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TimKerbin

how do you survive interplanetary re-entry?Any tips?

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One mundane mission turned out to be anything but. Here's how. FYI I'm running v1.1.

 

In one mission that turned out to be extraordinarily epic and took me at least 8 hours to finally finish (contracts to get science from orbitting Duna and Ike) I just couldn't get him to survive Kerbin re-entry, so I let him escape Kerbin and go into a parked orbit whose AP was just past Duna. Actually this was my first attempt at returning from an interplanetary mission in v 1.1; I thought I had planned ahead. I was wrong.

 

Because this pod had over 1000 science on it I wasn't going to give up easy. I also try to never, ever let a Kerbal die or go MIA. So I devised an interplanetary rescue mission (which worked first time to my amazement!) to salvage the pilot and his science. However, thinking I had learned my lesson from the last attempt at re-entry I had put several airbrakes and retractable radiators on this rescue pod, thinking it would work. But it didn't. On several attempts, at various altitudes, they burned up on re-entry.  To be honest this was my first time using airbrakes and radiators of this type, I just didn't know how to use them. Regardless, they heated up way past their tolerances and quickly burned up. I then realised I had enough delta-v in this final stage with its lv909 engine to slow down sufficiently to get into a parked Kerbin orbit. And I did. However, even at Kerbin orbital speeds none of my radiators or airbrakes survived, so I let them burn up in the hope they would bleed off enough speed to let my pod survive. And it did. The pod landed safely, after what was for me a totally exhausting mission, and I got over 1200 science from it.

 

So what the heck am I doing wrong here? How do you survive interplanetary re-entry?I know I could 'bleed' off speed but how do I do that when I burn up the moment I hit the upper atmosphere? Even if I try an aero-capture I have to survive long enough to do that, and I don't; I start encountering heat at about 58km and it just gets worse. I don't survive long enough to get captured. Believe me I tried everything, at all altitudes. It just wouldn't survive. For a one-man pod with a small tank and an lv909 engine there really isn't many options. I did think about using the 2.5m heat-shield...would that work better? Not only at deflecting heat but creating drag too.

 

Prior to aerodynamics and re-entry heat being added proper to KSP I've done countless interplanetary missions; however, now that I'm playing v1.1 it's meant I have to re-evaluate and even scrap my previous designs. I'd love to hear what the more advanced KSP players do to deal with this problem! All advice welcome.

 

Cheers,
Tim

 

UPDATE: After I wrote this, I tried recreating the entire mission in sandbox mode, this time using a single 2.5m ablator shield on my final stage, with no radiators. It worked brilliantly. Even with a Kerbin PE of 30km none of the instruments or other parts of the final stage experienced ANY heating at all, travelling at 3500m/s. My only issue now is working out the best way of incorporating the 2.5m shield into my 1.5m final stage design. Also, does this mean that if I want to send more than one Kerbal, and use a 2m pod, I will have to use an even bigger ablator shield? In the same way the 1m shield was useless, I'm guessing the 2.5m shield will be useless too. Unless the larger pods have a greater heat tolerance....

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What does your pod look like? A simple pod with a 1.25m heatshield can survive pretty much any reentry.

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Here's a pic.

 

https://ibb.co/cj2Hva

 

That lander-can is lighter and has a greater heat tolerance than the other one-man capsule which I could have used. I'm wondering if I put the shield in the wrong place - instead of placing it at the bottom of the science bay I should have put the science bay and other parts above the landercan and just put the shield on the landercan directly.

 

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Posted (edited)

1 hour ago, TimKerbin said:

I'm wondering if I put the shield in the wrong place - instead of placing it at the bottom of the science bay I should have put the science bay and other parts above the landercan and just put the shield on the landercan directly.

 

I would agree with that. I never use the service modules, but it looks like the heat is coming around the protected service module and hitting the lander can, which bulges out. I know the modules are useful, but I usually just attach parts radially (taking data from instruments just in case), and kinda hope they don't burn up. With a good angle, everything usually comes through fine. I don't think you need a large shield.

Edited by Cubfan

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On 3/3/2017 at 11:25 AM, TimKerbin said:

I had put several airbrakes and retractable radiators on this rescue pod, thinking it would work. But it didn't.

