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 one month ago
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    2,567 mjdfx150529

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

    +1 one month 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.

    one month 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?

    one month ago
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    2,567 mjdfx150529

    @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?

    one month 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 one month 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 one month 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 6 months ago
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    I don't understand, I'm no good at math xD

    +1 6 months ago
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    418 JoshMan

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

    +1 7 months ago
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    540 MyMessage

    I’m assuming you right

    +1 7 months ago
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    540 MyMessage

    Ok....

    +1 7 months ago

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