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xendelaar

Heat shields when coming in hot

Question

Hi there,

Just a quick question. Is it possible to aerocapture a vessel  with a direct transfer from  Kerbin to Laythe using heat shields? I've been trying this all night long, but the vessel keeps exploding the minute (nay, second!)  I enter Laythe's atmosphere! 

Below you'll find a picture of my vessel. As you can see, I'm coming in hot (orbital velocity is + 8000 m/s). The vessel consists out of 5 FL-T stacks with 5 large heat shields. Do I need more heat shields? Or maybe the larger, inflatable one? Or is it just impossible at these speeds.

V3fTgLp.jpg

 

If I need more heat shields, thus weight, I'll bet it's much more efficient to just fiddle around with gravity slings around Tylo to get a capture with Laythe.

 

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Yeah, heat shields tend to go foof at those speeds-- I doubt the inflatable would help you.  The problem is that they're overheating and exploding before they get down to the point where there's enough drag to actually slow the ship any, so adding more heat shields wouldn't really help, I expect.

Reverse gravity assist would probably be your best bet, though at those speeds I'm not sure how much velocity you'll be able to shed.

(Or, even better, set up a better encounter in the first place so you're not coming in at 8 km/s!)  :wink:

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The reason you're exploding the second you touch Laythe's atmosphere is because a critical part exceeded its maximum skin temperature. Parts can explode if they exceed either their skin maximum or their internals maximum.

Normally, a hot skin conducts heat to neighboring skins and to the part's internals. But skin thermal mass is extremely small, meaning even a small amount of heat will increase the temperature significantly in a very short time. In your case, the heat flux was so large that there was never a chance for any heat to be conducted away; the skin just melted instantly.

Heatshields have two additional layers of safety against this. For starters, the skin temperature maximum is absurdly high. Something like twice as high as other parts. Secondly, it carries ablator. This ablator is consumed through a process called pyrolysis - meaning, the heatshield actively consumes heat from its skin to remove ablator. This artificially cools the skin as long as ablator exists. The higher the skin temperature, the faster the rate of ablation, and therefore the more heat is consumed. Thus the heatshield tends to achieve an equilibrium temperature somewhere, even under extremely high heat load.

The first question you should ask is: which part is exploding? Is it actually one of the heatshields, or something else? You can press F3 after your disaster happens, and check the log for the first instance of "...was destroyed due to overheating". When this happens to be one of the heatshields, then yep, you're screwed - this kind of trajectory is just too aggressive. However, in my experience, it is extremely hard to kill a heatshield, even intentionally... so it's possible that something else went. Something that wasn't properly shielded. Something perhaps sitting directly behind that small hole in the center of your four heatshields... If that is the case, you need to figure out a way of protecting that part from the airstream. If you do that, you might even be able to aerobrake.

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I really recommend the new inflatable heatshield. I have done some seriously quick aerobraking without it batting an eyelid. 

It take a bit more to keep things prograde (air brakes at the rear are your friend here) but once you have that sorted it's pretty much indestructable. 

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On 17-6-2016 at 4:17 AM, StrandedonEarth said:

Have you tried "rotiserrie mode?" Spin it really fast, that may help spread the heat around. Or not. But it's fun to try!

How far out is your last quicksave? I seem to recall that 1.1.2 added backups of autosaves. I haven't looked for those, but perhaps that can get you farther back emough to get a more favorable encounter.

I did the rotiserrie thing but that didn't help. I eventually reverted to an older savegame, where I just left Kerbin's SOI. Here, I fiddled with the manoeuvre node until I had a proper encounter where my orbital velocity was only 3200 m/s... At this speed, re-entry was a cake walk! Thanks for all the help.

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Edited by xendelaar

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What @Snark said. Even if you were to cheat or turn off reentry heating, I'm fairly certain that your vessel would simply skip off of Laythe's atmosphere anyway at the speeds you are traveling at.

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1 minute ago, Andem said:

Even if you were to cheat or turn off reentry heating, I'm fairly certain that your vessel would simply skip off of Laythe's atmosphere anyway at the speeds you are traveling at.

