Working out Specific Impulse

[Iforgotthis]

Hello!

I have been reading about specific impulse and delta v and all that recently; it's captivated me. I want to work out the specific impulse of the mainsail engine, BUT I want to do it through the power of mathematics! I know what it should be, because the game says (280s ASL), and I know the equation, I just don't know how to apply the numbers to the equation. Could someone help?

Okay, so for the equation I need the thrust in newtons (which is 1500 kN) and the fuel consumption in kg/s. That's where a problem arises, because the fuel consumption is given in l/s, not kg/s. How do I convert between the two? Is it possible without knowing the density of the fuel? Am I just being an idiot?

I really want to get my head around this, I don't know why I just enjoy working these things out, even if they have already given me the answer!

[AlexinTokyo]

I'm working from memory here, but I think one unit of fuel (there's some discussion as to whether the units are liters or not) masses 5 kg.

Of course, you can look at the fuel mass of a tank (right click in the VAB for the extended data) and divide by the capacity.

[Kasuha]

fuel consumption is given in l/s, not kg/s

That's not really true, fuel unit is not specified (it was in earlier versions of KSP but is not now) and corresponds to 5 kg in case of liquid fuel and oxidizer.

It would be very strange fuel if its one litre was 5 kg anyway.

[Iforgotthis]

That's not really true, fuel unit is not specified (it was in earlier versions of KSP but is not now) and corresponds to 5 kg in case of liquid fuel and oxidizer.

It would be very strange fuel if its one litre was 5 kg anyway.

How right you are... I'm sorry for that mistake, I was reading from here: http://wiki.kerbalspaceprogram.com/wiki/Rockomax_%22Mainsail%22_Liquid_Engine

Which gives the fuel consumption in l/s. It's only now that I've checked the game that I see you are right, and no unit is provided.

So how are you supposed to work out specific impulse now? And more importantly, where did the ksp wiki get the number 109.11 l/s for fuel consumption from?

Edited April 15, 2014 by Iforgotthis

[Kasuha]

Wiki is not wrong in numbers, only in used units. Fuel consumption in there is in units per second.

For rocket engines, things work a bit in reverse in KSP compared to real world. The thrust setting and engine's maximum thrust give current thrust of the engine, atmospheric pressure is used to calculate Isp and from these two, fuel consumption is calculated.