Over 5,000 parts were refurbished for reuse after each flight. 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). This contract was subsequently transitioned to United Space Alliance, a limited liability company joint venture of Boeing and Lockheed Martin. The prime contractor for most other components of the SRBs, as well as for the integration of all the components and retrieval of the spent SRBs, was USBI, a subsidiary of Pratt and Whitney. The motor segments of the SRBs were manufactured by Thiokol of Brigham City, Utah, which was later purchased by ATK. With a combined mass of about 1,180 t (1,160 long tons 1,300 short tons), they comprised over half the mass of the Shuttle stack at liftoff. Each provided a maximum 14.7 MN (3,300,000 lbf) thrust, roughly double the most powerful single- combustion chamber liquid-propellant rocket engine ever flown, the Rocketdyne F-1. The Space Shuttle SRB was the most powerful solid rocket motor ever flown. After burnout, they were jettisoned and parachuted into the Atlantic Ocean where they were recovered, examined, refurbished, and reused. The Space Shuttle Solid Rocket Booster ( Space Shuttle SRB) was the first solid-propellant rocket to be used for primary propulsion on a vehicle used for human spaceflight and provided 85% of the Space Shuttle's thrust at liftoff and for the first two minutes of ascent. United Space Boosters Inc., Pratt and Whitney Two Space Shuttle SRBs on the crawler-transporter
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