Very hot re-entry

[Dungas Kerman]

I'm guessing this is a lost cause but figured I might as well ask. 

I'm having a wee bit of trouble returning to Kerbin from a Laythe mission. My reentry speed is so ludicrously high that I can't manage to have either of my pods survive. After exhausting all my fuel I'm still hitting the atmosphere at around 4900km/s and my heatshields (2,5m with full ablator) only survive for a couple seconds. I tried not decoupling and using my interplanetary tug as an improvised extra heat shield. It helps a little but as it burns up it starts to pitch and then my pods explode immediately since their heat shields can't do their jobs. I've fiddled around a little and with a periapsis of 41km and staging at the correct time I can make 1 pod survive but it won't slow down enough to be captured so it's hurled back into space.

 

Does anyone know if there is some technique I can use to make this infernal reentry? Or should I just accept that it's a lost cause and use my remaining deltaV for another encounter in a few years?

Any advice welcome, think of poor Valentina.

Thanks

 

[magnemoe]

4900 m/s I say you have a problem.  if the return module is very lightweight perhaps? Lightweight with the 10 meter inflatable sure. 
Now you have one advanced option: intercept, put something out at past Mun orbit who intercept the craft, match velocity, pick up the Valentina has the 10 meter inflatable or reduce velocity for reentry. 
Put an node on the return capsules trajectory just inside Kerbin SOI, Launch an rescue mission I would want 5-6 km/s in LKO for this. 
 

[Leganeski]

4900 m/s is well within the range of survivability, but it needs to be just a crew capsule and a heat shield (and maybe some other small parts, such as a parachute or an experiment storage unit). Any large parts except for a heat shield will increase the ballistic coefficient significantly and usually end up hurting more than they help.

If you need to recover two crew capsules, consider sending them down separately at different times, each with their own heat shield.

[king of nowhere]

9 hours ago, Leganeski said:

If you need to recover two crew capsules, consider sending them down separately at different times, each with their own heat shield.

to expand on that: you can only control one object at a time, and a ship going deep into the atmosphere and outside of physical range of the vessel you're controlling is authomatically deleted.

if it's not deep enough to be deleted, it will just move on without braking. in both cases, you lose it.

so you must separate your two pods and ensure they enter atmosphere at 20 minutes distance from each other.

 

if you can't find the problem, post pictures of your crew pods. 5 km/s is survivable, maybe there is some problem in the design

[Dungas Kerman]

Well I had a lander and a mothership so I have two mk3 pods. One with just a mk3 pod, heat shield and parachute, and the other the same but with an extra lander can underneath the mk3 pod (oh and 4 radial drogue chutes). 

In my first attempt I tried staggering the reentry so that I had more than 1 hour in between arrivals. That was easy enough since I've done it many times before without problems. The first pod to arrive was hitting Kerbin with a periapsis of about 21km and burned to a crisp before it even got there. I didn't bother trying the second one since I want all my Kerbonauts to survive and it is coming in at a similar speed anyway.

 

Here is a screenshot of what I'm working with. 

https://ibb.co/nb3PQGF

I would expect that lander can to be the weakness in this but oddly enough it's not. Its heatshield overheats and is detached before the lander can explodes so it really looks like it's just the shield that can't take it.

[king of nowhere]

33 minutes ago, Dungas Kerman said:

Well I had a lander and a mothership so I have two mk3 pods. One with just a mk3 pod, heat shield and parachute, and the other the same but with an extra lander can underneath the mk3 pod (oh and 4 radial drogue chutes). 

In my first attempt I tried staggering the reentry so that I had more than 1 hour in between arrivals. That was easy enough since I've done it many times before without problems. The first pod to arrive was hitting Kerbin with a periapsis of about 21km and burned to a crisp before it even got there. I didn't bother trying the second one since I want all my Kerbonauts to survive and it is coming in at a similar speed anyway.

 

Here is a screenshot of what I'm working with. 

https://ibb.co/nb3PQGF

I would expect that lander can to be the weakness in this but oddly enough it's not. Its heatshield overheats and is detached before the lander can explodes so it really looks like it's just the shield that can't take it.

those kind of reentry pods are the most resistant available. the mk2 lander pod can be a weakness, but you say it doesn't explode first, so it is fine against my every prediction.

but i see the problem: 21 km periapsis. way too low here. you get inside the denser atmosphere too fast. high speed reentry is a delicate balance, you have to stay as high as possible in the atmosphere to reduce heating, while still be deep enough to get captured. if the height required to get captured is lower than the height at which you explode, there's when you can't get a capture.

I once managed to save a high speed reentry by raising periapsis by a single km.

I suggest trying with 35 km periapsis first. that should be enough to capture safely. if you still explode, try 36 km, and so on until you don't explode. if that minimum level to not explode is not low enough for capture you're screwed, but it really should not be your case. in fact, eyeballing it i'd say the smallest pod should be fine anywhere between 30 and 40 km. the one with the lander can may be harder to land

Edited December 3, 2022 by king of nowhere

[The Aziz]

Can you use the Mun to your advantage? Set up the trajectory, preferably much, much earlier, so that the ship gets through Mun's SOI and make it so that it slows you down before atmosphere.

[Vanamonde]

You could also try making a first braking pass that is just enough to lower the apoapsis within Kerbin's SOI. It would then go around and make additional passes until the remaining speed is survivable. Oddly, though, multiple gentler passes can consume more ablator than one severe pass, so it can be tricky. 

[Dungas Kerman]

4 hours ago, king of nowhere said:

those kind of reentry pods are the most resistant available. the mk2 lander pod can be a weakness, but you say it doesn't explode first, so it is fine against my every prediction.

but i see the problem: 21 km periapsis. way too low here. you get inside the denser atmosphere too fast. high speed reentry is a delicate balance, you have to stay as high as possible in the atmosphere to reduce heating, while still be deep enough to get captured. if the height required to get captured is lower than the height at which you explode, there's when you can't get a capture.

