air balloon in there run for the google x prize. seems a low cost boost.

Seems like a good idea to me, and very cheap, which is good Only problem that I can see is the wind might affect the position of the balloon, thus affecting the orbit of the satellite.