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Realistic Densities and Mass - Realism and Gameplay


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Hello,

Note the title isnt Realism vs Gameplay. The Community Resource Project for which I asked for some help in starting this thread with, and didnt get much of a response (yet maybe they will show up here now), has a part in making the game realistic by using resources to add resource use in the game; this is all well and good for 'payloads'.

Resources are fine...but now there is a serious problem with 'fuel densities'.

Squad is trying to get into the resource game which is an added complexity when the 'lift and drag' issue have yet to be resolved.

Altho some formulas do not use derivatives, simple differential math is needed now because of fuel use; liquid fuel use.

I am assuming it was done; I dont think it was done correctly; why; because Squad's Liquid Fuel densities were pulled out of the perverbial hat just like the Comm.Res Proj did when they started out; then some of us pulled together to make resource densities more real-world like; even though the CRP (short for Community Resource Project) is off by a factor of 1 million, the values 'work' in game and there is a relationship.

All except for fuel densities...something is really wrong here. First liquid fuel is assumed to be liquid hydrogen and oxidizer is liquid oxygen...plugging in these real world values and I can even get rockets off the ground; changing them back to a 'heavier density' > 0.005 (for both !!) and my rockets fly again; that is actually backwards !! Lighter densities mean less mass > density and mass are directly proportional > D = M / V.

The game uses metric tonnes (< not tons!) I believe...and the game density as used by the CRP for water is 0.001; everything else real-world is based on this in-game setting.

Now the CRP has nothing to do with Squad liquid density values...but there is a problem here and I am addressing it.

I will be looking at this in more detail probly over the next few days (off and on as I am in career mode).

It looks like the densities were the same back to v23 (beyond that I dont know), and how they are related to the game...we could look at the fuel tanks I suppose and calculate volumes...then look at real world volumes and size for aesthetics...the important part is mass, dry and wet weight...and the derivative when mass changes due to the loss of fuel in a burn; now that that has changed drastically as well.

I am wondering if a fudge factor was introduced into the game along time ago and left to let it ride? I am a math nut; I thrive on this stuff; those in the in-game know...tell us about TWR changes...do they matter now that drag is in the game??? How does drag affect TWR and why is the liquid density of fuels so high to make the game work?

Cmdr Zeta

Edited by Cdr_Zeta
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Maybe if you reduce the density of the fuel, it reduces the burn rate (by mass), and therefore the thrust of the rocket? That could stop you from getting off the ground because the rocket components are way too heavy.

I, too, would love some realistic values here. These rockets and planes are far smaller than real-world craft yet are often actually heavier and tend to have much higher thrust values.

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Hello,

So...where is this discussion...dead? When I get into my resource mining in Career mode here I will be workin on my fuel production formulas and liquid fuel densities are out the door; I dont know about monoprop that probly is as well.

Now there is a fancy MOD stating Liquid Hydrogen as a seperate fuel using modified engines, probly negating whatever Squad is doing for it's engines; that and the fact that drag is added to parts (?)...

Cmdr Zeta

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All except for fuel densities...something is really wrong here. First liquid fuel is assumed to be liquid hydrogen and oxidizer is liquid oxygen...plugging in these real world values and I can even get rockets off the ground; changing them back to a 'heavier density' > 0.005 (for both !!) and my rockets fly again; that is actually backwards !! Lighter densities mean less mass > density and mass are directly proportional > D = M / V.

Why would you assume it's H2 and LOX? Those are cryo fuels and boil off over time, whereas KSP fuel and oxidizer does not. If anything, I'd guess it's some sort of hydrazine/NTO pair. Note that the density is 5 kg/l (or 5,000kg/m³ in more sensible units), rather high for a fluid (it's higher than titanium, and about five times that of a typical fluid).

Anyhow, the density is kinda irrelevant as the tanks are grossly wrong in terms of volume (an FL-T400 should be around 9200L, it's only 440L) and all calculations are using mass flow rather than volume.

The game uses metric tonnes (< not tons!) I believe...and the game density as used by the CRP for water is 0.001; everything else real-world is based on this in-game setting.

Yes, it's metric tonnes (1000 kg / 2204.62 lbs), although those are close to short tons (2000 lbs - 10% diference) and very, very close to long tons (2240 lbs - < 1%).

Maybe if you reduce the density of the fuel, it reduces the burn rate (by mass), and therefore the thrust of the rocket? That could stop you from getting off the ground because the rocket components are way too heavy.

The burn rate IS by mass. The volume is essentially meaningless in terms of burn rate. The capacity of the tanks ARE measured by volume for whatever stupid reason though. The tanks though appear to be wildly wrong in terms of liters - I think Squad just made up values for 'em.

As an example, the FL-T400 tank is (by specification) 1.25m in diameter, and I estimate it's length at about 1.875m (it's about 1.5 diameters long). That would give it a volume of roughly 9.2 m³ ; 9200 L. It's in-game volume is 440L. You could assume that as the tank is slightly narrower in the middle part (the ends flare out a bit), it would lose some volume, and also you could assume that the walls have some thickness to 'em and such, but that doesn't make up for the 20:1 discrepancy, unless the tank is a solid block of metal with just a tiny hole for fuel in it's middle..

I, too, would love some realistic values here. These rockets and planes are far smaller than real-world craft yet are often actually heavier and tend to have much higher thrust values.

