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# Fuel Cunsumption Question

## Question

Hello forum members! I was doing some dV calculations today and came across a problem. In a previous question I had asked how to calculate the dV of a stage with different engines that burn out at different times.

Using what @Red Iron Crown said about this, my rocket should burn 471.01 liquid fuel in the time it takes the solid rocket boosters to burn out. Now this leads to the question: What is liquid fuel? Is it defined as the liquid fuel, of which there is 180 of in the FL-T400 fuel tank?

My rocket design will be linked once I get home from school, but it is a basic orbiting rocket that I am hoping to send to the moon like the one that I described in another thread:

Mk16 Parachute

Mk1 Capsule

*science instruments

materials bay

Heatsheild

decoupler

FL-T400 fuel tank

Terrier engine

decoupler

4x FL-T400 fuel tank

Swivel

**2x BACC Thumper SRB

## Recommended Posts

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1 minute ago, Benjamin Kerman said:

What I am wondering is when the engine says that the fuel consumption is 12.73 fuel/second, I am wondering what the definition of fuel is.

You could verify it yourself using the equation from the other thread. Set t=1 and use thrust and Isp to determine propellant mass.

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2 hours ago, Benjamin Kerman said:

What I am wondering is when the engine says that the fuel consumption is 12.73 fuel/second, I am wondering what the definition of fuel is.

KSP gives you a volume flow rate for the engine consumption. Which is kind of awkward, since KSP doesn't have any real volume units. Just a number that is actually a multiple of a mass. When your FL-T400 contains 180 liquid fuel, what the game really means is not "180 liters" or "180 gallons" or the like, but rather "180 times 5 kg". Because 5 kg happens to be the base mass in which liquid fuel is counted, as defined in the game's resource definition config file. Different resources may or may not have different base masses (which the game calls "densities").

Of course, you can easily convert a volume flow rate back into a mass flow rate. If it isn't immediately obvious from reading the above: given that 1 liquid fuel is 5 kg, and the engine consumes 12.73 liquid fuel per second, that means the mass flow rate is simply 12.73/s * 5kg = 63.65 kg/s.

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Liquid engines' Isp rating considers both liquid fuel and oxidizer consumed. The units given in the tank stats for these resources each weigh 5kg. When doing your calculations, be sure to work in masses and not the other units for fuels.

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DV calculations are based on propellant, for most rocket engines that's LF+Oxidizer (the exceptions in stock being the nuke engine and the rapier in atmo mode... and the solid fuel rockets of course)

Edited by Kerbart
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1 minute ago, Red Iron Crown said:

Liquid engines' Isp rating considers both liquid fuel and oxidizer consumed. The units given in the tank stats for these resources each weigh 5kg. When doing your calculations, be sure to work in masses and not the other units for fuels.

What I am wondering is when the engine says that the fuel consumption is 12.73 fuel/second, I am wondering what the definition of fuel is.

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