Virgin Galactic has selected a polyamide-based fuel grain to power its hybrid rocket motor for the remainder of the test flight program and start of commercial operations.

This decision follows numerous ground test firings and is supported by data collected over an extensive development program.

In 2008, Scaled Composites, Virgin Galactic's primary contractor, appointed rocket propulsion specialist, Sierra Nevada Corporation (SNC) to develop the basic hybrid rocket motor design for SpaceShipTwo.

As a part of the program, Virgin Galactic has been developing two variants of this motor using two types of solid fuel grain: HTPB, a type of rubber (the fuel used in the SpaceShipOne rocket motor) and polyamide, a category of benign thermoplastic using Scaled engineering.

Both fuel grains were designed to be interchangeable with the hybrid motor, and both have been tested extensively.

Virgin Galactic has now determined it will use the polyamide version for its space flights. Both industrial partners will continue to support the motor program as the company progresses toward commercial service.

"Of the numerous challenges Virgin Galactic has faced and overcome in our unprecedented mission to create the world's first spaceline, the greatest engineering challenge has been to develop the world's largest operational hybrid rocket motor to power SpaceShipTwo and its occupants safely, regularly and efficiently to space," said Virgin Galactic CEO George Whitesides.

"It is a great credit to the work of our partners Scaled and SNC that we have completed this important milestone."

To view a ground-firing of the polyamide-fueled hybrid rocket motor, click here.