No, KSP rockets have significantly less thrust than their real-world counterparts. The Mainsail is close to the F-1 in mass (6t vs 8.3t), but nowhere near in terms of thrust (1.5MN vs 6.7MN). Of course, weird densities appear here as the Mainsail is only about half the length of the F-1..

(the jet engines have no real world counterparts at all)

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Those are cryo fuels and boil off over time, whereas KSP fuel and oxidizer does not.

By that logic - none of the materials in KSP have any real life equivalent, cause nothing here makes sense when you try to match it against real life. Even air doesn't.

So we might skip it all together. And we probably should.

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What is this thread about?

LiquidFuel, Oxidizer, SolidFuel, Monopropellant, Ore, these are all resources that are an abstraction; they have no real-world counterpart. IIRC the old assumption was that LiquidFuel was RP-1 which meant that the density worked out to roughly 4.5 liters per unit based on the capacity of certain tanks.

That's the problem, though: KSP runs on arbitrary units and not liters, or some other meaningful measurement. Real Fuels attempted to solve this by moving all unit measurements to liters and adjusting its densities appropriately, and then moving tank volume over to the usable volume inside a tank cylinder. This meant Realism Overhaul could actually be realistic instead of using arbitrary units.

So now you had Realism Overhaul using realistic volumes and densities, and a key mod or two moved to supporting that. Then along came CRP which intended to unify the mod community's resources so no one would stomp on each other. I can't remember entirely what direction the push came from but Real Fuels resources were to be integrated into CRP (because of TACLS, I think, which bridged the gap and used liters) which meant that all of CRP's resources would move over to liters for tankage with realistic densities. That, I think, is now how CRP is doing things because RF will be using it (and RF would only use it if it were realistic, that being its goal).

Now, CRP will not redefine any of Squad's resources, for good reason. RF doesn't use abstractions, inasmuch as possible, and so tosses Squad's resources right out the door (where they belong), and other mods generally try to work with stock systems and thus need to leave those resources be. However, everything else in CRP moved over to 1 liter per unit with realistic densities because it's something that we can understand and actually account for, and people making part models can use the actual volume of their tanks instead of just slapping whatever number "feels right" on their tanks.

I have no idea why Squad hasn't done that, but that's why CRP does it, to the best of my recollection.

Is this thread pretty much just "Why doesn't Squad use liters?"

Edited by regex
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It's also worth noting that the given mass specific heats for LF and OX are now pretty close to something Hydrazine-y (Aerozine 50?) and NTO. And the Isps work out too. The ratio is all wrong of course, though (11:9, should be something like 1.8:1) and the densities are equal (they should be something like .00725 and .0045), but if you think of the tank as a single mass, it almost makes sense now.

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The ratio is all wrong of course, though (11:9, should be something like 1.8:1).

It's not, really. NTO/UDMH mixture ratios vary from 1.9 to about 2.7 by mass, which is 1.0 to 1.45 by volume. Although the individual masses are wrong, the 1.22 volume ratio fits right in the middle of the range.

I don't think anyone actually runs a NTO/UDMH rocket at a 2.3 mixture ratio (what we have), but whatever. (Capsules and top stages sometimes use 1.9 so they can use the same size tanks for both propellants. Launch vehicles run as close to max combustion temperature as they can (around 2.6-2.7) since the engines are easy to cool and NTO/UDMH doesn't really burn all that hot compared to the alternatives.) As an average value the Kerbal mixture works out, and mixture ratio generally doesn't make more than a few percent difference in rocket performance anyway (design-wise, that is -- mixture going wrong during a mission is about as disastrous as you can get).

You probably can't run a jet engine on UDMH, but UDMH and kerosene are near enough the same density. We can just assume that "LiquidFuel" is whatever parts UDMH or Kerosene you need to run your rockets or jets for as long as you end up running them. (You can actually build a kerosene/NTO rocket, but you end up needing something like 70% NTO by volume -- so that's not it.) It's a bit more of a stretch, but an LFO tank with no Oxidizer in it gives us a fuel density close-ish to methane for the nuclear engines, too.

The showstoppers for realistic masses are the everything-elses, not the fuels. Capsules, cockpits, structural parts, and fuel tanks weigh 2-3x as much as they should. The fuel tanks are the easiest to compare: dry fuel tanks in KSP weigh 11% of their wet mass, while the tanks on most real-life launch vehicles (Ariane, Proton, and Soyuz at least) hover around 6%. Stuff like Atlas V and the Shuttle Super Lightweight External Tank are down to 3.5%.

Except it's worse than that. Kerbal masses were balanced to make the rockets "look right" taking off from tiny super-dense Kerbin, but the balance of each individual part is completely ad-hoc. You can't really divide all the masses by 2.5 and have something reasonable, especially once you start thinking about all the mod parts out there.

As an example, the FL-T400 tank is (by specification) 1.25m in diameter, and I estimate it's length at about 1.875m (it's about 1.5 diameters long). That would give it a volume of roughly 9.2 m³ ; 9200 L. It's in-game volume is 440L. You could assume that as the tank is slightly narrower in the middle part (the ends flare out a bit), it would lose some volume, and also you could assume that the walls have some thickness to 'em and such, but that doesn't make up for the 20:1 discrepancy, unless the tank is a solid block of metal with just a tiny hole for fuel in it's middle..

Check your math here, it's pi*r^2*h. If a unit is five liters, the FL-T400 volume (400 units) is pretty spot on. 2000 L capacity out of ~2300 L total volume.

Edited by NonWonderDog
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