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Reentry Questions


baldamundo
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So I remember atmospheric heating being a bit of a joke back when it was introduced in 1.0 - even with difficulty set to max and Deadly Reentry installed, it was child's play to survive reentry. However, I'm really struggling with the current version, and most of the threads I can find are either out-dated, unclear, or actually contradictory, so I have some questions.

FWIW I currently have re-entry heat set to 100% and the latest version of Deadly Re-entry installed (although I'm not clear on how much difference that makes!) and am playing in the stock Kerbol system.

  • How exactly does your trajectory relate to re-entry heating? The 'common sense' assumption seems to be that a shallower trajectory is safer, but I've also seen people argue that it exposes you to heating for longer, therefore total re-entry heating is greater. Is it basically the case that a steep trajectory minimises heating, while a shallow trajectory minimises speed and g-forces?
  • How is re-entry heat determined? Presumably the higher the speed and the denser the atmosphere are the key variables. How exactly do angle of attack and your vessel's aerodynamics affect heating? What about mass/weight?
  • Does having radiators affect  re-entry heating at all?
  • How exactly do heatshields work? Is there a certain temperature threshold at which they stop increasing in temperature and start burning ablator instead? What determines the rate of ablation? Why is it that once the ablator is used up, the part seems to explode instantaneously, rather than only when it reaches its temperature limit?
  • What's the deal with spaceplanes? If I ever get above about Mach 3.5, I start exploding. Is this a simple function of velocity vs altitude/pressure? If so, is there a guide somewhere as to what speeds are safe at which altitudes? Does angle of attack make a difference, and if so how? What about the shape of your plane?
  • Any tips for surviving EVE reentry?
  • Do people generally consider reentry balanced at 100% reentry heat setting with Deadly Reentry installed? Should I just give up and turn it down? Or should I uninstall Deadly Reentry? Does anyone have any recommended thermal settings?.

 

Really appreciate any help people can give, so thanks! Have a feeling there were some other questions I've forgotten as well...


EDIT: Forgot to ask, are there other decent strategies for plausible re-entry vehicles besides heatshielded capsules and shuttle-style spaceplanes?

Edited by baldamundo
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Moving to Add-on Discussions.

Was going to put it in Gameplay Questions, until I saw that you've got Deadly Reentry installed, which means that it's really about that mod, not about the game.

If you have a question specific to a particular mod, though, you may want to consider asking your question in that mod's thread, instead of here.  My guess is that you'd be more likely to get a helpful and/or prompt answer.

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If you can, try using pods that can offset their center of mass. This lets you do a lifting re-entry, which reduces G-forces while allowing you to use a slightly steeper trajectory through the uppermost atmosphere. 

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My general impressions on a couple of those:  First off, as far as I can tell heatshields are basically assumed to be made of ablator - once it's gone, they're done.  Secondly, re-entry profile depends a bit on how you've designed your landing profile, in my opinion.  If you're doing a powered landing, by all means come in steep to reduce heating time - and slow down as you need to to keep heat under control.  For areobrake landings, I prefer moderately shallow: Shed a good portion of your speed high in the atmosphere, where you won't heat up as much as you do it.  You shouldn't really be going more than Mach 4 at the maximum when you hit 30km - and that's probably stretching it: Mach 3 would be safer.  But in that case you're intentionally converting speed to heat, so you want manage that.

I do believe radiators will work - even though they shouldn't, really.  Just keep them out of the airstream, so they aren't trying to radiate their *own* heat.  I think that your spaceplane question is basically a ratio of pressure to velocity - higher velocity at higher pressure means more heat.  This goes to my rule of thumb of mach 3-4 by 30km - beyond that you're gaining heat to quickly for any management technique to help you.  A heatshield will hold for a bit - but you need to be below that speed and dissipating heat by the time you get done.

Also be aware that your *density* makes a difference - the denser you are, the more energy you need to dissipate, and if you're using atmospheric braking that means the more heat you'll be exposed to.

As for other reentry profiles, there's an interesting mod out there called 'inline balutes' - which work much like high-altitude parachutes.  With them you can slow yourself down far enough that you don't need a heatshield - though you'll still need some other way to slow your final descent, as they aren't designed for low altitudes.

 

Anyway, those are just my impressions.  I could well be wrong on lots of it. :wink:

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10 hours ago, baldamundo said:

So I remember atmospheric heating being a bit of a joke back when it was introduced in 1.0 - even with difficulty set to max and Deadly Reentry installed, it was child's play to survive reentry. However, I'm really struggling with the current version, and most of the threads I can find are either out-dated, unclear, or actually contradictory, so I have some questions.

