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Sovnheim

Surviving a Fast Atmospheric Reentry

Question

Hello everybody.

It so happens that every once in a while, my craft baaaaarely has enough fuel to make the trip to another body.

I often end up having just enough fuel to enter the atmosphere of a body, but not enough to decelerate to orbit and then enter the atmosphere (which is usually the safer option for me). We are talking about entry speeds along the lines of 2.5km/s to 3.5km/s. 

Even with a heat shield, something always ends up overheating (often the airbrakes) then explodes. I have tried many techniques but none seem to work and my crafts dramatically explodes in the atmosphere.

Any smart solutions that I might use ? 

Note, I usually have all the important stuff safely locked inside, such as MJ, batteries, antennas and so on. But there is always something outside (RCS engines, airbrakes) and it makes the crafts vulnerable. 

How do you people manage such situation ? 

Thanks for your time :)

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2.5 to 3.5 km/s is no problem.  The answer is simple:  use a heat shield.  Anything behind the heat shield will be just fine.  So just make sure that everything's behind the heat shield.

Your airbrakes are burning off because they aren't supposed to be used for reentry.  They're sticking out beyond the heatshield, and they have a low temperature tolerance.  It's not what they're for.  So don't use them for reentry, or at least, if you have them, don't use them until you're done with the hot part of reentry.

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One other thing to do is multiple passes- eg to get into Kerbin when coming in really hot try coming in with your PE at around 40,000m just to slow down and get captured, then at AP it only takes a little squirt of fuel to lower the PE to 25,000m to get fully captured and landed. I've brought back entire landers from Mun/Minmus doing this with no heat shields.

Having a pilot that can hold retrograde during reentry helps a lot, but be sure to have plenty of EC on hand because Val will drain your batteries fast!

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If you're entering VERY hot, sometimes an axial spin to evenly distribute heat will give you an edge, as well. And what Snark said on airbrakes - those are for slowing down once under 1300 m/s and not much else.

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4 minutes ago, Gojira1000 said:

If you're entering VERY hot, sometimes an axial spin to evenly distribute heat will give you an edge, as well. And what Snark said on airbrakes - those are for slowing down once under 1300 m/s and not much else.

Thanks to you both for your answers. 

I'll use bigger shields for starters, but sometimes they tend to spin in the wrong direction, and my shield faces upwards, where it is useless.

And yes, axial spins have proved to do the job, I noticed that ! 

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Axial spins are mainly helpful if you're coming in without a heatshield.

If you're using a heatshield, spinning generally doesn't help, because your only options are spinning around the roll axis (which doesn't do anything, since you're facing perfectly prograde) or around the pitch/yaw axis (which you can't do without exploding, because you have to keep the heatshield facing forwards).

Also, bear in mind that the zone of protection from a heatshield isn't a cylinder-- it's actually a narrow cone, getting slightly wider as you go aft from the heat shield.  So if you have a small component that sticks out from behind the heatshield, but not very much (like an RCS thruster), it's probably OK, as long as it's placed reasonably far back from the shield and isn't right behind it.

Also:  if you're having RCS woes during reentry, take a good hard look at the unidirectional RCS ports.  I've noticed that they seem to be rather under-used in the KSP community-- everyone seems to go for the 4-way thrusters.  The reason they're worth looking at is that they're a lot more heat-resistant.  The linear thrusters can handle temperatures up to 2600 K, whereas the 4-way thrusters only go up to 1500 K (which is a lot lower than most spacecraft parts).

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4 hours ago, Sovnheim said:

I'll use bigger shields for starters, but sometimes they tend to spin in the wrong direction, and my shield faces upwards, where it is useless.

Usual stuff - put high-mass parts closer to the heat shield. And also spam reaction wheels to get some more control.

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My eve assender did not have enough stability and would tip sideways just enough to start blowing off pieces.  Even though the com was very low and it never flipped any deviation from retrograde was fatal on eve.  Spinning in the roll access kept the craft pointing directly retrograde or at least keep heating low enough part didn't explode

 

4E07gPJ.png

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As much as I will be boo'ed off stage, I prefer to clip fuel tanks into the structure of the vehicle. That could technically be considered cheating, but I prefer it to ungainly vehicles. This also helps with keeping everything behind your main surface for entry, be it a heat shield, engine, or whatnot.

One question that was pointed out: Why does rolling help? That makes sense if you're changing what faces are exposes, but locked retrograde with no changing exposure STILL drastically reduces heating, to the point it's almost an exploit. You can have engines that would pop normally survive Kerbin re-entry with ease, just roll it.

