[El Gato]

I'm no expert, but here's my guess... the Solid Rocket Boosters are just as the name indicates. A solid chemical compound that is burned. That would most likely NOT be environmentally friendly.

That's pretty much correct.The SRB's combine a "rubber" laced with aluminum powder fuel and an ammonium perchlorate oxidizer. The exhaust from the SRBs is probably pretty nasty stuff, all sorts of hydrocarbons and aluminum and it's oxides. The SSME's, the main engines burn hydrogen and oxygen, with the exhaust being steam, that is water vapor. That's just about as environmentally friendly as you can get.

The original conceptual design for the shuttle would have had a fly-back booster, probably also burning LOX and H2, although possibly just jet fuel/kerosene and oxygen, which would probably still be more environmentally friendly than the witches brew that comes out of the SRBs. Yet another victim of budget cuts used to benefit the social welfare state.

[SauronOfMordor]

The original conceptual design for the shuttle would have had a fly-back booster, probably also burning LOX and H2, although possibly just jet fuel/kerosene and oxygen, which would probably still be more environmentally friendly than the witches brew that comes out of the SRBs. Yet another victim of budget cuts used to benefit the social welfare state.

During the initial stages, I remember a proposal that involved the shuttle riding piggy-back on a jet that would get it to altitude and speed, detach, and return (a lot like Burt Rutan's SpaceShip One concept)