What is Multistage Rocket and it's Advantages

A multistage (or multi-stage) rocket is a rocket that uses two or more stages, each of which contains its own engines and propellant. A stacked stage is mounted on top of another stage; a parallel stage is attached next to another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Taken together these are sometimes called a launch vehicle. 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. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height.

In stacked staging schemes, the first stage is at the bottom and is usually the largest, the second stage is above it and is usually the next largest. Subsequent upper stages are above those. In parallel staging schemes solid or liquid rocket boosters are used to assist with lift-off. 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. This process is repeated until the final stage's motor burns to completion.


Advantages:

The main reason for multi-stage rockets and boosters is that once the fuel is burnt, the space and structure which contained it and the motors themselves are useless and only add weight to the vehicle which slows down its future acceleration. By dropping the stages which are no longer useful, the rocket lightens itself. The thrust of the future stages is able to provide more acceleration than if the earlier stages were still attached, or than a single, large rocket would be capable of. When a stage drops off, the rest of the rocket is still travelling near to the speed that the whole assembly reached at burn-out time. This means that it needs less total fuel to reach a given velocity and/or altitude.

A further advantage is that each stage can use a different type of rocket motor, with each stage/motor tuned for the conditions in which it will operate. Thus the lower stage motors are designed for use at atmospheric pressure, while the upper stages can use motors suited to near vacuum conditions. Lower stages tend to require more structure than upper as they need to bear their own weight plus that of the stages above them, optimizing the structure of each stage decreases the weight of the total vehicle and provides further advantage.