More of This Rocket Engine Feature • Part I: Newton's Three Laws applied to Rockets • Part 2: Practical Rocketry • Part 3: Rocket Thrust and Control • Part 3a: Rocket Stability and Control Systems  • Part 3b: Rocket Mass

Further Rocket Engine Research • How a Firework Rocket Engine Works • How a Liquid Propellant Rocket Engine Works • How a Solid Propellant Rocket Engine Works Rockets Main Page

Liquid-propellant rocket engines control the thrust by varying the amount of propellant that enters the combustion chamber. A computer in the rocket's guidance system determines the amount of thrust that is needed and controls the propellant flow rate. On more complicated flights, such as going to the Moon, the rocket engines must be started and stopped several times. Liquid rocket engines do this by simply starting or stopping the flow of propellants into the combustion chamber.

Solid-propellant rockets are not as easy to control as liquid rockets. Once started, the propellants burn until they are gone. They are very difficult to stop or slow down part way into the burn. Sometimes fire extinguishers are built into the engine to stop the rocket in flight. But using them is a tricky procedure and doesn't always work. Some solid-fuel engines have hatches on their sides that can be cut loose by remote control to release the chamber pressure and terminate thrust.

The burn rate of solid propellants is carefully planned in advance. The hollow core running the length of the propellants can be made into a star shape. At first, there is a very large surface available for burning, but as the points of the star burn away, the surface area is reduced. For a time, less of the propellant burns, and this reduces thrust. The Space Shuttle uses this technique to reduce vibrations early in its flight into orbit.

NOTE: Although most rockets used by governments and research organizations are very reliable, there is still great danger associated with the building and firing of rocket engines. Individuals interested in rocketry should never attempt to build their own engines. Even the simplest-looking rocket engines are very complex. Case-wall bursting strength, propellant packing density, nozzle design, and propellant chemistry are all design problems beyond the scope of most amateurs. Many home-built rocket engines have exploded in the faces of their builders with tragic consequences. 

Next page > • Part 3: Rocket Stability and Control Systems 
• Part 4: Rocket Mass

Information and Images Provided by NASA