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Stage

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A stage is a subset of ship parts that are all activated simultaneously. Any stage is placed in order with other stages. A stage can consist of one or more parts on the spacecraft. Upon activation of the stage, all parts within become active. Engines will engage, parachutes will prime, decouplers will detach. When designing spaceships, it is essential to put all parts into their correct stages. An engine or a decoupler being activated out of step will most likely lead to a catastrophic failure. Stages are activated one after the other, from the lowest stage -the highest number- to the last stage -zero-.

Setup

The game can automatically set up or reset the stages of a craft, based on the assumption that decouplers serve to separate stages once they are out of fuel or otherwise outlived their usefulness. A new stage is created whenever a new set of decouplers is placed on a craft, with that new stage containing all of the parts attached to that decoupler. This logic doesn't work for all vessel designs, so it may be necessary to manually reorder parts to the correct stages, usually when building for multi-mission spacecraft. This happens by adding stages and rearranging the icons in the staging process list on the right side of the screen in the VAB. Here you can create or delete any number of stages with no restrictions other than that a single stage must remain.

It is also possible reorder parts to different stages on the launch pad or mid flight. However, staging should mostly be done in the VAB, where there is no pressure of time and it can be saved with the craft file.

Staging Design

There are three main staging types with different fuel flow behaviour.

Serial Staging

First using S2's engines and then S1's

In this type there is no fuel flow between the tanks and there are simply two stages which are executed. First all radial engines are activated which use up their fuel. After they run out of fuel those stages are jettisoned and the center stage starts burning. This way you can drop the outer tanks the center stage have to lift less mass.

Stage 1 2 3
Operation Start S2 Jettison S2 Start S1

Parallel Staging

All engines are running, but the S1's engine uses the fuel from the S2 engines

This staging is similar to the serial staging, but with the difference that the center stage is running all the time but using the fuel of the radial stages. By this staging type the radial stage will run out of fuel sooner, but when those stages get jettisoned the center tanks are still full. So this type can lift more mass with the same acceleration which could be more effective than the serial staging so you can escape the gravity faster.

Stage 1 2
Operation Start S2, S1 Jettison S2

Asparagus Staging

All engines are running and first using S4's fuel then S3's and so on

A more advanced staging type is asparagus staging. The difference to the parallel staging is, that also the radial stages are using the fuel of other radial stages. So all engines will use the fuel of 2 radial engines which deplete very fast. After they run out of fuel those 2 could be jettisoned and the engines use the next 2 radial engines. This repeats until the last radial stage is jettisoned and only the center stage is running. The feature of this type is that after jettisoning a radial stage all remaining tanks are still full so the rocket loses mass more quickly.

Stage 1 2 3 4
Operation Start S4, S3, S2, S1 Jettison S4 Jettison S3 Jettison S2

Basic Staging Mechanisms

Merging Stages

Decouplers are generally used to separate stages but you can create stages without decouplers in order to -for example- engage solid fuel boosters at just the right time without having to dump anything off your ship to do so. All you have to do is create a stage that contains only the boosters -or other parts you want engaged-. When such a stage is activated the previous stage will be merged into it, the booster now occupying the active stage with all the parts from the previous stage also remaining active. Once used up, the solid boosters can be decoupled by another stage containing only their decouplers leaving the rest of the active stage intact.

Gears, Ladders and Landing Legs

Not all functional parts are influenced by staging. Some of them can be activated and deactivated multiple times even if they are not in the active stage. As of version 0.17.1 these include the LT-2 Landing Strut, Small Gear Bay, Telus Mobility Enhancer and the Telus-LV Bay Mobility Enhancer. These parts serve a purpose that requires repeated activation and as such can be engaged and disengaged at any time.