fuel mass calculation

[draeath]

I've read that liquid fuel and oxidizer comes in at around 5kg per liter... but I have yet been unable to figure out what the relation between in-game fuel quantity and liters. Eg, if I right-click my tank and see those numbers - how do I go from that to liters? Is this actually consistent or does it seem to differ per tank?

I'm trying to do some number crunching to pre-plan things such as burn time and fuel expenditure for given delta-V values, and I think this is the last bit of knowledge that I need.

Alternatively I can give up on this part and not have any way to determine remaining/used fuel for a given maneuver, which isn't really necessary but it's certainly nice to know.

I am trying to reduce my dependence on Mechjeb's autopilots. It does give me fuel mass, vessel mass, and empty mass - but I have no way to ignore tanks/engines that would not be used during the burn - and it does not display them with the precision that I would like.

Edited January 9, 2014 by draeath

[sciguyCO]

I'm 95% sure that the numbers for fuel & oxidizer in the "resources" display and in the fuel right-click display is in Liters, regardless of tank. Plus that's what the wiki labels the tank capacity as.

I suppose one way to check would be to make a rocket with a command pod +FL-T100 + engine + launch clamps. Start by viewing the total fuel + oxidizer (should be 100) and initial mass (from the info box in map view). Fire the engine without releasing the clamps until you're out of fuel. If the {units} equals liters, then your final mass should be 0.5 tonnes less.

[draeath]

Ah, so if I'm ever in doubt about the actual capacity of a specific tank, I can just slap a pod on said tank, "launch" it, and view the resources display. Just so long as I remember the conversion "factor" for said tank, I can then easily determine exactly how many equivalent liters of fuel+oxy is in said tank.

[Smidge204]

The tanks have info in the parts menu that tell you how much fuel/ox they have, what their full mass is and what heir empty mass is. No need to launch anything.

=Smidge=

[sciguyCO]

Ah, so if I'm ever in doubt about the actual capacity of a specific tank, I can just slap a pod on said tank, "launch" it, and view the resources display. Just so long as I remember the conversion "factor" for said tank, I can then easily determine exactly how many equivalent liters of fuel+oxy is in said tank.

I'm not even sure launching is necessary. The fuel tanks should give that information in the VAB (maybe requiring a right-click in the part selection palette). It gives fuel and oxidizer separate, so they'll need to be added together.

The only thing I thought you were checking was whether those numbers were in liters or some other unit.

For most stock parts, you can even go by the tank's name. All the "FL-T###" tanks' capacity is their "###" value (so FL-T100 has 100 units total fuel+oxidizer, FL-T200 has 200, etc). The rockomax tanks are all "X200-#", where their capacity is 100 units per # (so the -8 has 800 units, -16 has 1600 units, etc).

[draeath]

The tanks have info in the parts menu that tell you how much fuel/ox they have, what their full mass is and what heir empty mass is. No need to launch anything.

=Smidge=

There are no units, and when I've looked up threads regarding them, I've found completely different results between tanks.

(eg not the full/dry mass, but the quantity value)

[draeath]

For most stock parts, you can even go by the tank's name. All the "FL-T###" tanks' capacity is their "###" value (so FL-T100 has 100 units total fuel+oxidizer, FL-T200 has 200, etc). The rockomax tanks are all "X200-#", where their capacity is 100 units per # (so the -8 has 800 units, -16 has 1600 units, etc).

What are these 'units' then? Also, I almost never use the stock tanks - so I'd need to know what unit the quantity is represented in. See the post just prior to this one.

[Dave Kerbin]

Liquid fuel+oxidizer has the same mass regardless of the container.

First there is the ratio of Liquid Fuel to Oxidizer, which is 9 parts liquid fuel to 11 parts oxidizer. Since you usually carry both (unless you are building a jet engine) I ignore this value.