Yup.  Radiators don't help in a meaningful way with reentry heat, either IRL or in KSP.  That's not what they're for.

Airbrakes aren't super useful for reentry because they have a low temperature tolerance.  They're designed for high-speed atmospheric flight, not orbital reentry.

I've done plenty of reentries in that speed range (you mentioned 3500 m/s) with small 1.25m craft using the 1.25m shield.  Don't bother with aerocapture, just set Pe to 30ish km and plow straight in.  Goes great.  :)

They work just fine, as long as two things are true:

  • The craft is aerodynamically stable during reentry, so that the heat shield stays facing forward
  • The craft isn't too massive (i.e. I haven't built a heavy javelin of a ship).

Reentry is easiest when your ship is lightest.  I usually design my reentry vehicles to keep them as small and light as possible-- usually just a command pod, heat shield, and parachute, that's about it.

A 1.25m heat shield should absolutely be plenty to protect a craft during reentry from Duna.  Is there a particular reason not to just strip down the reentry vehicle so that it ditches everything but the heat shield?

Was your whole craft exploding, or just the sticky-outy parts?

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Posted (edited)

TBH, the lander cans are only intended to be used in vacuum, and not for atmospheric entry.

 

Heat tolerance of parts is misleading for re-entry. 

For a direct interplanetary re-entry the heat load is so high that no part has the heat tolerance to just ignore it. A heat shield loses ablator when too hot, anything else explodes.

You'll see that the 1.25m lander can sticks out past the 1.25m heat shield. This means that it's not actually shielded at all, and can't be used for direct re-entry without a bigger shield.

 

Parts that fit entirely behind a heat shield don't experience much heating at all, so even low tolerance parts survive just fine. 

 

Basically, your design is wrong, because you're using a command pod never intended to re-enter kerbin.

Edited by Lelitu

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Don't use a lander can for re-entry at speed in Kerbin's atmosphere. Use a Mk1 capsule for that.

 

You best bet is for your final craft to be just the capsule, chute and a heatshield of matching size, from which you can usually remove all or most of the ablator at assembly. Decouple the engines, experiments (making sure all the science is in the capsule), electrics, etc before re-entry.

 

You will then be able re-enter from anywhere at any speed.  

 

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If you have the extra delta-V available, slowing into Kerbin orbit before beginning final descent can help a lot.   (I tend to load my designs with redundant delta-V anyway.  Not so great from the precision/perfectionism point of view, but it's how I prefer to do things...)

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I've had problems with the experiments and RCS quads burning off the exterior of a Mk. 1 command pod on reentry from Minmus, never mind further out -- but the capsule always survives.  I've even had one survive a retrograde reentry, from Minmus, where I went a little too low; almost lost the pilot to G force, but the ablator held up and the capsule was fine.  What I plan to start doing in the near future is using science storage parts -- there's a "material exposure bay" that fits between a Mk. 1 and a 1.25 m heat shield, and a Science Jr. that can store science data and connects at the small end of a Mk. 1.  These do require an EVA to take the data out of (for instance) a goo canister and store it in the Science Jr., but by the time you're ready to fly to Duna or Even you ought to be able to handle a short EVA without problems.

Small lander cans, unfortunately, are just a little bit bigger than the 1.25 m heat shield; they stick out into the multi-Mach blast, and the corners quickly melt (which the game depicts as an explosion).  There is no way to get a lander can down to Kerbin with that heat shield, unless you can enter slowly enough to almost completely avoid heating effects (hint: you'll use a BUNCH of fuel trying to hover down, but it will work if you can spare the fuel) -- and if you can do that, you don't need a heat shield at all.

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As suggested by others: lose the lander can. For a one-kerbal mission, your reentry vehicle should be a 1.25m capsule, a 1.25m heat shield and a parachute. Nothing else.

The capsules are designed to be aerodynamically stable when flying blunt-side first. You shouldn't even need SAS.

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16 hours ago, Zeiss Ikon said:

I've had problems with the experiments and RCS quads burning off the exterior of a Mk. 1 command pod

Science experiments are easily meltable, but I generally don't have a problem with them burning off the Mk1; just put them on the sloping sides of the pod, not too close to the bottom, and they're easily in the "shade" of the heat shield and have no problem.

