Heat shield exploding before ablating

[Kelderek]

I've got a vessel that is exploding in re-entry due to the heat shield exploding with almost all of the ablator remaining. This seems really odd to me.

I was attempting a very steep descent (I basically went straight up to about 400km and then fall back down to kerbin). I would expect the ablator to burn off before the part could explode. Here's a screenshot a fraction of a second before the explosion with some thermal data in KER:

Can someone explain to me what is happening with the heat here? Is the heat increasing faster than the ablator can burn off?

EDIT: I know that this descent is too steep for practical purposes, I'm just trying to understand how the heat is affecting the parts during re-entry.

Edited September 4, 2015 by Kelderek

[Randazzo]

I've got a vessel that is exploding in re-entry due to the heat shield exploding with almost all of the ablator remaining. This seems really odd to me.

I was attempting a very steep descent (I basically went straight up to about 400km and then fall back down to kerbin). I would expect the ablator to burn off before the part could explode. Here's a screenshot a fraction of a second before the explosion with some thermal data in KER:

http://i.imgur.com/a6ZQiQ4.png

Can someone explain to me what is happening with the heat here? Is the heat increasing faster than the ablator can burn off?

http://i.imgur.com/2ugDFv6.png

EDIT: I know that this descent is too steep for practical purposes, I'm just trying to understand how the heat is affecting the parts during re-entry.

You're going too fast, and the skin of the vessel is getting too hot. You have two temps that are measured against max temp now, skin and internal, and if either exceeds max temp, you go pop.

Ablator can only shed off so much heat (very little at that), and the heatshield is otherwise just a normal part with very low conduction triggered by convective flux reaching a certain point. If you overwhelm it's capability (which you are), it will explode like any other part.

So yes, the convective flux is far exceeding the heatshields ability to shed heat, and therefore it overheats and explodes.

edit: If you want to see the actual numbers, turn on thermal data in action menus via the alt+f12 menu. You can watch the convective flux climb as you start plowing through the atmosphere.

Edited September 3, 2015 by Randazzo

[Chaia]

Your Vessel will also explode if the aerodynamic forces are too strong, those will not burn off your ablator nor heat your vessel, they will just explode it. There is a thread with some research about aerodynamic failure in jools atmosphere, where the vessel was not burning, but exploding if it dips a few km too deep into the atmosphere

[Claw]

I agree that it looks like you are simply coming in to steep, and the heat shield isn't able to ablate fast enough.

Ther aero forces statement is interesting also, but it doesn't look like you've reached that level yet. You can confirm which is happening using the F3 flight log (if you haven't already).

Cheers,

-Claw

[Kobymaru]

I agree that it looks like you are simply coming in to steep, and the heat shield isn't able to ablate fast enough

Then what's the point of an ablator, really? This does not sound like it has a foundation in reality. If a heat shield with an ablator gets to hot, shouldn't just the ablator get burned off? I think Squad should raise the Heat shield max temperature to infinity as long as there is ablator left.

[Kelderek]

Your Vessel will also explode if the aerodynamic forces are too strong, those will not burn off your ablator nor heat your vessel, they will just explode it. There is a thread with some research about aerodynamic failure in jools atmosphere, where the vessel was not burning, but exploding if it dips a few km too deep into the atmosphere

The F3 report specifically says that the heat shield explodes due to overheating.

I was able to change my flight profile to complete my mission safely (suborbital hop to obtain near and high space science).

I guess I'm just boggled by heat shields in general for KSP. It seems like there is an extremely narrow range of usefulness for heat shields. Most of the time they are not needed at all as other parts can handle heat reasonably well. 9 times out of 10 when I use a heat shield only about 5% of the ablator is ever used. Then there is the other extreme like in this case where the heat shield doesn't help at all, it just overheats and explodes before being able to burn off ablator. I just think the numbers need to be tweaked so that they have a wider range of usefulness.

On a side note, I also think that heat shields that are still shrouded and have a stack below them should not be able to burn ablator at all until they become exposed. I have had many ships burn off ablator completely during aerobrake maneuvers before I even remove the stack beneath them, making them useless for landing later.

Edited September 3, 2015 by Kelderek
I fail at typing

[goldenpeach]

It happened to me when I tried to make en Eve impactor. Due to the launcher constraint, I decided to make it go in atmosphere directly from the sun orbit.

The heatshield didn't last ten second before over-heating, thus making the whole impactor unprotected against heat.

well, at least, I can say it was an impactor: it just hit the atmosphere instead of the ground

[MarvinKitFox]

Then what's the point of an ablator, really? This does not sound like it has a foundation in reality. If a heat shield with an ablator gets to hot, shouldn't just the ablator get burned off? I think Squad should raise the Heat shield max temperature to infinity as long as there is ablator left.

That's like complaining "Whats the use of my car's airbag? When I ram into a granite cliff at 315mph the airbag does nothing!"

[Kobymaru]

That's like complaining "Whats the use of my car's airbag? When I ram into a granite cliff at 315mph the airbag does nothing!"

