As the still-to-be-announced date of the first commercial flight of Virgin Galactic’s SpaceShipTwo approaches, more and more of the technology involved is getting nailed down. A case in point is the company's announcement that it has decided which fuel will be used in the first passenger-carrying flights of the suborbital spacecraft. The solid fuel grains that will fuel the world’s largest operational hybrid rocket will be a thermoset plastic similar to nylon.

When SpaceShipTwo launches after being dropped by the WhiteKnightTwo carrier aircraft, it will be powered into orbit by a hybrid rocket motor developed by Sierra Nevada Corporation that uses solid rocket fuel and nitrous oxide as an oxidizer. The gas oxidizer means that, unlike conventional solid rocket motors, it can be shut down at will.

Its predecessor, SpaceShipOne, used hydroxyl-terminated polybutadiene (HTPB), which is a classic fuel for solid rocket motors. But after reviewing data collected from static test firings, Virgin Galactic has decided on an alternative fuel for the remainder of the test flights of SpaceShipTwo and for the first commercial flights.

Instead of HTPB, the rocket motor will now use a fuel based on polyamide, a class of thermoset plastics of which the most well known is nylon. It is used in everything from clothing to airbags and industrial plastics, and has the added advantage of not requiring any redesign of the motor, which was built to regard the two fuel grains as interchangeable.

SpaceShipTwo is designed to take six paying passengers on a suborbital flight from the Virgin Galactic Spaceport America in the Mojave desert into space. Once there, the passengers can look back at Earth and enjoy several minutes of zero gravity. According to Virgin Galactic, the spacecraft can also be fitted with equipment racks to carry experiments and the technology used can also be adapted to launch micro-satellites into low Earth orbit (LEO).

The video below shows a static test firing of the rocket motor.

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