Stock Radiator physics help?

[MaverickSawyer]

Got a bit of a headscratcher here... I'm trying to use the stock radiators to reject some built-up heat in my 1.25m probe. They pull the heat out just fine, but they also spike up to over 1200K and just... stay there. I'm confused, as there's no parts on the probe anywhere near that temperature. Why do they spike so much, and is there a way to control that thermal spike?

[Streetwind]

In 1.0.5, the radiators gained the ability to cool parts below their own temperature, as opposed to just to their own temperature. That is kinda useful, because heat rejection ability scales strongly with radiator temperature. Trying to keep parts below certain temperatures becomes increasingly difficult the lower that target temperature is, if the radiator itself cannot exceed that temperature.

Radiators right now can get up to four times as hot as whatever parts they are trying to cool. Only then will the radiator stop taking heat from those parts. If your radiators are at 1200 K, that would imply that your other parts are at 300 K. Which admittedly confuses me slightly, since in 1.0.4 radiators would not even try to cool anything that was not above 400 K, but who knows, that might have changed.

If you want to know a little more about what's actually going on inside your ship, hit alt+F12, go to the "physics" tab, then to the "thermal" sub-tab, and activate display of thermal debug data in rightclick menus. Now you can rightclick on any part you want and see its temperature and thermal capacity, as well as incoming and outgoing thermal flux of various kinds.

[Geschosskopf]

7 hours ago, MaverickSawyer said:

Got a bit of a headscratcher here... I'm trying to use the stock radiators to reject some built-up heat in my 1.25m probe. They pull the heat out just fine, but they also spike up to over 1200K and just... stay there. I'm confused, as there's no parts on the probe anywhere near that temperature. Why do they spike so much, and is there a way to control that thermal spike?

Well, as long as the ship is continually getting hot for some reason (engines running for a long time, being close to the sun, etc.), then the radiators will continue to try to get rid of that heat.  They do this by sucking the heat from throughout the vessel and accumulating it in themselves, from which point it gets radiated away to the environment.  Heat is energy, so basically you're concentrating the energy in the radiators.  This is why radiators get hotter than the rest of the ship. 

Radiator temperatures change due to the rate at which heat is being generated by the ship or applied by the sun compared to how fast the radiators can expel heat.  The rate of expulsion depends on the size of the radiator, its orientation to the sun, and the ambient temperature.  Bigger radiators expel heat faster than small ones due to more surface area.  If the radiator is facing the sun, it's being heated by the sun as well as what it sucks from the ship so can't expel heat as quickly (or at all).  And heat only flows from hot to cold, and the greater the difference in temperature, the faster it flows.  Thus, radiators cannot get colder than the ambient temperature (even if not facing the sun) and the closer their temperature is to the ambient level, the slower they expel heat to the environment.

So, if your radiators are getting hot and staying hot, it's because they suck up heat from the ship faster than they can get rid of it, and aren't getting rid of it very fast.  This is probably due to one of the above factors.  That is, they're too small for the job and/or are facing the sun and/or the ship is close to the sun.

[Snark]

And just to be clear, it's good that the radiators are blazingly hot.  The hotter they are, the faster they dump heat.  IRL the rate at which a body radiates energy goes up with the fourth power of its absolute temperature; dunno if the game models that, but in general, the hotter the better if you want to dump heat.

Something to be aware of:  that nifty new "suck heat out of things that are colder than me" ability of radiators comes at a price.  Radiators now consume electricity while they're operating.  They consume electricity constantly, at a constant rate, regardless of whether there's actually any heat that they need to extract.  So be aware of this:  active radiator = electricity drain.

For the folding panels, that's not such a problem-- when you retract them, it turns them off (they stop cooling and stop drawing electricity).  However, the static radiator panels are always on, always drawing power, and there's no way to turn them off.  So be sure you have enough electrical supply if you load up a ship with the static panels.  Or else you can install this mod that *cough* I wrote to add an on/off toggle for the static radiators.

[sal_vager]

Oh by the way, radiators will saturate and stop absorbing heat when they get too hot, it's something like 1800 to 1900 degrees, it's to prevent them from just exploding so you have a chance to stop generating heat, cool down and use them again.

