Kethane as Nuke-Fuel

[KerbMav]

Update:

So I edited my nukes to run on Kethane only and tried to change the fuel consumption to give me the same dV for a Kethane tank as if I were using a LF/OX tank of the same size.

Now I fear I am cheating myself in regards to available dV here, because I think I got the ratio wrong and so I ask for a bit of help with my math.

Also: Should I have compared the mass of the tank instead of just saying 1 tank LF/OX = 1 tank Kethane?

(Is the density of LF/OX different than that of Kethane?)

----------

Lets say I wanted to make Kethane the fuel to be used by N-engines.

I think I would be able to edit the part.cfg of the engine to use Kethane instead of the L/O mix.

As 1000 units of Kethane are not 1000 units when converting, what amount of consumption should I set to keep the current efficiency? Relative to volume/size of the tanks or rather mass of the fuel in tank?

Edited May 23, 2014 by KerbMav
because

[Greys]

the efficiency of that sort of engine is defined by what I believe is "molar mass" of the fuel, the volume density of Kethane is double the volume density of LF/O as a rule and the mass density of Kethane is 0.002 tons per unit compared to 0.005 tons per unit for LF/O.

Lighter fuels provide less thrust but greater ISP, so you have twice as much fuel to burn compared to a same size tank; and that fuel is more efficient, but I don't think it's a straight line progression, and given all that you've got 4/5ths the fuel mass to push.

[KerbMav]

Hm, the FL-T400 fuel tank holds 180 LF - the same sized KE-TM30 Kethane Storage Apparatus holds 1000 K - the other comparable tanks also have a ratio of 1:5.5 - so to get the same output it should be somewhat like 5.5 usage?

Mass of both tank variants is the same for both kinds of fuel.

[KerbMav]

I still need confirmation or correction regarding my changes to the nukes - I am preparing for a big tour to all anomalies and do not wish to change the parameters of the game other than the name of the fuel I am using, thank you for thinking with/for me!

[Streetwind]

dV is related to mass, so you should probably think in terms of mass. If one tank happens to contain a larger amount of mass in the same volume than a different tank, that results in a smaller ship... but that small ship will have the exact same dV as a giant ship which has the same amount of fuel and dry mass, just packaged in a much larger container. Ultimately, every single volume unit in KSP is arbitrary, and they're not even consistent between different materials. One unit of monopropellant has a different volume than one unit of liquid fuel, for instance. It's like one thing is labeled in gallons and the next in liters and the next one again in pints and the final one in fillings of Kerbonaut helmets. In other words, if you're trying to balance something by volume, you're going to have a bad time. </meme>

As for changing an engine's fuel consumption - you can't. At least not directly. The volume flow rate is computed by the game dynamically depending on various factors that you can adjust. First, it computes a mass flow rate based on thrust and Isp; then, it looks up the arbitrary density of the resource in the resource definition and uses that to calculate a volume flow rate using the equally arbitrary volume units.

The relationship between thrust and Isp is a physical law; you cannot affect it. There is no way you will ever be able to keep thrust the same and also Isp the same, yet get a different mass flow rate. You could get a different volume flow rate by changing the resource density, but all that you do with that is exchange one arbitrary measurement for another. The actual fuel consumption in terms of mass per time remains identical.

This has a convenient effect: assuming you do not change the thrust, and you do not change the Isp, then you do not need to worry at all about adjusting the fuel consumption of the engine to a specific value. It is automatically correct, because the mass flow rate is always the same, and dV depends entirely on mass. If your kethane tank has the same wet and dry masses as your iquid fuel tank, and the engine has the same thrust and Isp, then you get the same dV. The propellant used or its density is irrelevant. If said kethane tank happens to contain 5.5 times as many arbitrary volume units, then the engine will also automatically display that it is consuming 5.5 times as many volume units per time. The underlying mass flow rate is still the same, though, and therefore the fuel consumption is the same.

Now, if for some reason you really wanted to make sure that a kethane tank of the same size (do the models even match?) as a liquid fuel tank has the same dV as it does, despite having a different wet and/or dry mass, then you need to change the base values that influence the mass flow rate. For example, if a kethane tank of the same size has a higher mass, then you need to lower the engine Isp. This will cause it to require more mass flow to achieve its rated thrust. I'm not sure about the exact math involved, but for starters, divide Isp by the mass difference between the tanks. For example, the kethane tank has 1.5 times the mass of the liquid fuel tank, then do 800 / 1.5. Or, if the kethane tank happens to be 20% lighter, then do 800 / 0.8.

However even if that hits it pretty well, it will never hit it exactly, because even if the ratio of wet mass to dry mass used by kethane tanks is the same as that used by stock liquid fuel tanks (for which there is abolutely no guarantee), then the fact that you had a higher or lower wet mass also means you have a higher or lower dry mass, and that will throw the numbers off. And if the tank's ratio of wet to dry mass is different, then things get even more screwy. In that case it might be easier and quicker to edit all the kethane tanks to match stock liquid fuel tanks in wet and dry masses than tring to fiddle with the LV-N.

[KerbMav]

Thank you!

Edit the tanks ... !!!

But what does ratio do for me then?

		PROPELLANT
		{
			name = Oxidizer
			ratio = 1.1
		}

How much mass it pulls from which fuel type?

Edited May 23, 2014 by KerbMav

[Streetwind]

Yes, it's a mass ratio.

For a single fuel, just put 1.0 there and call it a day

[KerbMav]

I hope I:

- switched the nukes to Kethane - working!

- set density of Kethane to that of LF/OX - should be correct!

- changed Kethane tanks to same dry mass as equivalent LF/OX tanks - third party verification requested

- changed Kethane tanks to hold the combined mass of LF and OX as equivalent tanks - third party verification requested

@PART[nuclearEngine]
{
	@MODULE[ModuleEngines*]
	{
	!PROPELLANT[LiquidFuel] {}

	!PROPELLANT[Oxidizer] {}

	PROPELLANT
		{
			name = Kethane
			ratio = 1.00
			DrawGauge = True
		}
	}
}

@PART[cl_large_nuclearEngine]
{
	@MODULE[ModuleEngines*]
	{
	!PROPELLANT[LiquidFuel] {}

	!PROPELLANT[Oxidizer] {}

		PROPELLANT
		{
			name = Kethane
        	ratio = 1.00
			DrawGauge = True
		}
	}
}

@RESOURCE_DEFINITION[Kethane]
{
	@density = 0.005
}

@PART[kethane_tank2mExtralarge]
{
	@mass = 4

	@RESOURCE[Kethane]
	{
		@maxAmount = 6400
	}
}

@PART[kethane_tank2mLarge]
{
	@mass = 2

	@RESOURCE[Kethane]
	{
		@maxAmount = 3200
	}
}

@PART[kethane_tank2mMedium]
{
	@mass = 1

	@RESOURCE[Kethane]
	{
		@maxAmount = 1600
	}
}

@PART[kethane_tank2mSmall]
{
	@mass = 0.5

	@RESOURCE[Kethane]
	{
		@maxAmount = 800
	}
}

@PART[kethane_tank1mStandard]
{
	@mass = 0.25

	@RESOURCE[Kethane]
	{
		@maxAmount = 400
	}
}

@PART[kethane_tankExternal]
{
	@mass = 0.1

	@RESOURCE[Kethane]
	{
		@maxAmount = 60
	}
}

Size of Kethane and LF/OX tanks is not the same/equivalent - but it is only marginal and I do not care.

Edited May 26, 2014 by KerbMav