SEATTLE - Microsoft cofounder Paul Allen and aerospace pioneer Burt Rutan are building the world's biggest plane to help launch cargo and astronauts into space, in the latest of several ventures fueled by technology tycoons clamoring to write America's next chapter in spaceflight.

Their plans, unveiled Tuesday, call for a twin-fuselage aircraft with wings longer than a football field to carry a rocket high into the atmosphere and drop it, avoiding the need for a launchpad and the expense of additional rocket fuel.

Allen, who teamed with Rutan in 2004 to send the first privately financed, manned spacecraft into space, said his new project would "keep America at the forefront of space exploration" and give a new generation of children something to dream about.

"We have plenty and many challenges ahead of us," he said at a news conference.

Allen and Rutan join a field crowded with Silicon Valley veterans who grew up on Star Trek and want to fill a void created with the retirement of NASA's space shuttle. Several companies are competing to develop spacecraft to deliver cargo and astronauts to the International Space Station.

Allen bemoaned the fact that government-sponsored spaceflight was waning.

"When I was growing up, America's space program was the symbol of aspiration," he said. "For me, the fascination with space never ended. I never stopped dreaming what might be possible."

Allen and Rutan last collaborated on the experimental SpaceShipOne, which was launched in the air from a special aircraft in 2004. It won the $10 million Ansari X Prize for the first privately financed, manned spaceflight.

Sir Richard Branson's Virgin Galactic licensed the technology and is developing SpaceShipTwo to carry tourists to space.

The new plane will have a wingspan of 380 feet - the world's largest. The plane will carry under its belly a space capsule with its own booster rocket; it will blast into orbit after the plane climbs high into the atmosphere.

This method saves money by not using rocket fuel to get off the ground. Another older rocket company, Orbital Sciences Corp., uses this method for unmanned rockets to launch satellites. An unmanned test is scheduled for 2016.