# Fuel tank

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A liquid fuel tank holds liquid fuel required for powering rocket engines and jet engines.

## Overview

A liquid fuel tank can provide fuel to any engine directly attached to it. When tanks are directly connected together in a stack they will feed fuel from the top tank first, automatically switching to the next tank as they empty. You can mount additional fuel tanks with radial decouplers, and the use fuel lines to provide fuel into the central tanks. This can be used to provide an asparagus staging system where outer tanks are drained first, then dumped leaving central tanks completely full. This can be tricky to get set up when dealing with very large rockets as the rocket engines of the outer layers need to be kept running long enough to ensure the rest of the rocket can get into orbit once they have stopped providing thrust.

### Real world application

In real spacecraft, the fuel tanks store both fuel, usually highly refined kerosene for the first stage and/or liquid hydrogen (LH2) for subsequent stages, and an oxidizing agent, such as liquid oxygen (LOX). Both the fuel and oxidizing agent are stored under slight pressure in special fuel tanks to keep them from vaporizing; this is because LH2 and LOX require temperatures below 20.28 K (−252.87 °C, −423.17 °F) and 90.19 K (−182.96 °C, −297.33 °F) respectively to remain in a liquid state. In comparison, jet aircraft carry only fuel as real jet engines can rely solely on atmospheric oxygen as an oxidizer. Most jet engines operate using a refined kerosene hydro-carbon, similar to diesel, which can actually be used in some diesel engines. However, it is not recommended, or legal in some places, to use jet fuel as a replacement for diesel due to its high lead content and lesser lubricating ability.

## Dedicated fuel tanks

Liquid fuel and oxidizer density are both 5 kg/unit Mass
(t)
Liquid
Fuel

()
Oxidizer
()
Image Part Radial size Cost
()
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
R-4 'Dumpling' External Tank Radial mounted 50
(39.90)
0.1238 0.0138 2 000 5 50 9.9 12.1
R-11 'Baguette' External Tank Radial mounted 50
(25.21)
0.3038 0.0338 2 000 5 50 24.3 29.7
R-12 'Doughnut' External Tank Small 147
(119.46)
0.3375 0.0375 2 000 6 50 27 33
Oscar-B Fuel Tank Tiny 70
(51.64)
0.225 0.025 2 000 6 50 18 22
FL-T100 Fuel Tank Small 150
(104.1)
0.5625 0.0625 2 000 6 50 45 55
FL-T200 Fuel Tank Small 275
(183.2)
1.125 0.125 2 000 6 50 90 110
FL-T400 Fuel Tank Small 500
(316.4)
2.25 0.25 2 000 6 50 180 220
FL-T800 Fuel Tank Small 800
(432.8)
4.5 0.5 2 000 6 50 360 440
Rockomax X200-8 Fuel Tank Large 800
(432.8)
4.5 0.5 2 000 6 50 360 440
Rockomax X200-16 Fuel Tank Large 1 550
(815.6)
9 1 2 000 6 50 720 880
Rockomax X200-32 Fuel Tank Large 3 000
(1 531.2)
18 2 2 000 6 50 1 440 1 760
Rockomax Jumbo-64 Fuel Tank Large 5 750
(2 812.4)
36 4 2 000 6 50 2 880 3 520
Kerbodyne S3-3600 Tank Extra large 3 250
(1 597.6)
20.25 2.25 2 000 6 50 1 620 1 980
Kerbodyne S3-7200 Tank Extra large 6 500
(3 195.2)
40.5 4.5 2 000 6 50 3 240 3 960
Kerbodyne S3-14400 Tank Extra large 13 000
(6 390.4)
81 9 2 000 6 50 6 480 7 920
Mk2 Rocket Fuel Fuselage Short Mk2 750
(566.4)
2.29 0.29 2 500 50 50 180 220
Mk2 Rocket Fuel Fuselage Mk2 1 450
(1 082.8)
4.57 0.57 2 500 50 50 360 440
Mk3 Rocket Fuel Fuselage Short Mk3 2 500
(1 352.5)
14.29 1.79 2 700 50 50 1 125 1 375
Mk3 Rocket Fuel Fuselage Mk3 5 000
(2 705)
28.57 3.57 2 700 50 50 2 250 2 750
Mk3 Rocket Fuel Fuselage Long Mk3 10 000
(5 410)
57.14 7.14 2 700 50 50 4 500 5 500
C7 Brand Adapter - 2.5m to 1.25m Small, Large 800
(433.0)
4.57 0.57 2 300 20 50 360 440
C7 Brand Adapter Slanted - 2.5m to 1.25m Small, Large 800
(433.0)
4.57 0.57 2 300 20 50 360 440
Mk2 to 1.25m Adapter Small, Mk2 550
(366.4)
2.29 0.29 2 500 50 50 180 220
Mk2 to 1.25m Adapter Long Small, Mk2 1 050
(682.8)
4.57 0.57 2 500 50 50 360 440
Mk2 Bicoupler Small, Mk2 x 2 860
(676.4)
2.29 0.29 2 500 50 50 180 220
2.5m to Mk2 Adapter Large, Mk2 800
(432.8)
4.57 0.57 2 500 50 50 360 440
Mk3 to Mk2 Adapter Mk2, Mk3 2 200
(1 282.0)
11.43 1.43 2 600 50 50 900 1 100
Mk3 to 2.5m Adapter Large, Mk3 2 500
(1 353.0)
14.29 1.79 2 600 50 50 1 125 1 375
Mk3 to 2.5m Adapter Slanted Large, Mk3 2 500
(1 353.0)
14.29 1.79 2 600 50 50 1 125 1 375
Mk3 to 3.75m Adapter Extra large, Mk3 2 500
(1 353.0)
14.29 1.79 2 600 50 50 1 125 1 375
Kerbodyne ADTP-2-3 Large, Extra large 1 623
(246.0)
16.88 1.88 2 000 6 50 1 350 1 650

