Branson: SS2 Will Be a Clean Lean Green Flying Machine has a Q&A with Richard Branson in which the British billionaire promotes the “greener” nature of the WhiteKnightTwo/SpaceShipTwo combo:

We should no longer rely on old, expensive, “dirty” technology to transport satellites into space. The industry must be modernised and made more sustainable, both for financial reasons and for the health of the planet. In response to this challenge, we have been developing a greener solution in Mojave, California — one that will have less impact on the environment and will be more cost effective. Our two-step launch process, which does not employ rockets until the aircraft reaches the stratosphere, uses less energy than other launch systems that rely on rockets to lift off from the ground.

The carrier plane, WhiteKnightTwo, flies like an airplane, lifting the secondary space plane to a height of 50,000 feet. It was built using a lightweight all-carbon-composite design and is powered by four Pratt and Whitney Canada PW308A engines; some of the most powerful, economic and efficient available. In time, as we learn more about how best to employ these technologies we are pioneering, we and other companies may be able to apply this knowledge in other areas of the space and airline industries.

In the second step of the launch, our rocket-powered space plane, SpaceShipTwo, detaches from the carrier plane at an altitude where the air is thinner and the space plane needs far less energy to reach suborbital space. The hybrid rocket motor is more efficient and flexible than previous models. The fuel is a solid rubber compound, and the oxidiser (the chemical that provides the oxygen to burn the fuel) is nitrous oxide in liquid form.

This combination of solid and liquid fuels powers an economical rocket engine that can be controlled and shut down more easily than the solid-fuel rockets used in the 20th century. Its byproducts of carbon dioxide, carbon monoxide, nitrogen and water vapour are preferable to those of the solid-fuel rockets, which burn ammonium compounds and aluminium, among other fuels.

Read Branson’s full response.