Rocket engines produce thrust—the force that moves the rocket. Chemical rockets are the most common type of rocket; they generate thrust by combustion. There are two components necessary for combustion: fuel (the substance that burns) and an oxidizer (to enable the burning). In the combustion chamber of the engine, the fuel and oxidizer chemically react to produce gas at high pressure. Liquid hydrogen (H₂) and liquid oxygen (O₂) are commonly used as propellants, or reactants, in rocket engines. In their combustion reaction, two hydrogen molecules combine with one oxygen molecule to form two water molecules (in gaseous form) and heat.

2 H₂(l) + O₂(l) = 2 H₂O(g) + heat

The acceleration of the combustion products through an exhaust nozzle creates thrust. Following Newton's third law of motion (for every action, there is an equal and opposite reaction), the force of the exhaust exiting the nozzle goes hand in hand with a force pushing the rocket in the other direction. This concept can be demonstrated by letting go of an untied balloon full of air; as the elasticity of the balloon restores it to its regular size, air is pushed out of the balloon mouthpiece. The balloon reacts by moving in the opposite direction from the air molecules exiting the mouthpiece. A common misconception is that rocket exhaust pushes against the launch platform to launch the rocket; however, rocket engines can still work in space, where there is nothing to push against. It is simply the action of throwing the mass of the exhaust backwards that causes the rocket to move forward.

The ratio of the oxidizer to fuel (O/F) is called the mixture ratio; it is based on the mass of the propellants. If the mixture ratio were stoichiometric, there would be exactly the right amount of oxidizer and fuel so that they could be consumed completely with no propellant remaining. However, most rockets are fuel-rich, meaning that there is more fuel than needed to react with the oxidizer. Altering the mixture ratio by making it fuel-rich can improve the overall system by improving nozzle efficiency (the size of the molecules in the exhaust affect the exhaust velocity) and making engine cooling easier.

The propellants of a chemical rocket can be liquid or solid. The measure of efficiency of a propellant is known as specific impulse; the higher the specific impulse, the larger the thrust generated by a given amount of propellant in a given amount of time. Solid propellants are easier to handle than liquid propellants, but they generally have a lower specific impulse. Liquid fuel engines are more complex than solid fuel engines, but offer other advantages as well. For example, liquid fuel engines can be throttled, stopped, and restarted.

There are many possible liquid propellants. Some liquid fuels are petroleum based, such as kerosene. Other propellants, such as liquid hydrogen and liquid oxygen, are cryogenic—they need to be stored at extremely low temperatures to maintain their liquid state. Hypergolic propellants, such as monomethyl hydrazine and nitrogen tetroxide, ignite on contact with each other; easy and reliable ignition makes hypergolic propellants desirable, but they are extremely toxic so must be handled with care.

Print Background Essay