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Nytheris

How to stop ship flipping over on re-entry?

Question

I'm trying to re-enter Kerbin's atmosphere after orbiting, and I can keep my heat shield facing the right way for the majority of the re-entry heat, then it will suddenly flip over for seemingly no reason and my ship will explode almost instantly.

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I'd recommend upgrading KSC enough (its the first upgrade on the kerbalnaut complex, 75k funds.) to do an EVA, then EVA, pick up the science and get back in the capsule.  Bonus: if you can get into space/orbit with Bob, you can get away with only one science jr.  [SAVE FIRST: EVAing takes some practice and you can easily get lost in space]. Run the experiment first up the first in the atmosphere. EVA to grab the science and reset the experiment.   Go back to the capsule and deposit the science.  Go back to the science jr and run the experiment again and grab the science and return to the capsule.  Don't forget to do an EVA report and get back in the capsule before hitting the atmosphere!  Note that if you use Bob, reset the thing and do the whole process over again when you land (even if you can't get back into the capsule, you can always grab the capsule from the tracking station.  PS. Obviously Bob can do this whether or not you upgrade the kerbanaut complex.

This method needs one less science jr (if you send Bob.  Jeb can do all the rest but needs two science jr.s since he can't reset the thing), and also picks up the EVA data (from "flying over KSC" on the launchpad, from space, from "flying over wherever you land", and from where ever you land.  "Flying over" just means holding on to the capsule.)  You don't get the funds from returning the science jr.s and other stuff, but it gets very frustrating to try to get the center of drag behind the center of mass.

Other things to upgrade right away (buying too many of them might risk running out of money and losing the game)

mission control (75k, 7 missions active) + tracking station (150k, patched conics visible on your map) together give you maneuver nodes.

launch pad (75k, 140ton rockets) [I suspect your rocket is on the edge of what needs a launch pad upgrade, although its been awhile since I started a new game].

 

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IMO, you are getting too greedy with the science. I will take one SciJr or one utility bay when I'm at that early stage of the career. Not two materials and a utility bay all stacked like that.

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It depends on the configuration of your craft. Please add a screenshot.

Generally, your CoM should be located closer to the front. If there are multiple fuel tanks in your vessel and there is some fuel inside, try to move the fuel towards the heat shield.

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This is on re-entry after getting science from orbit so I have no fuel at this point, this is all that's left.

y47zKu7l.jpg

I didn't think about CoM before but it makes sense, since it's quite high up in this case.

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The heatshield's abator is used/burned up during reentry, and it's part of the heatshield's weight. So as it is used up the heatshield becomes lighter, CoM moves towards your command pod, and flipping happens.

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All ships will naturally want to point with their center of mass in front of their center of drag, like a dart or a badminton cock. What's happening here is that the blunt face of the heat shield is very draggy while the capsule is aerodynamic (when pointing forward) and also very heavy compared to those Science Jr's. You're able to hold it when the atmosphere is thinner and so aerodynamic effects are less, and also like the other said because you're probably losing mass on your heat shield as it ablates away. Once you're in the thicker air, where there's more drag, and your center of mass has shifted back, flipping ensues.

 

To correct this, add 2 or 3 basic fins at the top of your capsule. You'll probably need to add more wings at the bottom of your lifter to not flip during launch, but it should work. The basic fins will also give you a tiny bit of lift so you can control your descent better (by pitching and yawing with the capsule's torque) as an added plus.

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I'd make a different suggestion: pick up that science using an EVA, and then you can use a decoupler to drop those lightweight science bays. Coming down with just the module and heat shield should be easy.

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13 minutes ago, technion said:

I'd make a different suggestion: pick up that science using an EVA, and then you can use a decoupler to drop those lightweight science bays. Coming down with just the module and heat shield should be easy.

In this stage of the game you don't always have the ability to do an EVA. (If this is in career mode of course)

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If you have nose cones you could put your labs on top of your command pod. Chutes should be on the labs. Add a decoupler under the nose code on the labs (or even no decoupler) or one big chute. A heat shield should be added under the pod.

This is a quite odd ship, but the heaviest part (the pod) will stay at the bottom and the ship should not flip

Sorry I can't put any pictures, but I'm not in front of KSP.

