AS A small boy Paul Allen, the co-founder of Microsoft, dreamed of going into space. He even tried to launch the hollow aluminium arm of a chair, stuffed with propellant, into orbit. It didn't work out. But his latest adventure in space travel—a joint venture with Burt Rutan, a famous designer of aircraft—looks more promising. Earlier this month, the two of them said they will build an air-launched orbital delivery system. To do this, Paul Allen's company Stratolaunch Systems will have to build the world's largest aeroplane.

The Stratolaunch, as the plane will be called, will be big. Really, really big. It will have six engines, a wingspan of 117 metres (385 feet) and weigh about 544 tonnes. (The wingspan of Boeing's 747 is around half that of the Stratolaunch.) Taking off will require 3.6km of runway, and the aircraft will launch its rocket—a shortened version of the Falcon 9 rocket, built by another private space firm called SpaceX—at around 9,100 metres. The whole contraption will be able to put about 6 tonnes of payload into low-earth orbit.

The idea is to offer a cheaper way of getting medium-sized payloads into orbit, and the system is designed to fill a niche that Boeing's Delta 2 rocket once served. Former NASA administrator, Mike Griffin, who now sits on the board of Stratolaunch, says that besides delivering cargo to the International Space Station, the Stratolaunch will tap a thriving market for launching small to middling communications satellites. There are also other customers in the form of NASA and the Department of Defense. Ultimately, however, Mr Allen wants to see the system launch humans into space.

Of course the obvious question is why not launch the rocket directly from the ground in the first place? It turns out that land-based rocket launches are greatly restricted by irritations such as where one's rocket pad is, and what the weather is like. Air launch, by contrast, makes orbital access to space much more flexible, a particular bonus for military applications. There will also be a small efficiency gain from launching the rocket from above much of the Earth's atmosphere. Mr Allen is being cautious about saying how much money he will put into the venture. All he will admit is that an effort of this size requires an "order of magnitude" more money than he invested into a previous collaboration with Mr Rutan, SpaceShipOne. This cost Mr Allen $25m.



Meanwhile, Mr Rutan's company, Scaled Composites in Mojave, will be doing what it is best at: scaling composites. It will be super-sizing its existing White Knight aeroplane, which can carry rockets—such as SpaceShipOne—of suborbital flight. Other components for the Stratolaunch will be scavenged from second-hand 747s. Mr Rutan plans to start work as soon as he has a hangar large enough to build the giant airframe. The current schedule foresees test flights in 2015, and an initial launch by 2016. But the spaceflight business is hard and unforgiving, and the schedule is likely to slip. Mr Rutan has come a long way since he built his first plane, the dinky two-seater VariViggen, in 1972. With its 6-metre wingspan, he would be able to fit 20 along the Stratolaunch.