Out of 270 SRBs launched over the Shuttle program, all but four were recovered – those from STS-4 (due to a parachute malfunction) and STS-51-L ("Challenger" disaster). Over 5,000 parts were refurbished for reuse after each flight. The final set of SRBs that launched STS-135 included parts that flew on 59 previous missions, including STS-1. Recovery also allowed post-flight examination of the boosters, identification of anomalies, and incremental design improvements.