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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. However, smaller parts have smaller attachment nodes, which result in weaker connections, so, say, putting a 1.25 meter part in between two 3.75 parts will make the stack wobbly. Matching radial sizes also serve 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

Radial size Fuel per m Typical crash tolerance Typical maximum temperature
Rocket fuel Liquid fuel Monopropellant
Tiny / 0.625m 115 n/a 213 6 m/s 2000°
Small / 1.25m 213 n/a 438 6 m/s 2000°
Large / 2.5m / Rockomax 853 n/a 750 6 m/s 2000°
Extra Large / 3.75m / Kerbodyne 1925 n/a n/a 6 m/s 2000°
Mk1 n/a 213 n/a 20 m/s 2000°
Mk2 213 213 213 40 m/s 2200°
Mk3 1000 1000 1000 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, and about the same relative size to kerbals as the Redstone's tanks to humans.
  • Large / 2.5m / Rockomax. This size is about the same relative size to kerbals as the Atlas V or Falcon 9's tanks to humans.
  • Extra Large / 3.75m / Kerbodyne. Styled after NASA's real-world Space Launch System, which uses Rocketdyne RS-25 and RL10 engines.

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.
  • 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, that is, two Mk3 tanks stacked and then offset into each other will stack seamlessly with a 3.75 meter tank.

Adapters

  • 1:1 parts between the same size are not shown.
  • Note that all parts can be reversed so a nosecone can serve as a tail and vice versa.
  • Fairings (Airstream Protective Shells) can be placed inline and thus serve as adapters as well as nosecones.
Fore size
Nose Tiny Small, Mk1 Mk 2 Large Mk 3 Extra Large
Aft size Tail Tail Connector A
Tail Connector B
Mk3 Cargo Ramp
Tiny Small Nose Cone
CH-J3 Fly-By-Wire Avionics Hub
Stayputnik Mk. 1
Small, Mk1 Advanced Nose Cone - Type A
Advanced Nose Cone - Type B
AE-FF1 Airstream Protective Shell (1.25m)
Aerodynamic Nose Cone
Crew Mk1 Cockpit
FL-A10 Adapter
FL-A5 Adapter
units of fuel NCS Adapter
Crew Mk1 Command Pod
TVR-200 Stack Bi-Coupler (2)
TVR-1180C Mk1 Stack Tri-Coupler (3)
TVR-2160C Mk2 Stack Quad-Coupler (4)
units of fuel Mk2 Bicoupler (2) Rockomax Brand Adapter 02
TVR-200L Stack Bi-Adapter (2)
TVR-300L Stack Tri-Adapter (3)
TVR-400L Stack Quad-Adapter (4)
Mk2 Crew Mk2 Cockpit units of fuel Mk2 to 1.25m Adapter
units of fuel Mk2 to 1.25m Adapter Long
Large AE-FF2 Airstream Protective Shell (2.5m)
Protective Rocket Nose Cone Mk7
units of fuel C7 Brand Adapter - 2.5m to 1.25m
units of fuel C7 Brand Adapter Slanted - 2.5m to 1.25m
Rockomax Brand Adapter
Crew Mk1-2 Command Pod
Crew PPD-12 Cupola Module
units of fuel 2.5m to Mk2 Adapter
Mk3 Crew Mk3 Cockpit units of fuel Mk3 to Mk2 Adapter units of fuel Mk3 to 2.5m Adapter
units of fuel Mk3 to 2.5m Adapter Slanted
Extra Large AE-FF3 Airstream Protective Shell (3.75m) Kerbodyne ADTP-2-3 units of fuel Mk3 to 3.75m Adapter

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.