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Capsule survives re-entry with 0 ablator on heatshield?


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Guys, this makes no sense to me. Why is it that on default Hard settings (heat 100%), I can put literally 0 ablator on my heatshield and survive re-entry, even directly from Eeloo to Kerbin?

How to reproduce problem

  • Sandbox, Default Hard settings
  • Make a Mk 1-3 capsule with 1 parachute and one 2.5 m heatshield, a solar panel, a decoupler, and under it a Rockomax X200-32 Fuel Tank and a Poodle engine.
  • Reduce ablator to 0.
  • Cheat into a high orbit. I decided to go as extreme as Eeloo
  • Maneuver for a 30,000 m periapsis at Kerbin, decouple, point retrograde for re-entry
  • Ablator reaches 0, temperature bar goes high on the heatshield but it doesn't fail.

I redid this test with 120% Re-Entry Heat and it still didn't burn up. I realize there's a mod called Deadly Reentry that aims to fix this, but I would like to understand the rationale behind vanilla stock behaving this way.

N0vZ2l3.jpg

(Screenshot shows my peak heating during re-entry. The capsule and its heatshield all survived)

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Ablator makes very little difference (thread link).   KSP includes radiative cooling in its thermodynamic simulation, and if you look with the alt-F12 thermal debug information, this mechanism dominates the cooling of heat shields when they start to get dangerously hot.  Radiative cooling increases quickly with temperature, T4, so 20% additional heat raises the temperature in Kelvin just 5%.  The heat shield survives a rather high 3300K, and has very low thermal conductivity so it protects further parts quite well.   Given the physics that is modeled, reducing the temperature at which heat-shields fail would be a more effective way to increase the difficulty in this area of the game.

I realize that that was just observations of the game, and you asked about rationale.  I think @NathanKell adjusted the thermal model when he was working with Squad, after having experience with his Real Heat mod among other things, so you might look in that mod's thread for insight.   I would not assume that he was able to put all that he wanted in to KSP, of course.  Also, there are a few independent mods and suggestions on the bug-tracker in this area, so you are not the only one who finds KSP imperfect in this area.

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Moving to Gameplay Questions.

On 5/24/2020 at 5:17 PM, Xavven said:

Why is it that on default Hard settings (heat 100%), I can put literally 0 ablator on my heatshield and survive re-entry, even directly from Eeloo to Kerbin?

Because heat shields are extremely tough and useful even without ablator-- very high temperature tolerance, and fairly low thermal conductivity.

And also because your ship has a very low ballistic coefficient, i.e. it's lightweight relative to the amount of drag it produces.  Lightweight, draggy designs don't take as much of a pounding from reentry heat.

If your ship had significantly more mass loaded up behind that 2.5m heat shield, then you might find yourself needing the ablator.

 

(Out of curiosity, how fast were you going when you hit atmosphere?  You're only 3500 m/s in that screenshot, but you're already deep in the atmosphere and decelerating hard.)

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I had two payloads going both at 7500 meters per second during their entry. One was a crew capsule and one was a heavy lander. The heavy lander almost blew up immediately while the crew capsule survived to tell the tale. I have been so confused at why but as previously mentioned by @Snark. The less payload, the less ablator. 

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7 hours ago, Snark said:

 

(Out of curiosity, how fast were you going when you hit atmosphere?  You're only 3500 m/s in that screenshot, but you're already deep in the atmosphere and decelerating hard.)

 

5529 m/s at 70,000 Kerbin altitude

Sun apoapsis of 133 billion meters (farther than Eeloo).

Quote

And also because your ship has a very low ballistic coefficient, i.e. it's lightweight relative to the amount of drag it produces.  Lightweight, draggy designs don't take as much of a pounding from reentry heat.

Ah, that explains it! Thanks, Snark! I will run some more tests with a heavy payload and 100% heat.
Edit: Confirmed! I found a case where if I add enough payload, it explodes with 0 ablator, but having some ablator helps it just enough to survive. This answers it. Thanks again.

Edited by Xavven
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