SRBs

Solid rocket boosters (SRBs) are rocket motors that provide a large portion of liftoff thrust for various orbital launch vehicles. They burn solid propellant and are mounted on the vehicle during the initial ascent, detaching after burnout while the core propulsion system continues to operate.

Each SRB consists of a rigid outer casing, usually steel, containing a solid propellant grain, and a nozzle at the aft end. Propellant is typically a cast ammonium perchlorate composite with a polymer binder and metal such as aluminum. Ignition is electric, and thrust is largely constant during the burn, which lasts from tens of seconds to a few minutes. The propellant is arranged in segments with joints sealed by elastomer O-rings, a vulnerable feature highlighted by historical failures and subsequently redesigned in many systems.

Advantages of SRBs include very high thrust, relative simplicity, and lower unit cost compared with liquid engines. Limitations include the inability to throttle or shut down the burn once it has begun, sensitivity to propellant temperature and geometry, and potential failure modes at field joints or the casing. Notable historical events, such as the Challenger disaster in 1986, underscored the importance of joint design and material reliability.

Historically, SRBs are most closely associated with the Space Shuttle program, which used two reusable boosters for each flight from 1981 to 2011. They have also been employed on other launch vehicles to augment lift, including various European, Russian, and Japanese designs, where they provide a reliable, high-thrust boost during the early phase of ascent.