The test pilots at Scaled Composites are in the midst of a busy flight test schedule, flying SpaceShipTwo on Tuesday for the first time this year after a period of relative quiet. Flight tests of SS2's mother ship, WhiteKnightTwo, were sporadic early in 2012, but the unusual, twin-fuselage carrier began flying regularly at the beginning of this month.

The rocket motor which will be used to power SpaceShipTwo – predictably called RocketMotorTwo – was also tested at the Mojave Air and Space Port on Tuesday with a full duration test fire lasting 55 seconds. The engine test was performed by Sierra Nevada Space Systems, the company designing and building the rocket engines for Virgin Galactic.

"We’ve had seven successful test flights and three full-scale rocket motor tests [since June 1]" Virgin Galactic CEO George Whitesides said in a press release. "We are on track for powered flight by the end of 2012.”

WhiteKnightTwo flies in front of the sun during the return to Mojave after releasing SpaceShipTwo from an altitude of 51,000 feet. Photo: Virgin Galactic/Chris Van Pelt

Since June 1, WhiteKnightTwo has made six flights and SpaceShipTwo one flight. Several of the flights on WK2 tested basic systems and served as pilot proficiency time, according to Scaled. But a few of the flights outlined were rehearsals for SpaceShipTwo flights, including a captive carry flight on June 8 and simulated SS2 approaches on June 21.

WhiteKnightTwo and SS2 share the exact same fuselage and the mothership can be configured to make approaches similar to that of the space ship providing a training simulator for pilots.

This marks the first time since last September that SpaceShipTwo took to the air, marking its 17th glide flight. Scaled Composites test pilot Pete Siebold was at the controls of SS2 for this week's flight with Mike Alsbury serving as co-pilot.

The flight of WK2 back on June 15 included "SS2 glide flight proficiency" practice with Virgin Galactic pilot David Mackay at the controls. Mackay is a former Royal Air Force test pilot who is the first captain for Virgin Galactic. The practice flight imples Mackay will make his first glide flight of SS2 in the near future. Up until now only Scaled Composites' own test pilots have flown the space ship, beginning with Siebold's first flight back in October 2010.

Scaled's other SS2 test pilot, Mark Stucky, made the last flight back in September of last year. During that flight the aircraft experienced "a downward pitch rate that caused a stall of the tails." The event was not planned and Stucky used the airplane's innovative "feather" mode to return to controlled flight and make a safe landing.

The episode showed once again why flight testing is important in the development of any new air or spacecraft even with modern computational fluid dynamics modeling.

Virgin Galactic has over 500 people signed up for sub-orbital rides aboard SpaceShipTwo. The schedule has slipped several times for the first passenger flights, but interest remains strong for the space tourism company. At the end of May the Federal Aviation Administration granted Scaled Composites its launch permit to allow rocket-powered flight testing of SS2 to happen before the end of the year according to Virgin Galactic.

*UPDATE 3:25 p.m. EDT - Virgin Galactic founder Richard Branson announced on his blog that the company's chief pilot, David Mackay made his first glide flight of SpaceShipTwo this morning. Mackay becomes the third person to pilot the spacecraft and the first non-Scaled Composites test pilot, and the flight marks the second of SpaceShipTwo this week. *