The first liquid propelled rocket was launched in 1926 by Robert Goddard. Liquid Rockets are not as simple as solid fueled rockets. The liquid fueled rocket uses a fuel and oxidizer as it propellent.

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 In a liquid propelled rocket the fuel and oxidizer are pumped into a combustion chamber. In the combustion chamber the oxidizer and fuel burn, in doing so they make high pressure and high velocity steam. These gases then go through the nozzle and in the nozzle they are accelerated even more.  

 

In the rocket the oxidizer and fuel are held in different two different tanks. this is because most liguid fuels burn on contact. In order to get the fuel and oxidizer to the combustion chamber they are pumped and controlled by valves. There are two ways to get the get the fuel and oxidizer to the combustion chamber; the first is a pressurized gas feed and the second is a turbo pump feed.

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The most widely used oxidizer is liquid oxygen. There are many other oxdizers used in rockets. Most of these oxidizers are gases but are cooled to become liquid. This is done because it gets the molecules closer, thus improving the oxidizers performance. Common fuels are liquid hydrogen, liquid ammonia, hydrazine, and kerosene.