Propellant volume vs ratio equation ?

[C3PO]

I'm not exactly sure what the point of this exercise is, but if it's to calculate the fuel mass in an Orbiter addon, making a spreadsheet to approximate the data would be trivial. Especially if we use hemispherical domes. We pull most of the specs out of thin air(or maybe Google) anyway.

 

[N_Molson]

LOX tank: 24x[0.3751/(1+0.3751)] = 6.55m
LH2 tank: 24x[1/(1+0.3751)] = 17,45m

Seems very good to me, thanks. :thumbup:

Space between the tanks can be estimated by looking at similar rockets (Delta 4) cutaways.

 

[Urwumpe]

I'm not exactly sure what the point of this exercise is, but if it's to calculate the fuel mass in an Orbiter addon, making a spreadsheet to approximate the data would be trivial. Especially if we use hemispherical domes. We pull most of the specs out of thin air(or maybe Google) anyway.

Well, yes. I always try to reduce the unobtainium in my designs over the iterations, sorry for thinking so complicated at times.

My idea of a great add-on is something that feels so real, that you wonder why such a good thing only exists in Orbiter, without actually being halfway real. Fiction, that you don't notice at first or second sight.

 

[N_Molson]

My idea of a great add-on is something that feels so real, that you wonder why such a good thing only exists in Orbiter, without actually being halfway real.

I totally agree with that. The point of this is to be as close as reality as possible.

OK this is what I get with those dimensions :

I hand-made the domes, the shape doesn't look too bad to me.

What do you think about the spacing between the tanks and the RS-25 (which dimensions should roughly be right - the shape of the nozzle is for mockup purposes, of course-) ?

The "blocks" are the attachment points for 8x GEM-60s.

 

[C3PO]

I had a go at that spreadsheet, and the results were interesting.

For hemispherical tank domes(h=d x 0.5):
LOX tank: 7.1 m
LH2 tank: 16.9 m
Propellant mass: 95,003 kg

For semi-ellipsoidal tank domes (h=d x 0.152):
LOX tank: 6.7 m
LH2 tank: 17,3 m
Propellant mass: 103,395 kg :thumbup:

The dome shape had a larger impact than I'd expected.

[Edit] A dome height of 0.152 x diameter is probably too low for any significant pressure.

 

[N_Molson]

According to the above pic, my "domes" have a height of 0.719 m.

So if I use the spherical cap volume formula, I get 5,64 more cubic meters for each tank (I actually added the domes to the tanks height provided by C3PO (6.55 and 17,45)).

So the volumes would be :

LOX tank : 80.26 + 5.64 = 85.9 m3
LH2 tank : 213.83 + 5.64 = 219.47 m3

But yeah, now I'm thinking to it, it messes the fuel to oxidizer ratio...

 

[C3PO]

But yeah, now I'm thinking to it, it messes the fuel to oxidizer ratio...

:thumbup:
You have to subtract the domes first and add them again later.

I tried to tidy up the spreadsheet just enough that other people could use it. I zipped up the xls file if anyone wants to have a peek.
View attachment Fueltanks.zip

 

[N_Molson]

Nice spreadsheet, pretty much what I needed !

To refine it, you could add the required volume/mass of He to keep the main tanks under operating pressure.

 

[C3PO]

Nice spreadsheet, pretty much what I needed !

To refine it, you could add the required volume/mass of He to keep the main tanks under operating pressure.

You're welcome.

The trick is to decide how to implement the various "stuff"(technical term):lol: You could accurately calculate the vol/mass of liquid He needed, but you'd probably need some variable to arrive at a realistic result. The question is how close will it get you to a real stage.

IMHO it's better to reverse engineer some actual rocket stages to get correction factors that fit the real hardware. For example you could enter diameter and propellant mass of a real stage and figure out the "length factor" to make the model fit the hardware. A similar method could be used to calculate the total mass. The hard part is digging around for the correct specs of the real hardware.

 

[Notebook]

I don't know if this is any help to N Molson? A thread I started sometime ago(wow, 7 years!). Its about how the cog of two tanks of fuel and oxidiser changes as the fuel is consumed. Ends with a load of graphs, but there may be some relevant stuff in there!

http://www.orbiter-forum.com/showthread.php?t=6705&highlight=tank+level+change

N.