Solid rocket booster

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NASA Image of the final solid rocket booster (right) being mated to a Delta II rocket (blue). Two boosters (white) can be seen already attached.

Solid rocket boosters (SRB) (or motors, SRM) are used to provide the main thrust in spacecraft launches from the launchpad up to burnout of the SRBs. Many launch vehicles include SRBs, including the Ariane 5, Atlas V, and the NASA Space Shuttle. The NASA Space Shuttle uses two Space Shuttle SRBs, which are the largest of their type.

The solid-fuel SRBs are advantageous for the purpose of boosting launches compared to liquid-fueled rockets because they provide greater thrust and do not have the refrigeration and insulation requirements of liquid-fueled rockets. Adding detachable SRBs to a vehicle also powered by liquid-fueled rockets eases the amount of liquid-fuel needed and lowers the launch vehicle mass. This is known as staging.

One example of the increased performance from SRBs is the Ariane 4 rocket. The basic 40 model with no boosters could lift 2,175 kilograms payload to Geostationary transfer orbit [1]. The 44P model with 4 solid boosters has a payload of 3,465 kg to the same orbit [2].

Solid boosters are usually cheaper to design, test, and produce compared to equivalent thrust liquid boosters. However, the costs on a per-flight basis tend to be equivalent.

Solid rocket motors cannot easily be turned off or have their thrust terminated during flight, which is a risk factor for manned spacecraft.

Solid rocket motors also present a significant handling risk on the ground. Once their propellant is poured into place and cured, they are always loaded and could catch fire or explode in an accident. Such an accident on August 22, 2003 killed 21 technicians at the Brazilian VLS rocket launch pad. [3]

The manned and unmanned Shuttle Derived Launch Vehicles currently planned as part of NASA's Vision for Space Exploration both rely heavily on modified versions of the current Space Shuttle solid rocket boosters; the manned vehicle would use a single, expanded solid rocket booster as its sole first stage.