Solid-fuel Rockets: Channel Configuration

When you read about advanced solid-fuel rockets like the Shuttle's solid rocket boosters, you often read things like:
The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star-shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure. [source: NASA]
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:

An 11-point star-shaped perforation in the channel of an advanced solid-fuel rocket.

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:

  • Simplicity
  • Low cost
  • Safety
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.