Prior to launch, the entire weight of the Space Shuttle is supported on the launch pad by two solid rocket boosters. Each booster is attached to the pad by four large bolts.
The heart of each booster is the motor, the largest solid rocket ever to be flown and the first designed for reuse. It is made of four factory prepared segments filled with propellant at the manufacturer's facility and assembled at the launch site. The segmented design permits ease of fabrication, transportation and handling.
The motor segments are located in pairs from one batch of propellant ingredients to minimize any thrust imbalances between boosters used for a single Shuttle flight. Propellant loading is also done in such a manner as to cause a reduction in thrust 55 seconds into the Shuttle flight to prevent overstressing the Shuttle vehicle during its critical phase of flight, the period of maximum dynamic pressure.
The exhaust nozzle in the aft segment of each motor, in conjunction with the orbiter main engines, steers the Shuttle during flight. It can be moved up to eight degrees by the booster thrust vector control system which is controlled by the orbiter guidance and control computer.
At burnout the two solid rocket boosters are separated from the external tank by pyrotechnic (explosive) devices and moved away from the Shuttle vehicle by eight separation motors - four housed in the forward compartment and four mounted on the aft skirt. The separation motors are fired by a command from the orbiter. The recovery system, in the forward section of the booster, consists of parachutes and a homing device. Following separation - at about 5.8 kilometers (19,000 feet) - the booster is slowed by a drogue parachute and finally by three main parachutes to impact water at a speed of about 25 meters/second (85 feet/sec), aft end first. By entering the water this way, the air in the empty booster is trapped and compressed, causing the booster to float with the forward end out of the water. After divers insert a nozzle closure and force the water from the booster using air pumps, the booster is towed to shore.
After recovery, the booster is disassembled and refurbished. The motor segments are shipped to the manufacturer for reload for another Shuttle flight. The other systems are refurbished either at the launch site or at the respective manufactures' locations .
The two solid rocket boosters are each 149.1 feet (45.4 meters) high and 12.2 feet (3.7 meters) in diameter. Each weighs 1,300,000 pounds (589,670 kilograms). Their solid propellant consists of a mixture of Aluminum powder, Ammonium Perchlorate powder, and a dash of Iron Oxide catalyst, held together with a polymer binder. They produce about 3.1 million pounds (13.8 million newtons) thrust each for the first few seconds after ignition, before gradually declining for the remainder of a two-minute burn. Tog ether with the three main engines on the orbiter, this provides a total thrust of over 7.3 million pounds (32.5 million newtons) at liftoff.
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