Why were the filament carbon SRB's never used? They were supposed to weigh 10, 000 lbs. less (I think).

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Perhaps because of the added expense and difficulty in repair in the case of any damage. I don't know the specifics, but given the circumstances, those seem to be the most likely reasons.

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That, and the fact was that the design introduce a whole new set of joints (i.e. failure points) to the SRB, connecting the steel joints to the carbon fiber barrels. A sensitive point post-Challenger.
The shuttle stack display at the KSC visitor's center has them.

The Filament Wound Case (FWC) SRBs were needed for polar launches from the Vandenberg, CA shuttle launch site. The launch direction (almost due south) doesn't benefit from earth's rotation, so payload capability is reduced.

The FWC SRBs would have mitigated some of that, allowing more useful military shuttle payloads.

After the Challenger loss, polar-inclination military payloads were redesignated to expendable launchers. Thus the west coast shuttle launch complex (SLC-6) wasn't completed, and the FWC SRBs weren't needed: http://en.wikipedia.org/wiki/SLC-6

It's all part of a cost-benefit analysis. Sure, payload weight is money, but so is manufacturing and design. Although lighter technology might be available, that doesn't make it more economically feasible, as the cost of the lighter technologie might outweigh it's cost/benefit.