Fuel tank

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A fuel tank is a dedicated part to contain fuels and provide it to the appropriate reaction engines or consumers. In the stock game there are tanks for four different fuel mixtures:

The distribution through the rocket depends on the type of fuel. While xenon gas and monopropellant are available everywhere in the craft without additional plumbing, engines using liquid fuel and oxidizer needs to be attached to the tank with only fuel crossing parts between them.

Overview

A liquid fuel tank can provide fuel to any engine with a valid fuel delivery path; in most rockets, a fuel tank will provide fuel to any engine in its stage.

Monopropellant tanks and xenon tanks will provide fuel to any engine attached to the rocket, regardless of fuel delivery path.

Real world application

In real spacecraft, the liquid fuel is usually highly refined kerosene for the first stage and/or liquid hydrogen (LH2) for subsequent stages, and the oxidizer is usually liquid oxygen (LOX). Both the fuel and oxidizer 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 liquid fuel, as jet engines can rely solely on atmospheric oxygen as 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. Ore is a simplification for some kind of chemical which could be mined from the ground on other planets, and converted to a rocket fuel.

Advanced usage

Asparagus staging

→ Main article: Tutorial: Asparagus Staging

It is possible to mount additional fuel tanks radially with or without decouplers, and to transfer their contents to a central tank using fuel lines. This can be used to construct an asparagus staging system where outer tanks are drained first and dropped once empty, while still keeping the central tank full. This can be tricky to construct when dealing with very large rockets as the rocket engines of the outer layers have to be kept running long enough to ensure the rest of the rocket can get into orbit once they have stopped providing thrust.

Flow priority

If Advanced Tweakables are enabled, then fuel tanks will have a "Flow Priority" option. If an engine can draw fuel from multiple fuel tanks, then tanks with a higher flow priority will be emptied first, followed by tanks with a lower flow priority.

For example, if three rocket fuel tanks with flow priority 30, 0, and -10 are all attached to one LV-909 "Terrier" Liquid Fuel Engine, then the Terrier engine will use fuel from the tank with flow priority 30. Once this tank is depleted, then it will draw fuel from the tank with priority 0, followed by the tank with priority -10.

Adjusting flow priority can allow players to control how a rocket's mass distribution changes as it uses fuel. In addition, it can allow for rockets to use methods such as asparagus staging without the use of fuel lines.

List of tanks

Rocket fuel tanks

These tanks contain both liquid fuel and oxidizer and are designed for use with liquid fuel engines. They can also be used for jet engines, however liquid fuel tanks are preferable because they don't contain oxidizer, which is not used by jet engines.

Their mass fully fuelled with liquid fuel and oxidizer is between 8 and 9 times higher than the dry mass. The lower ratio of 8 applies to all fuel-containing Mk2 and Mk3 fuselages and adapters, along with the C7 brand adapters (2.5 m to 1.25 m). All other tanks have the higher ratio of 9. The combined fuel and oxidizer mass compared to the dry mass is this ratio reduced by one. Thus the mass of the contents in a Mk3 tank is 7 times greater than the dry mass.