Not that it really matters all that much, other than recovering some of the cost of the instruments-- I always make sure to pull the science out of them and store them in the pod itself, so I can't lose any data.

As for the RCS quads:  yeah, those are curiously delicate when it comes to temperature tolerance.  (Kind of interesting that the mono-directional RCS ports are so much tougher than the quads.)  That generally doesn't come up for me, though, since I practically never have RCS thrusters on my reentry vehicle.

16 hours ago, Zeiss Ikon said:

can store science data and connects at the small end of a Mk. 1

...why not just store the science in the command pod itself, along with the pilot?  That way, you're ironclad safe:  it's not physically possible to lose the science without losing the whole ship.

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On 3/14/2017 at 11:38 AM, Snark said:

...why not just store the science in the command pod itself, along with the pilot?  That way, you're ironclad safe:  it's not physically possible to lose the science without losing the whole ship.

Apparently, there's a (fairly low) limit to the number of experiments' data you can store in the Mk. 1.  I've had problems in the past with losing science because (for instance) I couldn't bring a second barometer/thermometer reading back into the capsule; I had to either overwrite a stored one, or discard the one I had just pulled out of the instrument.  Even then, the recovery level is lower than bringing back the whole instrument, if I've understood that correctly.  My standard science load for Mun/Minmus (in this Science game) has been four goo canisters and three each barometers and thermometers (don't have any other science instruments as yet; I've been pushing the rocket engines and tanks because they get me further, sooner).

Where I've had problems with the science burning off is losing the goo canisters if my reentry is a little hot -- they're apparently even more fragile than the RCS quads, as I've got my capsule RCS mounted right at the rim, with the goo as high up the cone as I can get it without interfering with the parachute, and the goo burns off first.  Interestingly, the thermometers and barometers are tougher than the RCS; I've had a couple missions land with nothing left on the outside of the pod but a single thermometer/barometer pair.

My last reentry, I did a "rotisserie roll" after turning SAS off (neither needed nor helpful once the capsule become base-stable during reentry) and it seemed to help keep the goo from burning away -- but on that mission I hadn't done any goo experiments, it was a completely different mission reusing a capable vessel design.

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Posted (edited)

I don't think some of that is right.

I believe you can store as many experiments as you like in any of the capsules. What you can't do is store more than one of the same type from the same location. 

You get the same science from returning an experiment in the capsule as you do leaving it in the instrument and returning that. 

There is no reason to return science experiments unless you are doing a crewless mission and even then you can use an ESU. 

Edited by Foxster

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2 minutes ago, Zeiss Ikon said:

Apparently, there's a (fairly low) limit to the number of experiments' data you can store in the Mk. 1.

The limitation is that you can only store one copy of the same experiment from the same location in the same command pod. For example, if you do a mystery goo experiment when landed in the Mun's Highlands and store that experiment in the command pod, you can store mystery goo data from any other location in the same command pod, but you can't store a second copy from the Mun's Highlands.

This is only an issue if you want to get multiple copies of the same experiment. With low science returns, it can be worth getting multiple copies of the experiments that don't return 100% of the data (in stock, that means just the goo and the materials study, and I think also the surface sample). I think four copies will give you all of the data, but it's generally not worth getting more than two copies.

So how do you go about getting two copies of the goo/materials data? You add a science container. It's a small white box with black at the top and bottom, and is found in the "Science" tab in the VAB/SPH. It has the same restrictions as the command pod, so you'll need one for each extra copy of the data that you want (though usually the extra mission cost for the low science return means that it's only worth sending one and getting two copies of the data you want).

 

10 minutes ago, Zeiss Ikon said:

I've had problems in the past with losing science because (for instance) I couldn't bring a second barometer/thermometer reading back into the capsule; I had to either overwrite a stored one, or discard the one I had just pulled out of the instrument.

This is not how it works. If the science experiment asks you to overwrite/discard an existing experiment, it is because you haven't taken the data out of the science part. You need to EVA a kerbal, right click on the science part, click "collect data" (or something similar; the phrase is usually obvious), and then get back into the command pod. Alternatively, you can transfer that data to a science container by right-clicking the science container and clicking "store data".

