[1.8+] Real Fuels

[Northstar1989]

Northstar1989: Yes, thank you for that explanation. The problem is that if I simply declare boiloff to be the 4/9th power, it will mean that to get realistic boiloff for large tanks, it will be impossible to create fuel cells that don't boil everything off very quickly. In real life, insulation mass is going to be increased for the small tanks to prevent that, and I'm really unsure I want to go so far into the details as for RF to model insulation mass, *especially* given that it cannot know surface area except by the roughest approximation.

You *could* make boil-off just scale with the 2/3rd power for the cryogenic tanks (assume the smaller tanks are insulated to the same relative rate of heat leakage for their volume), and make the mass of the cryogenic tanks scale non-linearly (such that larger tanks weigh relatively less when cryogenic). You can assume fuselages and Service Modules are both insulated as well (in fact, highly pressurized tanks have comparatively VERY LOW rates of boil-off, as their surface area is very small relative to their propellant mass- which also makes them cost much less mass to insulate, and the structural components of their walls are much thicker...)

Non-cryogenic tanks (default and balloon tanks) *should* scale with the 4/9th power, such that it's almost impossible to have tiny tanks that don't boil-off very quickly: this is actually how it works in real life.

One way or another, there needs to be more of an incentive to use large fuel tanks. At first I suggested tank structural mass scale with the Square-Cube law, but it turns out that is completely inaccurate as fuel tanks are pressure vessel in space. The Square-Cube Law *does* apply to the relative amounts of insulation required for the same effect with different tank sizes, and the base rates of heat leakage, however- which means the required amount of insulation mass should scale with roughly the 2/3rd power of volume (actually, it's even more favorable for larger tanks than that, as the thicker walls require for structural integrity act as low-quality insulators in themselves...)

The also currently aren't enough ways to mitigate boil-off. Thermal fins are heavy, expensive, and currently almost impossible to use with FAR as their surface attachment is too weak, and they tend to tear off during ascent. Even if they make it to orbit, they seem to be bugged and not all that effective. Plus, why should they work? All a thermal fin can do is cool a rocket to ambient temperature (which actually should be quite high near Kerbin or any other planet, in comparison to cryogenic temperatures, due to IR give-off from the planet...) Active Cooling systems work MUCH better, because with enough power, they can cool a fuel tank to any given/desired temperature.

Creating a realistic (4/9th power) relationship between tank volume and boil-off is one way to mitigate that, as it would allow players to somewhat reduce the effect simply by making the tanks bigger. Allowing players to more heavily insulate (perhaps with a tweakable insulation slider- where insulation mass scales with the 2/3rd power of volume), and actively cool their fuel tanks would be another. One way or another, it shouldn't be harder to mitigate tank boil-off in KSP than it is in real life (in real life, you can make the tanks larger, better insulated, and actively cooled. You can also store propellants in chemically-stable low-boiloff forms, such as LH2/LOX as water.)

Regards,

Northstar

Edited October 30, 2014 by Northstar1989

[Starwaster]

The also currently aren't enough ways to mitigate boil-off. Thermal fins are heavy, expensive, and currently almost impossible to use with FAR as their surface attachment is too weak, and they tend to tear off during ascent. Even if they make it to orbit, they seem to be bugged and not all that effective. Plus, why should they work? All a thermal fin can do is cool a rocket to ambient temperature (which actually should be quite high near Kerbin or any other planet, in comparison to cryogenic temperatures, due to IR give-off from the planet...) Active Cooling systems work MUCH better, because with enough power, they can cool a fuel tank to any given/desired temperature.

Northrats, They're 40kg. That's not heavy.

And they are active cooling because they consume electricity. Just pretend that the fin is only the visible radiator part of it or something.

Here, try this.

@PART[ThermalFin]
{
	@node_attach = 0, -0.66, 0, 0, -1, 0, 1

	@MODULE[ModuleHeatPump]
	{
		@heatDissipation = 1.0
		@heatGain = 1.0
		@heatConductivity = 0.0
	}
}	

I think that still works in the current HeatPump class.

