Solid Fuel Rockets
"T minus 5...4...3...2...l...Liftoff of the space shuttle..."
As the shuttle rose from the launch pad, it was powered by three main engines and two solid rocket boosters (SRBs) attached to the side of the orange external fuel tank. The SRBs burned their fuel at a furious rate, consuming 2.2 million pounds of propellant in just over two minutes. Once they burned out, the boosters fell away and landed by parachute in the ocean for recovery and reuse.
Solid-Fuel Rocket Diagram
The space shuttle's boosters remain the largest solid-fuel rockets ever flown – and the first to be used as primary propulsion for a human spaceflight system. Weighing in at 2.6 million pounds, most of which was fuel, the rockets provided 83 percent of the liftoff thrust for the space shuttle.
Space Shuttle Solid Rockets Firing
The shuttle's SRBs are the descendents of solid-fuel rockets that have existed for almost 800 years. As early as the 13th century, the Chinese used primitive rockets powered by gunpowder in battle. In fact, all rockets were powered with solid fuel until the 20th century when engineers began to develop more versatile liquid-fuel and hybrid engines.
The advantage of solid-fuel rockets is that they can be safely stored for long periods of time and then launched quickly. This makes them ideal for use as military missiles. Model rocket hobbyists like them for the same reasons.
A solid rocket motor is a fairly simple design consisting of a casing, a nozzle, grain (propellant), and an igniter. Once the grain is lit, the fuel burns as a solid mass and produces exhaust gases that are expelled out the nozzle. Compared with a liquid-fuel rocket, the design was simple: no turbo pump or complicated plumbing required.
Alas, with simplicity come some drawbacks. The major one is that solid-fuel rockets have lower performance than do liquid boosters, which means they are not useful for sending medium to large payloads into space. Their effectiveness is limited to launching payloads of less than two tons into low Earth orbit. Most payloads are much heavier.
Simple solid-fuel rockets have several other disadvantages in relation to liquid rockets. They can't be test fired ahead of time, and once they are lit, they can't be throttled, stopped, or restarted. Meanwhile, the low-density of liquid fuels allows for lightweight fuel tanks, which saves weight and boosts a rocket's payload capacity.
Engineers have developed more advanced solid rocket motors that can be throttled, shutdown and reignited. However, the drawback of lower performance remains.
Given these limitations, solid fuel rockets are primarily used today as strap-on boosters to enhance the payload capacity of liquid rockets, as with the space shuttle and other launchers. Solid rockets are also used as spin-stabilized upper stages for launching satellites into higher orbits.
Solid-fuel rockets can fail if there are air pockets in the grain, the grain fractures, or the case bonding fails. These problems increase the burn surface area, resulting in excess exhaust gas and pressure that can rupture the rocket's casing.
The space shuttle Challenger and her seven-member crew were lost when a seal between two SRB segments failed. This breach allowed hot gases to escape and burn through the external tank, leading to an explosion.