The Stratolaunch aims to compete with SpaceX, enabling cheaper rockets to be launched from high altitude for placing satellites into low Earth orbit.
The world's largest airplane has moved one step closer to taking its maiden flight and eventually launching rockets from high altitude.
Known as the Stratolaunch, the enormous six-engine aircraft is said to have a wingspan of 385 feet -- more than 120 feet wider than an Airbus A380-800 and 65 feet beyond the Hughes 'Spruce Goose.'
Backed by Microsoft co-founder Paul Allen, Stratolaunch Systems aims to join SpaceX in reducing satellite launch costs. Carrying rockets to high altitude before igniting the engines requires less propellant than a sea-level launch, theoretically reducing costs and allowing heavier payloads relative to overall rocket weight.
Stratolaunch Systems and SpaceX collaborated briefly a few years ago during early development but parted ways.
During a Q&A session following a 2012 lecture, SpaceX chief Elon Musk argued that horizontal air launches only bring "maybe a five percent improvement in payload to orbit" and requires a "humongous" airplane that eventually limits the size of the rocket and payload.
"From SpaceX's standpoint, would it make more sense to have a gigantic plane or to increase the size of the first stage by five percent?" he said. "Uhh, I'll take option two."
SpaceX rival Orbital ATK has since partnered with Stratolaunch Systems to build the specialized air-launch rockets. The Stratolaunch is not expected to take its first flight until sometime next year.