[dreadicon]

I apologize if this has been mentioned prior, but what in the name of all that is kerbal is the black magic equation used to divine the volume of a given tank by it's dimensions? closest I have come up with is cubic meters * 1000(liter conv) * 0.25. ish. Is this right? is there a logic to it? Am I seeking that which does not and cannot exist?

I only ask because I am getting close to finishing fuel-filled Procedural Wings, and now that I am getting down to the nitty gritty of exact maths, I find myself perplexed.

Another question, which tank(s) would be most appropriate for a wet wing? Or if none of the existing tanks are, could someone with more knowledge of real life wet wings give some suggestions for a tank config? (utilized space, mass per liter, etc)

Thanks for a great mod as always! And to anyone who can lend some advice/comments!

EDIT: disregard about the black magic; I was tripping over the numbers somewhere. Maths all work out now!

Edited October 31, 2014 by dreadicon

[Starwaster]

I apologize if this has been mentioned prior, but what in the name of all that is kerbal is the black magic equation used to divine the volume of a given tank by it's dimensions? closest I have come up with is cubic meters * 1000(liter conv) * 0.25. ish. Is this right? is there a logic to it? Am I seeking that which does not and cannot exist?

I only ask because I am getting close to finishing fuel-filled Procedural Wings, and now that I am getting down to the nitty gritty of exact maths, I find myself perplexed.

Another question, which tank(s) would be most appropriate for a wet wing? Or if none of the existing tanks are, could someone with more knowledge of real life wet wings give some suggestions for a tank config? (utilized space, mass per liter, etc)

Thanks for a great mod as always! And to anyone who can lend some advice/comments!

No particular black magic, this is science. If you know how much of a structure in cubic meters can hold fuel then that * 1000 is your volume in liters. Problem solved. Not sure what the 0.25 in your equation is for unless that's your attempt to figure out how much of the wing can hold fuel..... it seems reasonable.

As a point of reference, try making a wing roughly the dimensions of a 747's wings. Here's a picture showing what one of those wings can hold (in gallons, liters, etc)

[dreadicon]

No particular black magic, this is science. If you know how much of a structure in cubic meters can hold fuel then that * 1000 is your volume in liters. Problem solved. Not sure what the 0.25 in your equation is for unless that's your attempt to figure out how much of the wing can hold fuel..... it seems reasonable.

As a point of reference, try making a wing roughly the dimensions of a 747's wings. Here's a picture showing what one of those wings can hold (in gallons, liters, etc)

http://i.imgur.com/aOtrqa6l.jpg

hey! thanks for the idea of using the 747 as a reference! And the picture provides a great reference for fuel contents!

After re-checking my numbers, I am not sure where I was going wrong. But the numbers all make sense now. I am having trouble tracking down the volume of the 747's wing itself though (not the fuel capacity). I could theoretically just base it off the weight, but.....not preferable.

[NathanKell]

Note that rocket propellants are placed in pressurized tanks. Yes, even pump-fed (vs. pressure fed) engines need pressurized tanks, but pressure-fed engines need highly pressurized tanks (which mass a lot more). This means two things:

1. Tanks mass more than your average car/airplane gas tank

2. Tanks generally are rounded, so the walls can be made strong with limited reinforcement (usually tanks are capsule / pill-shaped, though sometimes spherical or toroidal)

#2 in particular means you can't just take the (exterior) volume of an object and say "that's how much propellant it holds." The "tank" parts you see in KSP are really stages, inside which are two or more capsule tanks. This means a fair amount (somewhere between 10 and 25%) of the volume is not utilized by the tank. That's why the volume available doesn't correspond to a calculation of the volume of the part.

As for what tank type to use, I suggest probably Default. But you will need to handle interaction with FAR for mass...you should ignore any basemass and only increase above and beyond FAR's mass based on tank mass.