Note: Not only are the above four dedicated fuel tanks, but there is also a selection of fuselage parts which serve to hold fuel. These fuselage parts are intended for space planes and/or shuttles, but can be used as part of normal rocket if desired (see fuselage fuel tanks)

## Fuselage fuel tanks

Liquid Fuel Density is 5 kg/unit Mass
(t)
Liquid
Fuel

()
Image Part Radial size Cost
()
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Engine Nacelle[Note 1] Small 600
(480)
0.925 0.15 2 000 10 50 150
Engine Pre-cooler[Note 1] Small 1 650
(1 618)
0.375 0.15 2 000 10 50 40
Mk1 Diverterless Supersonic Intake[Note 1] Small 720
(560)
1.18 0.17 2 000 10 50 200
Mk0 Liquid Fuel Fuselage Tiny 200
(160)
0.275 0.025 2 000 10 50 50
Mk1 Liquid Fuel Fuselage Small 550
(230)
2.25 0.25 2 000 10 50 400
Mk2 Liquid Fuel Fuselage Short Mk2 750
(430)
2.29 0.29 2 500 50 50 400
Mk2 Liquid Fuel Fuselage Mk2 1 450
(810)
4.57 0.57 2 500 50 50 800
Mk3 Liquid Fuel Fuselage Short Mk3 4 300
(2 300)
14.29 1.79 2 700 50 50 2 500
Mk3 Liquid Fuel Fuselage Mk3 8 600
(4 600)
28.57 3.57 2 700 50 50 5 000
Mk3 Liquid Fuel Fuselage Long Mk3 17 200
(9 200)
57.14 7.14 2 700 50 50 10 000
NCS Adapter Small, Tiny 320
(256)
0.5 0.1 2 400 10 50 80
FAT-455 Aeroplane Main Wing[Note 2] Radial mounted 2 800
(2 320)
3.78 0.78 1 200 15 50 600
Big-S Wing Strake[Note 2] Radial mounted 1 000
(920)
0.6 0.1 2 400 15 50 100
Big-S Delta Wing[Note 2] Radial mounted 3 000
(2 760)
2.0 0.5 2 400 15 50 300
1. The Engine Nacelle, Engine Pre-cooler, and Mk1 Diverterless Supersonic Intake are a combination of air intake and liquid fuel tank. Only the tank properties are shown. They are located in the "Aerodynamic" category in the game.
2. The FAT-455 Aeroplane Main Wing, Big-S Wing Strake, and Big-S Delta Wing are a combination of lifting surface and liquid fuel tank. Only the tank properties are shown. They are located in the "Aerodynamic" category in the game.

Note: In general, fuselage fuel tanks weigh less and hold notably less fuel when compared to dedicated fuel tanks, but can be used to power most jet engines for more than a few minutes. On the other hand, a rocket engine will empty a fuselage fuel tank in mere seconds!

## Modder's resource

The fuel/dry/full mass of dedicated fuel tanks are associated with the following formulas. This assume that the game values are given in liters (1 L = 1 dm³ = 0.001 m³) and metric tons (1 t = 1 Mg = 1000 kg).

${\displaystyle {\text{mass}}_{\text{fuel}}=({\text{fuel}}+{\text{oxidizer}})\cdot 0.005{\frac {\text{t}}{\text{l}}}}$

${\displaystyle {\text{mass}}_{\text{dry}}={\text{mass}}_{\text{full}}-{\text{mass}}_{\text{fuel}}={\text{mass}}_{\text{full}}-({\text{fuel}}+{\text{oxidizer}})\cdot 0.005{\frac {\text{t}}{\text{l}}}}$

Take the Rockomax X200-16 Fuel Tank for example. Based on some rough in-game measurements it is approximately 0.67 meters tall with a radius of 1 meter. This would give the tank/stage a real world volume of around 2094.4 liters (2.0944 m³). Dividing the tank's actual in-game fuel capacity (0.72 m³ + 0.88 m³ = 1.6 m³) by its total volume we can find what percentage of the volume is used for fuel, around 76.39%, very close to the roughly 80% for the Saturn V's stages. The reason the entire volume is not used is because the actual fuel tank is a pressure vessel with rounded ends inside the cylindrical stage.

## Notes

• As of v0.17 all fuel tanks still provide fuel for both rocket engines and jet engines, being that there is currently only one type of fuel recognized in game.
• It should be noted though that RCS uses its own type of fuel that is separate to Rocket/Jet fuel.