Liquid fuel density

[KingPotato]

Hello, fellow kerbonauts!

I was playing kerbal space program the other day (surprising, huh?) where I sent an unmanned probe to the Mün.

When I landed there, I noticed that not much fuel was left. In order to know whether I would be able to get it safely back to Kerbin, I needed to know how much delta velocity the ship had.

As I like finding out how to calculate things myself, rather than just trying out or using an existing formula, I started doing some research on how I should calculate everything required to find out delta velocity. This included properties of a variety of rocket parts, and what specific impulse is. Everything went well, until the formula I made for calculating how long I could burn with the amount of fuel gave an invalid, unrealisic answer.

I could not figure out where the issue was, and started doing some tests on the launchpad, while observing different factors I used in the formulae. One of these, was the different mass of the spaceship relative to the amount of fuel inside. For the calculation of fuel mass, I used the information the KSP wiki gave me, including the density of liquid fuel and oxidiser (both of which appeared to be

ÃÂ=5 kg*L^-1). However, the fuel mass I found did not match those of the tests I performed. I googled some more, and other than the wiki, the actual density was nowhere to be found on the web.

So here's my question: Am I just overseeing something, and just being an idiot?

Or is the density stated on the wiki actually wrong? If so, can someone give me the correct density or a different way to find the wet mass and dry mass of a fuel tank (other than using addons e.g. mechjeb)?

Thanks in advance for your answers.

P.S. If any information required to answer this question is lacking or insufficient, please just say so in the comments.

P.S.S. I did not use any mods.

Edited January 17, 2014 by KingPotato

[WafflesToo]

200 units per ton SHOULD be the correct value, though I've noticed something screwy with the game's mass reporting lately. I wonder if they changed something?

[UmbralRaptor]

The ResourcesGeneric.cfg file still shows A density of .005 tonne/l (5 kg/l). Do you have any fuel-related mods, and what was your testing method?

[Smidge204]

I just use a ratio of total initial fuel mass to current fuel mass.

For example, the FL-T400 Fuel Tank holds 180 units of fuel and a complimentary amount of oxidizer. The full weight is 2.25 tons and the empty weight is 0.25 tons, meaning 180 units of fuel (and complimentary amount of oxidizer) is 2 tons.

If I have 30 units of fuel (and complimentary amount of oxidizer) left, I simply multiply 2 tons by 30/180 to get 0.333 tons of fuel remaining. That means my fuel tank with 30 units of fuel (and complimentary amount of oxidizer) has a dry mass of 0.25 tons and a wet mass of 0.585 tons.

No need to worry about the fuel density since that's implicit in the calculations, and heaven forbid they DO change something in the future...

The only time this might trip you up is if, for whatever reason, you do not have complimentary amounts of fuel and oxidizer. Then you have to break it down further and the math gets a little messier.

=Smidge=

[sciguyCO]

Well, the wet and dry mass values are given on the tanks in the VAB. I have always assumed that the fuel weight was the difference between those, and using that and the amount of Fuel+Oxidizer does give you the 5kg / L value (under the assumption that the fuel units are Liters, which may not be explicitly stated anywhere).

I set up a similar equation to what you describe, here's what I use:

Inputs:

• M: Ship's *current* mass (from map view, click the "i" button)
  
• F: Ship's current fuel + oxidizer (from resources tab) in "liters"
  
• I: Engine's ISP, From the engine's stats.
  

dV = I * 9.8 * ln(M / (M - (F * 0.005) ) )

That seems to give me reasonable numbers though I haven't verified against something like Kerbal Engineer. I used that calculation during a self-imposed ban on the flight engineer part (though I still used the one the VAB) to improve my non-instrument piloting.

What kind of numbers are you seeing in your testing? Are you using the ship mass info from the info screen, or something else?

[capi3101]

Five kilograms per unit is the correct density for both the liquid fuel and the oxidizer; to find their combined mass, add the two together and divide by 200. For all liquid fuel tanks except for the Oscar-B and Round 8, the ratio of liquid fuel to oxidizer is 9:11.

So what you do is take your current mass, stick it in the rocket equation as your total mass. Subtract your calculated fuel mass from your current mass - the difference is your dry mass. Solve as normal for your current engine combo.

How about it - what is your rocket's current mass (from the map screen), its current liquid fuel and oxidizer levels, and what kind of engines is it using?

EDIT: Kashua's right - it's five kilograms per unit. My bad...will fix that real quick so I can confuse the folks who post on this thread in the future.

Edited January 17, 2014 by capi3101

[Kasuha]

Liquid fuel / oxidizer unit is 5 kilograms. The unit is not litre. Fuel has no defined volume and there is no need to define it. All you need to know is its weight.

[Soda Popinski]

I also found my fuel took longer to burn than stated based on the wiki. I suspect the fuel usage may include both fuel and oxidizer.

Anybody know which one it is?

[KingPotato]

To all of those who said that the unit used for liquid fuel is undefined, I just presumed it was in litres because the liquid fuel tanks section on http://wiki.kerbalspaceprogram.com/wiki/Parts gives you the fuel and oxidiser capacity in (L).

With that information, I just put the amount of fuel I had at the time into the formula ÃÂ=m/V and turned it into m=ÃÂV.

In case the wiki is actually wrong, this should solve my problem, but I'm still not sure...

