StagesSome rockets use staging in order to reduce the weight of the rocket as it ascends. A multi-stage rocket discards a portion of the rocket once the propellant in that section has been consumed. This makes the remaining part of the rocket lighter and able to reach a greater altitude and speed.
There are two types of rocket staging, serial and parallel. The Saturn V rocket which launched men to the moon used serial staging. After the first stage of the rocket ran out of fuel and oxidizer, it dropped into the ocean as the second stage ignited to continue the flight.
The Space Shuttle uses parallel staging. At launch, the shuttle's main engine and its two Solid Rocket Boosters (SRBs) are ignited at the same time. Once the SRBs have used up all their propellant, they are discarded as the main engine continues to propel the shuttle into orbit.