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Aser

Were the heat shields intended to act this way?

Question

I came back to KSP for the new update (haven't played since 1.0 hit), and I've noticed that the heat shields do not behave themselves any more. I first noticed it on returning from my first orbit in a new career. I always try to return pretty gently (~75k ap and ~50k per). Everything seemed pretty normal until i reached around 20k. I'm blazing towards the ground and it seems like I'm coming in way too hot. Maybe I'll start slowing down? Nope. Ended up pulling my chutes at 1000m going 350m/s. Of course they ripped off and I slammed into the ground, killing Jeb.

I played around with it some more and did some reading here on the forum. Slashy recommended decoupling the heat shield after you're through with it. When I do that, I slow down just fine. Using KER I see that my rate of deceleration goes from ~2.4 G's to ~4 G's as soon as the shield is decoupled. Why is the heat shield so much more aerodynamic than the bottom of the pod?

Is this an intended behavior, or is this a bug?

Also, I seem to remember a mod that had heat shields with built in decouplers. Does any one know which one it is? Edited by Aser

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I don't know why this is but have the same problem. For now drogue parachutes would help a lot because they can deploy at 450 m/s and slow you quite a bit. I usually slap on one of the small radial mount ones just for safety.

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Aser,
You're not the only one who's had this happen and I've reported it as a bug. I haven't had any kerbals die from it, but I've had a couple close calls. As you can see, the fix of decoupling the heat shield solves the problem. It would be nice if we had small drogues to help with this early in the tech tree, but we don't.
As for the why...
I suspect it's a problem with the mass and lack of depth of the part. It works fine at supersonic speeds, but with the thinner lower atmosphere and KSP's subsonic physics, it just doesn't want to slow down. It masks everything behind it and there's no side drag to help.

Hopefully they'll get this fixed, and my condolences on Jeb!
-Slashy

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I can't say I have ever seen this particular bug happen. I never decouple my heat shields, but I can tell you it is certainly not normal. Though I will say that 50k Periapsis is a bit shallow and I've never re-entered that way. On Kerbin you can easily come in between 5 and 30km Periapsis from Minmus and still only use a fraction of your ablator. 5km is ideal, you will slow much faster and use less ablator that way. You want to come in steep, but not so steep you can't slow down in time to deploy your chutes. Edited by Alshain

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This is the problem when a thing that weighs what a Mercury capsule weighs has only 64% of the surface area it does in real life, and there's less air to slow you down than in real life on top of it.

Drogues are good, though I agree they are not placed early enough in career for use from the start (then again, heat shields aren't either--you don't _need_ shields at the start, nor for trans-Munar if you are very careful and/or do reentry on multiple passes).

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You want the shallowest re-entry angle possible, minimizing your vertical velocity and giving the atmosphere more time to bleed off your horizontal velocity before gravity pulls you down. It has lower G-loads, but at the same time higher heat loads. If you are using a heatshield you should be fine on the heat end assuming you are not trying to do a 1-pass re-entry from escape velocity.

De-coupling your heatshield once you slow below ~1000 m/s is a good idea anyways - you need a much smaller parachute to slow down and the aerodynamic force on the one you have is less for a lighter pod, meaning you can deploy it while moving somewhat faster.

If you have access to something small and heavy, you can stick it on the side of your pod and try for a lifting re-entry. That also will give you more time to slow-down.

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Thanks for the replies.

I guess I will continue to jettison the shield before it kills anymore kerbals.

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I haven't experienced this behaviour. Mine tend to be fine especially if I come in at a shallow angle. I haven't done many re-entries with just a heatshield and a pod though so that may be why. I did notice that heatshield were buggy for me a couple of versions ago and it made me stop playing for a while (the ablator didn't ablate and the heatshield just ended up exploding, followed by the capsule).

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[quote name='MaxL_1023']You want the shallowest re-entry angle possible, minimizing your vertical velocity and giving the atmosphere more time to bleed off your horizontal velocity before gravity pulls you down. It has lower G-loads, but at the same time higher heat loads. If you are using a heatshield you should be fine on the heat end assuming you are not trying to do a 1-pass re-entry from escape velocity.[/QUOTE]

Not true. If you come in too shallow you will spend more time slowing down and therefore more time burning. If you come in steeper (but not too steep) you will burn hotter, but nothing the shields can't handle and less time burning. You should come in not too steep but not too shallow either because they you will not slow down fast enough. Try coming in shallow on Eve and you will not survive.

[quote name='MaxL_1023']De-coupling your heatshield once you slow below ~1000 m/s is a good idea anyways - you need a much smaller parachute to slow down and the aerodynamic force on the one you have is less for a lighter pod, meaning you can deploy it while moving somewhat faster.[/QUOTE]

Most of the time the mass of the shield is irrelevant for the parachutes. The mass of the decoupler on your fuel efficiency and it's cost is actually a bigger concern. If you are coming down with nothing but a pod on Kerbin, the nose cone parachutes appropriate to that pod are more than enough.