[MaverickSawyer]

2 hours ago, Snark said:

Or else you can install this mod that *cough* I wrote to add an on/off toggle for the static radiators.

Yeah, I have that installed.

So, I tweakscaled the stock small fixed panels down to... Can't remember how much smaller. less than 50%, for sure, though. THey sucked the heat out of the overheating parts just fine... but they stayed very hot and were, in fact, slowly heating UP the parts they were near.

 

I wound up scrapping those radiators and switching to the ones from the Heat Management mod. They're not glowing red-hot anymore.

[Claw]

Are you using any other mods? There will likely be an initial spike in the radiators, though 1200 seems a bit high. After that spike, they should start cooling down if your probe truly is sitting there and doing nothing else.

 

Edit: Ah, ninja'd. Well, could you post a picture of the ship in question so we can have a look to see if it's a stock problem or a stock+addon interaction issue? Thanks!

[MaverickSawyer]

I'll reset that configuration and launch it again after work.

[Geschosskopf]

11 hours ago, Snark said:

For the folding panels, that's not such a problem-- when you retract them, it turns them off (they stop cooling and stop drawing electricity).  However, the static radiator panels are always on, always drawing power, and there's no way to turn them off.  So be sure you have enough electrical supply if you load up a ship with the static panels.  Or else you can install this mod that *cough* I wrote to add an on/off toggle for the static radiators.

That's a nifty mod, BTW :).

But to further elaborate, I recommend using folding radiators if at all possible instead of fixed ones, even with Snark's excellent mod.  The reason is because fixed radiators are fixed.  The folding ones rotate themselves edge-on to the sun, which minimized their exposure to solar heat, which maximizes their heat expulsion rate.  This is MUCH harder, if not impossible, to achieve with fixed radiators.  For instance, fixed radiators are often symmetrically arranged so some of them are probably always facing the sun and thus might be heating the ship up instead of coolling it down (heat can flow in both directions in the right circumstances).  Plus there are often constraints on the ship's orientation due to the need to thrust in a given direction, keep solar panels illuminated, etc., which force disadvantageous orientations on the radiators.

The net result is that if you use fixed radiators, you'll need more radiator capacity to provide a given amount of cooling than if you used retractable radiators.  For instance, keeping drills and ISRUs cool during operation.  Because the mining ship is on the ground at a fixed orientation to the sun (except for the passage of the sun overhead during the day), some of its fixed radiators will always be in sunshine so will be much less effective.   You thus need enough radiators on the shady side to do the whole job by themselves, and enough on what's currently the sunny side to do the job themselves later in the day once the sun as passes the zenith and they're in the shade in their turn.

[fourfa]

I'm not sure that it's actually modeled at that level of complexity.  I only know that I've been sending ships out a lot lately with one large ISRU (200kW), two large drills (2x100kW), and two large fixed radiators (2x200kW).  Day, night, Kerbin, Mun, Minmus, doesn't seem to make a difference.  The drills and ISRU just get up to temp and stay there.

The fixed panels are really light in comparison also.  And beware - I bumped a retractable radiator with a Kerbal on EVA and it shattered surprisingly easily. The always-on power draw - the always-on RTG covers the power draw of 30 radiators.  Just sayin.

[MaverickSawyer]

2 hours ago, Geschosskopf said:

But to further elaborate, I recommend using folding radiators if at all possible instead of fixed ones, even with Snark's excellent mod.  The reason is because fixed radiators are fixed.  The folding ones rotate themselves edge-on to the sun, which minimized their exposure to solar heat, which maximizes their heat expulsion rate.  This is MUCH harder, if not impossible, to achieve with fixed radiators.  For instance, fixed radiators are often symmetrically arranged so some of them are probably always facing the sun and thus might be heating the ship up instead of coolling it down (heat can flow in both directions in the right circumstances).  Plus there are often constraints on the ship's orientation due to the need to thrust in a given direction, keep solar panels illuminated, etc., which force disadvantageous orientations on the radiators.

Understood. The ship in question was oriented in such a way that the radiators were in the shade at the time I started them up.

I understand that they get hot, too. I've seen it with spikes up to around 900K on startup, but those gradually cool down. These radiators seemed to almost be trying to cool each other, and I somehow got into a thermal equivalent of a "perpetual motion" machine: little energy bleedoff, coupled with an input from the deployed solar panels...

