How to calculate how long a engine can run with a specific amount of fuel

[Metro]

I'm not sure whether it's because I have some strong migraine now, but I don't manage to calculate the time a engine needs to use a specific amount of fuel.

My rocket is in orbit around Kerbin and has 204.000 l of fuel in that stage left. The engine is the LV-T45 which has a fuel consumption of 11.01 l/s in vacuum.

To get the time in seconds the engine needs to "empty the tank", don't I simply have to calculate 204000 l / 11.01 l ?

I'm feeling like I'm doing a idiotic mistake there

Edited May 6, 2014 by Metro

[8MMW]

In the parentheses after deltaV for each stage the time is shown. It is the time it will take to get displayed dV (by burning all fuel available at that stage) (1001 m/s for current S2) at full throttle, (41.2 seconds in current S2 stage). Is this what you are looking for?

Edited May 4, 2014 by 8MMW

[UmbralRaptor]

It is 204/11.01 ~= 18.5 s, if you got that from the appropriate source. I'd check to make sure you're counting all tanks.

I am not familiar enough with KER or your craft to know where KER is getting its numbers from. Pics?

[Metro]

In the parentheses after deltaV for each stage the time is shown. It is the time it will take to get displayed dV (by burning all fuel available at that stage) (1001 m/s for current S2) at full throttle, (41.2 seconds in current S2 stage). Is this what you are looking for?

Yes, exactly. How much time it takes to get displayed dV. How would you calculate that manually?

It is 204/11.01 ~= 18.5 s, if you got that from the appropriate source. I'd check to make sure you're counting all tanks.

I am not familiar enough with KER or your craft to know where KER is getting its numbers from. Pics?

Oh, well. Here is how my rocket looks like now

Edited May 4, 2014 by Metro

[8MMW]

I just checked, and the 11.01 l/s refers to liquid fuel + oxidiser. Assuming you meant 204l of liquid, it should be about 370 liquid + oxidiser; but even then 370/11 = 34 s.

One thing I'm sure about is what KER displays (as said above), the time is displayed correctly, taking all fuel lines and asparagus into account, unless of course the fuel is manually transferred between tanks (Alt+RMB). I can't explain why it doesn't match simple calculations. You can always quicksave and check, it should be 41.2s.

Edit:

Now I see that you're still in atmosphere (below 70 000m), that changes the engine efficiency. While ascending (and changing air pressure) the engines' efficiency and fuel consumption will change, so it would be difficult to calculate it manually. KER should display it correctly however.

[DeMatt]

I just checked, and the 11.01 l/s refers to liquid fuel + oxidiser. Assuming you meant 204l of liquid, it should be about 370 liquid + oxidiser; but even then 370/11 = 34 s.

One thing I'm sure about is what KER displays (as said above), the time is displayed correctly, taking all fuel lines and asparagus into account, unless of course the fuel is manually transferred between tanks (Alt+RMB). I can't explain why it doesn't match simple calculations. You can always quicksave and check, it should be 41.2s.

It's 11 oxidizer to 9 fuel, so there's 249.33L of oxidizer involved. 453.33L combined divided by 11.01 L/s = 41.17 seconds burn time, exactly what KER says.

Edit:

Now I see that you're still in atmosphere (below 70 000m), that changes the engine efficiency. While ascending (and changing air pressure) the engines' efficiency and fuel consumption will change, so it would be difficult to calculate it manually. KER should display it correctly however.

Metro is high enough that the atmospheric effect on the Isp is smaller than our level of precision. You can tell by looking at KER's listed Specific Impulse figure... 370 seconds, to three decimals.

[KvickFlygarn87]

I think I could take a look at KER's source code.

[Padishar]

I think I could take a look at KER's source code.

That particular bit of the code (in VesselSimulator/Simulation.cs) is a little hard to follow as it works in terms of mass flow rates. Be assured the calculations are accurate, DeMatt gave the correct figures above or just look at the engine stats in the VAB where the fuel and oxidiser rates are given separately .

[draeath]

Have a look at the FireMarshal script in my sig - this might help you out.