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[quote name='Alshain']Not true. If you come in too shallow you will spend more time slowing down and therefore more time burning. If you come in steeper (but not too steep) you will burn hotter, but nothing the shields can't handle and less time burning. You should come in not too steep but not too shallow either because they you will not slow down fast enough. Try coming in shallow on Eve and you will not survive.



Most of the time the mass of the shield is irrelevant for the parachutes. The mass of the decoupler on your fuel efficiency and it's cost is actually a bigger concern. If you are coming down with nothing but a pod on Kerbin, the nose cone parachutes appropriate to that pod are more than enough.[/QUOTE]

I thought his problem wasn't burning hot, but not bleeding off enough speed before hitting the ground. A shallower angle gives your pod more time to slow down when your heatshield can take it.

Maybe I am used to the RP-0 heatshields - one of those is almost as heavy as your pod.

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[quote name='MaxL_1023']I thought his problem wasn't burning hot, but not bleeding off enough speed before hitting the ground. A shallower angle gives your pod more time to slow down when your heatshield can take it.

Maybe I am used to the RP-0 heatshields - one of those is almost as heavy as your pod.[/QUOTE]

I've read on another thread (the one discussing the eve re-entry i think), that there's always one problem with wrong entry angles:

Too steep: Obviously, you come in way to fast. You either burn up or hit hard, because the atmosphere cannot slow you down enough in that short time.
Too shallow: You spend too much time in the upper atmosphere: You might lose enough horizontal speed up here, but that happens way too high. Then, you get into the drop (where your prograde marker begins falling down significantly) pretty high from the ground and you start to gain vertical velocity. Once your falling speed exceeds 200m/s (and i guess this can happen while your overall speed is above safe chute deployment) and you don't have drougue chutes, you fall trough the lower atmosphere like a brick. A fast brick.

This explaination might be wrong, but it sounds intuitive for me. Some other User might correct me.

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[quote name='NathanKell']This is the problem when a thing that weighs what a Mercury capsule weighs has only 64% of the surface area it does in real life, and there's less air to slow you down than in real life on top of it.[/QUOTE]

The problem is not at all with the pod though. In fact the pod is the thing that works correctly, but the heatshield isn't. The thing that confuses people is that if there's a heatshield attached to the pod, the whole thing behaves like it has very little drag. But as soon as the heatshield is gone, and the pod itself is exposed to the air, drag actually [I]increases[/I] by a noticable amount.



[quote name='MaxL_1023']I thought his problem wasn't burning hot, but not bleeding off enough speed before hitting the ground. A shallower angle gives your pod more time to slow down when your heatshield can take it.[/QUOTE]

A shallower reentry often requires a deeper periapsis, though, which is probably why people are finding your statement confusing. If you use the highest possible periapsis that still works - i.e. the shallowest possible reentry angle - your vertical speed will be low only in the upper reaches of the atmosphere that do not slow you down measurably, but high and increasing when you finally encounter the lower reaches, causing high g-loads and high heating - and potentially failing to slow down in time.

What you really want is a shallow angle and low vertical speed [I]during transition from middle into lower atmosphere[/I], not elsewhere. That generally involves choosing a steeper reentry angle with a lower periapsis and possibly an elevated apoapsis, so your vertical speed is high while passing through the upper reaches but slows down as your craft levels off when approaching its periapsis inside the atmosphere. Edited by Streetwind

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Nope, you get exactly (perhaps even 1% larger, since there's a bit of extra side area) the same drag force with the shield or without the shield. What changes is the mass, and therefore the deceleration in m/s.

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Not saying you're wrong, but I'm still trying to understand the numbers Aser posted. He said he went from 2.4g to 4g deceleration on decoupling the heatshield.

pod + chute + decoupler + heatshield: 1.29 t... or 1.19 t if the heatshield lost for example half of its ablator.

pod + chute alone: 0.94 t

Is that roughly one third of mass difference enough to cause a roughly two thirds improvement in deceleration?

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As Slashy pointed out, the instant you decouple the heatshield you're effectively doubling the drag (because on decouple, the bottom of your pod is no longer occluded, so that makes drag, plus the heatshield/decoupler vessel produces its own drag which it passes on to you by collision, i.e. it's driven upwards into you and therefore slows you further).

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[quote name='NathanKell']As Slashy pointed out, the instant you decouple the heatshield you're effectively doubling the drag (because on decouple, the bottom of your pod is no longer occluded, so that makes drag, plus the heatshield/decoupler vessel produces its own drag which it passes on to you by collision, i.e. it's driven upwards into you and therefore slows you further).[/QUOTE]

Oooh, okay. That makes sense. So it's a result of how KSP models occlusion.