Well, the wet and dry mass values are given on the tanks in the VAB. I have always assumed that the fuel weight was the difference between those, and using that and the amount of Fuel+Oxidizer does give you the 5kg / L value (under the assumption that the fuel units are Liters, which may not be explicitly stated anywhere).

I set up a similar equation to what you describe, here's what I use:

Inputs:

• M: Ship's *current* mass (from map view, click the "i" button)
  
• F: Ship's current fuel + oxidizer (from resources tab) in "liters"
  
• I: Engine's ISP, From the engine's stats.
  

dV = I * 9.8 * ln(M / (M - (F * 0.005) ) )

That seems to give me reasonable numbers though I haven't verified against something like Kerbal Engineer.

This is actually the same formula used here: http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet

Because I intended the whole calculation I was doing, to be created by myself, I didn't want to use anything similar to that, seeing as how I haven't had logarithms in school yet

Edited January 17, 2014 by Specialist290
Merging sequential posts.

[Starwaster]

I also found my fuel took longer to burn than stated based on the wiki. I suspect the fuel usage may include both fuel and oxidizer.

Anybody know which one it is?

Resource consumption is all resources/propellants listed in the parts engine module and is the amount required to produce the amount of thrust specified based on the engines Isp (AtmosphereCurve)

This is pretty much how it works in real life with one important exception.

In real life thrust is variable according to atmospheric pressure as determined by Isp while consumption stays constant. In this game however thrust stays constant and fuel consumption is increased at sea level to compensate. This last point is likely responsible for any discrepancy you've noticed. The mod Real Fuels provides a more realistic experience where thrust is reduced at sea level increasing as you get closer to vacuum.

[tavert]

The unit is not litre. Fuel has no defined volume and there is no need to define it. All you need to know is its weight.

Could be liters. Used to be explicitly labeled as liters. Leads to some unrealistic densities, but so does everything else in KSP.

[Starwaster]

Could be liters. Used to be explicitly labeled as liters. Leads to some unrealistic densities, but so does everything else in KSP.

When you look at the size of the tank parts and apply some math to determine volume, it's pretty apparent that it can't be liters.

To really make the tank volumes and resource densities metric requires increasing tank volume and decreasing resource density. (assuming liquid resources such as LOX, LH2 or RP1. forget how this affects compressed gaseous products)

This is currently done with the Real Fuels mod which does indeed assume liters. RF is based on Modular Fuel Tanks, earlier incarnations assumed a base of 6.25 liters, which would make 1 volume unit 6.25 m3 or 6.25 kl.

[Kasuha]

Could be liters. Used to be explicitly labeled as liters. Leads to some unrealistic densities, but so does everything else in KSP.

Leads to unrealistic densities and unrealistic utilization of fuel tank internal space. Besides, volume unit for the fuel is irrelevant. Everything in the game, including eventual fuel tank buoyancy in water comes from fuel mass, not volume. Parts don't grow bigger by volume of fuel in them.

Come on, just because Squad took liter as a placeholder name of fuel unit initially does not mean we need to stick to it forever.

[tavert]

When you look at the size of the tank parts and apply some math to determine volume, it's pretty apparent that it can't be liters.

To really make the tank volumes and resource densities metric requires increasing tank volume and decreasing resource density. (assuming liquid resources such as LOX, LH2 or RP1. forget how this affects compressed gaseous products)

Leads to unrealistic densities and unrealistic utilization of fuel tank internal space. Besides, volume unit for the fuel is irrelevant. Everything in the game, including eventual fuel tank buoyancy in water comes from fuel mass, not volume. Parts don't grow bigger by volume of fuel in them.

Come on, just because Squad took liter as a placeholder name of fuel unit initially does not mean we need to stick to it forever.

You guys appear to have missed my point about densities. There's no point in trying to apply real-life fuel densities and calculating internal volumes when we're taking off from a planet that is several times denser than any known element. If you're using realism mods then it matters, but not in the stock game.

I agree with Kasuha that the volume unit is irrelevant, if the resource units are even supposed to be measuring volume. It's quite arbitrary, but calling them liters and assuming resource units are in terms of volume is for the most part inherited from older versions of the game.

[kahlzun]

calling them liters and assuming resource units are in terms of volume is for the most part inherited from older versions of the game.

Liquids are measured in volume: 1 millilitre is the fluid that fits in 1 cm3

[tavert]

Liquids are measured in volume

Volume is one way to measure a quantity of a liquid, but mass is more useful in most contexts (particularly for stock KSP where we don't really have to worry about tank construction or different-density fuel types). In common usage mass and volume can be used roughly interchangeably since most liquids we commonly deal with are approximately incompressible across the temperature and pressure ranges we normally encounter.

The resource quantities are also not just liquids - electric charge certainly isn't in units of liters (or any other volume unit) and never has been, the liters tooltips were removed before electric charge was implemented as a resource.

[Kasuha]

Liquids are measured in volume

In chemistry, even liquids are often measured in mass. Among others also because liquid's volume changes with temperature while mass doesn't.

And of course even solids can be measured in volume. Wood is often measured in cubic meters, for instance, because it's important to know how big car you need to send for it.

In general, units which are the most convenient for the purpose are used in real world. So I don't see any reason to stick to dogmas like "liquids are measured in volume" in KSP.