How Rocket Engines Work

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:

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.