The Space Shuttle Solid Rocket Booster (SRB) was the first solid-propellant rocket to be used for primary propulsion on a vehicle used for human spaceflight. The two SRBs provided 85% of the Space Shuttle's thrust at liftoff and for the first two minutes of ascent. After burnout, they were jettisoned, and parachuted into the Atlantic Ocean, where they were recovered, examined, refurbished, and reused.
The Space Shuttle SRBs were the most powerful solid rocket motors ever flown at the time of their debut. The Space Launch System (SLS) SRBs, adapted from the shuttle, surpassed it as the most powerful solid rocket motors ever flown, after the launch of the Artemis 1 mission in 2022. Each Space Shuttle SRB provided a maximum thrust, roughly double the most powerful single-combustion chamber liquid-propellant rocket engine ever flown, the Rocketdyne F-1. With a combined mass of about , they comprised over half the mass of the Shuttle stack at liftoff.
The motor segments of the SRBs were manufactured by Thiokol of Brigham City, Utah, which was later purchased by Alliant Techsystems (ATK). The prime contractor for the integration of all the components and retrieval of the spent SRBs, was United Space Boosters Inc., a subsidiary of Pratt & Whitney. The contract was subsequently transitioned to United Space Alliance, a joint venture of Boeing and Lockheed Martin.
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 (destroyed by the range safety officer during the 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 had flown on 59 previous missions, including STS-1. Recovery also allowed post-flight examination of the boosters, identification of anomalies, and incremental design improvements. Refurbished segments have been used on the solid rocket boosters of the Space Launch System.
Overview
The two reusable SRBs provided the main thrust to lift the shuttle off the launch pad and up to an altitude of about . While on the pad, the two SRBs carried the entire weight of the external tank and orbiter and transmitted the weight load through their structure to the mobile launcher platform. Each booster had a liftoff thrust of approximately at sea level, increasing shortly after liftoff to . They were ignited after the three RS-25 main engines' thrust level was verified. Seventy-five seconds after SRB separation, SRB apogee occurred at an altitude of approximately ; parachutes were then deployed and impact occurred in the ocean approximately downrange, after which the two SRBs were recovered. The SRBs helped take the Space Shuttle to an altitude of and a speed of along with the main engines.
The SRBs committed the shuttle to liftoff and ascent, without the possibility of launch abort, until both motors had fully consumed their propellants and had simultaneously been jettisoned by explosive bolts and thrusters to push them away from the Shuttle. Only then could any conceivable set of launch or post-liftoff abort procedures be contemplated. In addition, failure of an individual SRB's thrust output or ability to adhere to the designed performance profile was probably not survivable.
The SRBs were the largest solid-propellant motors ever flown until 2022 and the first solid-propellant rockets designed for reuse. Each is long and in diameter. Each SRB weighed approximately at launch. The two SRBs constituted about 69% of the total lift-off mass. The primary propellants were ammonium perchlorate as the oxidizer along with aluminum powder and PBAN as fuel. The total propellant load for each solid rocket motor weighed approximately (see ). The inert weight of each SRB was approximately .
Primary elements of each booster were the motor (including case, propellant, igniter, and nozzle), structure, separation systems, operational flight instrumentation, recovery avionics, pyrotechnics, deceleration system, thrust vector control system, and range safety destruct system.
While the terms solid rocket motor and solid rocket booster are often used interchangeably, in technical use they have specific meanings. The term solid rocket motor applied to the propellant, case, igniter and nozzle. Solid rocket booster applied to the entire rocket assembly, which included the rocket motor as well as the recovery parachutes, electronic instrumentation, separation rockets, range safety destruct system, and thrust vector control.
Each booster was attached to the external tank at the SRB's aft frame by two lateral sway braces and a diagonal attachment. The forward end of each SRB was attached to the external tank at the forward end of the SRB's forward skirt. On the launch pad, each booster also was attached to the mobile launcher platform at the aft skirt by four holddown studs, with frangible nuts that were severed at liftoff.
The boosters were composed of seven individually manufactured steel segments. These were assembled in pairs by the manufacturer and then shipped to Kennedy Space Center by rail for final assembly. The segments were fixed together using circumferential tang, clevis, and clevis pin fastening, and sealed with O-rings (originally two, changed to three after the Challenger Disaster in 1986) and heat-resistant putty.
Components
Hold-down posts Each solid rocket booster had four hold-down posts that fit into corresponding support posts on the mobile launcher platform. Hold-down studs held the SRB and launcher platform posts together. Each stud had a nut at each end, the top one being a frangible nut. The top nut contained two explosive charges initiated by NASA standard detonators (NSDs), which were ignited at solid rocket motor ignition commands.
When the two NSDs were ignited at each hold down, the frangible nut fractured, releasing the hold-down stud. The stud traveled downward because of the release of tension in the stud (pretensioned before launch), NSD gas pressure and gravity. The stud was stopped by the stud deceleration stand, which contained sand. The hold-down stud was long and in diameter. The frangible nut was captured in a blast container mounted on the aft skirt of the SRB.
