by Douglas Messier

Managing Editor

The preliminaries are over. And now the moment of truth has arrived for Richard Branson’s Virgin Galactic.

Almost 3.5 years after SpaceShipTwo Enterprise broke up during a flight test on Halloween 2014, the company is scheduled to conduct the first powered flight of SpaceShipTwo Unity later this morning from the Mojave Air and Space Port in California. The test was preceded by seven glide flights.

I’ll be providing live updates on the flight on Twitter @spacecom.

The stakes are very high for a program that has seen two fatal accidents and four deaths since Richard Branson launched it in September 2014. It’s not clear whether Virgin Galactic would survive a third catastrophic accident.

While Enterprise was built and flight tested by Scaled Composites, Unity is a product of Virgin Galactic and The Spaceship Company, which is also owned by Branson’s Virgin Group.

Word has it that Unity will burn its hybrid nitrous oxide-rubber engine for 30 seconds after being dropped from the WhiteKnightTwo carrier aircraft at around 50,000 feet. This will be 50 percent longer than the 20-seconds engine burns performed by Enterprise on its second and third flight tests in 2013 and 2014.

It’s not clear exactly what altitude the spacecraft will reach, but previous flights provide some clues. Enterprise reached 71,000 feet with a 20-second engine burn on its third flight. The plan was to reach 135,000 to 138,000 feet with a 38-second burn on the fourth flight before the vehicle broke up about 12 seconds after engine ignition.

So, a 30-second burn should probably get Unity somewhere north of 100,000 feet. How much heavier (or lighter) Unity is compared with its predecessor, and the degree to which engineers have been able to improved performance of the hybrid engine, will affect the ship’s performance.

The spacecraft will go supersonic and will use its feather system — which re-configures the vehicle’s twin tail booms to slow the vehicle down — during its descent toward the spaceport.

The premature unlocking and deployment of the feather system during powered ascent caused Enterprise to break up. Unity has been redesigned to prevent a recurrence of that fatal error.

The main uncertainty for today’s flight is the weather, specifically the spring winds that have been battering Mojave over the past week. On the final flight of Enterprise, the spacecraft was limited to ground-level crosswinds of only 10 mph (16 kph) for landing. Upper level winds could also be an issue today.