Hmm. I have a pair of rather large panels installed, but that's so that I can get enough power at long distances from the sun. Maybe that's part of the problem?

 

EDIT: Nope. Nighttime launch revealed no differences. It's probably the fact that I scaled the panels down to 25% of standard size.

Edited December 11, 2015 by MaverickSawyer

[Claw]

FYI: The fixed radiators only pull heat from up to two parts away, while the retractable ones pull from the whole ship.

[MaverickSawyer]

1 hour ago, Claw said:

FYI: The fixed radiators only pull heat from up to two parts away, while the retractable ones pull from the whole ship.

Oh, really? Very interesting, and good to know. I wouldn't have guessed that.

[Claw]

Yes, that's a recent change. They used to be completely passive, which meant (practically speaking) they only pulled from the part they were attached to (and weren't very good at it). They were changed to "active" radiators but given a part reach of two. That way you can use them to cool something like an engine, which doesn't allow direct attachment.

[Geschosskopf]

6 hours ago, MaverickSawyer said:

Oh, really? Very interesting, and good to know. I wouldn't have guessed that.

There's something else to keep in mind.  Heat naturally flows from part to part within a ship even without radiators.  It goes from hotter parts to colder parts and only stops when all parts of the ship reach the same average temperature (assuming no more heat is being added).  This process, though, is relatively slow compared to the rate at which (for example) nuclear engines can create heat.  Also, every part radiates heat away to space using the same general mechanics as radiators, only way slower because these other parts ain't radiators.  This radiation process is even slower than the rate the parts absorb heat from nearby hot parts.

So, for example, you have a ship with a nuclear engine and no radiators, and you do a burn that gets the engine nice and toasty but not dangerously hot.  During the burn, only the engine (and maybe the part it's attached to) will get hot because heat can't seep out of the engine nearly as fast as the engine is creating it.  Then over time after the burn,  the engine's heat gradually works its way into all parts of the ship, raising their temperatures (perhaps significantly) while the engine itself cools down.  Eventually the ship reach thermal equilibrium with all  parts the same temperature, and then all parts very gradually cool off to ambient via radiation.  This is all fun to watch happen with the HotSpot mod :).

Now add fixed radiators to this, but put them more than 2 parts away from the engines.  Because the radiators can't reach the engine heat directly, they won't have much effect on the total heat energy in the ship until a significant portion of the initially concentrated engine heat seeps into parts the fixed radiator can reach.  And even when that happens, the rate at which the radiators reduce overall ship heat will be limited by the rate at which heat seeps between parts naturally.  Actually, it will happen a bit quicker than that because the radiators will be cooling whatever is within reach, increasing the thermal gradient between those parts and the hot parts, thus increasing the rate of heat seepage.  But it won't be by very much.  Still, the ship will eventually cool back down to ambient somewhat faster than having no radiators at all because the radiators are faster at throwing heat overboard than non-radiator parts.

If the fixed radiators are within 2 parts of the engine, then they should be cooling the engine directly,  This will prevent the engine from getting so hot so fast, and will result in less total heat build-up in the ship during the burn, so it will take considerably less time to cool back to ambient than otherwise.

Of course, with any fixed radiator, a lot depends on the orientation of the ship.

[Snark]

12 hours ago, Claw said:

FYI: The fixed radiators only pull heat from up to two parts away, while the retractable ones pull from the whole ship.

 

11 hours ago, Claw said:

They were changed to "active" radiators but given a part reach of two. That way you can use them to cool something like an engine, which doesn't allow direct attachment.

Can someone please clear this up for me?

I see statements like the above from Claw.  And I see that if I go and look at the static radiator panels' .cfg file, it does indeed have a "maxLinksAway" property on its ModuleActiveRadiator, which is set to 2.  It's clear that they're meant to be cooling only the two-links-away parts.

However, when I use panels in-game, I swear I see them cooling the whole ship.  I'll launch a ship with some static panels up top, and SRBs down at the bottom, and they're nowhere near "two links away"... and yet the radiators are glowing cherry-red very quickly after activating the SRBs, it's clear that they're pulling heat from them.  As far as I can tell, the static panels are cooling the whole ship, same as the active panels.  What gives?

