LOS ANGELES – Aerospace engineers have been holed up in a Mojave Desert hangar for four years, fashioning a commercial spaceship to loft rich tourists some 62 miles above Earth. Now the wraps come partially off the top-secret project.

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

The rollout – a year after a deadly accident at Rutan’s test site – marks the start of a rigorous flight test program that space tourism advocates hope will climax with the first suborbital joy rides by the end of the decade. More than 250 wannabe astronauts have paid $200,000 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 Branson’s Virgin Galactic.

Among the new space entrepreneurs is the swashbuckling Branson, who teamed with Rutan’s publicity-shy Scaled Composites LLC to commercialize SpaceShipOne. Its successor, SpaceShipTwo, is being designed out of the public eye, along with the carrier aircraft White Knight Two.

Though technical details remain guarded, tidbits about the vehicles have trickled out: The twin-fuselage White Knight Two will have the same wingspan – 140 feet – as the Boeing B-29 Superfortress, the World War II bomber.

It will launch SpaceShipTwo, which will be the size of a corporate Gulfstream capable of carrying six passengers and two pilots. Both will be built wholly from ultra-light composite materials.

Only White Knight Two will be unveiled at Monday’s rollout, expected to be attended by politicians, government regulators and space tourism customers. Flight testing is slated for the end of September after ground tests in August.

Meanwhile, SpaceShipTwo is only about 70 percent complete, said Virgin Galactic president Will Whitehorn.

Virgin Galactic has pledged more than $250 million toward the project; about $100 million has been spent so far, 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.

First, though, it needs to emerge from its secret hangar before it can get off the ground.