FWIW I currently have re-entry heat set to 100% and the latest version of Deadly Re-entry installed (although I'm not clear on how much difference that makes!) and am playing in the stock Kerbol system.

  • How exactly does your trajectory relate to re-entry heating? The 'common sense' assumption seems to be that a shallower trajectory is safer, but I've also seen people argue that it exposes you to heating for longer, therefore total re-entry heating is greater. Is it basically the case that a steep trajectory minimises heating, while a shallow trajectory minimises speed and g-forces?
  • How is re-entry heat determined? Presumably the higher the speed and the denser the atmosphere are the key variables. How exactly do angle of attack and your vessel's aerodynamics affect heating? What about mass/weight?
  • Does having radiators affect  re-entry heating at all?
  • How exactly do heatshields work? Is there a certain temperature threshold at which they stop increasing in temperature and start burning ablator instead? What determines the rate of ablation? Why is it that once the ablator is used up, the part seems to explode instantaneously, rather than only when it reaches its temperature limit?
  • What's the deal with spaceplanes? If I ever get above about Mach 3.5, I start exploding. Is this a simple function of velocity vs altitude/pressure? If so, is there a guide somewhere as to what speeds are safe at which altitudes? Does angle of attack make a difference, and if so how? What about the shape of your plane?
  • Any tips for surviving EVE reentry?
  • Do people generally consider reentry balanced at 100% reentry heat setting with Deadly Reentry installed? Should I just give up and turn it down? Or should I uninstall Deadly Reentry? Does anyone have any recommended thermal settings?.

 

Really appreciate any help people can give, so thanks! Have a feeling there were some other questions I've forgotten as well...


EDIT: Forgot to ask, are there other decent strategies for plausible re-entry vehicles besides heatshielded capsules and shuttle-style spaceplanes?

  • The temperature of the reentry shockwave is based on velocity cubed. How much of that heat is transferred to your craft is a function of atmospheric density. Assuming a simple craft like a pod + heat shield, the optimal periapsis is 20km. As to shallow vs steep, there's a bit of a corridor where it's possible to go either too shallow or too steep.
  • Too steep can have one of two effects: Either parts of your craft (or the whole craft) burn up because you didn't slow down enough before hitting the denser parts of the atmosphere where you're getting maximal amounts of heat and your shield is overwhelmed. OR you suffer enough g-forces to render your crew unconscious or kill them outright. (crew blackout is handled by stock, not DRE. g-force related death is DRE, not stock). 
  • If you go too shallow, it is possible to cook off too much of your shield so you wont have any shielding when you descend at high speed into the lower part of the atmosphere. Or you can be so shallow that you don't brake enough and fly back out of the atmosphere or fail to capture on an aerocapture.
  • Stock radiators are not effective in reentry as they will not cool any part unless that part is hotter than the external temperature.
  • Ablative heat shields start ablating at 500k. The ablated material removes heat from the shield. The faster it ablates, the faster heat is removed. Contrary to what some people believe, heat shields do have limits even in real life though the mechanism of failure differs from real life. In Deadly Reentry, heat shields that run out of ablator have a reduced maxTemp, which is probably why your shield is exploding as soon as its ablator is depleted. 
  • Spaceplanes: You didn't say what your altitude was nor did you say if you were cruising or reentering and it matters. It would be highly unrealistic to be able to fly a plane much faster than Mach 3.5. Only the SR71 has ever flown that fast and that's faster than its operational speed of Mach 3.2. They were allowed to push it higher and its official max speed was 3.3 though 3.5 is its record. The shuttle on the other hand came in at Mach 25 but that was a shallow reentry. With stock Kerbin you will be coming in MUCH slower than Mach 25. I don't personally do spaceplanes that much but IIRC, the right periapsis was something like 40-50 km. You will want a pitch angle of about 35-40 degrees. (I will also mention here that with stock Mk3 parts I have survived RSS reentries. i.e. a full Mach 25 reentry so it is possible, at least with the RO physics configs to scale convection heating down to Earth  reentries)
  • Eve: For capsules, I think I used 40-60. I don't remember the exact periapsis/angle I used but it's not hard at all. Make that dense atmosphere work FOR you not AGAINST you. Do as much braking in the upper atmosphere as possible. If you use the inflatable shield, you want your altitude even higher as is basically heat resistant cloth. The point of the inflatable is to increase your surface area which means more drag which means more braking higher up.