The above comments are right though, a heat shield will generally survive any speed you throw at it, to the point I usually only roll with half the ablator since it's so effective.

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21 hours ago, Waxing_Kibbous said:

One other thing to do is multiple passes- eg to get into Kerbin when coming in really hot try coming in with your PE at around 40,000m just to slow down and get captured, then at AP it only takes a little squirt of fuel to lower the PE to 25,000m to get fully captured and landed. I've brought back entire landers from Mun/Minmus doing this with no heat shields.

Having a pilot that can hold retrograde during reentry helps a lot, but be sure to have plenty of EC on hand because Val will drain your batteries fast!

I have to second this.  Some things first:

  • not recommended with life support mods (this is the danger of "Apollo skipping off the atmosphere"
  • try to make it through with your engine still attached
  • have solar panels if possible (so you can keep your ship pointed engine first)
  • try to keep things retrograde yourself, then hit SRS when needed (it will drain EC)
  • don't overdo your retrothrusters.  They are for two things, getting your PE down, and cutting your velocity to allow parachutes.  A PE less than 1000 (when you hit 1000) isn't helpful.
  • you shouldn't need them for allowing parachutes (you can come in hotter without a heat shield)
  • If your PE is less than 70,000 (and you ejected your engines) you *will* come down.  You might want to read a book for awhile (can you play in a window?  You could always cruise the forums...).
  • The only time you need a specific amount of delta-v in an atmosphere is during aero-capture.  And that is just needed to get below escape velocity in one pass (assuming you won't burn fuel).  After that you can do as many passes as you want for low orbit or landing (although the delta-v you just did in one pass should be enough to land on.  No need for more than  one or two more passes).

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2 hours ago, Raideur Ng said:

As much as I will be boo'ed off stage, I prefer to clip fuel tanks into the structure of the vehicle. That could technically be considered cheating, but I prefer it to ungainly vehicles. This also helps with keeping everything behind your main surface for entry, be it a heat shield, engine, or whatnot.

One question that was pointed out: Why does rolling help? That makes sense if you're changing what faces are exposes, but locked retrograde with no changing exposure STILL drastically reduces heating, to the point it's almost an exploit. You can have engines that would pop normally survive Kerbin re-entry with ease, just roll it.

The above comments are right though, a heat shield will generally survive any speed you throw at it, to the point I usually only roll with half the ablator since it's so effective.

Yes, I agree, there is something weird about it.

Whatever though, rolling most certainly does help to avoid overheat, even if it's just the experiments, solar panels or batteries on the sides of a command pod.

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Aside from what  everyone else has said, at some point, you might just have to accept one simple fact:  If you're coming in that fast, anything which is NOT behind a heatshield is vulnerable and likely to explode.  So you have to make a few decisions.  First of all, do you really NEED to bring that part back in the first place?  Obviously for most missions, you need to bring your crew back.  This also means you may need a few other things like parachutes and a battery.  You probably want to bring back whatever science you collected(but in some cases, you may be able to have the crew collect the data and not need to bring back the actual experiment itself).  A lot of times though, almost everything else is optional.  Sure, the extra funds recovered are nice, but not generally enough to make or break you.  Secondly, if you do decide you want to try and bring some stuff back, it seems like even if some of those parts do end up exploding, they won't hurt the rest of the ship.  I've lost mystery goo pods or batteries quite a few times with no damage to anything else important  But if something is exploding on reentry and it IS causing damage to the rest of your ship, then either make sure it's behind the heatshield and/or in a service bay where it will be better protected...or just don't bring it back at all if you don't really need to.

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5 hours ago, nater662 said:

Aerobrake into slow yourself down. Also, as said above the air brakes are not for entry but for slowing down a plane

This calculator his helpful. 

http://alterbaron.github.io/ksp_aerocalc/

I said something about this in another similar thread. I use air brakes for re-entry and they are extremely effective. You do not have to restrict yourself to only using them for aircraft. Not only do they slow the craft but they do the other important job of increasing drag at the back and so keeping the heat shield orientated into the wind.  

You just have to make sure is that your air brakes when deployed (and anything else you don't want to lose) are within an imaginary tube extending back from the heat shield. You can achieve this by careful placement and rotation of the air brakes at build time and does not necessarily involved clipping.  

