Virgin Galactic's second SpaceShipTwo, VSS Unity, glides to a landing during an Aug. 4 test flight in the skies above Mojave, California. Credit: Virgin Galactic

WASHINGTON — Virgin Galactic performed the latest glide flight of its second SpaceShipTwo suborbital spaceplane Aug. 4, calling it a “dry run” for upcoming powered test flights.

SpaceShipTwo, carried aloft by its WhiteKnightTwo aircraft, separated from the plane about an hour after its 11:58 a.m. Eastern takeoff from Mojave Air and Space Port in California. The spaceplane landed back in Mojave ten minutes later.

The glide flight was the sixth for this SpaceShipTwo vehicle, named VSS Unity, and the first in two months. As on two previous glide flights, pilots tested the feathering system that raises the vehicle’s twin tail booms for a safe reentry, then returned the tail booms to their normal position for landing.

This flight was the first to carry most of the hybrid propulsion system that will be used for future powered test flights. The spacecraft’s oxidizer tank was filled with nitrous oxide and a helium pressurant tank was also loaded. The only major component missing was the solid fuel casing, which was replaced on this flight with a ballast tank filled with water.

The test, the company said in a statement after the flight, was “essentially a dry run for rocket-powered flights.” Prior to release from WhiteKnightTwo, the SpaceShipTwo pilots tested venting of nitrous oxide from the tank. The ballast tank also allowed pilots to test landing of the vehicle with a heaviver weight and different center of gravity.

“We are really pleased with what we saw today,” David Mackay, Virgin Galactic chief test pilot and one of the two pilots on this latest test flight, said in a statement. “We collected hundreds of gigabytes of data for us to review, and from the pilots’ point of view, it felt really wonderful.”

While calling this flight a “dry run” for future powered tests, the company didn’t indicate if additional glide flights are planned. “Lots of data to examine now, but a great day’s work and an important step towards powered flights,” the company said in its statement.

Virgin Galactic officials have said in the past that there is not a set number of glide flights planned, only a series of test milestones that the company hopes to achieve in a test flight campaign. Company president Mike Moses said in an October 2016 interview that the company anticipated needing about 10 glide flights to achieve all its goals, but added that it could be completed in fewer flights, or require additional flights, depending on how well the test campaign went.

Similarly, the company has not set a timetable for either beginning powered test flights or full-fledged commercial service, flying tourists and research payloads on brief suborbital spaceflights. There is a widespread expectation in the industry, though, that Virgin Galactic will try to begin powered test flights by the end of this year to keep in on track for beginning commercial service some time in 2018.

Company founder Sir Richard Branson declined to give a schedule in an April interview here, despite suggesting in other recent interviews that he would be “very disappointed” if SpaceShipTwo is not in commercial service by the end of 2018. “I’ve made the mistake of giving dates before and being wrong,” he said.

Development of SpaceShipTwo has suffered from extensive delays due to both technical issues and fatal accidents. A test stand explosion in July 2007 during testing of the vehicle’s hybrid engine killed three Scaled Composites employees and injured three others.

The first SpaceShipTwo, VSS Enterprise, broke apart during a powered test flight in October 2014 when the vehicle’s co-pilot, Michael Alsbury, prematurely unlocked the feathering system as the spaceplane passed Mach 1. Alsbury died in the accident and the vehicle’s pilot, Peter Siebold, was injured.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...