Heatshield destruction temperature too low

[lajoswinkler]

Why is the destruction temperature of stock heatshields set so low? IMO, it should be a lot higher, so high it's not realistically achievable unless you approach at speeds over at least 10 km/s.

Let the high temperatures destroy the ablator at exponentially higher rate as the temperature climbs, but the shield should not detonate before its ablator is depleted.

Sometimes the reentry is so intense at the very begining of it so that the ablator is still almost 100% because the other factors (pressure, drag, etc.) are low, but the maximum temperature is exceeded in a fraction of a second so the thing just gets instantly destroyed, followed by whatever it was shielding. It presents a very annoying problem when you launch probes into Eve, just to mention a common example. Soviet Venera probes did not circularize their orbits. It was always a direct interplanetary collision.

After all, a heatshield is nothing but a passive piece of equipment made out of a refractory material with a temperature probe here and there.

[Aetharan]

You're talking about a part with a max temp of 3,400º that comes with the benefit of carrying a lot of heat away from the craft (losing ablator) instead of absorbing all of that energy into itself and conducting it up into your probe / command pod / whatever.  It does its job, and does it well, but it's not a magical "If I have this, I can ignore the effects of atmospheric compression in front of my ship" tool.  The problem is not in how the part works.  You're coming in too steep and too fast.

Edit:  Just for reference, in one of the challenge threads I've taken part in, I brought a ship back in from Duna-height Kerbolar orbit to Kerbin re-entry with a PE on the first pass of 45 km and no heat-shield at all-- tail-first entry presenting a Spark, which has a max temp of only 2,000º.  That ship lost nothing to heat or atmospheric forces.

Edited December 28, 2015 by Aetharan

[lajoswinkler]

Yes, first pass on Kerbin. I don't come in too steep cause I can return from Duna in a single reentry procedure, but I'm not talking about that.

I'm talking about doing that with Eve and doing it in a single pass. Or doing it with gas giants in Outer Planets Mod. It should be possible because that's what people did in real life. Soviets smacked their probes right into Venus and they survived. Probes were thrown into Jupiter and Titan at incredible speeds, they survived.

There is absolutely no reason for the heatshield to explode before it loses its ablator. I mean, it's fairly easy to change the max temp value in the config file, but I'm just saying it would be better if it behaved that way by itself.

[FancyMouse]

KSP is already scaled so that entry into Kerbin is similar to entry into Earth, so derived numbers like your 10km/s doesn't compare to RL numbers.

Eve entry is possible, both in 1.0.4 and in 1.0.5. Yes I'm talking about single pass aerocapture and yes I'm talking about interplanetary speed (~5.5km/s entry speed, a little higher than optimal). My testing shows 1.0.5 is even easier due to more powerful heat shield but periapsis needs to be a little lower than 1.0.4. If you have a problem, post your picture and ask.

Keep in mind KSP is a game that if you find something doesn't work, it's better to ask around (and possibly learn something new) instead of blaming the game.

And no. Ablators don't magically just convert heat to ablation. There is a procedure to do that, and that isn't instant. I'm fine with using a max temp explosion to model the case when the condition is so extreme that even ablator can't handle it.

Edited December 28, 2015 by FancyMouse

[Starwaster]

 

6 hours ago, lajoswinkler said:

Why is the destruction temperature of stock heatshields set so low? IMO, it should be a lot higher, so high it's not realistically achievable unless you approach at speeds over at least 10 km/s.

Let the high temperatures destroy the ablator at exponentially higher rate as the temperature climbs, but the shield should not detonate before its ablator is depleted.

Sometimes the reentry is so intense at the very begining of it so that the ablator is still almost 100% because the other factors (pressure, drag, etc.) are low, but the maximum temperature is exceeded in a fraction of a second so the thing just gets instantly destroyed, followed by whatever it was shielding. It presents a very annoying problem when you launch probes into Eve, just to mention a common example. Soviet Venera probes did not circularize their orbits. It was always a direct interplanetary collision.

After all, a heatshield is nothing but a passive piece of equipment made out of a refractory material with a temperature probe here and there.

 

As mentioned previously, scale is an issue as well as balance. The more balanced a shield is for Earth, the more overpowered it would be for Kerbin.

That said, looking at the config for stock shields, 3300 K is a pretty reasonable temperature for an ablating, charring heat shield.  

Bottom line is that it is your expectations that need resolving. If you can't survive an Eve reentry then you're doing it wrong. Modify your approach until you find a solution that works. Venera might have been steep but it was still enough of an angle for aerobraking to  come into play so decrease your reentry angle.

[OhioBob]

I recently performed a bunch of entry and aerocapture tests to study vehicle temperature and ablator loss.  The tests were performed at Eve, Duna, and Jool.  At Duna and Jool I found no overheating problems as long as entry occurred at normally expected velocities and entry angles.  I suppose the peak temperature could exceed that of a heat shield if you came in at an excessive velocity or at a suicidal angle, but then I would expect the vehicle to overheat and be destroyed.  I didn't find any behavior that I thought was out of balance.  Eve was more of a challenge and was the only planet where I found that destructive overheating could be a likely problem.  With a low ballistic coefficient I found little problem, though temperature could get up to 2500-3000 K.  It was only with very high ballistic coefficients that that I had to be really careful.  If I had a combination of high BC and high entry velocity, then things started to get iffy.  Under those conditions the safe entry corridor gets pretty narrow, and may even close all together.  You really have to think about what you're doing and design appropriately, but that's a good thing.  We don't want to take the danger and difficulty completely out of the game.