[Starman4308]

Hey Nathan, would you mind mentioning on the first page that liquid ammonia is the only non-cryogenic* nuclear fuel? It's kind of an important consideration for interplanetary missions.

*Unless a minute of max timewarp isn't enough to see boiloff. But even if there is slow boil-off, it's so slow that it wouldn't matter for 99% of missions.

[Starwaster]

Hey Nathan, would you mind mentioning on the first page that liquid ammonia is the only non-cryogenic* nuclear fuel? It's kind of an important consideration for interplanetary missions.

*Unless a minute of max timewarp isn't enough to see boiloff. But even if there is slow boil-off, it's so slow that it wouldn't matter for 99% of missions.

Actually, it IS cryogenic. It just happens to have a higher boiling temperature than most cryogenics. Because the temperature in space (in KSP, both stock and in RSS) doesn't actually get that high (-77C) You probably won't see a tank of it actually reach its boiling temperature even without cooling apparatus. (which is fortunate...). But even if you do reach its boiling point, again, it's so high that your boiloff rate is going to be really low. Even on the ground.

And (sorry, nitpick time), it is NOT a fuel. The only thing that could be considered fuel in a solid core NTR is the uranium fuel rods. Ammonia, hydrogen, methane and water are all propellants. Or 'working fluid'. Both are acceptable terms

Trivia: Discovery from the movie 2001: A Space Odyssey used ammonia in its nuclear engines. That's why its tanks hadn't all boiled off by the time Leonov arrived in 2010.

Edit:

Actually.... well this is interesting. A government website I just looked at (OSHA) lists its boiling point as -33 C (actually -28 F because we're Americans and we don't use metric. Bastards.)

Engineering Toolbox also lists it as -33C but also provides a lower range of temperatures starting at -60F (-51C) and lists increasingly higher pressure ratings as the temperature increases. My guess is that -33C represents the highest acceptable temperature that it can be kept at without being hazardous.. I'm not sure where Real Fuels is getting a temperature of -78. The closest thing that corresponds to is Ammonia's freezing point of -77 C. So the TANK_DEFINITION values are wrong for ammonia. I'm the one who originally wrote up ammonia for Real Fuels so I hope that's not my number..... if it is, my fault, it's wrong. If someone else changed it then they're wrong, their fault. (I hope it was someone else)

Edited November 1, 2014 by Starwaster

[Starman4308]

Actually, it IS cryogenic. It just happens to have a higher boiling temperature than most cryogenics. Because the temperature in space (in KSP, both stock and in RSS) doesn't actually get that high (-77C) You probably won't see a tank of it actually reach its boiling temperature even without cooling apparatus. (which is fortunate...). But even if you do reach its boiling point, again, it's so high that your boiloff rate is going to be really low. Even on the ground.

And (sorry, nitpick time), it is NOT a fuel. The only thing that could be considered fuel in a solid core NTR is the uranium fuel rods. Ammonia, hydrogen, methane and water are all propellants. Or 'working fluid'. Both are acceptable terms

Trivia: Discovery from the movie 2001: A Space Odyssey used ammonia in its nuclear engines. That's why its tanks hadn't all boiled off by the time Leonov arrived in 2010.

Okay, sorry, liquid ammonia propellant is not, for practical purposes, subject to boil-off once you get it to orbit.

Also, is sub-boiling evaporation/sublimation modeled? The ambient temperature in LKO is -200C, which should be colder than methane's boiling point (its melting point too, for that matter*), but I did see loss of methane from that tank in LKO.

*There's probably somebody who'd get a kick out of having to melt frozen propellants before use.

[Starwaster]

Okay, sorry, liquid ammonia propellant is not, for practical purposes, subject to boil-off once you get it to orbit.

Also, is sub-boiling evaporation/sublimation modeled? The ambient temperature in LKO is -200C, which should be colder than methane's boiling point (its melting point too, for that matter*), but I did see loss of methane from that tank in LKO.

