When you read about advanced solid-fuel rockets like the Shuttle's solid rocket boosters, you often read things like:
This paragraph discusses not only the fuel mixture but also the configuration of the channel drilled in the center of the fuel. An "11-point star-shaped perforation" might look like this:
The idea is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust. As the fuel burns, the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.
Solid-fuel rocket engines have three important advantages:
- Low cost
They also have two disadvantages:
- Thrust cannot be controlled.
- Once ignited, the engine cannot be stopped or restarted.
The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system. We'll learn about those and other possibilities next.