The program then scales to 5 million tons per year. Their main research involved engaging directly with commercial aerospace vendors to elicit what current and near-term technology platforms can achieve at what cost. We have met or corresponded directly with: Airbus, Boeing, Bombardier, Gulfstream, Lockheed Martin, Northrup Grumman; GE Engines, Rolls Royce Engines; Atlas Air, Near Space Corporation, Scaled Composites, The Spaceship Company, Virgin Orbit, and NASA, the latter in respect of its high-altitude research aircraft fleet.
They eliminated aerostats and hoses because the technology is not ready or untested. Modified business jets, noted prominently in McClellan et al (2010, 2012) study, are incapable of reaching altitudes above ~16 km. High payload, high altitude aerostats have been hypothesized but not yet successfully tested, and in all events, are operationally fragile, unable to operate in adverse weather conditions.