*There's probably somebody who'd get a kick out of having to melt frozen propellants before use.

On the subject of frozen propellants, see my edit:

The tank definition in RF lists ammonia's boiloff point as -78 which is why I cited that (typo actually). But even as I was typing it, the number just felt wrong. So I looked it up on a few different websites and that's way too low a temperature value. Ammonia freezes at -77.77 so -78 is frozen ammonia. As I mentioned in my edit there's a chance that that's my mistake because I wrote up the original tank definitions for it but I just don't remember if that's my number or if someone else came along and changed it. (you can guess which one I'm hoping for....)

(oh and, no, evaporation isn't modeled. Boiloff is the tank venting so that it doesn't burst after the contents started boiling)

[NathanKell]

Liquid Ammonia is indeed cryogenic; boiling point of -78C. That said, I'm reworking how boiloff works due to Northstar1989's info...

Edit: ninja'd by Starwaster; will fix that too.

[Starman4308]

(oh and, no, evaporation isn't modeled. Boiloff is the tank venting so that it doesn't burst after the contents started boiling)

Figured out what was going on with the liquid methane boil-off. #1: I didn't account for the fact that very low Kerbin orbit is warmer: up to about -60 degrees Celsius, enough to boil methane. #2: there are weirdnesses with temperature calculation during time warp: it doesn't properly update, so if start timewarp at very low orbit, the tank/ambient temperature will persist through timewarp, even if most of your orbit is in the -200C zone.

Ergo, for now, it should suffice for me to lift up my methane-propelled rockets to the -200C zone, and kill time warp once I'm out of the boiling zone.

[FreeThinker]

Hey, is there anyway to change the waste produced by TAC LS electrolysis be change into hydrogen?

[Northstar1989]

Hey, is there anyway to change the waste produced by TAC LS electrolysis be change into hydrogen?

Dreadicon was working on a config to fix that a while back.

Speaking of which, how *is* that effort going Dreadicon?

Regards,

Northstar

[Northstar1989]

Also, I hate to double-post, but this is an entirely different subject...

@NathanKell

If the Thermal Fins are actually working correctly in the latest version (speaking of which, have you received any further word from Dreadicon about the Interstellar integration-config he was working on? There were still a few odds-and-ends we didn't get around to dealing with, but I think Dreadicon won't be able to fix those particular issues for quite a while, and I didn't even quite understand what some of the things he wanted to fix were- so it might be good enough in its current state for now...) I was wondering...

What are the chances you might be able to add more aerodynamic Thermal Fins?

As I discussed before, the Thermal fins currently have a tendency to *rip off* when they are attached to the exterior of a rocket with FAR installed (especially one with high TWR- but they seem *quite* flimsy, and even rip off low-TWR rockets with fairly vertical ascent trajectories in my experience...) This would probably be significantly alleviated if you created longer versions of the Thermal Fins, with a more streamlined shape...

I was particularly thinking that instead of having to attach two sets of Thermal Fins in 8x symmetry around the exterior of some of my larger fuel tanks (greatly inflating part-count), which inherently creates a LOT of drag in FAR, maybe you could add an 8x or 10x length version of the Thermal Fin that is correspondingly heavier and more effective, and also has a stronger surface-attachment if that's possible (to account for the greater area of contact with the surface it is attached to).

A longer Thermal Fin could also have a more streamlined, triangular shape, which would generate a *LOT* less drag in FAR that the current Thermal Fins, which just tend to jut out rather abruptly (that is, their top-side slope is quite abrupt compared to what could be achieved with a much longer Thermal Fin). And it would (realistically) also be an opportunity to offer a bulk-deal on radiator surface area, with a 10x length fin costing LESS than 10x as much as a standard-sized Thermal Fin...