Actually, if you cheat and turn off reentry heating, you'd probably be okay.

I recall one occasion in 0.90 (i.e. before reentry heat was a thing) when for various stupid reasons I had a ship that arrived at Eve going over 25 km/sec.  Aerobraked with no problem.  (It was... dramatic.)

Of course, that was before the new aero, too, but I doubt that would make all that much difference in this case.  If you approach the planet with a low enough Pe, you'll shed that velocity in a hurry.  But yeah, it would require cheating and turning off the heat.

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Wow, that's quite a high intercept velocity.  Are you hitting Laythe head on?  That is, is Laythe in the part of its orbit that it is moving toward you as you're approaching it.  If that's the case, then this will greatly increase the relative velocity.  To decrease the velocity, you want to try to intercept Laythe so that you're coming up on it from behind.  Laythe orbits Jool at about 3.2 km/s, so there's a difference of 6.4 km/s between a head on intercept and a trailing intercept.

I don't know if the above is your problem, and, if it is, it may be too late to do anything about it.  Laythe orbits Jool every 2.4 days, so if there is anyway to adjust your trajectory so that you arrive a day sooner or a day later, you can turn a head on intercept into a trailing intercept.  I've never tried to adjust my arrival time, so I don't know how hard or easy that is, or even if it is practical at all.

 

Edited by OhioBob
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It sounds as if this is a sandbox mission in order to test possibilities, before you try it for reals. So the above advice is the stuff to follow. I just wanted to tell you that I have heard of two occasions of players trying out the inflatable heatshield for fun on 8km/s aerobrakes, and it worked beautifully.

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6 hours ago, Snark said:

Actually, if you cheat and turn off reentry heating, you'd probably be okay.

I recall one occasion in 0.90 (i.e. before reentry heat was a thing) when for various stupid reasons I had a ship that arrived at Eve going over 25 km/sec.  Aerobraked with no problem.  (It was... dramatic.)

Of course, that was before the new aero, too, but I doubt that would make all that much difference in this case.  If you approach the planet with a low enough Pe, you'll shed that velocity in a hurry.  But yeah, it would require cheating and turning off the heat.

Wouldn't your Kerbals turn to goo if you hit that fast?

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2 minutes ago, Loren Pechtel said:

Wouldn't your Kerbals turn to goo if you hit that fast?

Nope, they have essentially infinite acceleration tolerance.

In 1.1.2 I've done 40-g reentries with the inflatable heatshield, no problem.  Hit Kerbin's atmosphere going 3600 m/s STRAIGHT DOWN.  Try it, it's fun. :)

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Just now, Snark said:

Nope, they have essentially infinite acceleration tolerance.

In 1.1.2 I've done 40-g reentries with the inflatable heatshield, no problem.  Hit Kerbin's atmosphere going 3600 m/s STRAIGHT DOWN.  Try it, it's fun. :)

The g-meter has a red zone.  I thought you could squash them.

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@OhioBob I have to admit that my encounter was more or less "sub-optimal" hehe. I thought it didn't matter because the heat shields would, well... shield me against the heat... As you can see, I hardly ever use heat shields haha.


@Andem and @Snark: Out of curiosity I turned off the max temperature in the cheat menu and did land on Laythe without any problem. I reloaded the game afterwards of course!

@bewing :This was a bold (read: silly) move in my current "all sliders on 20%"-carreer, So that's 220.000 kerbucks down the drain. I hope I can save my kerbal by spending all my return trip fuel trying to circularize around Laythe, but to be honest, I don't think I packed enough fuel for this manoeuvre. Else, Bob Kerman will definitely be toast. :(

So if I try to summarize:

  • It's not possible to shed 8000 m/s velocity at any given altitude in Laythe while using only heat shields :)

My next question is then of course:

  • What's the maximum atmospheric re-entry velocity at which heat shields don't go poof yet?
Edited by xendelaar

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14 minutes ago, xendelaar said:

I have to admit that my encounter was more or less "sub-optimal" hehe. I thought it didn't matter because the heat shields would, well... shield me against the heat.

