Based on @Joshman 's calculations

Link to Joshman's post: https://www.simplerockets.com/Forums/View/1123/My-Fuel-Density-Calculations-Seem-Weird

To get the density of the RP-1 fuel:
Fuel consumption of Apex 1: 1350L / s
Specific Impulse of Apex 1: 298s
Thrust of Apex 1: 3200000N

gravitational acceleration at surface of planet: 9.81m/s
We could get exhaust velocity of the engine as 298 * 9.81 = 2923m/s
Then we could get mass flow rate as 3200000 / 2923 = 1095kg / s
We divide mass flow rate by fuel flow rate to get density 0.81kg / L, or 810kg / m^3
Density of RP-1 is 810kg / m^3

Jundroo did get the density right.

To get the density of the RP-1 fuel using Joshman's way (he had a value wrong which I hope I did correct):
Using the height Joshman gave in his post: 1.17m
The radius of the command pod is 0.75m
I got a volume of 1.77m^3 (I have no idea how Joshman got such a large number Edit: Joshman took diameter instead of radius for the volume calculations so he got fore times the correct answer)
Assuming that all is fuel, density is 1797 / 1.77 = 1015kg / m^3
Quite close to RP-1's 810kg / m^3

To get the drymass of the fuel tank:
Using RP-1's density as 810kg / m^3
Time it by the fuel's ideal volume of 1.77m^3 (The value that I got) to get mass of 1434kg
Subtract that from the fuel tank's total mass of 1797kg to get 363kg as the fuel tank's dry mass.
However, this is not accurate, since I assumed that all volume in the fuel tank is fuel, which is not true (as we now know that the fuel tank has mass), but it will make for quite a good estimation.

To get the relationship between the fueltank's dimensions and dry mass.
The volume of the tank is 1.77m^3, we divide mass by volume to get the fuel tank's empty density as 205kg / m^3
However, the mass of the fuel tank should be determined by its surface area, not its volume (since it is a container). So, dividing the mass by the tank's surface area (2 * 0.75^2 * 3.14 + 1.5 * 3.14 * 1.17 = 9m^2) to get mass per surface area as 40kg / m^2
40 is a much nicer value than 205, so I think that the mass of an empty tank is determined by its surface area, not volume.

To get the mass ratio:
Mass of fuel tank and fuel: 1797kg
Mass of empty tank: 363kg
We divide wet mass by dry mass to get 4.95
Quite bad.

Thanks for reading!


12 Comments

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    @mjdfx150529 then it gives you 2 buttons. If 3 different engines then 3.

    +1 3 months ago
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    3,752 Kell

    @AnotherFireFox what if two different engines are drawing from the same rank?

    +1 3 months ago
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    @AndrewGarrison For you information, Real Fuels mod of KSP gives you a button to fill a fuel tank with O/F in exact ratio attached engines want. All you have to do is just slab an engine on a fuel tank and put the button. Tada! The mod fills the tank perfectly among the hundreds of fuels and oxidizers. No worry and hassle to chose right type of fuel.

    3 months ago
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    @AndrewGarrison Nooo! Don't make it any simpler, it's already too simple for players like me T_T Just name jet fuel JP-8 and leave RP-1 alone. To be honest I'm dead thirst for separated Oxidizer. BTW, where's the Fuel configs? Why can't we modify it as we modify parts.txt?

    3 months ago
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    3,752 Kell

    @AndrewGarrison This is interesting. As in KSP jet fuel is a component of rocket fuel. So how do you plan on adding nuclear engines with a new propellant type called nuclear propellant?

    3 months ago
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    @AnotherFireFox We changed the density after that comment. Density of our RP-1 now includes a ratio of 2.56 units of LOX to 1 unit of RP1, which puts it at 1.1kg/L.

    I am currently working on adding Jet Engines, so I'll need to add a new fuel for that. To avoid confusion, I'm thinking about adding a new fuel type called Jet Fuel and renaming RP-1 to Rocket Fuel.

    +2 3 months ago
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    @AndrewGarrison Wait, what? So you guys just threw the oxidizer away? I've thought you guys would've treated RP1 fuel as a mixture of Lox/RP1

    +1 3 months ago
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    I was interested in yours and @JoshMan 's posts and wanted to take this opportunity to double-check our calculations and make sure we aren't too far off with the dry mass / wet mass ratio, so I ran some numbers on a fuel tank of height 2m and radius 0.8m. Here's what the game currently spits out with that configuration:

    Volume: 4.0m^3
    Height: 2.0m
    Radius: 0.8m
    Fuel Density: 0.81g/mL
    Wet Mass: 3382.7kg
    Dry Mass: 210.3kg
    Ratio: 16.1

    +5 9 months ago
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    I don't understand, I'm no good at math xD

    +1 9 months ago
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    418 JoshMan

    Wow! Great work! Im jealous of all the redundent calculations! :)

    +1 9 months ago
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    542 MyMessage

    I’m assuming you right

    +1 9 months ago
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    542 MyMessage

    Ok....

    +1 9 months ago

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