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

(Units of fuel)
Oxidizer
(Units of fuel)
Image Part Radial size Cost
(Funds)
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
R-4 FT.png
R-4 'Dumpling' External Tank X 50
(39.90)
0.1238 0.0138 2 000 5 50 9.9 12.1
R-11 FT.png
R-11 'Baguette' External Tank X 50
(25.21)
0.3038 0.0338 2 000 5 50 24.3 29.7
R-12 FT.png
R-12 'Doughnut' External Tank Small 147
(119.46)
0.3375 0.0375 2 000 6 50 27 33
Oscar-B FT.png
Oscar-B Fuel Tank Tiny 70
(51.64)
0.225 0.025 2 000 6 50 18 22
FL-T100 Dark.png
FL-T100 Fuel Tank Small 150
(104.1)
0.5625 0.0625 2 000 6 50 45 55
FL-T200 Dark.png
FL-T200 Fuel Tank Small 275
(183.2)
1.125 0.125 2 000 6 50 90 110
FL-T400 Dark.png
FL-T400 Fuel Tank Small 500
(316.4)
2.25 0.25 2 000 6 50 180 220
FL-T800 Dark.png
FL-T800 Fuel Tank Small 800
(432.8)
4.5 0.5 2 000 6 50 360 440
X200-8 White.png
Rockomax X200-8 Fuel Tank Large 800
(432.8)
4.5 0.5 2 000 6 50 360 440
X200-16 White.png
Rockomax X200-16 Fuel Tank Large 1 550
(815.6)
9 1 2 000 6 50 720 880
X200-32 White.png
Rockomax X200-32 Fuel Tank Large 3 000
(1 531.2)
18 2 2 000 6 50 1 440 1 760
Jumbo-64 White.png
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.png
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.png
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.png
Kerbodyne S3-14400 Tank Extra large 13 000
(6 390.4)
81 9 2 000 6 50 6 480 7 920
Mk2 LF+O Fuselage Short.png
Mk2 Rocket Fuel Fuselage Short Mk2 750
(566.4)
2.29 0.29 2 500 50 50 180 220
Mk2 LF+O Fuselage.png
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.png
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.png
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.png
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.png
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.png
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.png
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.png
Mk2 to 1.25m Adapter Long Small, Mk2 1 050
(682.8)
4.57 0.57 2 500 50 50 360 440
Mk2 Bicoupler.png
Mk2 Bicoupler Small, Mk2 x 2 860
(676.4)
2.29 0.29 2 500 50 50 180 220
2.5m to Mk2 Adapter.png
2.5m to Mk2 Adapter Large, Mk2 800
(432.8)
4.57 0.57 2 500 50 50 360 440
Mk3 to Mk2 Adapter.png
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.png
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.png
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.png
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
ADTP-2-3 Gray.png
Kerbodyne ADTP-2-3 Large, Extra large 1 623
(246.0)
16.88 1.88 2 000 6 50 1 350 1 650


Liquid fuel tanks

Liquid fuel tanks can only be used with jet engines and the LV-N "Nerv" Atomic Rocket Motor, as they don't contain the oxidizer required to operate rocket engines.

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

(Units of fuel)
Image Part Radial size Cost
(Funds)
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Engine Nacelle.png
Engine Nacelle[Note 1] Small 600
(480)
0.925 0.15 2 000 10 50 150
Engine Pre-cooler.png
Engine Pre-cooler[Note 1] Small 1 650
(1 618)
0.375 0.15 2 000 10 50 40
Mk1 Fuselage-Intake.png
Mk1 Diverterless Supersonic Intake[Note 1] Small 720
(560)
1.18 0.17 2 000 10 50 200
Mk0LiquidFuelFuselage.png
Mk0 Liquid Fuel Fuselage Tiny 200
(160)
0.275 0.025 2 000 10 50 50
Mk1 Liquid Fuel Fuselage.png
Mk1 Liquid Fuel Fuselage Small 550
(230)
2.25 0.25 2 000 10 50 400
Mk2 Liquid Fuselage Short.png
Mk2 Liquid Fuel Fuselage Short Mk2 750
(430)
2.29 0.29 2 500 50 50 400
Mk2 FT.png
Mk2 Liquid Fuel Fuselage Mk2 1 450
(810)
4.57 0.57 2 500 50 50 800
Mk3 Liquid Fuel Fuselage Short.png
Mk3 Liquid Fuel Fuselage Short Mk3 4 300
(2 300)
14.29 1.79 2 700 50 50 2 500
Mk3 Liquid Fuel Fuselage.png
Mk3 Liquid Fuel Fuselage Mk3 8 600
(4 600)
28.57 3.57 2 700 50 50 5 000
Mk3 Liquid Fuel Fuselage Long.png
Mk3 Liquid Fuel Fuselage Long Mk3 17 200
(9 200)
57.14 7.14 2 700 50 50 10 000
NCS Adapter.png
NCS Adapter Small, Tiny 320
(256)
0.5 0.1 2 400 10 50 80
FAT455AirplaneWing.png
FAT-455 Aeroplane Main Wing[Note 2] X 2 800
(2 320)
3.78 0.78 1 200 15 50 600
BigSWingStrake.png
Big-S Wing Strake[Note 2] X 1 000
(920)
0.6 0.1 2 400 15 50 100
BigSDeltaWing.png
Big-S Delta Wing[Note 2] X 3 000
(2 760)
2.0 0.5 2 400 15 50 300
  1. 1.0 1.1 1.2 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. 2.0 2.1 2.2 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.