[lajoswinkler]

Why would the shield explode before its ablating material is lost? Do you guys know how shield work? The rammed air in front of them radiatively heats them up and they sublimate away. The greater the heat radiated onto them, the higher the ablating rate is. The heat is not causing its temperature to rise, but is instead spent on sublimation.

Tolerating what happens in KSP is like saying that if you heat a cup of water fast enough, it would just get broken to its subatomic particles instead of first turning into gas. A cup of water will remain at the same temperature while it's boiling (100 °C) because the heat we supply is spent on changing its phase. It's very similar with heat shields. That's how they keep the vessel from heating up.

I'd be ok with the KSP shields, in the event of a wild reentry, losing ablator very fast and then exploding, but I'm sometimes seeing them detonate with basically all of the ablator still on them. It's stupid and unrealistic.

Edited December 28, 2015 by lajoswinkler

[FancyMouse]

This is already discussed before. As I said earlier, it's an opportunity to learn, not to blame the game or pretend you know something.

[Starwaster]

1 hour ago, lajoswinkler said:

Why would the shield explode before its ablating material is lost? Do you guys know how shield work? The rammed air in front of them radiatively heats them up and they sublimate away. The greater the heat radiated onto them, the higher the ablating rate is. The heat is not causing its temperature to rise, but is instead spent on sublimation.

Tolerating what happens in KSP is like saying that if you heat a cup of water fast enough, it would just get broken to its subatomic particles instead of first turning into gas. A cup of water will remain at the same temperature while it's boiling (100 °C) because the heat we supply is spent on changing its phase. It's very similar with heat shields. That's how they keep the vessel from heating up.

I'd be ok with the KSP shields, in the event of a wild reentry, losing ablator very fast and then exploding, but I'm sometimes seeing them detonate with basically all of the ablator still on them. It's stupid and unrealistic.

Heat shields have  maximum heating rate that they are qualified for. That's in real life that I'm talking about; in KSP that's not something that's hard coded into the shield or exposed to the player. But if it heats up faster than the ablator can keep up with then it can fail. Again, in real life, that's the sort of thing that has to be taken into consideration when planning a mission and deciding how to design the shield and what kind of reentry profile it will fly. 

Not sure what you're trying to say there above. Do you think the shield shouldn't heat above the point at which it starts sublimating? If so that is WRONG. It starts sublimating at 400 and continues increasing. Apollo's shield peaked at about 2750 C. 

You also say it still had all its ablator on it? That's not even plausible unless you're hitting a bug that causes the shield to not ablate at all. Or you're exaggerating and think that the amount of ablation you're seeing isn't enough. If it's the former and you really are seeing no ablation at all then you need to move this to the support threads (modded or unmodded as appropriate). If it's the latter then I'm sorry but your expectations need adjusting. 

[MaxL_1023]

No ablator is perfect. Besides heat flux to the rest of the craft, you need to consider the mechanical durability of the shield itself. At those temperatures, extreme ablation will burn through in some areas and break the shield apart in others - the act of sublimation itself adds local forces to the heatshield body. 

The Apollo missions needed special heatshields rated for Lunar returns (10-11 km/s as opposed to 7.5 km/s), lifting re-entries (very hard to do manually in KSP but possible) to reduce the aerodynamic loads and still had a re-entry window which I heard described as hitting a bump on a basketball.

Basically, you are asking too much of your heatshield. It already takes a badly-aimed ballistic re-entry from an interplanetary trajectory to burn one out on Kerbin - they are fine for Eve orbit entries or even aerocapture passes if you keep your angle of approach reasonable. You also might just have too much weight behind your shield - if your mass/drag ratio favors mass you will maintain speed further into the atmosphere and end up with higher heat loads. Try making your craft wider (2.5m shield on a 1.25m stack can work with an adapter) and you won't have as much trouble. 

[Guest]

I have no problem with changing how the game is played with new updates. However, I wish it was made more obvious when you start the game that changes were made. I was incredibly frustrated when I first found out that my ships were exploding. It looks like a bug if you don't study reentry effects or visit this forum regularly. I mean, not everyone that enjoys this game spends time reading wikipedia about reentry and shock waves. Interesting stuff, and I'm glad that I know this stuff now, but still frustrating when people claim it's your fault for being ignorant of real life reentry, and that the game was changed to simulate said effects. I think a quick introductory video could be added at game start to educate players on reentry before just blowing up your crafts. This would stop some people like me from making false assumptions and exposing their ignorance on a public forum and save them from ridicule.

Edited December 28, 2015 by Otis

[Starwaster]

Since the ablation rate itself has been questioned repeatedly, @lajoswinkler, and you also question everyone else's knowledge in the face of some perfectly reasonable answers, I have to ask: how much do YOU really know about shields? You seem to think the rate itself should be much higher but do you know how much they really ablate? I thought I did until @NathanKell and others pointed me at some literature on Apollo that indicated that the actual amount of shield that was ablated away was less than 1/100th of what I thought it should be. That's why the stock shields have such low ablation rates. 

If you want higher ablation rates, install Deadly Reentry. I intentionally (and extremely unrealistically) increased the ablation rates in order to keep players on their toes and sweating into their keyboards.

[NathanKell]

Mercury lost about two to three pounds of ablator on reentry, based off the sources I found. One unit of ablator in KSP is 2.2lb (1kg).

 

EDIT: And I really have no idea where you're getting your stuff here @lajoswinkler. Radiative heating really isn't much of an issue except at BLEO (lunar, interplanetary) reentry velocities. The shockwave compresses the air, yes, and then you get convective (or I guess you could consider it, given high Q, conductive) heating. And you most certainly do get a high wall temp, although of course the backing doesn't heat up--but the skin temperature isn't tracking the backing material, it *is* tracking the wall temp.