Stage

A stage is a subset of ship systems that are activated simultaneously. A stage can consist of a single part or even all the parts on the spacecraft, but all parts must belong to a stage. When the stage is activated all the functional parts contained within will become active. Engines will engage, parachutes will prime, decouplers will explode. When designing your spaceship putting every part into the correct stage is essential. An engine or a decoupler being activated out of step will most likely lead to a catastrophic failure. Staging happens in a linear fashion in Kerbal Space Program, meaning the stages can only be activated one after the other, from the lowest stage -the highest number- to the last or 'lander' stage -number one-.

Decouplers serve to seperate stages once they are out of fuel or otherwise outlived their usefulness. When constructing a new ship the game will automatically create a new stage when you place a decoupler or seperator on your craft. However it can't follow the logic of the design process so by the time you finish you may have to manually reorder parts to the correct stages you want them to belong to. This happens by rearranging the icons in the staging process list on the right of the screen in the VAB. Here you can also create or delete any number of stages with no restrictions other than that a single stage must remain. You can also reorder parts to different stages on the launch pad or even mid flight, as well as create or delete stages. That said, it's better to get the staging sorted out in the VAB. Trying to get your decouplers untangled while your SRBs are burning is a hazardous endeavor.

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 seperate 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.