FLASHBACK: The White Knight, with SpaceShipOne underneath, on the runway at Mojave Airport in September 2004. On Monday, Mr Richard Branson and aerospace designer Burt Rutan will show off White Knight Two. -- PHOTO: AP

Aerospace engineers have been holed up in a Mojave Desert hangar, fashioning the commercial spaceship for the past four years.

But come Monday, British billionaire Richard Branson and American aerospace designer Burt Rutan will show off their mothership, which is designed to air launch a passenger-toting spaceship out of the atmosphere.

More than 250 wannabe astronauts have each paid US$200,000 (S$270,000) or put down deposits for a chance to float weightless for a mere five minutes.

'Having invested all my faith in it, I'm so excited to see the actual thing,' said artist Namira Salim, a customer who is lined up for a ride on the world's first spaceline.

In 2004, throngs of spectators gathered at the high desert north of Los Angeles to witness the launch of SpaceShipOne, which was the first private manned craft to reach space.

Its successor, SpaceShipTwo, which can carry up to six passengers and two pilots, is being designed out of the public eye, along with the carrier aircraft White Knight Two.

Only White Knight Two, however, will be unveiled at Monday's roll-out, which is expected to be attended by politicians, government regulators and space tourism customers.

Flight testing is slated for the end of September, after ground tests next month.

Observers of the infant private spaceflight industry are encouraged by the progress, but note that the main attraction - the actual spaceship that will carry passengers - is yet to come.

Exactly when tourists will experience zero gravity or get a glimpse of Earth's curvature is unknown, but the project already lags Virgin Galactic's 2004 prediction that passengers would be in space last year.

Virgin Galactic president Will Whitehorn declined to set a date for commercial travel, but did say the earliest flights to space could be late next year or early 2010.

The two-hour trip is expected to include about five minutes of weightlessness. Unlike the space shuttle that orbits Earth, early space tourism plans involve flights that simply go up and come back down.

Virgin Galactic has pledged more than US$250 million towards the project; about US$100 million has been spent so far, Mr Whitehorn said.

Virgin Galactic already has lofty plans for White Knight Two, besides space tourism.

Company executives envision the aircraft can be used as a launcher of small satellites into low Earth orbit. With the proper permits, the craft can also be adapted to fight wildfires or be used as an emergency rescue vehicle.

Monday's unveiling comes a year after an explosion at a test site during the development of SpaceShipTwo's propellant system. The blast killed three technicians.

The company, now owned by Northrop Grumman Corp, is appealing against a state fine of US$28,870 for workplace violations in connection with that blast.

ASSOCIATED PRESS