(And I don't think it's just me. I'm pretty sure I've seen someone else in the forums mention observing the same thing, at some point.)

Is this a bug?

Edited December 11, 2015 by Snark

[Claw]

6 hours ago, Snark said:

Is this a bug?

Quite possibly. Posting a screenshot or sharing a .craft would help. The radiators received a lot of attention last go-around, but it's possible things are still not as intended.

[sal_vager]

I think you ran into this.

[Snark]

32 minutes ago, sal_vager said:

I think you ran into this.

Yep, that sounds like it.  Okay, you guys are on it, 'nuff said. 

[Nightshift83]

Hey there, been playing KSP since 0.7 or something like that, but never understood how radiators exactly work since they were introduced.

I mean if static ones can only cool up to two parts away, why should it matter ? Since as stated above temperature should distribute itself evenly across the whole ship.

Do they have any effect during atmospheric entry ? If yes how does it work.

And how does it work in real life btw ? Since they operate in (almost) vacuum how do they get rid of the heat they acumulated before ?

 

Oh and great community for a great game! Keep it up!

I will try to leave my sleeper role and try to be more active from now on

[Geschosskopf]

1 hour ago, Nightshift83 said:

Hey there, been playing KSP since 0.7 or something like that, but never understood how radiators exactly work since they were introduced.

That's not surprising, considering that radiators came into existence after the original heat model, more have been added since, and in the meantime the heat model itself has changed fundamentally 4 or 5 times, and the radiators mechanics have also changed at least once

1 hour ago, Nightshift83 said:

I mean if static ones can only cool up to two parts away, why should it matter ? Since as stated above temperature should distribute itself evenly across the whole ship.

Heat doesn't flow very fast by itself throughout the ship.  This is why engines can overheat and explode---they make heat faster than they pass it to other parts.  Thus, without radiators, it's possible that the engines will remain quite hot for a long time after a burn.  If you have to use the engine again soon, because it's already pretty hot to start with, the 2nd burn might be enough to blow it up.

1 hour ago, Nightshift83 said:

Do they have any effect during atmospheric entry ? If yes how does it work.

Radiators are best thought of as doors through which heat moves between the ship and the environment.  Heat only flows from hot to cold, so these doors will work in both directions.  If the environment is hotter than the ship, then the radiators will pull heat from the environment into the ship.

This is what SHOULD be happening during reentry.  The radiators are in the plasma which is way hotter than the ship, so should be making reentry even worse.  They certainly shouldn't be cooling the ship, and in fact should be doing the opposite.  Whether the game actually works this way or not, however, is a question I've never investigated.  I've never tried it because it SHOULD NOT work, and if it does work, it's a bit too much of an exploit for me.

1 hour ago, Nightshift83 said:

And how does it work in real life btw ? Since they operate in (almost) vacuum how do they get rid of the heat they acumulated before ?

Heat moves by 3 mechanisms:

• Conduction:  This is heat moving from 1 thing to another (or throughout a big thing) due to direct physical contact.  This is why heat spreads (eventually) from Part A to Part B of a ship, and why if you stick a metal spoon in a fire, pretty soon the end you're holding gets hot.
• Convection:  This only happens in atmospheres and oceans.  The hot object heats up the surrounding fluid.  That little packet of fluid therefore expands, which decreases its density, so that it rises with respect to the rest of the fluid surrounding everything.  This is why smoke rises and why your hand gets hotter faster if you hold it over a flame than beside the flame.
• Radiation:  Heat is, at the bottom line, just energy.  Energy is, at the quantum level, photons.  So any object hotter than its surroundings will spew out photons in all directions at the speed of light, each of these carrying away some of the object's heat.  This process is called "radiation" and is why radiators are called "radiators".  It's also how incandescent lightbulbs work, they're just tuned so that some of the photons they emit are at visible wavelengths.

Radiated heat is a MAJOR thing.  It's why you can't stand close to a big fire, it's why a burning house can ignite the one next door even when that's 50 feet away on the upwind side.  And it's why radiators work in space.

 

[Snark]

1 hour ago, Nightshift83 said:

I mean if static ones can only cool up to two parts away, why should it matter ? Since as stated above temperature should distribute itself evenly across the whole ship.