Remember, I'm the current developer of DRE. I use the same DRE you guys have :wink: . My thermal settings are pretty typical for stock except that I often lower my conduction to RO levels so I can test Real Fuels boiloff. The effect that has on reentries actually makes them hotter and I still survive just fine. (makes them hotter because the skin can't conduct its heat as quickly into the interior)

One last thing about you running out of ablator. Sounds like TOO shallow and baking it off. I said that earlier but just to revisit that, a typical reentry should only use about 25-35%. (maybe less on a suborbital. Mun or Minmus would use more but you should still have plenty.

And I NEVER turn the difficulty slider below %100. I might raise it but never lower it.

Edited by Starwaster
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Thanks for the detailed replies, everyone - a lot of useful info!

On 12/05/2017 at 2:09 AM, DStaal said:

My general impressions on a couple of those:  First off, as far as I can tell heatshields are basically assumed to be made of ablator - once it's gone, they're done.  Secondly, re-entry profile depends a bit on how you've designed your landing profile, in my opinion.  If you're doing a powered landing, by all means come in steep to reduce heating time - and slow down as you need to to keep heat under control.  For areobrake landings, I prefer moderately shallow: Shed a good portion of your speed high in the atmosphere, where you won't heat up as much as you do it.  You shouldn't really be going more than Mach 4 at the maximum when you hit 30km - and that's probably stretching it: Mach 3 would be safer.  But in that case you're intentionally converting speed to heat, so you want manage that.

I think the real difficulty that I'm having is that I lose huge amounts of ablator in the upper atmosphere, but it barely seems to slow me down at all.

 

On 12/05/2017 at 6:57 AM, Starwaster said:
  • The temperature of the reentry shockwave is based on velocity cubed. How much of that heat is transferred to your craft is a function of atmospheric density. Assuming a simple craft like a pod + heat shield, the optimal periapsis is 20km. As to shallow vs steep, there's a bit of a corridor where it's possible to go either too shallow or too steep.
  • Too steep can have one of two effects: Either parts of your craft (or the whole craft) burn up because you didn't slow down enough before hitting the denser parts of the atmosphere where you're getting maximal amounts of heat and your shield is overwhelmed. OR you suffer enough g-forces to render your crew unconscious or kill them outright. (crew blackout is handled by stock, not DRE. g-force related death is DRE, not stock). 
  • If you go too shallow, it is possible to cook off too much of your shield so you wont have any shielding when you descend at high speed into the lower part of the atmosphere. Or you can be so shallow that you don't brake enough and fly back out of the atmosphere or fail to capture on an aerocapture.

You say that 20km periapsis is best for simple craft like a pod with heatshield, is that regardless of what apoapsis you're returning from? And does that imply it's not ideal for larger or heavier craft?

Quote
  • Spaceplanes: You didn't say what your altitude was nor did you say if you were cruising or reentering and it matters. It would be highly unrealistic to be able to fly a plane much faster than Mach 3.5. Only the SR71 has ever flown that fast and that's faster than its operational speed of Mach 3.2. They were allowed to push it higher and its official max speed was 3.3 though 3.5 is its record. The shuttle on the other hand came in at Mach 25 but that was a shallow reentry. With stock Kerbin you will be coming in MUCH slower than Mach 25. I don't personally do spaceplanes that much but IIRC, the right periapsis was something like 40-50 km. You will want a pitch angle of about 35-40 degrees. (I will also mention here that with stock Mk3 parts I have survived RSS reentries. i.e. a full Mach 25 reentry so it is possible, at least with the RO physics configs to scale convection heating down to Earth  reentries)

Wasn't the SR71's top speed more affected by the limitations of jet engines than atmospheric heating? The rocket powered X-15 reached almost Mach 7 iirc. Would you agree with DStaal's advice that you shouldn't be going faster than around Mach 4 below 30km?

And am I right in understanding that the difference with spaceplanes compared to heatshields, is that since they aren't ablating, they can survive being heated for longer, but are more vulnerable to rapid heating, which is why they use a shallow reentry? And is the pitch angle a balance between trying to maximise drag but without overly compromising stability?

Quote
  • Eve: For capsules, I think I used 40-60. I don't remember the exact periapsis/angle I used but it's not hard at all. Make that dense atmosphere work FOR you not AGAINST you. Do as much braking in the upper atmosphere as possible. If you use the inflatable shield, you want your altitude even higher as is basically heat resistant cloth. The point of the inflatable is to increase your surface area which means more drag which means more braking higher up.

What sort of apoapsis would that be a return from? Doing some testing with a 2.5t probe (not including the heatshield) coming in from a very low orbit, and there's no way I can survive a 40km periapsis without stacking multiple heatshields. With a 1.875m heatshield, all 300 ablator is used up by 70km, and with a 2.5m heatshield, all 800 is gone by 63km.


