Engines and Ablator

[TiktaalikDreaming]

I'm messing with a massive expansion deflection engine for a mod that was designed to use ablative cooling rather than fuel based regenerative cooling for the nozzle. I was thinking that the ablative module might be useful if it reduces heat while ablating. AKA, set it so it starts ablating at a fairly low temp and see if I can balance the numbers so it's fine until it exhausts the ablator resource. And then it should overheat pretty fast. But it doesn't seem to do anything about heat, it just deminishes if the temp is over X, and makes no change to temperature changes.

Does anyone know how the ModuleAblator works? It seems like basically a clock that starts once the temp exceeds it's start value, and the existence or not of the resource makes no difference. :-/

[Guest]

Ablator is a very weird resource. To put it plainly, you'd be better with radiators, and call it a different cooling method. The main thing with ablator is that it keeps the part with ablation cool, not the parts around it. In reentry for example, the "fire" hits the heat shield, and the heat shield keeps itself cool, and prevents the rest of the craft from touching the reentry head. If you have the smallest hole between or in heat shields, the part it is covering will overheat. I hope this is helpful. Also, ablator is a resource that you can't transfer(you can't have ablator tanks.)

[TiktaalikDreaming]

Ablator is a very weird resource. To put it plainly, you'd be better with radiators, and call it a different cooling method. The main thing with ablator is that it keeps the part with ablation cool, not the parts around it. In reentry for example, the "fire" hits the heat shield, and the heat shield keeps itself cool, and prevents the rest of the craft from touching the reentry head. If you have the smallest hole between or in heat shields, the part it is covering will overheat. I hope this is helpful. Also, ablator is a resource that you can't transfer(you can't have ablator tanks.)

In this case I'm intending to use it to model the evapourative cooling provided by sacrificial layers of (usually copper) metal on the inside of an engine nozzle.

Rockets are typically ablatively cooled or regeneratively cooled. Regen cooling usually means passing a cryo propellant through pipes around the nozzle before feeding it into the engine. It preheats the propellant and cools the nozzle. Ablative cooling is where the inside of the nozzle is made of a material that will melt/evapourate and the phase change will provide cooling to the material in the nozzle that has yet to boil off. So, the ablator resource should be mostly just cooling the part in question. And once it's gone, the nozzle should heat up dramatically.

Basic testing suggests that the cooling isn't actually happening. In particular, I suspect it won't work unless it's "re-entry heat". But it may be to do with how I've set up the rather confusing ModuleAblator. I was hoping someone had more info on the settings or if the ModuleAblator actually does do what I think it does.

[Guest]

In this case I'm intending to use it to model the evapourative cooling provided by sacrificial layers of (usually copper) metal on the inside of an engine nozzle.

Rockets are typically ablatively cooled or regeneratively cooled. Regen cooling usually means passing a cryo propellant through pipes around the nozzle before feeding it into the engine. It preheats the propellant and cools the nozzle. Ablative cooling is where the inside of the nozzle is made of a material that will melt/evapourate and the phase change will provide cooling to the material in the nozzle that has yet to boil off. So, the ablator resource should be mostly just cooling the part in question. And once it's gone, the nozzle should heat up dramatically.

Basic testing suggests that the cooling isn't actually happening. In particular, I suspect it won't work unless it's "re-entry heat". But it may be to do with how I've set up the rather confusing ModuleAblator. I was hoping someone had more info on the settings or if the ModuleAblator actually does do what I think it does.

you are correct in saying that it isn't really "cooling," but just preventing heat. Could you provide the files so I know exactly what you've done?

[NathanKell]

ModuleAblator affects skin temperature, not internal temperature. So it's not really going to work for this application.

Short primer:

When part skin temperature > threshold, add a negative flux to the part's skin equal to e^(part_internal_temperature / lossexp) * lossConst * ablator.amount * ablator.density * ablator.hsp * pyrolysisLoss. The loss rate (in units) is all but the last three.

[problemecium]

I like this concept! Good luck making it work~!

[Guest]

ModuleAblator affects skin temperature, not internal temperature. So it's not really going to work for this application.

Short primer:

When part skin temperature > threshold, add a negative flux to the part's skin equal to e^(part_internal_temperature / lossexp) * lossConst * ablator.amount * ablator.density * ablator.hsp * pyrolysisLoss. The loss rate (in units) is all but the last three.

Good point

[TiktaalikDreaming]

ModuleAblator affects skin temperature, not internal temperature. So it's not really going to work for this application.

Short primer:

When part skin temperature > threshold, add a negative flux to the part's skin equal to e^(part_internal_temperature / lossexp) * lossConst * ablator.amount * ablator.density * ablator.hsp * pyrolysisLoss. The loss rate (in units) is all but the last three.

Ah. See, I wasn't even aware things had a skin and core temp. :-/ Oh well. Back to ablator being a propellant.

[Guest]

Ah. See, I wasn't even aware things had a skin and core temp. :-/ Oh well. Back to ablator being a propellant.

Just remember, you can change the heat displacement values to keep it cool. You could say that you have a radiator inside the cryogenic fuel tank.

[NathanKell]

Use an alternator module for it, instead of a propellant. If it's a propellant it'll count for Isp, if it's an alternator it'll just consume the resource. Use a negative generation rate to use the resource up.