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Thank you for the help and suggestions everyone. I think I'm going to go with doing an EVA to collect the science and decouple the labs. I was worried that I would mess up the EVA but I guess I have to practice it at some point.

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25 minutes ago, Nytheris said:

Thank you for the help and suggestions everyone. I think I'm going to go with doing an EVA to collect the science and decouple the labs. I was worried that I would mess up the EVA but I guess I have to practice it at some point.

You can practice before launch, on the launchpad. Losing grip of the ladders and falling is a good way of simulating failure in space - your goal should be to never lose grip, EVA thrusters should be a last resort measure.

If you go with the same model, make sure to place some ladders on the nearest lab to get close enough to the further away one to get the data. All should be well without having to risk a Kerbal drfting off in cold lonely space.

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Another way to go: put the heat shield up top. A bit more drag on the way up, sure, but when you come down it'll be easy to keep your attitude.

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I would guess you are running out of electric charge and the reaction wheels stop working. Also, from my experience the danger zone is between 1500-1000m/s for potential control loss on re-entry- be careful during those times.

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On 12/21/2015 at 10:54 PM, A_name said:

All ships will naturally want to point with their center of mass in front of their center of drag, like a dart or a badminton cock. What's happening here is that the blunt face of the heat shield is very draggy while the capsule is aerodynamic (when pointing forward) and also very heavy compared to those Science Jr's. You're able to hold it when the atmosphere is thinner and so aerodynamic effects are less, and also like the other said because you're probably losing mass on your heat shield as it ablates away. Once you're in the thicker air, where there's more drag, and your center of mass has shifted back, flipping ensues.

 

To correct this, add 2 or 3 basic fins at the top of your capsule. You'll probably need to add more wings at the bottom of your lifter to not flip during launch, but it should work. The basic fins will also give you a tiny bit of lift so you can control your descent better (by pitching and yawing with the capsule's torque) as an added plus.

I believe this is a very good answer. worked for me after I removed the 3-kerbal pod from the top of my duna lander and replaced it with lander cans. craft stopped flipping.

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On 12/21/2015 at 10:21 AM, Nytheris said:

Thank you for the help and suggestions everyone. I think I'm going to go with doing an EVA to collect the science and decouple the labs. I was worried that I would mess up the EVA but I guess I have to practice it at some point.

By the way, this is just a very peripheral side note, but I've seen a couple of people in this thread refer to those gizmos as "labs".  Be careful about getting in the habit of calling them that, because the game doesn't.  When it refers to them in chatty contract descriptions, it calls them "materials bay."

I only bring it up because I've seen posts in this forum where people got bitten because they saw a contract that said "have a lab on the station" and they thought it meant the Science Jr. when what it actually meant was the Mobile Processing Lab (the big two-kerbal research lab).

Easiest way to avoid getting bitten by this is to just get in the habit of mentally referring to them as "materials bays."  ;)

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For returning packages like yours, I fit 3-4 small fins at the nose. Use the small ones and place them so far up they won't get overheated on the return.

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I'd also suggest just putting the science can's on the top of the pod, rather than underneath - sure it looks bloody stupid, but it will mean the CoM is sorted. Otherwise, as others have said, fins will work, or putting the heat shield on the other end.  Though, to be honest, at this point I'd actually be looking at upgrading the KSC so I can EVA and fly this thing with one Sci Jnr, and a scientist so I can rinse and repeat the science, and ditch the experiments and come home in just the capsule.

Wemb

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one other tip, just minor, but if you don't have RCS thrusters, you should empty the minoprop out of your command pod... that would slightly help with the COM issues.

If you aren't using monoprop thrusters, it also give you at least a little more dV

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Ok. Here is the problem. First, you need a SAS wheel. It stabilizes your ship. It requires power though. Second, your center of mass is most likely too high. To fix this, try adding a small fuel tank to the bottom. Add some RCS thrusters and a RCS tank also, to make sure you can maintain your reentry trajectory until you hit the atmosphere. To make the center of mass even lower, try adding a rcs tank to the bottom as well. Further, try moving that service bay to the bottom. Hope this helps. :)

Edited by InfiniteAtom
Removed accidental capslock

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