Monopropellant fuel tanks

→ Main article: Monopropellant

Monopropellant tanks store monopropellant, which is consumed by RCS maneuvering thrusters.

In real life, the term "monopropellant" is used to refer to fuels that do not require additional reactants to effect energy: although liquid fuel requires oxidizer to burn to produce thrust, monopropellants require no additional chemicals to work. A well-known real life monopropellant is hydrazine.

RCS Fuel Density is 4 kg/unit Mass
(t)
Monopropellant
(Units of fuel)
Image Part Radial size Cost
(Funds)
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
FL-R10.png
FL-R20 RCS Fuel Tank Tiny 200
(176)
0.10 0.02 2 000 12 50 20
FL-R25 FT.png
FL-R120 RCS Fuel Tank Small 330
(186)
0.56 0.08 2 000 12 50 120
FL-R1 Yellow.png
FL-R750 RCS Fuel Tank Large 1 800
(900)
3.4 0.4 2 000 12 50 750
Mk2 Monopropellant Tank.png
Mk2 Monopropellant Tank Mk2 750
(270)
1.89 0.29 2 500 50 50 400
Mk3 Monopropellant Tank.png
Mk3 Monopropellant Tank Mk3 5 040
(2 520)
9.8 1.4 2 700 50 50 2 100
Stratus-v roundified monopropellant tank.png
Stratus-V Roundified Monopropellant Tank X 200
(176)
0.10 0.02 2 000 12 50 20
Stratus-V Cylindrified.png
Stratus-V Cylindrified Monopropellant Tank X 250
(190)
0.23 0.03 2 000 12 50 50


Xenon gas tanks

→ Main article: Xenon gas

Xenon gas tanks store xenon gas, which can be used by the IX-6315 "Dawn" Electric Propulsion System.

Xenon density is 0.1 kg/unit Mass
(t)
Xenon
(Xenon unit)
Image Part Radial size Cost
(Funds)
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
PB-X50R.png
PB-X50R Xenon Container Radial mounted 2 220
(600)
0.054 0.014 2 000 12 50 405
Pb-x150 xenon container.png
PB-X150 Xenon Container Tiny 3 680
(800)
0.100 0.024 2 000 6 50 720
PB-X750 Xenon Container.png
PB-X750 Xenon Container Small 24 300
(1 500)
0.76 0.19 2 000 6 50 5 700


Ore tanks

→ Main article: Ore

Ore tanks can be used to hold ore mined by the 'Drill-O-Matic' Mining Excavator or the 'Drill-O-Matic Junior' Mining Excavator, to be processed in a converter. By default, any ore tanks on a craft will start empty.

Ore density is 10 kg/unit Mass
(t)
Ore
(Units of fuel)
Image Part Radial size Cost
(Funds)
Full Empty Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
RadialHoldingTank.png
Radial Holding Tank Radial mounted 299
(300)[Note 1]
0.875 0.125 2 000 7 50 75
Small Holding Tank.png
Small Holding Tank Small 994
(1000)[Note 1]
3.5 0.5 2 000 7 50 300
Large Holding Tank.png
Large Holding Tank Large 2 970
(3 000)[Note 1]
17.0 2.0 2 000 7 50 1 500
  1. 1.0 1.1 1.2 Unlike the fuel tanks, these tanks are empty by default, and can be manually filled during vehicle assembly.


