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# Heat shield/re entry mechanics

## Question

So here is a question I have been trying to find an answer to but have had no luck. How does heat on the ship during re entry work? In the instance of heat shields or just standard non heat shielded parts, is it the lead point that takes 100% of the heat or is it heating any point at all that is not parallel to entry angle. Basically here is the back story that leads to my question. I am using stage recovery mod because I like the realism of being able to recover stages. Because of this each stage has parachutes and other things designed to slow down my stages. On my main stage (4 nerv's a orange fuel tank and some struts) I put 1.25s on top of the mark1 liquids and a 2.5 on top of the orange. during aero breaking with my pod attached the 1.25s that are set back a few meters from my pod but open to the elements were taking no heat (no ablator loss) but my lead point on the pod was. Is this how it works? To simplify I have a new stage I'm making (screen shot attached) will the engine take all of the heat and not the shields, or will the shields take the heat since they are also at a perpendicular angle to entry? Thanks in advance.

Edited by Nickel AP

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Considering that you are reentering this suborbital on Kerbin:

if you had an incredibly sensitive thermometer you would notice that during reentry, the engine would actually get up to a temperature where it was almost warm to the touch. And you might notice that the heatshields actually would register a temperature above absolute zero.

If you were to reenter the thing directly from the Mun, or interplanetary with about twice as much speed -- then the lead point is going to take maybe 80% of the heat, and things dangling off the sides take maybe 20%.

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I don't actually know the precise extent to which KSP attempts a physically correct airflow simulation, but I do know that the way it works is roughly the following:

- Lead parts fully occlude parts in the same stack as they are, provided they are of the same size or smaller as they are. If the stack flares out, then the point of widening may also take heat (and in turn occlude everything below itself), depending on how much it flares out and how much of a bow shock you have.
- Lead parts create a bow shock, which helps to protect parts radially attached to the stack and allowing leeway for mathematically non-perfect prograde orientations. It would suck if your flank started overheating the moment your heatshield was just 0.0001° off of prograde, right?
- The more a part sticks out from the stack, the less well it will be protected by the bow shock.
- The further away a part is from the lead part, the less well it will be protected by the bow shock.
- Parts surface-attached to the lead part are not protected by the bow shock at all. In fact, they might become the lead part themselves. Which, depending on the part, may have a desirable outcome... or a very undesirable one.

In your example screenshot, the engine will take heat, and all of the heatshields will take heat as well. The radial stacks are too large to fit into the shadow of the engine's bow shock. However, the radial drogues at the top of the radial stacks will be protected by the bow shocks of the heatshields.

@bewing - Do you really think a sarcastic quip is a proper answer to a genuine knowledge question?

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OK, yeah, that sounds a lot more sarcastic than I meant. I meant it in a more humorous way -- my bad (humor on the internet can easily be misinterpreted, and it is therefore dangerous). My basic point is that it's way overengineered. Removing all the heatshields will save lots of weight, and will probably make it reenter better, because of the extra drag. It will probably also launch better because of the lower weight. Heat is not going to be an important factor for this part's reentry. The angle of the bow shock depends on the speed. The faster you are going the narrower the bow shock. Most parts can take quite a lot of heat. Heatshields can take a huge amount of heat, but they have quite low drag and they are fairly massive -- so adding them to a RV can easily kill you.

My second point was that the lead part takes a slightly unrealistic amount of the heat, especially if that lead part is off to the side (because of attitude or construction). Beyond that, heat is inevitable -- you can't hide from it forever. So it's a useful thing to try your designs without the heatshields and see what happens. You can learn a lot just by watching the heat bars.

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1 hour ago, Streetwind said:

I don't actually know the precise extent to which KSP attempts a physically correct airflow simulation, but I do know that the way it works is roughly the following:

- Lead parts fully occlude parts in the same stack as they are, provided they are of the same size or smaller as they are. If the stack flares out, then the point of widening may also take heat (and in turn occlude everything below itself), depending on how much it flares out and how much of a bow shock you have.
- Lead parts create a bow shock, which helps to protect parts radially attached to the stack and allowing leeway for mathematically non-perfect prograde orientations. It would suck if your flank started overheating the moment your heatshield was just 0.0001° off of prograde, right?
- The more a part sticks out from the stack, the less well it will be protected by the bow shock.
- The further away a part is from the lead part, the less well it will be protected by the bow shock.
- Parts surface-attached to the lead part are not protected by the bow shock at all. In fact, they might become the lead part themselves. Which, depending on the part, may have a desirable outcome... or a very undesirable one.

In your example screenshot, the engine will take heat, and all of the heatshields will take heat as well. The radial stacks are too large to fit into the shadow of the engine's bow shock. However, the radial drogues at the top of the radial stacks will be protected by the bow shocks of the heatshields.

