LOS ANGELES — Scaled Composites is in final preparations for powered flight tests of Virgin Galactic’s SpaceShipTwo (SS2), following completion of the glide-flight envelope at Mojave, Calif.

The milestone means the suborbital spacecraft remains on track for the start of rocket-powered flights in the last quarter of this year, with passenger flights provisionally planned to begin by the end of 2013. The envelope clearance for airspeed, angle-of-attack, center-of-gravity (CG) and structural loads was completed during a final round of six flights between late June and mid-August.

Scaled Composites opted to re-clear parts of the glide flight envelope after modifying the tails of SS2 for additional stall margin at low angles of attack. The changes, which came in the wake of a tail stall during a flight in September 2011, included replacing a pair of smaller strakes on the inboard side of each vertical tail with a larger, one-piece, horizontal strake.

Final glide envelope clearance was achieved with the 22nd flight of SS2 since the first drop test from the WhiteKnightTwo (WK2) carrier aircraft in October 2010. The busy sequence to complete the pre-powered phase began with a “return to flight” test in late June following the tail modification, and included control surface flutter, aerodynamic stability and speed brake checks. Significantly, the flight also included an airborne functional check of the spacecraft’s reaction control system.

Further flights focused on tests of the new strake at aft CG and in a feathered position, as well as pushing up the airspeed to higher Mach numbers and landing at heavier weights. Elevon dampers and rudder locks were also evaluated, while other flights tested angle-of-attack and the feathering mechanism with the CG further aft. One of the tests, conducted on Aug. 2, included checks to make sure that a thermal protection system, now added to the horizontal tail, has no effect on handling.

The final glide flight, conducted Aug. 11, achieved the maximum airspeed to date and completed load expansion tests of the horizontal tail. The sortie also marked the 93rd flight of WK2, which was airborne for 1.5 hr.

Although some components of the Sierra Nevada-developed RM2 rocket motor have been flying on SS2, the vehicle is not thought to have flown with an entire propulsion system installed. Virgin Galactic currently plans to attempt the first rocket-powered atmospheric flight tests of SS2 “toward the end of the year” following the granting of an experimental launch permit by the FAA in May.