Heating - what is it about the 60s?

[hatterson]

Again, the issue is not necessarily that the 60-65k range (or high atmo in general) produces too much heat (I'd argue it does, but that's a separate argument), but rather that it produces too much heat relative to the low atmosphere because you're getting a ton of heat without getting any drag/slow down effects. *That* doesn't make sense as a good gameplay experience.

Here is a simple example. Which of these two events should be more thermally intensive:

1. A reentry with a PE of 59.5km entering atmosphere at 2105m/s
2. A reentry with a PE of 5.5km entering the atmosphere with a speed of 3750m/s

I can tell you from experimentation is that the first scenario ablates .14 tons out of .8 tons of ablator and the second only ablates .04 out of .8 tons despite the latter have a *much* more impressive re-entry visual. What that communicates to the player is that steeper entries are better, which is a weird gameplay incentive.

[Guest]

Yeah this doesn’t seem right to me either! It’s especially apparent with spaceplanes, they just disintegrate at that altitude, even Mk2 parts which ought to be able to handle it.

[Bej Kerman]

On 1/6/2024 at 2:44 PM, The Aziz said:

But considering that Kerbin atmo ends precisely at 70km, what we're experiencing is ionosphere (part of thermosphere with enough ionized particles to cause heating, but not enough to cause a ball of plasma around the vehicle). I'd still say that's fairly accurate considering the limitations of hardcoded atmo border.

Citation needed

Edited January 9, 2024 by Bej Kerman

[Pthigrivi]

What's impossible to tell is how this diagram is mapped onto Kerbin's atmosphere currently, mainly whether and how much it's been scaled down relative to atmospheric pressure. My guess is that they've basically applied a 70% scaling so the Karman line is at 70km rather than 100km and abruptly cuts off. In reality atmospheric heating begins above the Karman line, closer to 120km up, so you could scale 58% it to deliberately ease-in this initial heating event. What might make for a smoother transition would be instead to scale it so the mesopause is aligned at 70km (82% scale) so you don't have a sudden temperature spike as soon as you enter the atmosphere. All of these solutions are a little artificial of course because kerbin's scaled so much differently than earth. For my money the latter solution would make for the most intuitive gameplay while still incurring a solid dose of reality. 

Or maybe Im wrong and at these altitudes air temp doesn't matter compared to density and friction?

Edited January 9, 2024 by Pthigrivi

[PDCWolf]

15 minutes ago, Pthigrivi said:

What's impossible to tell is how this diagram is mapped onto Kerbin's atmosphere currently, mainly whether and how much it's been scaled down relative to atmospheric pressure. My guess is that they've basically applied a 70% scaling so the Karman line is at 70km rather than 100km and abruptly cuts off. In reality atmospheric heating begins above the Karman line, closer to 120km up, so you could scale 58% it to deliberately ease-in this initial heating event. What might make for a smoother transition would be instead to scale it so the mesopause is aligned at 70km (82% scale) so you don't have a sudden temperature spike as soon as you enter the atmosphere. All of these solutions are a little artificial of course because kerbin's scaled so much differently than earth. For my money the latter solution would make for the most intuitive gameplay while still incurring a solid dose of reality. 

Or maybe Im wrong and at these altitudes air temp doesn't matter compared to density and friction?

What translates into heating is having enough air density for the shock bow to actually compress enough air molecules into heating up. As the atmosphere gets less dense, obviously there's less air molecules, in fact not enough of them to compress into each other, which is why you have the ISS being able to generate lift, or adopt a posture where it reduces drag, but not heating up. In reality what KSP does is close to ignoring anything above the stratosphere, so you go from stratosphere to perfect vacuum as soon violently as just changing from 69.999 to 70.000 meters. Once you're below 70.000 meters, there's automatically enough air molecules for them to compress and heat up, although very lightly.

What we'd need to examine this in enough depth is a diagram that relates, at a fixed altitude, speed to temperature. From then we can discuss if it's too much or too little, but anything else apart from that has no fix without manipulating the whole atmospheric setup. IRL, even if you ignore all the other atmospheric components, you will absolutely burn up if you do what OP did (hanging around too long at high temperature). What we need to look at is that if at a certain speed, the maximum heating experienced at a fixed altitude makes sense.

[rjbvre]

The 70% scaling would make sense to me. According to the wiki article:

While Earth entry interface is considered to take place at the Kármán line 100 kilometres (330,000 ft), the main heating during controlled entry takes place at altitudes of 65 to 35 kilometres (213,000 to 115,000 ft), peaking at 58 kilometres (190,000 ft).

This would mean most of the heating should be between about 45 and 25km, with the most intense at 40. Unshielded sensitive parts no doubt exploding well above that. Almost all my re-entries have followed this pretty closely.

I do agree that things quickly blowing up just under 70km shouldn't be the goal, but some people are arguing for things that definitely shouldn't be possible on here, which is fair enough it's a game for fun, but at that point you should change your own settings rather than the base game.

Edited January 9, 2024 by rjbvre