Longer, more streamlined Thermal Fins, with stronger surface-attachments would be GREAT for my super-large fuel depots and ultra-heavy lifters (which otherwise suffer from part-count spam and excessive drag from the current short-length Thermal Fins), and also much less likely to tear off during ascent. They would also help to make up for the fact that larger fuel-tanks don't have any kind of an advantage over smaller tanks in terms of boil-off rate in RealFuels like they do in real-life... (ESPECIALLY if the larger fins were more cost-effective...)

Regards,

Northstar

P.S. Longer radiator fins also make more sense for two other reasons. First, in real life you don't want radiators radiating heat onto each other- which is what would happen if you placed radiators every 22.5 degrees around the outside of a fuel tank (i.e. two sets in 8x symmetry). Second, longer fins create a LOT less drag compared to a comparable radiator area of short, stubby fins- which is particularly important for larger rockets (which need a LOT of radiator area to be effective, and normally benefit from a MUCH better ballistic coefficient than smaller rockets- something spamming stubby thermal fins works strongly against). Longer, more streamlined thermal fins help avoid both problems.

Edited November 11, 2014 by Northstar1989

[Starwaster]

Making the fins longer won't make FAR less prone to ripping them off, but increasing the attach node size should. Did you try the config I offered you? It did in fact increase the surface attachment size (so, yes, surface attachment nodes can have their size increased)

[jrandom]

What we need is some talented modder to make a tweakscalable stack radiator that extends fins laterally from inside, so they can be deployed once safely out of the atmosphere...

[Starwaster]

What we need is some talented modder to make a tweakscalable stack radiator that extends fins laterally from inside, so they can be deployed once safely out of the atmosphere...

Didn't someone make one for another mod? If not Interstellar, then another?

[jrandom]

Didn't someone make one for another mod? If not Interstellar, then another?

Oh man, I haven't looked at Interstellar in ages. I'll give it another check, see if there's useful parts I want.

[Northstar1989]

Making the fins longer won't make FAR less prone to ripping them off, but increasing the attach node size should. Did you try the config I offered you? It did in fact increase the surface attachment size (so, yes, surface attachment nodes can have their size increased)

Increasing fin length *on its own* wouldn't. But increasing length, mass AND node strength/size would- as the magnitude of aerodynamic forces on the fins less than doubles with a doubling of length (reducing the overall acceleration on the fins away from the main rocket), *especially* if the longer fins were given a more streamlined shape; and a double length fin justifies a double-strength attachment...

Let's say you have two fins, one that juts out 1 meter from the rocket body at its base (having a triangular shape and constant slope out from the rocket body), is 1 meter long, and weighs 10 kg. The other juts out 1 meter at its base, is 2 meters long (thus having a more gradual slope), weighs 20 kg, and has twice the surface node attachment-strength (realistic considering it has twice the area in contact with the surface). Which do you think would generate more drag for its mass and radiator area? Which do you think would be more prone to ripping off during flight?

Regards,

Northstar

Edited November 11, 2014 by Northstar1989

[Northstar1989]

Didn't someone make one for another mod? If not Interstellar, then another?

Interstellar has its own system of radiators, completely separate from RealFuels (which include non-deployable inline radiators). AFAIK, you could *SPAM* ten dozen radiators from Interstellar on a RealFuels craft, and it wouldn't affect boil-off rate *at all*.

Regards,

Northstar

[Starwaster]

<Two paragraphs worth of diatribe SNIPPED!>

I asked a very specific question to which I have to take the answer as no. It had to do with whether you'd tried out a solution I recommended that addressed two different problems that you've posed. I have to take this to mean that you weren't really interested in a solution.

Interstellar has its own system of radiators, completely separate from RealFuels (which include non-deployable inline radiators). AFAIK, you could *SPAM* ten dozen radiators from Interstellar on a RealFuels craft, and it wouldn't affect boil-off rate *at all*.

1. Totally missing the point, the discussion is about models and not radiator implementations.
   
2. RF radiators definitely have some issues (which sadly will not see a fix that was coded and pushed) but your assessment is totally wrong, especially using the solution I offered which was in the form of a Module Manager patch. Very easy to use.
   

