Rocket Propulsion/Rocket Propulsion Using Liquid Propellant
Liquid Fuel Rockets are rockets that contain a liquid fuel as well as a liquid oxidiser. In pre-launch the liquid fuel would burn slowly until the oxidiser is released creating a very rapid burn with enough force for liftoff. Liquid fuel rockets can also be controlled through limiting or regulating the fuel and oxygen supplies. The Space Shuttle's large orange tank (ET - External Tank) contains liquid hydrogen and liquid oxygen that is used by the Orbiter upon take-off.
Liquid propellants have the advantage of better performance than solid rockets, often at the cost of handling cryogenic materials.
In the mid-80's NASA looked at using the Centaur upper stage for Galileo and Ulysses planetary probes to take advantage of that higher performance.
"Hypergolic" propellants combust upon contact with each other. This makes them attractive for use in smaller thrusters, such as those used on the Shuttle Orbital Maneuvering System and Reaction Control System.
Some satellites use monopropellant thrusters for station-keeping.
[edit] Water Rockets
Great sport (and serious study) can be had using plastic soda bottles, water and compressed air.
