Rockets have to carry an enormous amount of fuel in order to make the journey into orbit. So much fuel is needed that most rockets are made up of enormous chambers in which to store the liquid hydrogen and oxygen. Rockets such as the Arianne need maximum thrust for the first few seconds of take-off, so the main engine and two extra boosters fire at full power. After about two minutes, all the fuel in the two boosters has been used up, so they are jettisoned in order to lighten the payload. The less a rocket weighs the less fuel it has to use.
A multistage rocket, is a launch vehicle that uses two or more rocket stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.
By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. Each successive stage can also be optimized for its specific operating conditions, such as decreased atmospheric pressure at higher altitudes. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height.
In serial or tandem staging schemes, the first stage is at the bottom and is usually the largest, the second stage and subsequent upper stages are above it, usually decreasing in size. In parallel staging schemes solid or liquid rocket boosters are used to assist with launch. These are sometimes referred to as “stage 0”. In the typical case, the first-stage and booster engines fire to propel the entire rocket upwards. When the boosters run out of fuel, they are detached from the rest of the rocket (usually with some kind of small explosive charge) and fall away. The first stage then burns to completion and falls off. This leaves a smaller rocket, with the second stage on the bottom, which then fires. Known in rocketry circles as staging, this process is repeated until the desired final velocity is achieved. In some cases with serial staging, the upper stage ignites before the separation—the interstate ring is designed with this in mind, and the thrust is used to help positively separate the two vehicles.
A multistage rocket is required to reach orbit speed. Single-stage-to-orbit designs are sought, but have not yet been demonstrated.
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