Fuel consumption rate question.

[Joe32320]

What is the formula needed to work out the time it would take an engine to use a given amount of fuel/oxidizer?

I'm able to calculate the fuel flow rate given by the dialog box that appears by right clicking on the engine by the formula thrust/(ISP*g). Which means for a mainsail at full throttle near the ground with a ISP of 281 gives a fuel flow 0.5436U accordingly.

Now in the stage that I'm testing, I have 3600 units of liquid fuel plus the oxidizer it needs, which Flight Engineer predicts should run out after 73.5 seconds, however I can't find out how it is able to calculate this since the fuel flow rate I calculated above doesn't intuitively give me the answer flight engineer suggests I should be getting.

The reason I'm asking is because I'm using kOS to automate my launches, and I need to know the time require for the circularisation burn using the data accessible by kOS.

[sciguyCO]

If I understand correctly, the fuel flow is in units "tonne / second", and counts both fuel and oxidizer.

Fuel and oxidizer both mass at 1 tonne per 200 units, and are consumed at a 9:11 ratio. So burning one tonne of fuel + oxidizer = 90 units of fuel + 110 units of oxidizer

So your burn time should = unitsFuel / (90 * flowRate)

Plugging in 3600 units of fuel and 0.5436 flow rate comes out to 73.58 seconds, which matches Flight Engineer.

Does that help?

[Jimbimbibble]

You need to take into account the density of the fuel because the equation you used gives you the mass flow rate, not volumetric flow rate. In KSP, the fuel (both components) has a density of 5kg/U. The mass flow rate you calculate includes both the fuel and oxidizer by the way. Thrust is given as kN in KSP so watch your units when you calculate. As an example calculation, your mass flow rate is

dm/dt=F/Isp*g=1,500,000/(280*9.81)=546.1kg/s

dV/dt=dm/dt /density=546.1/5=109.2U/s

You said your rocket has 3600U of fuel, so the total volume of fuel and oxidizer is 3600*2/.9=8000U. So the burn time would be

T=V/ dV/dt=8000/109.2=73.2s

[Joe32320]

Thank you both, that answers my question perfectly