Quote:
Originally Posted by ZDan
Heh heh, you don't "throttle up" SRBs! They're full on until they are done...
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Yup. They're like a bottle rocket. There are only three settings: off, on, and burned out. Even liquid fuel rockets rarely have much throttle range (usually like 95%-100%). A big reason that SpaceX's rockets are so cool is because they can throttle the engines across a large range (40-100%), which is why they're able to balance the thrust perfectly for landings.
SRBs are just lit or unlit, and once you light them you can't shut them off. (There are a few rare exceptions where they can be extinguished early, but not the Space Shuttle's SRBs and I don't think any of those rocket motors are used in manned flight either.) In any case, it's completely on/off and they don't re-light.