Haha! Such naivete! :wink:

Quote

This was a bold (read: silly) move in my current "all sliders on 20%"-carreer, So that's 220.000 kerbucks down the drain. I hope I can save my kerbal by spending all my return trip fuel trying to circularize around Laythe, but to be honest, I don't think I packed enough fuel for this manoeuvre. Else, Bob Kerman will definitely be toast. :(

Owww! 20%???!? I can't stand going lower than 50% myself, and I test all my rockets in sandbox, first.

 

Quote

So if I try to summarize:

  • It's not possible to shed 8000 m/s velocity at any given altitude in Laythe while using only heat shields :)

My next questions is then of course:

  • What's the maximum atmospheric re-entry velocity at which heat shields don't go poof yet?

It depends a little too much on the atmospheric thickness to really answer that. The number for Eve is very different from the Laythe number. I could try doing some quickie tests in the sandbox, though. :wink: I assume that you have to dump the heatshields before you can retroburn?

Edited by bewing

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14 hours ago, bewing said:

It depends a little too much on the atmospheric thickness to really answer that. The number for Eve is very different from the Laythe number. I could try doing some quickie tests in the sandbox, though. :wink: I assume that you have to dump the heatshields before you can retroburn?

I was talking about laythe and kerbin. I forgot about Eve (and Duna for that matter). Of course the density of the atmosphere is important. :) I forgot to mention that. 

You're right: I have to dump the Shields in order to retro burn. Why do you ask? 

Btw. The reason i don't experiment in sandbox mode is because I like to make these mistakes in career mode. this gives me more cool rescue missions when I screw up. it's a way of prolonging the length of a career game. For me, the career ends after I unlock the tech tree and perform a grand tour and/or jool 5 mission. 

13 hours ago, Streetwind said:

 

The first question you should ask is: which part is exploding? Is it actually one of the heatshields, or something else? You can press F3 after your disaster happens, and check the log for the first instance of "...was destroyed due to overheating". When this happens to be one of the heatshields, then yep, you're screwed - this kind of trajectory is just too aggressive. However, in my experience, it is extremely hard to kill a heatshield, even intentionally... so it's possible that something else went. Something that wasn't properly shielded. Something perhaps sitting directly behind that small hole in the center of your four heatshields... If that is the case, you need to figure out a way of protecting that part from the airstream. If you do that, you might even be able to aerobrake.

This gives me a bit of hope! Tonight i will check the crash data when i get home. Then we'll see which part insta-melts first. 

Behind the small hole is another heat shield.

Edited by xendelaar

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5 minutes ago, xendelaar said:

You're right: I have to dump the Shields in order retro burn. Why do you ask? 

Just a vague hope that you could retroburn halfway and then the heat shields could handle the rest.

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Ow that would be a grand idea indeed! But alas.. the heat shields have to go in order to retro burn . :)

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7 hours ago, Loren Pechtel said:

The g-meter has a red zone.  I thought you could squash them.

The red zone on the g-meter doesn't actually mean anything (other than, perhaps, "gosh, that's a lot")-- it's just window dressing.

Kerbals inside a crewed part are completely invulnerable to anything that doesn't destroy the part. You can do a 40-gee reentry and it doesn't affect them in the slightest, other than the less brave ones making uh-oh faces.

(Then the lander can drops into the water at 8 m/s and instantly vaporizes from the impact, along with the crew...)

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4 hours ago, Snark said:

The red zone on the g-meter doesn't actually mean anything (other than, perhaps, "gosh, that's a lot")-- it's just window dressing.

Kerbals inside a crewed part are completely invulnerable to anything that doesn't destroy the part. You can do a 40-gee reentry and it doesn't affect them in the slightest, other than the less brave ones making uh-oh faces.

Don't you think, though, that squashing may be a potential future feature for KSP?

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28 minutes ago, bewing said:

Don't you think, though, that squashing may be a potential future feature for KSP?

