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 to show off their mother ship today, 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.

The last time there was this level of buzz in the high desert north of Los Angeles was in 2004, when throngs of spectators gathered to witness SpaceShipOne capture the $10 million Ansari X Prize by becoming the first private, manned craft to reach space. It was designed by Rutan and bankrolled by Microsoft Corp. co-founder Paul Allen.

SpaceShipOne ushered in a new space age dominated by deep-pocketed entrepreneurs with dreams of making space voyages as mundane as airplane travel. That vision remains unfulfilled.

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.

"They've been hyping this and selling tickets," said Alan Radecki, a helicopter mechanic and aviation photographer who follows the private space race. "This is the first time they're going to have hardware to show people."

Branson previously heralded 2008 as the "Year of the Spaceship." In January, he and Rutan offered a sneak peek of their commercial partnership, showing off scale models of the mother ship and the spacecraft it will launch.

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, a 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 today'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.

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 see Earth's curvature is unknown, but the project already lags Virgin Galactic's 2004 prediction that passengers would be in space last year.

Whitehorn declined to set a date for commercial travel, but he said the earliest flights to space could be late 2009 or early 2010. The maiden voyage has been reserved for Branson and his family; Virgin Galactic plans to rename the aircraft "Eve" after Branson's mother, a former glider pilot instructor and flight attendant.

Plans call for White Knight Two to carry SpaceShipTwo 50,000 feet in the air, tucked beneath its single 140-foot wing, before releasing it. SpaceShipTwo will then power its hybrid rocket and climb into space. Before gliding back to Earth, it will use a Rutan-designed "feathering" technique — in which the wings are rotated upward from the fuselage to reduce the heat of re-entry.

The 2 1/2-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 $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.