Dry mass and wet mass

In the part configuration the dry mass is given and the resources are given in a volumetric unit. To calculate the mass of the fuel and the mass fully fueled, wet mass, the density is also required. In general the mass of one type of fuel is:

Where:
  • is the mass of the fuel
  • is the volume of the fuel
  • is the density of the fuel

The total mass of the fuel is then simply the sum of all the fuels in the tank. And the wet mass is then the fuel masses added the dry mass.

Where:
  • is the fuel mass
  • and are the wet and dry mass
  • and are the volume and density of each fuel

Because all stock tanks, except the liquid fuel and oxidizer tanks, use only one type of fuel the sum of the different fuels in the tank is not required. And because the liquid fuel and oxidizer have the same density, the volumes can be added before converted into mass.

Where:
  • is the fuel mass of the liquid fuel/oxidizer mixture
  • is the wet mass of a tank with liquid fuel/oxidizer mixture
  • is the dry mass
  • and are the volume of liquid fuel and oxidizer
  • is the density of liquid fuel and oxidizer (they are the same)

Volume and density

Note: in Kerbal Space Program all fuels have different densities in different tanks, which can be attributed to balancing and the need for nice-looking fuel capacity numbers.

Volume examples

The volume of a cylindrical fuel tank can be calculated from this formula:

Where:
  • is the volume in litres
  • is the height in meters
  • is the radius in meters

The Rockomax X200-16 Fuel Tank has a height of 1.84 meters tall with a radius of 1.25 meters according to its config file. This translates to a total cylindrical volume (not accounting for the casing and empty space) of 9032.1 litres (9.0321 m³). The FL-T200 Fuel Tank, being 1.1105 meters tall and having a radius of 0.625 meters, has a cylindrical volume of 1362.8 litres (1.3628 m³),

Dividing the in-game fuel capacity by the volume results in 177.15 units/m³ for the Rockomax X200-16 and 146.78 units/m³ for the FL-T200.

Estimating actual density

As for fuel density, we should assume a volumetric usage efficiency. The reason the entire volume is not used is because the actual fuel tank is a pressure vessel with rounded ends inside a cylindrical stage with thick walls. The tanks themselves can also be insulated to keep the cryogenic fuels from boiling away too quickly.

A real life example is 80% volumetric usage in Saturn V's stages. We use this formula:

Where:
  • is the estimated fuel density in litres per unit
  • is the volumetric usage (80% in our case)
  • is the fuel tank volume in litres
  • is the total in-game fuel capacity in units

For the Rockomax X200-16 this is 4.516 litres/unit, while for the FL-T200 it is 5.451 litres/unit. While this is similar with other fuel tanks, a notable exception is the 'Doughnut' toroidal tank having less-dense fuel: 3.75 l/u at 100% volumetric usage or 3.41 l/u at 80%.

Comparison

The table does only contain tanks, which contain both liquid fuel and oxidizer. The height is units, where one unit is the height of the FL-T100 Fuel Tank.

Height (units) 1 2 4 8 16
Small FL-T100 Fuel Tank FL-T200 Fuel Tank FL-T400 Fuel Tank FL-T800 Fuel Tank
Large Rockomax X200-8 Fuel Tank Rockomax X200-16 Fuel Tank Rockomax X200-32 Fuel Tank Rockomax Jumbo-64 Fuel Tank
Extra large Kerbodyne S3-3600 Tank Kerbodyne S3-7200 Tank Kerbodyne S3-14400 Tank

Trivia

  • Before v0.18, both rocket engines and jet engines only used liquid fuel; this meant that all fuel tanks could provide fuel for both types of engines. Since v0.18, rocket engines use oxidizer, so liquid fuel tanks lacking the necessary oxidizer cannot be used to power rocket engines anymore.

See also