Well, pretty much anything is a "potential" feature.  :)  But I've certainly never heard anyone at Squad ever mentioning any such possibility.  And the "red zone" has been around forever, so I seriously doubt it was any sort of forward-looking preparation for a "crew acceleration tolerance" feature; almost certainly it was just a visually appealing bit of graphics back when they first designed the navball.  If they had already been planning for such a feature that long ago, surely they'd have done it by now-- it would be a pretty cheap and quick feature to implement.

And in any case, that sort of feature seems to me to go together with other "bio-realism" features such as life support, or crew happiness, or living space, etc.-- none of which has Squad ever addressed even slightly, or given any hints that they plan to do so.  My take on it is that they've simply made a conscious decision that "this game is about rockets more than kerbals" and kept it simple on purpose.

So, my personal guess would be that they have no plans to add such a feature, but of course I don't have any more of a crystal ball than you do.  But even if they did add it in some future version, you don't need to worry about it now, so might as well have fun.  :)

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Just a small update.  I did some tests and i managed to increase my atmosphere time from 0.1 seconds to 3 seconds. 

The shields heat up really quickly and then the energy dissipates towards the fuel tanks behind them, resulting in a big kaboom. Here are some pictures if anybody is interested:

 

I repeated the experiment several times and every time something else blew up first. Sometimes the struts went first and another time the tank was the first one to go poof. In one occasion a heat  shield was the first the go.

I thinkbob's days are numbered... but I haven't given up on him yet.  

Edited by xendelaar
Inserted some pictures

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Do you have to use Laythe direct?  Can you use Jool to capture/redirect to set up a more favorable encounter with Laythe?

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12 hours ago, Snark said:

The red zone on the g-meter doesn't actually mean anything (other than, perhaps, "gosh, that's a lot")-- it's just window dressing.

Kerbals inside a crewed part are completely invulnerable to anything that doesn't destroy the part. You can do a 40-gee reentry and it doesn't affect them in the slightest, other than the less brave ones making uh-oh faces.

(Then the lander can drops into the water at 8 m/s and instantly vaporizes from the impact, along with the crew...)

I'm curious--some of the landing gear has some very high impact resistance.  If you're on land does that mean you can hit really hard and survive?

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14 minutes ago, Loren Pechtel said:

I'm curious--some of the landing gear has some very high impact resistance.  If you're on land does that mean you can hit really hard and survive?

[Before testing]  No, of course not, don't be silly.  When it says "impact tolerance 70 m/s" that doesn't mean it can take impact straight down into terrain at that speed, it's just some way of expressing the fact that it can land while the aircraft is traveling quickly.

[After testing]  Huh.  I'll be damned, you're right.  Just made a small, lightweight test craft that came down on four of the big landing gear, vertically at over 50 m/s.  Landed without a problem, no damage.

So of course this raises the "but I thought they were breakable now?" question.  I think the key, here, is that I had a very lightweight craft, so it had extremely low mass that the very heavy-duty shock absorbers in the landing gear could handle.  If I had had a few dozen tons of rocket riding on them, I expect it would have been a very different story.

But yeah, looks like we've discovered a new exploit.  Oh joy.

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5 hours ago, xendelaar said:

Just a small update.  I did some tests and i managed to increase my atmosphere time from 0.1 seconds to 3 seconds. 

The shields heat up really quickly and then the energy dissipates towards the fuel tanks behind them, resulting in a big kaboom. Here are some pictures if anybody is interested:

 

I repeated the experiment several times and every time something else blew up first. Sometimes the struts went first and another time the tank was the first one to go poof. In one occasion a heat  shield was the first the go.

I thinkbob's days are numbered... but I haven't given up on him yet.  

Have you tried "rotiserrie mode?" Spin it really fast, that may help spread the heat around. Or not. But it's fun to try!

How far out is your last quicksave? I seem to recall that 1.1.2 added backups of autosaves. I haven't looked for those, but perhaps that can get you farther back emough to get a more favorable encounter.

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4 hours ago, Snark said:

But yeah, looks like we've discovered a new exploit.  Oh joy.

New?

People have been lithobraking landers on modular girders since the dawn of time... :P

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