Here, the three sections are joined together during Shuttle assembly, and the field joints between the segments contain two rubber O-rings. Under the heat generated by the burning propellant within the boosters, these rubber seals expand to fill the field joints and prevent the hot exhaust from escaping.

The two SRBs provide the majority of the thrust needed to propel the Shuttle into orbit. Although solid rockets are less complex and cheaper than liquid-fuel rocket engines, like those used on the Orbiter itself, their primary disdvantage is that they cannot be throttled or turned off. Once ignited, the SRBs continue to burn full force until they exhaust their fuel supply. After they have performed their job and lifted the Shuttle to an altitude of about 150,000 ft (45,760 m),the SRBs are jettisoned using small explosive charges. The SRBs then deploy parachutes and fall into the ocean. Here, they are recovered by tugboats that tow the boosters back to shore to be refurbished for use on a future flight.

The purpose of the External Tank, the single largest piece of the STS, is to carry the liquid fuel needed by the three main engines located in the aft section of the Orbiter. In addition to the SRBs, these three liquid fuel engines provide the thrust needed to reach orbit. The main engines require a large amount of liquid propellant over the nearly nine minutes they are in operation. This propellant is carried within the External Tank. The lower two-thirds of the tank is filled with liquid hydrogen while