You may be right but in my experience there is no bowed shock wave effect. I find that you get a cylinder of occlusion extending backwards from the circumference of the leading part. Not to be confused with the Aerodynamics FX setting on max, where it looks like there is a bowed shock wave - this is, as far as I am aware, purely visual and does not reflect what is really happening.

For instance, if you don't keep this craft almost exactly retrograde during re-entry then the Hitchhiker can will cook. There is a little over-sizing with the heat-shield that allows a small amount of leeway.

To the OP: I'd say that is a pretty good place to use the big inflatable heat-shield.

In my experience, each stack has its own cylinder of occlusion. In your craft, each of the tanks should be protected by their heat-shields but not the centre stack and it won't take all the heat for the whole craft.

Edited by Foxster

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Just now, Foxster said:

You may be right but in my experience there is no bowed shock wave effect. I find that you get a cylinder of occlusion extending backwards from the circumference of the leading part.

For instance, if you don't keep this craft almost exactly retrograde during re-entry then the Hitchhiker can will cook. There is a little over-sizing with the heat-shield that allows a small amount of leeway.

This is probably due to the way the effect is implemented. KSP tries to act as if there was a bow shock, but doesn't actually physically simulate it. Instead an algorithm tries to guess how parts should be occluded when, and potentially guesses wrong sometimes. I have also experienced instances where I had to turn off prograde quite a lot to get stacked parts heated, and other instances where it was super sensitive like your example. In fact, I think the Hitchhiker was part of multiple of those latter cases. Maybe the algorithm doesn't like it very much for some reason?

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Yes, I've seen some variation in parts, with the Hitchhiker being one of the worst. I wonder if that ring of grips at the top takes the part out to the absolute max of the part size.

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39 minutes ago, Foxster said:

... I wonder if that ring of grips at the top takes the part out to the absolute max of the part size.

That might very well be the case. Good thinking.

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It's the ring of grips at the bottom that's the problem. The heat occlusion zone is not a cylinder -- the game models it as a narrow cone. But the part right behind the shield can't have any "sticky-outy bits" to quote snark.

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5 minutes ago, bewing said:

The heat occlusion zone is not a cylinder -- the game models it as a narrow cone. But the part right behind the shield can't have any "sticky-outy bits" to quote snark.

Ooooh! Extremely interesting (but only to a KSP geek). Numbers?

Also, when you say "narrow", do you mean slightly tapered or sharply tapered.

(Also - I know you are part of Big-S now but are you speaking from authority or just your previous well-informed guestimation?)

Edited by Foxster

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Thanks for all of the replies. I definitely agree its over engineered, it was designed that way to experiment with the way heat transferred through stages built this way. also its my interplanetary pod/reserve fuel for an interplanetary tug I built with NERVS. I have since removed the engine completely in an effort to get the heat shields to slow the craft and absorb the heat. My main ship that suffers this question is my refueler. its a big orage with 4 nervs, 4 mark1 liquids with heat shields on top (covered by clam shell until over 70k, my goal is to get it to enter prograde with heatshields on the front, having the 4 nervs at COM causes them to take no heat (ablator loss). My goal is to get it to be capable of slowing down during reentry on its own using the heat shields. I'm thinking I can engineer some supports forward then out to bring the 4 nervs parallel with the 2.5 heat shield. hopefully that way all take the heat and wind and will slow the craft down. Again, over engineered to the 9s but that's what makes this game so fun. Re: the recommendation to remove the heat shields, does having heat shields add more or less resistance? I assumed heat shield = slower. Is it only there to be safer for heat but ends up being more aerodynamic?  on a side note, would air brakes at the aft keep it pointed prograde through atmosphere? it doesn't seem like it does which surprised me.

Edited by Nickel AP
incorrect description of ship

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1 hour ago, Nickel AP said:

on a side note, would air brakes at the aft keep it pointed prograde through atmosphere?

Well, weight up front, drag at the rear its what keep something pointing forward in the atmosphere. Air brakes may help, however it's reported to break/melt easily while reentering.

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Sometimes you need quite a few air brakes to be effective, maybe even a dozen or so. Just need to keep adding them until it works.

They won't overheat or break if you make sure they are wholly inside the occlusion cylinder from the circumpherence of the leading heat shield.

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3 hours ago, Nickel AP said:

Re: the recommendation to remove the heat shields, does having heat shields add more or less resistance? I assumed heat shield = slower. Is it only there to be safer for heat but ends up being more aerodynamic?

Safer for heat and more aerodynamic is correct. Heatshields have very low drag. Adding them reduces air resistance a lot. So heat shield = faster, always.

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