Configuring Engine Resource Consumption

[CaptainKipard]

1. This is from the rapier. What would happen if I typed 1.8 and 2.2? Would that double the amount of resources the engine uses or are these numbers only used for a ratio?

PROPELLANT
{
name = LiquidFuel
ratio = 0.9
DrawGauge = True
}
PROPELLANT
{
name = Oxidizer
ratio = 1.1
}

2. I know Isp determines fuel efficiency. Are there any other values that contribute to it? How do I define the rate at which all resources are used.

3. I'm having a lot of fun figuring out how to make stock resources work with modded resources and one resource that afaik is not present yet in any of the mods. I need to make liquid hydrogen and liquid oxygen burn proportionally, and make IntakeAir burn with liquid hydrogen. Is there any rhyme or reason to how the stock resources work? The rapier has a fuel to intakeAir ratio of 1:15, but when I calculate the volume of liquid hydrogen and the volume of the earths atmosphere, which would need to react realistically, I get a ratio of ~1:1889

[Sage]

1. I'm pretty sure those define only a ratio. Changing a that number in a solid motor, for example, has no effect

2. Nope. ISP is the time that an engine can sustain a newton of thrust with a kilogram (mass, not weight-kilos) of fuel. This define the mass used per second. Let's say that an engine has an ISP of 300s: if we keep the throttlr to a minimum of 1N the engine consume 1/300 kilogram of fuel per second; if we throttle up to the max, let's say it's 600N, the engine will eat 2Kg/s.

EDIT: the ratio should enter in the calculation here: logically speaking, every resource would be multiplied by its ratio, but has I've said, that should mean that ina monopropellant engine changing that value would affect the consumption, and that didn't work when I tried. Maybe I broke it, or the system put the every resource at ratio(n)/ratio(1). So the first resorce get always a multiplier of 1.

3. I don't really know here. Have you tried to look the b9 SABRE? Stock values are a lot of times nonsensical.

Edited November 16, 2014 by Sage

[RedParadize]

Humm, outside electic engine, The ratio doesn't affect much. But keep in mind that some people will use these engine with other fuel tanks. They might be surprised a bit. Thinking about it, if the engine burn more oxidizer, it would compencate for the liquid fuel that got burn with air intake during atmosphere flight. Is it why you want to change it ?

[CaptainKipard]

The reason I need to use non-stock resources is because the real Skylon is designed for them. The only way I can think of to use stock resources in this is to cheat, which in turn would make the parts OP in one way or another. I have to balance this for stock mechanics.

Btw, yes. Skylon has more moles of hydrogen than oxygen because it has hybrid engines.

The LOX:LH2 ratio is not a problem, it's just that LH2 already exists in this mod, so I really have to stick to that definition. I worked off of it to get the values for LOX, buuuut I still have to use IntakeAir for the air-breathing mode, and here were the problem lies. The stupid numbers that Squad came up with just don't seem to make sense. It's just so gamey.

B9 Sabres are 1 LiquidFuel : 9 IntakeAir, which is even worse.

Edited November 16, 2014 by Cpt. Kipard

[RedParadize]

Anything that make SSTO is OP by definition. Hey every KSP player have build one these at one point. Presonatly. I have a set of custom mod that project me directly in the world of science fiction. I currently have several fully autonomous single stage to everywhere design that can land on every planets and moon of the x6.4 solar system without coming back.

Thanks super danse fuel tanks.

Thanks OP nuke engine.

Thanks karbonite extraction.

Thanks Deep freeze.

Thanks Hooligans labs.

Thanks tweakscale TtW scaling factor.

And above all, Thanks overlaping part!

[Mecripp]

You could just come up with a funny gas for the intakeair ?

[NathanKell]

An engine will consume a set amount of mass per second, as mentioned, though the above explanation is missing the factor of g0 (because it *is* actually weight, not mass, when Isp is given in seconds, it's mass when Isp is given as exhaust velocity). How that mass is converted to units is as follows:

for each PROPELLANT, multiply ratio * density.

Sum all that up.

Divide mass consumed per second by that sum. This grants you a multiplier

Now, multiply a fuel's ratio by that multiplier; that's the number of units per second consumed of that fuel.

As you can see, saying LF=9 and Ox=11 is the same as 0.9/11 or 54 / 66.

In stock KSP, the math is very simple because LF, Ox, and IntakeAir are *all* density 0.005 (0.005 tons/unit). That means you don't have to worry about density, which simplifies the above calculation. If you're using a resource that has a different density, then the volume ratio (the ratios specified in PROPELLANT nodes) will no longer equal the mass ratio.

[CaptainKipard]

Instructions unclear. I tore a hole in the universe.

You could just come up with a funny gas for the intakeair ?

