Solid rocket booster
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Image:SRB-Stardust.JPG Solid rocket boosters (SRB) are used to provide the main thrust in spacecraft launches from the launchpad up to an altitude of about 45 kilometres. The NASA Space Shuttle uses two 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. Once the fuel is expended the casings are discarded and — if launched from Cape Canaveral — parachute into the shallow waters of the Carribean Sea to later be picked up by the U.S. Navy. Relieving the rocket or shuttle of this weight 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]