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!

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

+1 one month ago