Solid rocket boosters

Solid rocket boosters (SRBs) are large solid propellant motors used to provide thrust in spacecraft launches from initial launch through the first ascent stage. Many launch vehicles, including the Ariane 5, Atlas V, and the NASA Space Shuttle, use SRBs to give launch vehicles much of the thrust required to escape Earth's gravitational pull. The NASA Space Shuttle used two Space Shuttle SRBs, which were the largest solid propellant motors ever built and the first designed for recovery and reuse. The propellant for each solid rocket motor on the Space Shuttle weighed approximately 500,000 kilograms.

Compared to liquid propellant rockets, the solid-propellant SRBs are capable of providing large amounts of thrust with a relatively simple design. They provide greater thrust without significant refrigeration and insulation requirements. Adding detachable SRBs to a vehicle also powered by liquid-propelled rockets (known as staging) reduces the amount of liquid propellant needed and lowers the launch vehicle mass. Solid boosters are usually cheaper to design, test, and produce in the long run compared to the equivalent liquid propellant boosters. Reusability of components across multiple flights, as in the Shuttle assembly, also decreases hardware costs. However, SRB costs on a per-flight basis tend to be equivalent to those of their liquid counterparts.

One example of increased performance provided by SRBs is the Ariane 4 rocket. The basic 40 model with no additional boosters is capable of lifting a 2,175 kilogram payload to Geostationary transfer orbit. The 44P model with 4 solid boosters has a payload of 3,465 kg to the same orbit.

Solid propellant boosters are not controllable and must generally burn until exhaustion after ignition, unlike liquid propellant or cold-gas propulsion systems. However, launch abort systems and range safety destruct systems can attempt to cut off propellant flow by using shaped charges. Estimates for SRB failure rates range from 1 in 1,000 to 1 in 100,000. SRB assemblies can fail suddenly and catastrophically. Nozzle blocking or deformation can lead to overpressure or a reduction in thrust, while defects in the booster's casing or stage couplings can cause the assembly to break apart by increasing aerodynamic stresses. Additional failure modes include bore choking and combustion instability. Failure of an O-ring seal on the Space Shuttle Challenger's right solid rocket booster led to its disintegration shortly after liftoff.

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