Microsoft billionaire Paul Allen has announced plans with Scaled Composites to build the world's largest airplane, with a wingspan of 120 meters. This huge bird would look like the Wings Over the World aircraft in HG Wells' THINGS TO COME. It would be first stage of an air-launched orbital booster system developed with SpaceX. The 5-engine rocket looks like SpaceX's cancelled Falcon 5/Dragon, which would have a launch weight of 130 tons. In theory such an air-launched booster offers many advantages, as described in the 3-minute video.
Paul Allen has spent money encouraging Space before, as when he put up funds for the 10 million dollar X-Prize. This project would be 2-3 orders of magnitude bigger. As Howard Hughes and others have found, building a plane this size from scratch is incredibly expensive. (The Airbus A-380 cost about 15 billion to develop). The hangar door in the animation is nearly 150 meters wide! Before building the 747, Boeing first needed to construct the world's largest building in Everett, Washington as an assembly line.
The aircraft design in this animation looks like it has not been completely developed, unlike the stunningly original aircraft that Scaled is known for. For example, a pressurised aircraft would not have straight fuselage sides. The center section may need to gain area. A larger center section provides more lift, structural strength, and access between the hulls. Perhaps the designers will consider using two existing 747 or C-5 Galaxy hulls and twinning them with a new center section.
NASA studied such twinned aircraft in the 1970's. A major issue is runway width. The standard airport runway is 45 meters wide. Runway 12/30 at Scaled Composites' home in Mojave is 60 meters wide, but rated for aircraft of no more than 120,000 pounds. The concrete strip in the animation appears to be about 90 meters wide and very long. Building one such runway would cost around a billion dollars, and you would need one every place this bird would land. Emergency landings at other fields would be unthinkable.
Since the new 747-8 freighter has a payload capacity of 135 tons, they could also consider launching their 130 ton rocket from atop one. It will not be the first time a 747 has launched a Space Shuttle! In the 1970's NASA considered using giant twin airplanes as Shuttle carrier aircraft, but settled on the familiar 747. Developing an all-new aircraft was considered too expensive.
The dreamers behind this project deserve every encouragement. Hopefully they realise the challenges of such an enormous project. NASA has studied giant twin-hulled aircraft since the 1970's. We can hope that Paul Allen and allies can improve on the past.