Solid Rocket Boosters

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Solid Rocket Boosters

Mounted on either side of the space shuttle's external fuel tank are a pair of giant rockets with a single, two-minute purpose: to get the shuttle off the launch pad. The rockets are called the shuttle's solid rocket boosters (SRBs) because they contain solid, as opposed to liquid, propellant. Each booster has a thrust of about 3.3 million pounds of force at launch, enough power to propel the shuttle, its external fuel tank, the boosters themselves, and the shuttle's cargo and crew into the air.

The boosters ignite 6.6 seconds after the shuttle's main engines start. If the shuttle engines are performing properly, computer commands are automatically relayed to ignite the boosters and fire explosives to break open four 71-centimeter-long (28-inch-long), 8.9-centimeter-diameter (3.5-inch-diameter) bolts that attach each booster to the launch platform. The shuttle then leaps off the launch pad in a dramatic and heart-stopping display of pyrotechnics . Trailing pillars of flame and smoke, the boosters fly the shuttle into the sky to an altitude of about 45,700 meters (150,000 feet). The boosters push the shuttle to speeds of more than 4,825 kilometers per hour (3,000 miles per hour). Meanwhile, temperatures inside the boosters soar to nearly 3,300°C (6,000°F), which is nearly two-thirds the temperature of the Sun's surfaceand hot enough to not only melt steel, but also boil it.

About 123.6 seconds after liftoff, computer commands are relayed for another set of explosive bolts to detonate and separate the boosters from the orbiter's external fuel tank. The shuttle's three main engines continue burning to carry the spaceship into orbit. The boosters, however, have completed their mission. They continue to fly solo another 21,300 meters (70,000 feet) or so before their fuel is fully consumed, and the now-empty canisters begin falling back down toward the ocean.

Parachutes slow the boosters' descent and cushion their crash into the Atlantic Ocean. The spent boosters splash down about 227 kilometers (141 miles) from the launch site. They are retrieved by two special ships waiting in the area, and towed back to the Kennedy Space Center in Florida, where they are processed and returned to the manufacturer. The segmented motors are disassembled, and the cylindrical cases are cleaned, reinsulated, and refilled with propellant. The exhaust nozzles are refurbished, and other components are replaced as needed. Nose cone and aft skirt assemblies are added to turn the motor into a completed booster.

Measuring 45.4 meters (149 feet) tall and 3.7 meters (12 feet) in diameter, the shuttle's solid rocket boosters are the largest solid propellant motors ever flown. They are also the first that were designed to be reusable. The boosters are filled with a special mixture consisting of ammonium perchlorate, which is an oxidizer ; aluminum for fuel; iron oxide, which is a polymer to bind the ingredients together; and an epoxy curing agent. This mixture is liquid when poured into the segments that form each motor. The propellant is cured over a period of four days until it solidifies. When it hardens, it has the color and consistency of a pencil eraser.

At launch, each booster weighs 590,200 kilograms (1.3 million pounds), which includes 499,400 kilograms (1.1 million pounds) of propellant. The other parts of the booster are the cases, igniters, nozzles, separation systems, flight instruments, recovery avionics, pyrotechnics, deceleration systems, steering equipment, and range safety destruct systems . Each booster is made up of four solid rocket motor segments, which are transported by special railcars to the shuttle's launch site at the Kennedy Space Center.

The boosters were redesigned after the 1986 Challenger disaster, which claimed the lives of seven astronauts and destroyed a $2 billion orbiter. The disaster primarily was blamed on a faulty joint between two of the solid rocket fuel segments on the shuttle's right booster. A special commission that investigated the tragedy concluded that the joint had design flaws, which were exacerbated by the cold temperatures in the hours before Challenger's liftoff. A rubber O-ring seal leaked, allowing hot gases to escape and to trigger the explosion of the shuttle's fuel tank and the loss of the vehicle and the crew.

see also External Tank (volume 3); Rocket Engines (volume 1); Rockets (volume 3); Space Shuttle (volume 3).

Irene Brown


Furniss, Tim. Space Shuttle Log. New York: Jane's, 1986.

National Aeronautics and Space Administration. National Space Transportation System Reference. Washington, DC: Author, 1988.

Internet Resources

Space Shuttle RSRM. Thiokol Propulsion. <>.