Fuel tank

A fuel tank is a dedicated part to contain of fuels and provide it to the appropriate reaction engines. 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 for 11 parts oxidizer
• Liquid fuel alone usage in oxygenated atmospheres
• Monopropellant without any oxidizer usually used for RCS
• Xenon gas for ion engines

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 directly attached to it. When tanks are directly connected together in a stack they will feed their contents from the top to the bottom of the stack. 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.

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.

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. The oxidizer/liquid fuel ratio is exact except for the Oscar-B Fuel Tank.

Their mass fully fuelled with liquid fuel and oxidizer is between 5.245 and 9.333 times higher than the dry mass. The fuel-containing Mk3 fuselages and adapters have the highest factor with 9.33 times, followed by the FL-T and Rockomax tanks at 9 times. Third are the Kerbodyne extra large tanks which have a wet mass 8.2 times that of their dry mass. The ROUND-8 Torodial Fuel Tank has a relatively low factor of only 5.44 times the dry mass and the Oscar-B Fuel Tank is the most inefficient with a mass factor of 5.245. The combined fuel and oxidizer mass compared to the dry mass is the factor reduced by one. Thus the mass of the contents in a FL-T or Rockomax tank is 8 times higher 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

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

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

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)

The Rockomax X200-16 Fuel Tank for example: Based on some rough in-game measurements it is approximately 0.67·1.25 meters tall with a radius of 1·1.25 meters (1.25 is the rescale factor!). This would give the tank/stage a real world volume of around 4110.9 liters (4.1109 m³). Dividing the tank's actual in-game fuel capacity (720 unit + 880 unit = 1600 unit) with this volume, the result is 389.2 unit/m³. If we count with 0.8 ratio for volumetric usage efficiency (real life example: 80% for the Saturn V's stages), then 1 unit = 2.0555 liters. 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. The tanks itself are also insulated to keep the cryogenic fuels from boiling away to quickly. As the tanks of the liquid fuels and solid fuels has better volumetric usage efficiency, we can estimate the volume of 1 unit is about 2-2.5 liters.

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

Notes

• Before v0.18 all fuel tanks provided fuel for both rocket engines and jet engines, because there was only one type of fuel recognized in game.

• Since v0.18 fuselages cannot be used to power rocket engines anymore, as they don't contain the necessary oxidizer.

See also

• Reaction engine
• Jet engine
• Liquid fuel
• Oxidizer
• Liquid hydrogen on Wikipedia
• Liquid oxygen on Wikipedia
• RP-1 on Wikipedia