[TiktaalikDreaming]

Just remember, you can change the heat displacement values to keep it cool. You could say that you have a radiator inside the cryogenic fuel tank.

The point is that it's not cooled by the fuel, like a regular engine. The cooling mechanism eats away at the inside of the nozzle and should drop the mass as it goes. So, to model the performance, the engine needs to drop mass. To model the heat exchange, it looks like I can't use ModuleAblator. I'll release with dodgy work-around and put it on my list of modules to look at when/if I ever get around to installing visual studio.

- - - Updated - - -

Use an alternator module for it, instead of a propellant. If it's a propellant it'll count for Isp, if it's an alternator it'll just consume the resource. Use a negative generation rate to use the resource up.

Oh. Yes. Good thinking. :-)

[TiktaalikDreaming]

Use an alternator module for it, instead of a propellant. If it's a propellant it'll count for Isp, if it's an alternator it'll just consume the resource. Use a negative generation rate to use the resource up.

OK, weirdest thing. It looks like if I use ModuleAlternator, the resource value is automagically set to start at 0, regardless of what I set. It carries through the max storable, but starting ablator is set to zero. In case it was a processing order thing, I moved the resource tag to after the module, but it still has the same result. I'll probably release using propellant but keep working on a suitable replacement.

- - - Updated - - -

OK, weirdest thing. It looks like if I use ModuleAlternator, the resource value is automagically set to start at 0, regardless of what I set. It carries through the max storable, but starting ablator is set to zero. In case it was a processing order thing, I moved the resource tag to after the module, but it still has the same result. I'll probably release using propellant but keep working on a suitable replacement.

Ooo.... Multimode engine. With or without using ablator. Different heat and Isp values. :-)

[NathanKell]

Oh crap you're right. I forgot about that quirk of ModuleAlternator :\

[TiktaalikDreaming]

Oh crap you're right. I forgot about that quirk of ModuleAlternator :\

No worries. The idea made sense, even if the module doesn't work as expected. And it forced me to look at alternatives, which drove me to the multimode engine. Multimode acheives the goal of overheating once out of ablator, plus a decreasing mass during burn. I really should calculate the propellant mass fraction for ablator to adjust the Isp correctly. I just guessed for release. :-) It may be wildly too efficient if I guessed wrong.

OK, I was wildly optimistic on how little mass it would add to the exhaust. 24% Holey crap.

Edited August 25, 2015 by TiktaalikDreaming
updated with numbers

[linuxgurugamer]

On 8/23/2015 at 2:00 PM, NathanKell said:

ModuleAblator affects skin temperature, not internal temperature. So it's not really going to work for this application.

Short primer:

When part skin temperature > threshold, add a negative flux to the part's skin equal to e^(part_internal_temperature / lossexp) * lossConst * ablator.amount * ablator.density * ablator.hsp * pyrolysisLoss. The loss rate (in units) is all but the last three.

@NathanKell

Can you explain what these values are?  What is "e"?   I see the following in one of the heat shields, but don't understand it:

MODULE
	{
		name = ModuleAblator
		ablativeResource = Ablator
		lossExp = -7500
		lossConst = 0.1
		pyrolysisLossFactor = 6000
		reentryConductivity = 0.01
		ablationTempThresh = 500
	}

 

[Guest]

21 minutes ago, linuxgurugamer said:

Can you explain what these values are?  What is "e"?

I believe that it is being used as the variable for how much energy is present.  (Think e=mc²)

[NathanKell]

https://en.wikipedia.org/wiki/E_(mathematical_constant)

 

The formula quoted explains how each of the values is used, excepting two. ablationTempThresh sets the threshold at which ablation begins; below that, no ablation occurs (the loss evaluation is skipped). reentryConductivity is used rather than the part's own heatConductivity when the part has nothing attached to its bottom node, IIRC (this is to prevent heat shields conducting too much to other parts).

 

Really not sure what else you're asking--it's a simple mathematical formula, and the names of everything match what's in the cfg.

[linuxgurugamer]

23 hours ago, NathanKell said:

https://en.wikipedia.org/wiki/E_(mathematical_constant)

 

The formula quoted explains how each of the values is used, excepting two. ablationTempThresh sets the threshold at which ablation begins; below that, no ablation occurs (the loss evaluation is skipped). reentryConductivity is used rather than the part's own heatConductivity when the part has nothing attached to its bottom node, IIRC (this is to prevent heat shields conducting too much to other parts).

 

Really not sure what else you're asking--it's a simple mathematical formula, and the names of everything match what's in the cfg.

That answers most of it.  Where did the values come from?  I saw that for all three heat shields the values were the same, the only difference I saw was the ablator resource amounts.

So, if two parts have the same amount of ablator, but (for example) one is twice as large as the other, wouldn't the ablator be thinner on the larger one?  And wouldn't it erode faster?  How is that taken into account?

Thanks

[NathanKell]

Cool

 

Convective heating goes with the area in the airflow, so a shield with greater area will take in more heat. That'll cause the temperature to rise faster / require a greater rate of ablator mass flux to counteract the influx to reach a steady thermal state. Twice the diameter = 4x the area = 4x the heat intake = 4x the ablation required (and given, since ablation is proportional to temperature) to maintain a steady temperature--the resting temperature will be higher.