 

Basically, so long as you have one command pod, it is always possible to return one copy of every experiment from every location where you do that experiment. More copies will require science containers but you never need to return the actual science parts.

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Posted (edited)

20 minutes ago, Zeiss Ikon said:

My last reentry, I did a "rotisserie roll" after turning SAS off (neither needed nor helpful once the capsule become base-stable during reentry) and it seemed to help keep the goo from burning away -- but on that mission I hadn't done any goo experiments, it was a completely different mission reusing a capable vessel design.

Yes, this technique works especially well for saving lots of little parts:  science, RCS thrusters, solar panels, etc.  Heck I've even saved engines (and thus potentially the entire ship) this way on multi-engine craft, on very fast reentries (e.g. coming screaming in directly back from Minmus).

Make sure to turn on Temperature Gauges (F10) before doing this... then if you see lots of red indicators, start rotating!  It's fun to watch them oscillate between "oh crap" and "ahhhh".

If you start high enough before the atmosphere kicks in too much you can actually get a pretty wild spin going to help exaggerate the effect... it's not realistic, and it's maybe even a little cheaty, but it's very Kerbal.

 

 

Edited by paulprogart
reorg

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1 hour ago, paulprogart said:

If you start high enough before the atmosphere kicks in too much you can actually get a pretty wild spin going to help exaggerate the effect... it's not realistic, and it's maybe even a little cheaty, but it's very Kerbal.

I think that's more of a tumble -- you can't start one of those after your get into heavy heating, at least with a simple command pod, because it's too stable in the base-first position.  I'm talking about a roll (E or Q keys), which seems to even out the heat a bit and can by started after the capsule is too stable to do more than tip a bit under pitch or yaw command.

1 hour ago, eloquentJane said:

Basically, so long as you have one command pod, it is always possible to return one copy of every experiment from every location where you do that experiment. More copies will require science containers but you never need to return the actual science parts.

Okay, I may have gotten that from trying to bring in a second goo canister or thermometer reading from the same location -- and then quit trying because I was trying not to lose the science I'd gone clear to the Mun to get.  I'll try bringing the science inside next time, so I don't chance burning it up.

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Posted (edited)

46 minutes ago, Zeiss Ikon said:

I think that's more of a tumble -- you can't start one of those after your get into heavy heating, at least with a simple command pod, because it's too stable in the base-first position.  I'm talking about a roll (E or Q keys), which seems to even out the heat a bit and can by started after the capsule is too stable to do more than tip a bit under pitch or yaw command.

Sorry I got ahead of myself... I was referring to somewhat larger craft where it can be hard to generate enough angular momentum to counter the atmosphere even with reaction wheels...  Especially in the case of twin-engine designs where the twin barrels have to move a lot more air to allow the craft to roll.

It was meant more as advice for the scaling up the "rotisserie" technique in the future, in case you find yourself with a design that turns out to be less atmosphere-friendly than you'd hoped.  (In my case I often ended up deciding to returning crafts that were never seriously designed for reentry, just because I wanted to at least try to reclaim something rather than let them burn up.)

But more on topic, yeah a small craft with a Mk1 pod will certainly have no trouble generating a roll.  I find a nice gentle spin of 15-30RPM usually works well, and is certainly a lot nicer for the passengers than what I described.  :)  (In fairness, I only did that with automated craft...)

 

Edited by paulprogart
clarification

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On 3/3/2017 at 2:54 PM, Snark said:

Yup.  Radiators don't help in a meaningful way with reentry heat, either IRL or in KSP.  That's not what they're for.

I don't think radiators work in the atmosphere, period.  I had a science mission flirting with the very edge of Eve's atmosphere.  It was an orbital mission but I was dipping a few hundred meters into the atmosphere to collect "Flying High" science.  It was just light flirting--my solar panels didn't burn off, my Kerbal in a command chair was unharmed.  (I stuck him out there to do EVA reports.)

I come back out of the atmosphere and suddenly my radiators are hot.  (Not dangerously so.)  The only thing that makes any sense is they weren't radiating when there were a few thin whisps of atmosphere around.

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