Basic Rocket Propellants

The thrust of a rocket is measured by specific impulse. Though this is similar to measuring a car’s power by its miles per gallon or a foods nutritional value by its calories. Specific impulse is a unit measured in time that a given amount of propellant changes a body’s momentum. Since we know how much momentum is needed to achieve a determined altitude we therefore can use the specific impulse value as a way of finding how much propellant we need.

There are 3 types of rocket propellants; liquid, solid, and hybrid. While their names can give a general sense of what they are liquid fueled rockets have 3 sub varieties; petroleum, cryogenic, and hypergolic. All rocket engines produce thrust through exothermic reactions (burning) between a fuel and an oxidizer. While liquid propellants are pumped into the combustion chamber for the reaction to occur solid fueled rockets have the fuel and oxidizer combined into a solid fuel which once ignited must burn itself out. Hybrid rocket motors generally have a solid fuel which has a liquid oxidizer pumped onto it, allowing the rocket to be stopped and restarted like a liquid fueled rocket motor.

Petroleum was one of the first rocket fuels and continues to be in use today. Mostly just highly refined clean burning kerosene combined with liquid oxygen as the oxidizer. Both American and Soviet space programs relied heavily on petroleum fueled rockets. Today petroleum is used in the Atlas and Delta II launch vehicles as well as the privately funded Falcon 9 program.

Cryogenic fuels include liquid Hydrogen and liquid methane. However liquid methane rockets have never been used in actual launches. Liquid fluorine is also widely considered to be a liquid fuel however fluorine is in reality an oxidizer, while liquid fluorine is theoretically a very powerful propellant its high toxicity has kept it out of use.

Hypergolic fuels do not require an ignition source to begin their exothermic reactions, when two (or more) hypergolic compounds are brought together they spontaneously combust. This type of propellant is in use on the American Titan family of launch vehicles as well as the burgeoning Indian space program.

Hybrid rocket engines are something relatively new and have made a big splash already as the engines that powered the SpaceShipOne to capture the Ansari X prize. This type of engine will also be used on the following family of space planes developed by Scaled Composites.