Multistage rockets are spacecraft/satellite launch vehicles that consist of two or more rocket stages with each stage containing its engines and propellants which are detached post-launch one after the other till the payload reaches its intended orbit or destination. Multistage rockets are predominantly used as they are much more cost-effective when compared to single-stage rockets and they will help the payload to reach its maximum speed by getting rid of any additional weight. The Saturn V rocket that was used for the Apollo Mission was a three-stage rocket with the payload located at the top.

The structure of the launch vehicle consists of two or more rocket stages, with each stage containing its engines and propellants. The fuel required for a launch vehicle is large, hence, the fuel is stored in multiple stages. The payload (satellite or spacecraft) is kept at the top of the launch vehicle. Post-launch, if the fuel of any stage is exhausted, the empty fuel tank (stage) is detached from the launch vehicle body & the fuel in the next stage is ignited. This process is repeated till the payload is deployed or separated. By using the separation process which takes place from bottom to top, the overall weight of the launch vehicle reduces which helps the satellite or the spacecraft to gather more speed. Additionally, the overall fuel consumption is also reduced.

Multistage rockets are separated using explosive bolts. The point of contact that holds the two stages of a multistage rocket consists of holes (refer to figure below) when observed from a cross-sectional perspective. These holes are filled with explosive bolts. There are close to 300 points that can be filled with explosive bolts for each stage. A fire is ignited from the upper stage of a rocket that acts as a catalyst for the explosive bolt which goes off all around the stage and separates the lower stage that has exhausted its fuel and is of no use from the upper stage which separates and keeps burning.

Multistage rockets can be developed in different configurations. The method of deciding the configuration of a multistage rocket is known as staging. The combination of several rocket sections, or stages, that fire in a specific order and then detach, so a payload can penetrate Earth’s atmosphere and reach space.

There are three different types of staging that are used for a multistage rocket.

Serial Staging

In a serial stage configuration, the stages are attached one on top of the other or stacked. The first stage ignites at launch and burns through the fuel until it's completely exhausting. Now the first stage acts as an empty fuel tank and is dead weight, the first stage is detached, and immediately the second stage is ignited. Depending on the rocket & its mission, the second stage may get the payload into orbit or may require a third or fourth stage to deliver the payload into space or its intended destination.

Parallel Staging

Whereas the serial staging configuration involves stacked stages, parallel staging features multiple boosters staged that are strapped to a central sustainer. The boosters that are attached to the sustainer are also ignited at launch which helps the rocket attain the required thrust that is needed to break free from the Earth’s gravity. When the fuel of the boosters runs out, they are detached from the central sustainer. The central sustainer engine keeps burning and takes the payload to a high altitude near space.

Serial & Parallel Staging

This configuration includes a sustainer stage attached to boosters, that is detached once they are exhausted. The sustainer takes the payload to a considerable height after which it detaches itself and other stages that are serially stacked are ignited one after the other till the payload reaches its intended orbit. The Titan III rocket uses both serial & parallel staging. It used a two-stage sustainer in a parallel configuration and added two rocket stages in a serial configuration that detached themselves once they were exhausted.

Multistage rockets once discarded from each stage after it has served their purpose are made to crash towards the earth’s surface where they will burn up on re-entry. However, only the lower stages can be made to crash towards the earth. Upper rocket stages will still pose a risk to contribute towards space debris. To tackle this problem, the upper rocket stages after detaching conserve some fuel so that they can be redirected towards the graveyard orbit. However, this solution is not optimal, as the fuel conserved might not be enough to take the upper rocket stage to the graveyard orbit. Various space organizations are working on a novel solution to discard the upper rocket stages efficiently to negate the problem of space debris.

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