No I have to use the atmosphere

[Gaalidas]

I had a feeling while reading this thread that it would come down to resource density at some point. So the question, for us mathematically challenged out here, is this: To make less fuel be consumed per second of maximum thrust for a given engine be to reduce the resource density or increase it? A similar question exists for fuel tanks. How will this affect the amount of fuel we can carry with a given tank? In the end, with the same size tanks, we may end up still using the same amount of fuel compared to the maximum that you can carry. I am curious to know if there is any real benefit to changing any of these numbers to increase fuel efficiency without sacrificing performance at launch time.

[Mecripp]

But they can't change the resource density of intakeAir and use a Community Resource Pack then everything that used it would be wacky.

[NathanKell]

Gaalidas: Since Isp determines how much *mass* of fuel is consumed, changing a fuel's density will not change how many kilograms of it are consumed by an engine per second. It will just change how many units of fuel are consumed, but units are essentially meaningless in KSP; they don't correspond to any actual measure of volume.

If you increase density, then a tank that holds 440 units of the resource will still hold 440 units, but where once that meant 2.2 tonnes, now it means, say, 4.4 tonnes.

The rocket equation doesn't care about what size or shape your rocket is, all it cares about is mass_gross / mass_dry, and Isp.

[CaptainKipard]

What I'm getting from all this is that I'll just have to cheat and pretend that the atmosphere is providing 100 times more oxygen than is possible by artificially shrinking the IntakeAir ratio to something similar as in all other engines.

[EatVacuum]

What I'm getting from all this is that I'll just have to cheat and pretend that the atmosphere is providing 100 times more oxygen than is possible by artificially shrinking the IntakeAir ratio to something similar as in all other engines.

I'm not sure where you are coming from when you say that you need to fake the intake ratio to provide 100x more oxygen than is being provide, or for that matter your earlier comment about a 1:1889 fuel to air ratio. It's been a while since university chemistry and physics, but here's my calculations to get the numbers right;

By molecule -

2 LH2 + 1 O2 = 2 H2O + energy (heat/thrust) simple for a closed cycle rocket

[Starwaster]

I'm not sure where you are coming from when you say that you need to fake the intake ratio to provide 100x more oxygen than is being provide, or for that matter your earlier comment about a 1:1889 fuel to air ratio. It's been a while since university chemistry and physics, but here's my calculations to get the numbers right;

By molecule -

2 LH2 + 1 O2 = 2 H2O + energy (heat/thrust) simple for a closed cycle rocket

I think you're confused about a few things.

IRL, propellants are seldom mixed like that. That would be, what? A 16-1 mix ratio by mass? 5-1 is more likely. Other mix ratios are used as well. (see http://www.braeunig.us/space/propel.htm)

And KSP uses volume in its engine mix ratio configuration and not mass.

But, that aside, I think his main concern is when it's in operating as a jet and not when it's in closed cycle mode.

[CaptainKipard]

It's like this:

1k moles of LH2 is 2kg

1k moles of LO2 is 32kg

1k moles of gaseous O2 is also 32kg

As you said, you need 2 moles of H2 for each mole of O2 so for that reaction that's a 1:8 mass ratio.

Density of LH2 is ~ 70.85 kg/m³

Density of LO2 is ~ 1141 kg/m³

Density of Gaseous O2 is ~ 1.429 kg/m³ - That's 3 orders of magnitude less dense than liquid, and after all IntakeAir provides oxygen as a gas.

KSP uses volumetric measurements for the tanks and engine ratios, so:

Volume = mass/density

The volume of 2k moles of LH2 is ~ 0.056457...m³ or 56.457 Litres

The volume of 1k moles of LO2 is ~ 0.028045...m³ or 28.045 Litres

The volume of 1k moles of O2 is ~ 22.393282...m³ or 22393.282 Litres

But that's not all. The oxygen in the Earth's atmosphere accounts for only 21% of all the gasses so in fact we need:

22393.282 * 100/21 = 106634.67626... Litres of Air or IntakeAir.

SO the volume ratio of Liquid Hydrogen to IntakeAir that should react completely is:

56.457/56.457 = 1

to

106634.67626/56.457 ≈ 1888

Hmmm. That ratio is exactly twice as big as I previously calculated, so I made a mistake somewhere(either here, or before), but you get the point.

1:1888 is faaaaaar from what stock jet engines and other mods use.

edit

Fixed my calculations. I forgot to multiply the mass of LH2 by the number of moles; *2

Edited November 21, 2014 by Cpt. Kipard

[NathanKell]

You can't use real chemistry with stock resources. It just doesn't work. If you want to make something for stock, do it so it "feels" right. If you want real math, use Real Fuels.

[EatVacuum]

My apologies, that previous was a partial post that I didn't think went up. I had put in the molecular ratios for air intake version as well as the calculations for ratios by weight and volume for closed cycles and air breathing modes. But I guess that got lost. My mistake to do it on a smartphone.

[Gaalidas]

So, in simple terms, it is how it is and we should really just accept it and move on. No easy way to make an engine take up less fuel without over-increasing the thrust and making a fuel tank that's unrealistically light and unrealistically full.