Radial size

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Comparison of radial sizes with a Kerbal for scale. From top to bottom: Tiny (Size 0), Small, Large, and Extra Large (Size 3).

The radial size is the diameter or mounting position for most parts. If the mounting position is not radial, then the part is either round or one of two special forms. Such rectangular parts as the M-1x1 Structural Panel lack a radial size or it's a size to the next bigger diameter like the Probodobodyne QBE. Four radial sizes exist with varying names. The size scale looks like the internal definition as the Mk2 to size 1 Adapter uses this term and starts with 0 and many values in computer programming are starting from 0 rather than 1.

Any stackable part can be connected to any other stackable part. Matching radial sizes only serves to give a rocket a cleaner visual appearance, and several adapter parts exist to allow transitions between radial sizes while maintaining an unbroken surface.

Sizes

We will use the FL-T100 Fuel Tank as the baseline length unit.

Radial size Fuel per length Typical crash tolerance Typical maximum temperature
Rocket fuel Liquid fuel Monopropellant
Tiny / 0.625m 6 m/s 2000°
Small / 1.25m 6 m/s 2000°
Large / 2.5m / Rockomax 6 m/s 2000°
Extra Large / 3.125m / Kerbodyne 6 m/s 2000°
Mk1 20 m/s 2000°
Mk2 40 m/s 2200°
Mk3 50 m/s 2300°

Rocket

The typical mass ratio for rocket fuel is 9.

Tiny / 0.625m

Typically used for probes and nose electronics.

Small / 1.25m

The most common size, and the primary size available at the beginning of the game.

Large / 2.5m / Rockomax

Extra Large / 3.75m / Kerbodyne

Only rocket fuel is available at this size.

Spaceplane

Spaceplane parts may carry either rocket fuel or liquid fuel. Compared to rocket parts they tend to have a poorer mass ratio (usually about 8), but better crash tolerance and maximum temperature.

Mk1

Like the 1.25m rocket size, this size is a cylinder. All inline air intakes are of this size, but there are no rocket fuel tanks. Crash tolerance is typically 20 m/s and maximum temperature is typically .

Mk2

With its side chines, this spaceplane size is reminiscent of the real-world Lockheed SR-71 Blackbird. It is between the 1.25m and 2.5m sizes in cross-sectional area. These parts feature a small amount of lift.

Mk3

Reminiscent of the real-world Space Shuttle. The cross-sectional area is similar to the 3.75m rocket size.

Assets

When creating assets, the correct diameter for a desired model can be found in the table below. The "meter scale" refers to the actual model's diameter, which is then typically scaled up via the default rescaleFactor of 1.25, to give the "actual" in-game diameter. Size scale is used to denote the standard when selecting a node size.

Conversion table between the different definitions
Name Size scale Meter scale Actual size Example
Tiny 0 0.5 m 0.625 m Rockomax 48-7S
Small 1 1 m 1.25 m LV-T30 Liquid Fuel Engine
Large 2 2 m 2.5 m Rockomax "Mainsail" Liquid Engine
Extra large 3 3 m 3.75 m Kerbodyne KR-2L Advanced Engine

As a special radial size Radial mounted appears. Parts like Rockomax 24-77 so lack general metric counterparts as to eliminate confusion.

Non-cylindrical sizes named “Mk2”, and “Mk3” exist and are used by fuselage parts intended for spaceplanes.

Additional sizes

Several addons add additional, usually larger, radial sizes which are roughly following the stock size scale. There are up to 9 meter parts in the B9 Aerospace mod and mods like Tweakscale can scale parts more freely.