The airbags resource is the gas/pressure inside. If I ram my head in with a certain speed, it will catch me - there was enough pressure. If I ram my head in faster, I might break my head because it the air inside couldn't stop me and my head goes into the car. If it were bigger, with more air and pressure, it would have more head-stopping capacity. (I mean I'd still die from the rest of the car sqashing me, but we don't have thermal analog in this metaphor for that)

Now the shields clearly have a capacity which is dependant on the ablator ressource - except apparently, it explodes before it's consumed.

In your analagy, the airbag would pop before it is fully compressed, which really shouldn't happen.

Just think about how ablative heat shields work in reality: if they are heated very very much, the outermost layer evaporates (slightly cools the rest) and gets blown away. Then the next layer, then the next, then the next. But why would the whole thing suddenly explode or melt, if the heat comes from one direction only?

https://en.wikipedia.org/wiki/Atmospheric_entry#Ablative

Edited September 3, 2015 by Kobymaru

[NathanKell]

Because you're getting a spot of burn through. Ablative shielding *does* have a maximum flux it can protect against; beyond that point you'll get local spots of burnthrough.

[mrclucks]

Only times I've been able to go to space and straight down is literally going straight up to less than 100km.

All other times you're gonna want a periapsis of at least 20-30km so the upper atmosphere can bleed off speed before you get lower

[Tannerz]

But why would the whole thing suddenly explode or melt, if the heat comes from one direction only?

you are getting too wrapped up in a time function. Explosions are just things burning at different rates. The ablator most certainly did all burn off, it just happened in milliseconds instead of minutes due to the incredibly high heat over a sustained period. Think of it like a flash point with flammable vapor.

[Kelderek]

Because you're getting a spot of burn through. Ablative shielding *does* have a maximum flux it can protect against; beyond that point you'll get local spots of burnthrough.

I think my main issue is that the numbers really should be tweaked further. Here's what the range looks like right now:

Heat shield blows up |---------<+>----------| No heat shield needed

But here is what the range should be:

Heat shield blows up |-------<+++++>-------| No heat shield needed

It actually bugs me that a lot of vessels don't need heat shields at all. I would just like the devs to take another look at the numbers to see if they can be balanced better.

[Claw]

Okay. I'll take it more as a piece of balance feedback to look at. So, in that regard, can you provide a clearer picture of your craft (for replication) and a description of your orbit/reentry? That makes it easier to give it a try on my end to see what you are seeing.

Also, balancing heat shields window is hard on Kerbin, because using up all (or most) of the ablator on a Kerbin reentry means a lot of difficulty landing somewhere like Eve.

Cheers,

-Claw

[Kelderek]

Thanks, here's a craft file. The flight profile is to just pitch over east about 10 degrees max, I usually end up over 400km Ap. The main objective of this mission was to complete the early career "escape the atmosphere" contract and also obtain both near and high space science without going into orbit. The heat shield will overheat and explode after using less than 5% of the ablator.

Since I only needed to reach 250km for high space science, I was able to pitch over further toward the east (more like 45 degrees) and still get a high enough Ap for a suborbital trajectory and I was able to land safely.

Maybe this is all as intended, but it just didn't make any sense to me that the heat shield could explode from HEAT without burning off all or at least more of the ablator. This could just be a system too complex for me to wrap my brain around (lol, that has happened before).

As for how heat shields work on Eve compared to Kerbin, I would expect heat shields to ALWAYS be required for any re-entry at Kerbin from >70km to sea level. You could have it so you only need a smaller amount of ablator on Kerbin, but on Eve you would need all of it (thankfully we already have a slider for this on the part). The heat shield would need more or less ablator depending on your angle of re-entry, but if you had no heat shield at all then your ship should explode.

In my opinion, a heat shield should save your ship from >70km to sea level for any speed less than the escape velocity for kerbin's SOI, but if you are coming from outside Kerbin's SOI (returning from an interplanetary voyage), then you would have to slow down first for it to be safe.

I appreciate this discussion, I'd really like to better understand how all of this works.

[Claw]

Maybe this is all as intended, but it just didn't make any sense to me that the heat shield could explode from HEAT without burning off all or at least more of the ablator. This could just be a system too complex for me to wrap my brain around (lol, that has happened before).

Hmm... Well, maybe think about heat this way. Instead of thinking of "temperature" think of it as Energy. When an object passes through the atmosphere at high speed, the friction from the air passes energy into the object. Each object also radiates energy away. The rate that it adds energy is a matter of how fast that object is moving (i.e. more friction). The amount of energy that radiates off is more of a function of temperature differental between the object and the surroundings.

So, if you have an object that's passing through the atmosphere really fast, it heats up because it can't shed energy as fast as it's gaining it.

Ablator works on the same principle. It can only ablate so quickly, and hence can only shed energy so quickly...which is faster than shedding energy by pure radiation, but is still a finite rate. So, if you come in too steep, the heat shield can't shed energy fast enough to prevent an excessive energy build up.

