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SpaceShipTwo straps on its engine

Luke Colby / Virgin Galactic

Virgin Galactic's SpaceShipTwo rocket plane glides over its Mojave test range in California with its rocket motor components installed for the first time.

Virgin Galactic's SpaceShipTwo rocket plane has gone through more than 20 unpowered glide test flights, but today's test was special: It marked the first free flight during which the actual rocket motor components were installed.

"It was also the first flight with thermal protection applied to the spaceship's leading edges," the company said in today's status report. "It followed an equally successful test flight last Friday which saw SpaceShipTwo fly in this configuration, but remain mated to its WhiteKnight carrier aircraft."

Virgin Galactic said all the objectives of both flights were successfully met. At least two more such glide flights are expected to take place before the spacecraft's builders move on to the next phase of testing: powered flights, which call for the plane to fire up the hybrid rocket engine for its ascent.

SpaceShipTwo is being developed at California's Mojave Air and Space Port, with the aim of beginning passenger spaceflights as early as next year. The multimillion-dollar project follows up on the flights of SpaceShipTwo's predecessor, SpaceShipOne, which won the $10 million Ansari X Prize for private-sector spaceflight in 2004.

The new plane's hybrid rocket motor, built by Sierra Nevada Corp., is powered by a rubber-based fuel and nitrous oxide — much like SpaceShipOne's engine was. Virgin Galactic said the propellant tanks were installed on SpaceShipTwo for the latest tests, but did not indicate that the tanks were fueled up ... yet.

SpaceShipTwo's flight profile calls for the plane to ride on the WhiteKnight mothership to an altitude of 50,000 feet, then drop away and light up the rocket. That blast would power the craft to a height beyond 62 miles — giving up to six passengers a few minutes of weightlessness and a great view of the curving Earth beneath the black sky of space. The plane would then re-enter the atmosphere and glide back down to a runway landing.

Although the test program is being conducted in California, Virgin Galactic's business plan calls for commercial spaceflights to originate from Spaceport America in New Mexico. In recent months, some observers have questioned whether the $209 million New Mexico space project will match initial expectations on the economic development front. Spaceport officials insist, however, that the futuristic-looking facility will be ready for Virgin Galactic as well as other tenants.

Virgin Galactic was founded by British billionaire Richard Branson. The company says more than 500 people, including celebrities such as Ashton Kutcher, have signed up for the $200,000 suborbital space tour.

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Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.