Fuel:Oxidizer = 9:11

The next important value is mass. 200 L of combined fuel/oxidizer is equal to 1 ton, however we usually don't work with a combined figure. Looking at it from a Liquid Fuel perspective you have 90 L of liquid fuel (plus the required oxidizer) is 1 ton.

90 L Liquid Fuel = 1 ton (assuming matching oxidizer is included)

Finally you have the tank weight. With the exception of the two tiny tanks (the Round8 inner tube and the tiny Oscar) all the stock tanks have the same ratio of fuel to container mass, which is for every 8 tons of fuel there is 1 ton of metal container.

Fuel:Container mass = 8:1

I use these regularly since I've got Excel instead of Engineer redux, though I remember the ratios using slightly different numbers:

A 9 ton tank

Contains 8 tons of fuel

Which includes 720 L of liquid fuel

From those numbers it is easy to determine wet/dry values. For example if I write that I need 180 L of fuel, then I get a wet mass of 180/720*9 (2.25), and a dry mass of 180/720 (0.25).

For the Oscar and Round8 you just need to get the values from the VAB or Wiki. Both of them have worse fuel:tank ratios (about 5:1) and use very small non-rounded numbers for the fuel so they can be a pain to work with.

[draeath]

So the fuel quantities exposed in the VAB and right-clicking tanks appears to be in liters, going from your "200 = 1ton" figure above. Just to make sure my brain isn't, well, not braining... can you confirm if I'm correct on that?

[Dave Kerbin]

So the fuel quantities exposed in the VAB and right-clicking tanks appears to be in liters, going from your "200 = 1ton" figure above. Just to make sure my brain isn't, well, not braining... can you confirm if I'm correct on that?

Correct.

[Dave Kerbin]

Ironically I'm pretty sure all the units of measurement where shown in the old tooltips (before they made the right click for more info in .23) with the exception of the crash tolerance which was just a mysterious number to most people. Now the crash tolerance is the only figure that actually states the unit of measurement and everything else is a mysterious number.

[draeath]

Indeed. In the physics realm units are of dire importance, they really should go through and make sure units are, if not consistent, at least labeled everywhere.

[RadHazard]

Actually, there's been some debate as to whether the stock resource units are in fact liters, given how extraordinarily dense that would make liquid fuel and oxidizer compared to most known rocket fuels. There was a thread in the mod discussion board debating it when trying to come up with a standard for mod resources.

However, I can say for certainty that yes, 1 unit of liquid fuel weighs exactly 5 kg. Oxidizer weighs the same as liquid fuel.

[draeath]

Thank you guys very much!

The fruits of my (and thus yours, by proxy): FireMarshal!

[UmbralRaptor]

What are these 'units' then? Also, I almost never use the stock tanks - so I'd need to know what unit the quantity is represented in. See the post just prior to this one.

Arbitrary fuel units, called liters by convention. They could be moles, rather than something volumetric, but liters make sense (aside perhaps from density)...

So the fuel quantities exposed in the VAB and right-clicking tanks appears to be in liters, going from your "200 = 1ton" figure above. Just to make sure my brain isn't, well, not braining... can you confirm if I'm correct on that?

Yeah -- you can also see fuel, oxidizer, etc. densities in /KSP/GameData/Squad/Resources/ResourcesGeneric.cfg

[Dave Kerbin]

At the end of the day it doesn't actually matter what unit they are since the game doesn't those quantities in the physics simulation.

Instead the fuel and other quantities are multiplied by a constant (5kg for fuel and oxydizer) and added to the mass of the part before the physics engine gets a look at it. As far as the physics simulation is concerned a fuel tank is just a solid object that mysteriously changes mass between calculations, all the quantities in litres we have talked about are just window dressing. This becomes apparent when you start shifting fuel around on a spinning ship - you can actually change course without any apparent forces acting on the ship because the fuel is magic mass that can be swapped around without obeying the conservation of energy. Scott Manley (whose else) made a

demonstrating this effect a long time ago.