Eventually, yes.  However, the passive heat flow when it conducts between adjacent parts is quite slow.  If your ship has parts that get hot, it's iffy (at best) to rely on their conducting heat to neighboring components (especially since those components then get hot).  Radiators pull the heat out much more quickly and efficiently.

1 hour ago, Nightshift83 said:

Do they have any effect during atmospheric entry ? If yes how does it work.

I never bother with 'em for reentry, since I use heatshields which are far more effective.  Heatshields work for me because I'm not a spaceplane guy.

I've seen posts from folks who use spaceplanes, and apparently they use them to mitigate reentry heat, but that's just second-hand hearsay from me.  I would guess that they put the panels on top of the plane where they're out of the airflow during reentry. 

So I'd say that (tentatively) the answer to your question is yes, though it would be good to confirm that with someone who actually uses them for that purpose.

1 hour ago, Nightshift83 said:

And how does it work in real life btw ? Since they operate in (almost) vacuum how do they get rid of the heat they acumulated before ?

Simple, they radiate.  Thus the name.

It's the same as when you step out in the sunshine and you can feel it warming you up, even though you aren't touching the sun and there's vacuum between you and it.  Any time an object is surrounded by an ambient environment that's colder than itself, it will radiate to that environment.

Spacecraft with radiators rely on the fact that the ambient temperature of space is cold.  Really, super-duper cold, as in the temperature of the cosmic background radiation, which is (IIRC) less than three degrees K.

So anything that's exposed to the blackness of space will radiate furiously to it.

Of course, spacecraft are also usually in the sunshine, and the sun is radiating heat, so they get hot that way.

The key, then, is to minimize the heat they absorb from the sun, while maximizing the heat they radiate to space.  They do this by making the parts that are exposed to sunshine as shiny as possible (bright white paint; gold foil) so that they reflect the sunshine rather than absorbing it; and then arranging the geometry of the ship so that the radiators are either shaded from the sun, or else turned edge-on to it so that they don't pick up much sunshine.

You can see that latter effect in KSP in the folding radiator panels, which automatically rotate to keep themselves edge-on to the sun, just alike real spacecraft do.

[Nightshift83]

Thanks alot for the above answers. The heat=energy=photon explanation solved that mystery for me eventually. Kind of embarrassing, since it's not like I've never read Stephen Hawkins or the likes but I just couldn't think of it somehow (and I really thought about it quite a while).

About ingame simulation (reentry) I think I tried it once or twice but it didn't have any extreme effect as far as I can remember (post 1.0). But yeah I guess it would be quite fatal in real life.

Anyways thx again for the informations.. now checking: https://www.nasa.gov/mission_pages/station/structure/elements/radiators.html

[PTNLemay]

On 12/10/2015 at 4:26 AM, Streetwind said:

If you want to know a little more about what's actually going on inside your ship, hit alt+F12, go to the "physics" tab, then to the "thermal" sub-tab, and activate display of thermal debug data in rightclick menus. Now you can rightclick on any part you want and see its temperature and thermal capacity, as well as incoming and outgoing thermal flux of various kinds.

In my opinion, this should be a normal (non-cheated) feature in the game.  Like... an advanced heat-monitoring system part that you attach onto the rocket and it can give you full-thermal info.  It could be late in the tech tree, since I do agree it's a bit overpowered.   But... eventually being able to have it would be super convenient.  And sticking little thermometers all over the place becomes annoying after a while.

Edited January 8, 2016 by PTNLemay

[Geschosskopf]

2 hours ago, PTNLemay said:

In my opinion, this should be a normal (non-cheated) feature in the game.  Like... an advanced heat-monitoring system part that you attach onto the rocket and it can give you full-thermal info.  It could be late in the tech tree, since I do agree it's a bit overpowered.   But... eventually being able to have it would be super convenient.  And sticking little thermometers all over the place becomes annoying after a while.

Or you could get the HotSpot mod, that VASTLY improves the normal F11 heat color diagram, plus inserts detailed temperature readouts into the right-click menus of every part.  The latter is often "too much infoirmation" because it forces most of the buttons way down on a vastly taller pop-up, but sometimes it's useful.  I mostly put up with it for the F11 view.  But no extra parts, no tech tree problems, available at once.

Don't get me started on the basic, bare-bones instrumentation mods that should be stock.  I've got a very long list