Rocket staging

What Is Rocket Staging?

Staging provides method of increasing an efficiency of a rocket. 

Most of the weight of the whole rocket is the weight of its actual propellant. As the propellants are burned off during liftoff, a larger proportion of the weight of the vehicle becomes the nearly empty tank and structure. 

In order to lighten the weight of the rocket to achieve an orbital velocity, most rockets discard its own part in a process called staging.

A multistage rocket is a launch vehicle that has two or more rocket stages, with each of them containing its own engines and propellants. The lowest stage, or the first stage, ignites and lifts the vehicle at increasing velocity until complete exhaustion of its propellant. At this point the first stage drops off and the second stage ignites and accelerates the vehicle further. Most rockets has 3 stages.

credit: NASA

At the beginning of the flight, only the bottom or the first stage pushes the whole rocket. After burning out, the stage drops off and second stage takes over. It goes on until the top stage engine shuts down and the satellite is placed into an orbit. 

First stage can take over 50 % of the overall rocket liftoff mass, in most of the cases. 

 

There are two types of rocket staging, serial and parallel.

source: NASA

In serial staging, shown in Figure above, there is a small, second stage rocket that is placed on top of a larger first stage rocket. The first stage is ignited at launch and burns through the ascent until its propellants are exhausted. The first stage engine is then switched off, the second stage separates from the first stage, and the second stage engine is ignited. 

In this case, the payload is carried atop the second stage into orbit. 

Serial staging was for example used on the Saturn V rockets. The Saturn V had three stages, which performed two staging maneuvers on its way to earth orbit. Those stages has never been retrieved. 

source: NASA

In parallel staging, shown in Figure above, several small first stages - boosters are strapped onto to a central sustainer rocket. At launch, all of the engines are ignited to help the rocket attain the required thrust that is needed to break free from the Earth's gravity. When the propellants in the strap-on are extinguished, the strap-on rockets are detached from the central sustainer. The sustainer keeps burning and takes the payload into orbit. 

Parallel staging was used on the Space Shuttle. The discarded solid rocket boosters were retrieved from the ocean, re-filled with propellant, and used again on the Space Shuttle.

In case of Serial & Parallel staging, this configuration includes a sustainer stage attached to boosters, that is detached once they are empty. The sustainer takes the vehicle to certain altitude after which it detaches itself from other stages that are serially stacked and ignited one after another until payload reaches intended altitude. 

Good example of the combine staging is Titan III rocket. It contains two stage sustainer in a parallel configuration and two rocket stages in a serial configuration. 

source: Wiki

Space Launch System (SLS) is American super heavy-lift expendable launch vehicle. The first stage is powered by one central core stage and two parallel solid rocket boosters. Second stage is serially attached upper stage which is the interim cryogenic propulsion stage, flown on Artemis 1. It is planned to fly on Artemis 2 and 3. The Exploration Upper Stage is planned to fly on Artemis 4.