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I have had to change the way that I re-enter for 1.0.5. I had been finding that I was coming in too fast.

I now try to make sure that I pretty much only have a capsule and parachute for the final re-entry - no engine, no tanks and no heatshield. I think that more mass occluded behind the same cross-sectional area made it more difficult to slow down.

The other more kerbally way to try is to fly sideways to increase your area that is hitting atmosphere...

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Reality may have to be sacrificed for gameplay to "fix" this, as the pod with heatshield seems to be working correctly compared to a pod without.

The greater mass means there's more inertia for the aerodynamic drag to overcome.

Reducing the pod and heatshield mass will have negative effects in other areas unfortunately, such as moving the rockets center of mass downward, reducing stability.

And accounting for occlusion from parts in collision would eliminate the double drag, causing you to fall faster, and that may end up being a bad thing depending on your angle.

Plus the occlusion checks are computationally expensive, additional checks for collided parts are better avoided.

Artificially increasing the size of the heatshield drag cubes might work, drag cubes can already be overridden and this is done for the cargobays and fairings (cue bug reports that heatshields are unrealistically draggy).

I don't mind the extra decoupler to drop the heatshield on a Mk1 pod, the separator on the Mk1-2 doesn't look bad either and I can use the next size down and the offset tool to hide them later in career.

So there is a workaround, and we can adapt, we can't expect nothing to change with each update to KSP.

Maybe heatshields do need built-in decouplers though as that'd encourage players to drop them, and the double drag may actually come in handy to save your Kerbals on occasion if you're lucky enough to have your heatshield stay pressed against your pod by aero forces during descent. Edited by sal_vager

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For anybody who wants it, I've made a quick ModuleManager config to add a decoupler to the stock heat shields.

[url]http://forum.kerbalspaceprogram.com/threads/117530-[/url]

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[quote name='sal_vager']Reality may have to be sacrificed for gameplay to "fix" this, as the pod with heatshield seems to be working correctly compared to a pod without.

The greater mass means there's more inertia for the aerodynamic drag to overcome, this is why real space capsules drop their heat shields once they have slowed enough that the atmosphere in front of them is no longer being compressed to the point that it's heating up.

Reducing the pod and heatshield mass will have negative effects in other areas unfortunately, such as moving the rockets center of mass downward, reducing stability.

And accounting for occlusion from parts in collision would eliminate the double drag, causing you to fall faster, and that may end up being a bad thing depending on your angle.

Plus the occlusion checks are computationally expensive, additional checks for collided parts are better avoided.

Artificially increasing the size of the heatshield drag cubes might work, drag cubes can already be overridden and this is done for the cargobays and fairings (cue bug reports that heatshields are unrealistically draggy).

I don't mind the extra decoupler to drop the heatshield on a Mk1 pod, the separator on the Mk1-2 doesn't look bad either and I can use the next size down and the offset tool to hide them later in career.

So there is a workaround, and we can adapt, we can't expect nothing to change with each update to KSP.

Maybe heatshields do need built-in decouplers though as that'd encourage players to drop them, and the double drag may actually come in handy to save your Kerbals on occasion if you're lucky enough to have your heatshield stay pressed against your pod by aero forces during descent.[/QUOTE]

Why not just make ablator a tweakable resource in the VAB or SPH that can be added to crewed parts? If I need extra shielding for a particular mission, I could just add ablator to the pod or cabin as needed, instead of adding more parts. Also, I wouldn't have to rely on luck to keep my Kerbals alive.

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Well if it were only for crewed parts we couldn't use them to protect probes or other equipment.

I would think a combination of adding decouplers and moving drogues up in the tech tree should get us there. Edited by Pthigrivi

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All,
The official word on this is it's on the known bugs list, but we shouldn't expect a fix any time soon. We will have to work around it, but that's okay because we know how.

In the meantime, I recommend:
- Certify your capsule for reentry before putting a kerbal in it.
- Don't bother using the heat shield for LKO or suborbital flights.
- When using a heat shield, either use a decoupler or 5thHorseman's modified heat shields.

Best,
-Slashy

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Could you just point at prograde for awhile, after the burning stops, to put the HS behind you to slow down?

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The heatshield is a bit heavy isn't it? The Mk1 capsule is 0.84t and the 1.25m heat shield is 0.3t, which is more than 1/3rd of the capsule. Seems a bit excessive?

For Apollo, the CM was 12,250lb with the heat shield 1,870lb of that, according to [url]https://en.wikipedia.org/wiki/Apollo_Command/Service_Module[/url]

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