I once managed to save a high speed reentry by raising periapsis by a single km.

I suggest trying with 35 km periapsis first. that should be enough to capture safely. if you still explode, try 36 km, and so on until you don't explode. if that minimum level to not explode is not low enough for capture you're screwed, but it really should not be your case. in fact, eyeballing it i'd say the smallest pod should be fine anywhere between 30 and 40 km. the one with the lander can may be harder to land

What you are saying makes total sense but even the 21km periapsis doesn't slow me down enough to get captured. I managed to get one pod survive the explosion and cruise through (heat shield maxed out but held for some reason) but then it flew straight through the atmosphere and went back on an escape trajectory. 

 

I think I just stuffed up and should cut my losses at this point. Thanks for the suggestions though.

3 hours ago, The Aziz said:

Can you use the Mun to your advantage? Set up the trajectory, preferably much, much earlier, so that the ship gets through Mun's SOI and make it so that it slows you down before atmosphere.

I tried that but the gravity is too weak to make a difference at 4,9km/s. It may work if I planned much earlier as you said.

[king of nowhere]

15 hours ago, Dungas Kerman said:

What you are saying makes total sense but even the 21km periapsis doesn't slow me down enough to get captured. I managed to get one pod survive the explosion and cruise through (heat shield maxed out but held for some reason) but then it flew straight through the atmosphere and went back on an escape trajectory.

....

this makes no sense at all.

here I survive a reentry at 8 km/s

with a pod very similar to your own

 

and here at 9.7 km/s. this pod was a bit sturdier thanks to the mk3 crew pod

this is the one where 1 km periapsis made the difference between exploding and surviving. as you can see, a 30 km periapsis was already enough to slow down and get captured from 10 km/s.

by the way, those heat shields are not maxxed out. i discovered in the first landing that half the ablator is not even consumed, so i removed half the ablator from subsequent pods to make them lighter.

how is it possible that your pod at half that speed is not slowed down enough? it should be an easy reentry.

 

wait, last chance: that pod that survived at 21 km, were you controlling it? or were you using some other vessel at the time?

because, as I said previously, if you are not controlling directly the pod, it won't register the atmosphere at all. it will just pass by as if it was all space. this is the only way I can think of that your pod could both survive at 21 km and remain in an escape trajectory. or perhaps you were time warping hard? happens occasionally that the game does not stop time warp and this messes up with the atmosphere, skipping most of it.

if it's none of those things, I'd conclude it's a bug of some sort.

Edited December 4, 2022 by king of nowhere

[Dungas Kerman]

On 12/4/2022 at 1:46 PM, king of nowhere said:

....

this makes no sense at all.

here I survive a reentry at 8 km/s

with a pod very similar to your own

 

and here at 9.7 km/s. this pod was a bit sturdier thanks to the mk3 crew pod

this is the one where 1 km periapsis made the difference between exploding and surviving. as you can see, a 30 km periapsis was already enough to slow down and get captured from 10 km/s.

by the way, those heat shields are not maxxed out. i discovered in the first landing that half the ablator is not even consumed, so i removed half the ablator from subsequent pods to make them lighter.

how is it possible that your pod at half that speed is not slowed down enough? it should be an easy reentry.

 

wait, last chance: that pod that survived at 21 km, were you controlling it? or were you using some other vessel at the time?

because, as I said previously, if you are not controlling directly the pod, it won't register the atmosphere at all. it will just pass by as if it was all space. this is the only way I can think of that your pod could both survive at 21 km and remain in an escape trajectory. or perhaps you were time warping hard? happens occasionally that the game does not stop time warp and this messes up with the atmosphere, skipping most of it.

if it's none of those things, I'd conclude it's a bug of some sort.

I tried a lot of things.

First I tried the normal, staggered entry. I disconnected the two pods so they would arrive at Kerbin SOI some 2 days apart and had the loose uncontrollable one enter at around 20km periapsis. I burnt and exploded before reaching 30km I think. The ablator on that one was at full 800 (the other pod 320 because I also noticed it barely ever takes over 200 off). The next attempt was with the two pods together, and a higher entry altitude. Same result. Then I tried to just ram the entire ship in the atmosphere to hopefully slow down enough before the heat shields needed to take over. As I said before, in this case I can make one pod survive but it will shoot straight through on an escape trajectory. 

 

I'm also puzzled because I have made similar returns from the Joolian system and they weren't nearly this bad.

[Dungas Kerman]

Guys thanks for the help but I do think this is a bug so I just cheated myself out of this pickle. 

Turned the max heat off and tripled the gravity. Even then I didn't get a direct capture, I went into orbit first and landed on the second pass.

 

Edit: It was almost definitely a bug. I had a similar ship in Jool orbit so just to check I tried to return to Kerbin with it. 4600m/s entry velocity, 24km periapsis and 320 ablator. Went like a breeze. The heat warning didn't even get into the red and I got captured instantly.

Edited December 6, 2022 by Dungas Kerman
Additional test

[king of nowhere]

1 hour ago, Dungas Kerman said:

Guys thanks for the help but I do think this is a bug so I just cheated myself out of this pickle. 

Turned the max heat off and tripled the gravity. Even then I didn't get a direct capture, I went into orbit first and landed on the second pass.

 

Edit: It was almost definitely a bug. I had a similar ship in Jool orbit so just to check I tried to return to Kerbin with it. 4600m/s entry velocity, 24km periapsis and 320 ablator. Went like a breeze. The heat warning didn't even get into the red and I got captured instantly.

yeah, agree on the bug thing. there's no way something can pass with a periapsis of 24 km and not get captured.