Fuel tank

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:

• Liquid fuel and oxidizer in the usual ratio of 9 parts liquid fuel to 11 parts oxidizer. Used by Liquid Fuel Engines
• Liquid fuel alone, for use by jet engines in oxygenated atmospheres, or by the "Nerv" Atomic Rocket Motor
• Monopropellant (without any oxidizer) usually used for RCS
• Xenon gas for ion engines
• Ore; while not a fuel mixture, it can be processed into liquid fuel, liquid fuel and oxidizer, or monopropellant with a converter

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 ()	Oxidizer ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	R-4 'Dumpling' External Tank	X	50 (39.90)	0.1238	0.0138	2 000	5	50	9.9	12.1
	R-11 'Baguette' External Tank	X	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

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 ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	Engine Nacelle	Small	600 (480)	0.925	0.15	2 000	10	50	150
	Engine Pre-cooler	Small	1 650 (1 618)	0.375	0.15	2 000	10	50	40
	Mk1 Diverterless Supersonic Intake	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	X	2 800 (2 320)	3.78	0.78	1 200	15	50	600
	Big-S Wing Strake	X	1 000 (920)	0.6	0.1	2 400	15	50	100
	Big-S Delta Wing	X	3 000 (2 760)	2.0	0.5	2 400	15	50	300

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 ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	FL-R20 RCS Fuel Tank	Tiny	200 (176)	0.10	0.02	2 000	12	50	20
	FL-R120 RCS Fuel Tank	Small	330 (186)	0.56	0.08	2 000	12	50	120
	FL-R750 RCS Fuel Tank	Large	1 800 (900)	3.4	0.4	2 000	12	50	750
	Mk2 Monopropellant Tank	Mk2	750 (270)	1.89	0.29	2 500	50	50	400
	Mk3 Monopropellant Tank	Mk3	5 040 (2 520)	9.8	1.4	2 700	50	50	2 100
	Stratus-V Roundified Monopropellant Tank	X	200 (176)	0.10	0.02	2 000	12	50	20
	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 ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	PB-X50R Xenon Container	Radial mounted	2 220 (600)	0.054	0.014	2 000	12	50	405
	PB-X150 Xenon Container	Tiny	3 680 (800)	0.100	0.024	2 000	6	50	720
	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 ()
Image	Part	Radial size	Cost ()	Full	Empty	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)
	Radial Holding Tank	Radial mounted	299 (300)	0.875	0.125	2 000	7	50	75
	Small Holding Tank	Small	994 (1000)	3.5	0.5	2 000	7	50	300
	Large Holding Tank	Large	2 970 (3 000)	17.0	2.0	2 000	7	50	1 500

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:

$m_{\text{fuel}}=V_{\text{fuel}}\cdot \rho _{\text{fuel}}$

Where:

• $m_{\text{fuel}}$ is the mass of the fuel
• $V_{\text{fuel}}$ is the volume of the fuel
• $\rho _{\text{fuel}}$ 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.

${\begin{aligned}m_{\text{fuel}}&=\sum \limits _{i}(V_{i}\cdot \rho _{i})\\m_{\text{wet}}&=m_{\text{dry}}+m_{\text{fuel}}=m_{\text{dry}}+\sum \limits _{i}(V_{i}\cdot \rho _{i})\end{aligned}}$

Where:

• $m_{\text{fuel}}$ is the fuel mass
• $m_{\text{wet}}$ and $m_{\text{dry}}$ are the wet and dry mass
• $V_{i}$ and $\rho _{i}$ 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.

${\begin{aligned}m_{\text{fuel lf/ox}}&=(V_{\text{lf}}+V_{\text{ox}})\cdot \rho _{\text{lf/ox}}=(V_{\text{lf}}+V_{\text{ox}})\cdot 0.005{\frac {\text{t}}{\text{unit}}}\\m_{\text{wet lf/ox}}&=m_{\text{dry}}+m_{\text{fuel lf/ox}}=m_{\text{dry}}+(V_{\text{lf}}+V_{\text{ox}})\cdot 0.005{\frac {\text{t}}{\text{unit}}}\end{aligned}}$

Where:

• $m_{\text{fuel lf/ox}}$ is the fuel mass of the liquid fuel/oxidizer mixture
• $m_{\text{wet lf/ox}}$ is the wet mass of a tank with liquid fuel/oxidizer mixture
• $m_{\text{dry}}$ is the dry mass
• $V_{\text{lf}}$ and $V_{\text{ox}}$ are the volume of liquid fuel and oxidizer
• $\rho _{\text{lf/ox}}$ 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:

$V_{l}=h\cdot \pi \cdot r^{2}\cdot 1000$

Where:

• $V_{l}$ is the volume in litres
• $h$ is the height in meters
• $r$ 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:

$\rho _{\text{est}}=U_{\text{vol}}\cdot {\frac {V_{l}}{C}}$

Where:

• $\rho _{\text{est}}$ is the estimated fuel density in litres per unit
• $U_{\text{vol}}$ is the volumetric usage (80% in our case)
• $V_{l}$ is the fuel tank volume in litres
• $C$ 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

• Reaction engine
• Jet engine
• Liquid fuel
• Oxidizer
• ore
• Converter
• In-situ resource utilization on Wikipedia
• Liquid hydrogen on Wikipedia
• Liquid oxygen on Wikipedia
• RP-1 on Wikipedia