My real problem seems to be that I start losing significant amounts of ablator from about 85km, but my velocity is barely affected before I hit 60km.

Quote

Remember, I'm the current developer of DRE. I use the same DRE you guys have :wink: . My thermal settings are pretty typical for stock except that I often lower my conduction to RO levels so I can test Real Fuels boiloff. The effect that has on reentries actually makes them hotter and I still survive just fine. (makes them hotter because the skin can't conduct its heat as quickly into the interior)

One last thing about you running out of ablator. Sounds like TOO shallow and baking it off. I said that earlier but just to revisit that, a typical reentry should only use about 25-35%. (maybe less on a suborbital. Mun or Minmus would use more but you should still have plenty.

And I NEVER turn the difficulty slider below %100. I might raise it but never lower it.

Only using up 25% of your ablator?? Clearly I'm doing something horribly wrong in that case.

 

BTW, what sort of pitch angles are sensible if you're trying to reduce g-forces on a capsule with a lifting re-entry?

EDIT: Done some more tests with the Eve lander. With 0km and -90km periapses, I still get through an entire 2.5m heatshield, but with a -150km periapsis, it survives with 75/800 ablator remaining.

Edited by baldamundo
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1 hour ago, baldamundo said:

Thanks for the detailed replies, everyone - a lot of useful info!

I think the real difficulty that I'm having is that I lose huge amounts of ablator in the upper atmosphere, but it barely seems to slow me down at all.

 

You say that 20km periapsis is best for simple craft like a pod with heatshield, is that regardless of what apoapsis you're returning from? And does that imply it's not ideal for larger or heavier craft?

Wasn't the SR71's top speed more affected by the limitations of jet engines than atmospheric heating? The rocket powered X-15 reached almost Mach 7 iirc. Would you agree with DStaal's advice that you shouldn't be going faster than around Mach 4 below 30km?

And am I right in understanding that the difference with spaceplanes compared to heatshields, is that since they aren't ablating, they can survive being heated for longer, but are more vulnerable to rapid heating, which is why they use a shallow reentry? And is the pitch angle a balance between trying to maximise drag but without overly compromising stability?

What sort of apoapsis would that be a return from? Doing some testing with a 2.5t probe (not including the heatshield) coming in from a very low orbit, and there's no way I can survive a 40km periapsis without stacking multiple heatshields. With a 1.875m heatshield, all 300 ablator is used up by 70km, and with a 2.5m heatshield, all 800 is gone by 63km.


My real problem seems to be that I start losing significant amounts of ablator from about 85km, but my velocity is barely affected before I hit 60km.

Only using up 25% of your ablator?? Clearly I'm doing something horribly wrong in that case.

 

BTW, what sort of pitch angles are sensible if you're trying to reduce g-forces on a capsule with a lifting re-entry?

EDIT: Done some more tests with the Eve lander. With 0km and -90km periapses, I still get through an entire 2.5m heatshield, but with a -150km periapsis, it survives with 75/800 ablator remaining.

I feel bad about this but I forgot about something when I was composing my post: My most commonly used heatshield is the one from the SDHI mod which by the author's design is supposed to be more resilient (given that he was modeling it after the PICA shields which are supposed to be reusable). I use the SDHI shield mainly because it fits the SDHI service module snugly. The stock one clips.

So I did a quick run with the stock shield and at 20 pe, on a 2.5m pod I end up with 168 ablator left. I don't find the orbit I'm returning form to have much impact because coming in faster means generating more drag. The temperature is higher but so is the drag (velocity squared). The Minmus reentry left me with 140 ablator left.

I'l comment on Eve later

About spaceplanes: essentially correct. DRE remodels spaceplanes to mimic shuttle tiles as best as I could. The skin of each spaceplane part is configured to hold very little heat, and very low conductivity to the interior of the part. They have high emission to help them re-radiate the heat that they absorb. (this is in contrast to the stock parts which increase thermal mass so they store more heat). The skin-skin conductivity is INCREASED to try to evenly distribute the heat to adjacent parts. The shuttle came in at a high pitch to maximize drag, which also served to reduce heating due to the blunt body principle.

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Ok, back to the Eve issue, is that 2.5t landing mass? What are you putting down that is weighs so much?

I think going with the larger heat shield is the way to go since it increases your drag and that's what you're going to need for something that large. The inflatable would be even better but you DEFINITELY will need a shallow reentry for that. If you have thermal debugging turned on then you will see that each part is given a heating rate/cm2. Keep the inflatable at 15-20W /cm2 If you have to have the ablative shield (is it career? inflatable not available yet?) then go with the 2.5m shield and make it steeper not shallower. 

Edited by Starwaster
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