The Space Shuttle Solid Rocket Boosters (SRBs) are the pair of large solid rockets used by the Space Shuttle during the first two minutes of powered flight. They are located on either side of the orange external propellant tank. Each SRB produces 1.8 times more liftoff thrust than one F-1 engine – the most powerful single-chamber liquid-fueled rocket engine ever flown – 5 of which powered the Saturn V "moon rocket's" first stage. The SRBs are the largest solid-fuel rocket motors ever flown, and the first to be used for primary propulsion on human spaceflight missions. The spent SRBs are recovered from the ocean, refurbished, reloaded with propellant, and reused for several missions.
Overview
The two reusable SRBs provide the main thrust to lift the Space Shuttle off the pad and up to an altitude of about 150,000 feet (45.7 km). In addition, the two SRBs carry the entire weight of the external tank and orbiter and transmit the weight load through their structure to the mobile launch platform. Each booster has a (sea level) liftoff thrust of approximately 2,800,000 lbf (12.5 MN), and shortly after liftoff the thrust increases to about 3,100,000 lbf (13.8 MN). They are ignited after the three space shuttle main engines' thrust level is verified. Seventy five seconds after SRB separation, SRB apogee occurs at an altitude of approximately 220,000 feet (67 km); parachutes are then deployed and impact occurs in the ocean approximately 122 nautical miles (226 km) downrange, after which the two SRBs are recovered.
The SRBs are the largest solid-propellant motors ever flown and the first of such large rockets designed for reuse. Each is 149.16 feet (45.5 m) long and 12.17 feet (3.7 m) in diameter.
Each SRB weighs approximately 1,300,000 pounds (590,000 kg) at launch. The two SRBs constitute about 60% of the total lift-off mass. The propellant for each solid rocket motor weighs approximately 1,100,000 pounds (499,000 kg). The inert weight of each SRB is approximately 200,000 pounds (91,000 kg).
Primary elements of each booster are 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. 'Solid rocket booster' applies to the entire rocket assembly, which includes the recovery parachutes, electronic instrumentation, separation rockets, range safety destruct system, and thrust vector control. The term 'solid rocket motor' applies to the propellant, case, igniter and nozzle.
Each booster is 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 is attached to the external tank at the forward end of the SRB's forward skirt. On the launch pad, each booster also is attached to the mobile launcher platform at the aft skirt by four frangible nuts that are severed at lift-off.
The boosters are composed of seven individually manufactured steel segments. These are assembled in pairs by the manufacturer, and then shipped to KSC by rail for final assembly. The segments are fixed together using circumferential tang, clevis, and clevis pin fastening, and sealed with three O-rings (two prior to the Challenger Disaster in 1986) and heat-resistant putty.
Components
Hold-down posts
Each solid rocket booster has four hold-down posts that fit into corresponding support posts on the mobile launcher platform. Hold-down bolts hold the SRB and launcher platform posts together. Each bolt has a nut at each end, the top one being a frangible nut. The top nut contains two NASA standard detonators (NSDs), which are ignited at solid rocket motor ignition commands.
When the two NSDs are ignited at each hold down, the hold-down bolt travels downward because of the release of tension in the bolt (pretensioned before launch), NSD gas pressure and gravity. The bolt is stopped by the stud deceleration stand, which contains sand. The SRB bolt is 28 inches (711 mm) long and is 3.5 inches (90 mm) in diameter. The frangible nut is captured in a blast container. In the event of a hold down failure the thrust from SRB ignition is enough to break the bolts, freeing the vehicle.
The solid rocket motor ignition commands are 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 provide the ignition to the hold-down NSDs. The launch processing system monitors the SRB hold- down PICs for low voltage during the last 16 seconds before launch. PIC low voltage will initiate a launch hold.
Electrical power distribution
Electrical power distribution in each SRB consists 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 supply main DC bus power to corresponding SRB buses A, B and C. In addition, orbiter main DC bus C supplies backup power to SRB buses A and B, and orbiter bus B supplies backup power to SRB bus C. This electrical power distribution arrangement allows all SRB buses to remain powered in the event one orbiter main bus fails.
The nominal operating voltage is 28±4 volts DC.
Hydraulic power units
There are two self-contained, independent Hydraulic Power Units (HPUs) on each SRB. Each HPU consists of an auxiliary power unit (APU), fuel supply module, hydraulic pump, hydraulic reservoir and hydraulic fluid manifold assembly. The APUs are fueled by hydrazine and generate mechanical shaft power to drive a hydraulic pump that produces hydraulic pressure for the SRB hydraulic system. The two separate HPUs and two hydraulic systems are located on the aft end of each SRB between the SRB nozzle and aft skirt. The HPU components are mounted on the aft skirt between the rock and tilt actuators. The two systems operate from T minus 28 seconds until SRB separation from the orbiter and external tank. The two independent hydraulic systems are connected to the rock and tilt servoactuators.
The HPU controller electronics are located in the SRB aft integrated electronic assemblies on the aft external tank attach rings.
The HPUs and their fuel systems are isolated from each other. Each fuel supply module (tank) contains 22 pounds (10 kg) of hydrazine. The fuel tank is pressurized with gaseous nitrogen at 400 lbf/in² (2.8 MPa), which provides 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 boosts the hydrazine pressure and feeds it to a gas generator. The gas generator catalytically decomposes the hydrazine into hot, high-pressure gas, and a two-stage turbine converts this into mechanical power, driving a gearbox. The waste gas, now cooler and at low pressure, is passed back over the gas generator housing to cool it before being dumped overboard. The gearbox drives the fuel pump, its own lubrication pump, and the HPU hydraulic pump. As described so far, the system could not self-start, since the the fuel pump is driven by the turbine it supplies fuel to. Accordingly, a bypass line goes around the pump and feeds the gas generator using the nitrogen tank pressure until the APU speed is such that the fuel pump outlet pressure exceeds that of the bypass line, at which point all the fuel is supplied to the fuel pump.
When the APU speed reaches 100%, the APU primary control valve closes, and the APU speed is controlled by the APU controller electronics. If the primary control valve logic fails to the open state, the secondary control valve assumes control of the APU at 112% speed.
Each HPU on an SRB is connected to both servoactuators on that SRB. One HPU serves as the primary hydraulic source for the servoactuator, and the other HPU serves as the secondary hydraulics for the servoactuator. Each servoactuator has a switching valve that allows the secondary hydraulics to power the actuator if the primary hydraulic pressure drops below 2,050 lbf/in² (14 MPa). A switch contact on the switching valve will close when the valve is in the secondary position. When the valve is closed, a signal is sent to the APU controller that inhibits the 100% APU speed control logic and enables the 112% APU speed control logic. The 100-percent APU speed enables one APU/HPU to supply sufficient operating hydraulic pressure to both servoactuators of that SRB.
The APU 100-percent speed corresponds to 72,000 rpm, 110% to 79,200 rpm, and 112% to 80,640 rpm.
The hydraulic pump speed is 3,600 rpm and supplies hydraulic pressure of 3,050±50 lbf/in² (21±0.345 MPa). A high pressure relief valve provides overpressure protection to the hydraulic system and relieves at 3,750 lbf/in² (26 MPa).
The APUs/HPUs and hydraulic systems are reusable for 20 missions.
Thrust vector control
Each SRB has two hydraulic gimbal servoactuators: one for rock and one for tilt. The servoactuators provide the force and
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