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 aeroplanes - 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.