The solid rocket motor ignition commands were issued by the orbiter's computers through the master events controllers to the hold-down pyrotechnic initiator controllers (PICs) on the mobile launcher platform. They provided the ignition to the hold-down NSDs. The launch processing system monitored the SRB hold-down PICs for low voltage during the last 16 seconds before launch. PIC low voltage would initiate a launch hold.
Electrical power distribution Electrical power distribution in each SRB consisted of orbiter-supplied main DC bus power to each SRB via SRB buses labeled A, B and C. Orbiter main DC buses A, B and C supplied main DC bus power to corresponding SRB buses A, B and C. In addition, orbiter main DC bus C supplied backup power to SRB buses A and B, and orbiter bus B supplied backup power to SRB bus C. This electrical power distribution arrangement allowed all SRB buses to remain powered in the event one orbiter main bus failed.
The nominal operating voltage was DC.
Hydraulic power units Each SRB consists of two self-contained, independent Hydraulic Power Units (HPUs), used to actuate the thrust vector control (TVC) system. Each HPU consisted of an auxiliary power unit (APU), fuel supply module, hydraulic pump, hydraulic reservoir and hydraulic fluid manifold assembly. The APUs were fueled by hydrazine and generated mechanical shaft power to drive a hydraulic pump that produced hydraulic pressure for the SRB hydraulic system. The two separate HPUs and two hydraulic systems were located on the aft end of each SRB between the SRB nozzle and aft skirt. The HPU components were mounted on the aft skirt between the rock and tilt actuators. The two systems operated from T minus 28 seconds until SRB separation from the orbiter and external tank. The two independent hydraulic systems were connected to the nozzle rock and tilt servoactuators.
The HPU controller electronics were located in the SRB aft integrated electronic assemblies (IEAs) on the aft external tank attach rings.
The HPUs and their fuel systems were isolated from each other. Each fuel supply module (tank) contained of hydrazine. The fuel tank was pressurized with gaseous nitrogen at , which provided the force to expel (positive expulsion) the fuel from the tank to the fuel distribution line, maintaining a positive fuel supply to the APU throughout its operation.
In the APU, a fuel pump boosted the hydrazine pressure and fed it to a gas generator. The gas generator catalytically decomposed the hydrazine into hot, high-pressure gas; a two-stage turbine converted this into mechanical power, driving a gearbox. The waste gas, now cooler and at low pressure, was passed back over the gas generator housing to cool it before being dumped overboard. The gearbox drove the fuel pump, its own lubrication pump, and the HPU hydraulic pump. A startup bypass line went around the pump and fed the gas generator using the nitrogen tank pressure until the APU speed was such that the fuel pump outlet pressure exceeded that of the bypass line, at which point all the fuel was supplied to the fuel pump.
When the APU speed reached 100%, the APU primary control valve closed, and the APU speed was controlled by the APU controller electronics. If the primary control valve logic failed to the open state, the secondary control valve assumed control of the APU at 112% speed.
Each HPU on an SRB was connected to both servoactuators on that SRB by a switching valve that allowed the hydraulic power to be distributed from either HPU to both actuators if necessary. Each HPU served as the primary hydraulic source for one servoactuator, and a secondary source for the other servoactuator. Each HPU possessed the capacity to provide hydraulic power to both servoactuators within 115% operational limits in the event that hydraulic pressure from the other HPU should drop below . A switch contact on the switching valve closed when the valve was in the secondary position. When the valve was closed, a signal was sent to the APU controller, that inhibited the 100% APU speed control logic and enabled the 112% APU speed control logic. The 100-percent APU speed enabled one APU/HPU to supply sufficient operating hydraulic pressure to both servoactuators of that SRB.
The APU 100-percent speed corresponded to 72,000 rpm, 110% to 79,200 rpm, and 112% to 80,640 rpm.
The hydraulic pump speed was 3,600 rpm and supplied hydraulic pressure of . A high pressure relief valve provided overpressure protection to the hydraulic system and relieved at .
The APUs/HPUs and hydraulic systems were reusable for 20 missions.
Thrust vector control
Each SRB had two hydraulic gimbal servoactuators, to move the nozzle up/down and side-to-side. This provided thrust vectoring to help control the vehicle in all three axes (roll, pitch, and yaw).
The ascent thrust vector control portion of the flight control system directed the thrust of the three shuttle main engines and the two SRB nozzles to control shuttle attitude and trajectory during lift-off and ascent. Commands from the guidance system were transmitted to the Ascent Thrust Vector Control (ATVC) drivers, which transmitted signals proportional to the commands to each servoactuator of the main engines and SRBs. Four independent flight control system channels and four ATVC channels controlled six main engine and four SRB ATVC drivers, with each driver controlling one hydraulic port on each main and SRB servoactuator.