Why does the Shuttle have Solid Rocket Boosters?

[Ian Curtis]

Originally Posted by Don Rotolo

The SRBs are compact, simple in operation and stable in handling.

Liquid boosters need complex engines and support systems (pumps, storage bottles, valves, etc.) and have to be filled with relatively volatile stuff shortly before launch. While you can get more thrust from a liquid booster, it is physically larger.

So the advantages of a SRB include reliability and simplicity of use. Disadvantage as you mentioned is lack of throttle control.

There are proposals to switch to liquid boosters, but highly doubtful this generation of shuttles will ever consider them.

If solid propellants are so reliable and simple, why aren't they used exclusively for the shuttle? They generate more thrust than the external tank, and from this discussion seem relatively safe too. I understand the reasoning about being able to turn them on and off, but the external tank falls away too, and the shuttle uses the Orbiter engines to change orbits, doesn't it?

I think that's why I find this so confusing is it seems very at odds with itself. Why would they use liquid fuels for launch (from the earth to LEO at any rate) if the SRBs work so well?