Edited November 12, 2014 by Starwaster

[Spanier]

Are you planning to add variety to solid fuels?

[Northstar1989]

I asked a very specific question to which I have to take the answer as no. It had to do with whether you'd tried out a solution I recommended that addressed two different problems that you've posed. I have to take this to mean that you weren't really interested in a solution.

No, I'm just not interested in modding my mods any further than I'm already doing (trying to develop an updated KSP-Interstellar/RealFuels integration config in combination with Dreadicon) just to get what should be a rather core/essential part of the mod working. If the surface attachment points are too weak *for the current size of the thermal fins*, then NathanKell should push a fix to the base mod. However, what *I* suggested is larger/longer thermal fins, which would kill two birds with one stone- both the problem of needing to spam thermal fins for larger fuel tanks, and the issues with thermal fins ripping off (once again, a fin with twice the length, the same radius at the base, twice the mass, and twice the surface attachment area would be far less prone to ripping off...)

Your "fix" only solves one of these issues- and not in a very realistic/satisfactory manner either (the reason the fins rip off is because they're not shaped very aerodynamically- not just because the surface attachment points are too weak. Longer fins can be made MUCH more aerodynamic- consider the differences between a 1x1 right triangle and a 1x8 right triangle, and tell me which is more aerodynamic...)

Totally missing the point, the discussion is about models and not radiator implementations.

You asked if KSP-Interstellar had radiators- I answered your question, and then went on to point out that they don't work with RealFuels. I missed nothing. Why are you being so abrasive/critical lately?

RF radiators definitely have some issues (which sadly will not see a fix that was coded and pushed) but your assessment is totally wrong, especially using the solution I offered which was in the form of a Module Manager patch. Very easy to use.

You've completely confused me by writing there would be no fix, and then crossing that out without a real clarification. Are there going to be any fixes to the Thermal Fins in the base RealFuels mod or not?

The current Thermal Fins are far from usable/acceptable (besides being too small, thus requiring part-spam; and too blunt- thus ripping off with FAR; they also aren't nearly powerful enough compared to real-life active-cooling systems, and *in aggregate* are heavier and more expensive when you're using dozens of fins for the same effect a single device would accomplish in real life, *especially* for the larger fuel tanks, which benefit heavily from not having to miniaturize cooling technology as much...)

Regards,

Northstar

Edited November 13, 2014 by Northstar1989

[Hancer]

Dear All, I have a question:

I am using RSS and RO incl all the required mods. I am quite sufficient with the info and know-how and able to realise GEO orbits and such with these mods. But I am very poor at chemistry and fuel types and get sometimes confused. Could you please answer me these questions:

1-Procedural fuel tanks: There is a tank type selection such as fuselage, default, baloon..etc. Could you please tell me which tanks are for which fuel? Because when comparing delta v, default tank seems to get 200-300 m/s more with a default tank than cyro..etc and I see no difference in operating. However, sometimes by J2 engines wont ignite in space, so I am sure its about selecting a proper tank for the fuels. So please guide me which tanks I should be using for vacuum, fuel type and surface.

2-I am using Engine Ignitors: I still couldnt figure out how to take extra electrical ignitions with the tool box. It doesnt add additional electrical ignitons no matter where I place it.

3- "Need settling down": I am using ullage to get some g before ignitions but sometimes they work sometimes they dont. I think related with question 1.

4-Which type of fuels should I be using for most efficient 1st and 2nd stage with TWR around 1.25-13.0 at take off (to get around 4000-5000 delta v). Generally kerosene and LOX at main stage and lox and lh2 at second stage?

5-No matter what I try, I cannot get pass 16k delta v at RSS and RO with a payload of 12tonnes. So I think its impossible to have a manned flight to mars in rss/ro with saturn v type rockets.

Sorry for all these noobish questions about fuels.