Edited by Foxster

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12 hours ago, nater662 said:

Aerobrake into slow yourself down. Also, as said above the air brakes are not for entry but for slowing down a plane

This calculator his helpful. 

http://alterbaron.github.io/ksp_aerocalc/

I think this tool is obsolete. It worked for beta 0.9 and before.

The MK1-2 pod can reenter at 3100m/s without any heatshield. With a heatshield and 1/4 ablator allow 4800m/s reentry (tested). but the altitude have to be correctly chosen. It's very dangerous to dive lower than 31km on the first pass. You may blow even without and heat warning. If you go too high, you don't aerocapture and find yourself into Kerbol SOI.

As said, airbrakes are useless (they were very efficient in 1.0.4 though). Drogue chutes may be needed if your vehicle is not flat enough

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Thanks for your feedback.
I simulated some re-entries using HyperEdit at speeds up to 5km/s which worked without major problems. 

Considering the protection of the heatshield as a cone helped me improve my designs. I made shorter vehicles with a larger shield, and it works wonders. 

That said, I tried to enter Kerbin atmosphere by falling straight down (no horizontal velocity) to push the limits of the designs. It was a bit scary as the craft hit the thick layers of the atmosphere hard but definitely thrilling.

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I re-enter from Minmus on airplane parts no problem, but it takes several passes: set Pe to about 50km, set SAS to point normal, burn through, and maybe roll if things are getting hot. Each pass I lose about 200 m/s so it takes about 5 passes to drop to LKO altitudes. With life support this wouldn't be the optimal way.

Radiators can help you bleed off more heat, which means fewer passes.

I never use heat shields for trips that stay within the Kerbin system.

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maybe use sepratronsto bleed a few metres/seconds of speed

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2 hours ago, Reaper_Works said:

maybe use sepratronsto bleed a few metres/seconds of speed

Probably not. Separatrons are not efficient, they weight a lot for a very little dV. It's better to keep some fuel with a space engine (poodle, terrier or LVN) then burn all you have (but try to keep you PE reasonable), THEN dump the stage and finish the reentry with the pod + heatshield (maybe even not needed).

When I'm very fast (>4500m/s), I usually reduce speed before setting my PE. Anyway, your reentry vehicle should always be "rated" for speed reentry, you must know what your ship is capable of dealing with. When you return from Mun, you can always do multipass, but when you come from an interplanetary trip, you don't have the luxury of bleeding speed off 200m/s each orbit... You have no choice but to dip down into atmosphere to get captured.

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1 minute ago, Warzouz said:

Probably not. Separatrons are not efficient, they weight a lot for a very little dV. It's better to keep some fuel with a space engine (poodle, terrier or LVN) then burn all you have (but try to keep you PE reasonable), THEN dump the stage and finish the reentry with the pod + heatshield (maybe even not needed).

When I'm very fast (>4500m/s), I usually reduce speed before setting my PE. Anyway, your reentry vehicle should always be "rated" for speed reentry, you must know what your ship is capable of dealing with. When you return from Mun, you can always do multipass, but when you come from an interplanetary trip, you don't have the luxury of bleeding speed off 200m/s each orbit... You have no choice but to dip down into atmosphere to get captured.

Just for a little speed like if you are slightly to sharp angle or that stuff. also i think they are physics less

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21 minutes ago, Reaper_Works said:

Just for a little speed like if you are slightly to sharp angle or that stuff. also i think they are physics less

I don't think they are, but even they were, that doesn't change anything. The only advantage physicless parts have since 1.0 is that you can place them randomly anywhere. Don't bother about COM, they add their mass to their parent part (as drag). They aren't massless anymore.

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On 4/11/2016 at 10:00 AM, Warzouz said:

As said, airbrakes are useless (they were very efficient in 1.0.4 though). Drogue chutes may be needed if your vehicle is not flat enough

That is profoundly irritating.  I loved how overpowered airbrakes were (at least up through 1.0.4) and especially liked to use them for reentry.  One great trick that they could do was to "invert" the aerodynamics of a booster.  Put them down near the engines, and put a heat shield up at the top, and once you opened the brakes, you could convince the thing to come heat shield down (instead of engine down).

Any guesses if engines can survive re-entry, or will we need to create new tricks (presumably involving heat shields in odd places) to bring back boosters?

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I just survived entry at 9500 m/s . Aerobrake set periapsis for 35km and i slowed down enough to have a decaying orbit 

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Welcome to the forum, @jefalger. :D Congrats on your mission, but I'd suggest you post about it in your own thread since this one is rather old and there have been changes to the game's aerodynamics since then. 

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