Ablator: conditions of use?

[SinBad]

Hello,

Under what conditions is ablator depleted from a heat shield in ksp? I made a ship to do a 5,000,000km flyby of the sun then re-enter kerbin atmosphere. All went well, but as i was planning a heavy, steep and fast reentry i used the next size up heat shield (2.5m for a  16 ton 1.25m re-entry package) with the default amount of ablative material (all of it).

As i crossed 70km coming home, i noticed that half my ablator had disapeared before i hit atmosphere. I flew the mission again and watched the ablator levels. I didnt loose any on ascent, but it did slowly drain away while i was outside kerbins soi.

I have a hefty heat source on the ship in the form of an NFE reactor, i believe its adequetly cooled (radiator dont exceed 50%).

Could it be the reactor heat causing this, or heat from the sun, or maybe the game models solar wind ablation now? Has anyone else seen it, or have i discovered a wierd one off interaction between 80+ different mods?

[Laie]

I think (but won't guarantee) that ablator is used depending on the (surface?) temperature of your heatshield.

A few versions ago I did the "Milkrun" challenge on reddit: get to Minmus and back in under six hours. I was running Nervas continuously for like 60 minutes. While the vessel itself was never at risk of blowing up, I did notice that the heat would start nibbling at my ablator, beginning at a temperature of ~480-500K. Really really slow at first, but over the course of an hour it adds up.

I guess that something similar happened to you. Both your power source and close proximity to the sun are possible reasons. Timewarp may also have played into this, heat acts a little strange at 10x and 100x warp levels.

In my case, it turned out that no amount of radiators would suffice to keep the ablator safe. The best I could do was play for time, a complicated vessel design to keep the heat away from the ablator for as long as possible.

[SinBad]

Thanks, Laie, good info. Ill have to experiment a bit. Maybe the mechanic, like so many others, is disabled when shielded? Ill put an inline fairing over that part and see if that does anything on the next run. My return vehicle did make it to landing with a little bit of ablator left. This re-entry profile is using a little under half of it up. If I can solve the 'ceramic boil off' issue, I might be able save a lot of mass overall by not launching the ablator I don't need.

[linuxgurugamer]

Actually sounds like a bug.  Ablator should only be used inside atmosphere

[Snark]

3 hours ago, linuxgurugamer said:

Actually sounds like a bug.  Ablator should only be used inside atmosphere

Not a bug, it's by design, I'm pretty sure.

Ablator has a skin threshold temperature above which it starts to vaporize.  You can see the temperature in the part config.  For the stock shields, that temperature is 500 degrees.  So if the part's temperature is raised higher than that somehow (e.g. by flying really close to the sun), then it'll drain away.

500 seems a bit low to me (bear in mind that this is Kelvin temperature, so it's only a couple of hundred degrees warmer than Kerbin ambient)... seems to me that something like 800 would have been more reasonable.

 

[James Kerman]

Ablator also gets eaten away on an ISRU vessel without adequate cooling.

[Norcalplanner]

I've lost ablator on a vigorous ascent even when it was fully occluded by the rest of the rocket.  I guess it's the convection mechanic doing its thing in a predictably unpredictable Kerbal way..

[Streetwind]

A running NFE reactor doesn't heat up a spacecraft at all if it is provided with adequate cooling; it is very well insulated. I'd put my money on the Sun eating your ablator. It gets quite warm as you approach it.

Edited September 1, 2016 by Streetwind

[TheDestroyer111]

8 hours ago, linuxgurugamer said:

Actually sounds like a bug.  Ablator should only be used inside atmosphere

This is not a bug. This is real world physics - an ablator is a material that vaporizes under high temperatures to take away the heat. It doesn't matter if the temperature is in atmosphere or close to the sun, or from a reactor. It's still heat after all.

[Sharpy]

9 hours ago, linuxgurugamer said:

Actually sounds like a bug.  Ablator should only be used inside atmosphere

Ablator IRL evaporates/burns away due to heat. It really doesn't care what the heat source is.

But yeah, 500K is rather low.

[Laie]

1 hour ago, TheDestroyer111 said:

This is not a bug. This is real world physics

Yes and no. Yes because I agree that the presence of an atmosphere shouldn't matter.

OTOH, If ablator starts sizzling at 480K, and radiators won't cool your vessel to below 500K, this qualifies as a bug of sorts. Numbers pulled out of my nose: I suspect that it may be so, but didn't run any actual tests to confirm my suspicion.

[linuxgurugamer]

3 hours ago, TheDestroyer111 said:

This is not a bug. This is real world physics - an ablator is a material that vaporizes under high temperatures to take away the heat. It doesn't matter if the temperature is in atmosphere or close to the sun, or from a reactor. It's still heat after all.

The bug, then, is the extremely low temperature at which the ablator vaporizes.  Given that, it would be (IMHO) an acceptable compromise to not allow it to vaporize in space, since the actual temperature might be too high for the crafts to survive