[thackney]

The main motors on the shuttle could be throttled as they used both liquid fuel (I forget the name) and liquid oxygen.

Liquid Hydrogen was the fuel.

http://www.space.com/11970-nasa-final-shuttle-mission-atlantis-sts135-tanking-test.html

[Blueflag]

interesting. I knew it was hydrazine in the thrusters. Couldn’t recall the main motors’ fuel.

ZERO ‘pollution’ using H2. Ya just make water vapor. Oh wait, THE *MOST* powerful greenhouse gas is water vapor ;-)