This is also true in real life. If you've ever seen Apollo 13, there's a piece in there with news commentary showing a basketball and baseball , representing the Earth and the Moon. The commentator states that the capsule has to hit a reentry wedge as thick as a piece of paper. To shallow and the craft skips off and away. To steep and it'll burn up.

Is that fun in KSP? Not wholly. KSP actually has a bit bigger of a window than that, but turns out that sub-orbitals can be dangerous if they come straight back down from too high up. The artificially small-kerbin size also means it has an equally small atmosphere. So the atmospheric density ramps up quicker than in real life, which doesn't allow as much time for the capsule to slow down and prevent excess energy buildup.

As for how heat shields work on Eve compared to Kerbin, I would expect heat shields to ALWAYS be required for any re-entry at Kerbin from >70km to sea level.

Well, heat shields aren't always required for one main reason...KSP parts have a slightly high heat tolerance. The reason being because you can use those parts on all manner of vessels, including planes. Because of the lego style of building, having lower temperature tolerant parts would make space planes nearly impossible, due to the amount of time they spend in the atmosphere while heading to space.

Now, all that being said, KSP is still a video game subject to game balance. So I'm not trying to be dismissive of your feedback, but trying to first explain real world physics. I also used to do the "launch straight up to 250km+ and obtain science" in my career games. That doesn't work as well anymore, especially early game. Even if the pod survives the heat, it often doesn't slow down fast enough to prevent impact.

I'll give your profile a poke and see. I can't guess off hand what the safe reentry speeds are, but I see that you're hypersonic at 15/16km, which is pretty fast. The steep angle isn't giving much time for the ablator or energy to shed.

Cheers,

~Claw

Edited September 4, 2015 by Claw

[Reactordrone]

Personally I preferred the drag balance in 1.0.2 but even that was much slicker than FAR. The blunt body drag seems a little low for the capsules.

[Kelderek]

So, if you have an object that's passing through the atmosphere really fast, it heats up because it can't shed energy as fast as it's gaining it.

Ablator works on the same principle. It can only ablate so quickly, and hence can only shed energy so quickly...which is faster than shedding energy by pure radiation, but is still a finite rate. So, if you come in too steep, the heat shield can't shed energy fast enough to prevent an excessive energy build up.

I think this is the part I hadn't really considered before, but the way you describe it makes sense. Thanks for the detailed response!

[NathanKell]

Reactordrone: the drag for capsules returning bottom-first did not change at all since 1.0.2.

[Warzouz]

I find heat shipd useless, even on Eve, they don't do much. Using airbrakes is much much more efficient to control temperature. I even had ship explosions while cooling down...

[Edax]

That's like complaining "Whats the use of my car's airbag? When I ram into a granite cliff at 315mph the airbag does nothing!"

Or complaining that your refrigerator can't freeze nitrogen into liquid, or that your rocket engine can't take you to the speed of light.

[sal_vager]

Gentlemen, as humorous as you may find these comparisons to be they are not adding anything useful to this discussion.

[Clipperride]

In real life, reentry was a big problem during the 1950s. It was a genius move to turn the problem on its head, as it were, and experiment with Blunt end First. The shock wave creates a much cooler area behind the heat shield which the payload sits in. Laika, the first and hopefully only dog in space, was always going to be a one way ticket as the USSR hadn't solved the problem of aheding all the energy of launch. In tests of heatsink vs heat shield it was found that the ablative shield was the best protection per kg of mass added.

I do agree there should perhaps be Kerbal and Interplanetary versions of the heat shield. 15% of the current thickness would probably be enough for most trips back from the Moons.

The early Mercury flights had some issues, less with heat from such a sharp angle of reentry, but the g force on the human occupant. Luckily we don't have to worry about that!

[lost_in_space]

It seems like there is an extremely narrow range of usefulness for heat shields. Most of the time they are not needed at all as other parts can handle heat reasonably well.

I would have agreed prior to 1.04, but not now.

With 1.02, I never used heat shields. Just packed all my science stuff in a service bay and let that protect them.

I tried to do that in 1.04 and all my experiments exploded. I always use heat shields now.

[Norcalplanner]

In real life, reentry was a big problem during the 1950s. It was a genius move to turn the problem on its head, as it were, and experiment with Blunt end First. The shock wave creates a much cooler area behind the heat shield which the payload sits in. Laika, the first and hopefully only dog in space, was always going to be a one way ticket as the USSR hadn't solved the problem of aheding all the energy of launch. In tests of heatsink vs heat shield it was found that the ablative shield was the best protection per kg of mass added.

I do agree there should perhaps be Kerbal and Interplanetary versions of the heat shield. 15% of the current thickness would probably be enough for most trips back from the Moons.

The early Mercury flights had some issues, less with heat from such a sharp angle of reentry, but the g force on the human occupant. Luckily we don't have to worry about that!

Ablator is a tweakable that can be changed in the VAB. I regularly remove a majority of the Ablator for Mun and Minmus missions.