Can't get reentry right in 1.05

[Zalx]

I seem to be having trouble getting back onto Kerbin. For some reason all of my designs seem to flip over between 20 - 30K and kablooie.

I've messed a little with the weight distribution but would love some pointers as I'm not amazing at the game.

Thanks!

[Snark]

2 hours ago, Zalx said:

I seem to be having trouble getting back onto Kerbin. For some reason all of my designs seem to flip over between 20 - 30K and kablooie.

I've messed a little with the weight distribution but would love some pointers as I'm not amazing at the game.

Thanks!

Are you talking about a spaceplane, or a "traditional" rocket-style reentry?

I'll assume for the moment that you're talking about just a regular reentry.  The main thing is that your rocket's going to want to rotate so that the center of mass is in front of the center of drag.  For example, if you have a heavy capsule, on top of something big and light, it's going to tend to want to put the capsule end first.

There's not any special "secret", just make sure the CoM is at the end that you want to be in front while you're reentering.

Hard to be more specific than that, though, without knowing what your ship looks like.  Could you post a screenshot?

 

[Vegetal]

All of your designs? Just pods? Crew cabins? Space planes? Space stations? What?

A lot of factors come into play during reentry, especially with the different designs possible in KSP, if you don't get more specific, we can't help ya.

[Zalx]

30 minutes ago, Vegetal said:

All of your designs? Just pods? Crew cabins? Space planes? Space stations? What?

A lot of factors come into play during reentry, especially with the different designs possible in KSP, if you don't get more specific, we can't help ya.

 

12 hours ago, Snark said:

Are you talking about a spaceplane, or a "traditional" rocket-style reentry?

I'll assume for the moment that you're talking about just a regular reentry.  The main thing is that your rocket's going to want to rotate so that the center of mass is in front of the center of drag.  For example, if you have a heavy capsule, on top of something big and light, it's going to tend to want to put the capsule end first.

There's not any special "secret", just make sure the CoM is at the end that you want to be in front while you're reentering.

Hard to be more specific than that, though, without knowing what your ship looks like.  Could you post a screenshot?

 

 

Sorry, should have been more specific, was about 4am so I sort of flew through the question. Basically I've started career mode and I've got all of the parts before they cost 90 science. I'm building just a basic Mun flyby rocket and getting my COM down is what's killing me for the return capsule.

I don't seem to have any parts that are heavy enough to counterbalance the heavy MK1 cockpit at the end. Any help in that direction?

[sal_vager]

I'll assume you mean the Mk1 pod, If you jettison the rest of the craft so it's just the pod, a parachute, maybe a drogue chute, a heat shield and the science stuff on the pod you should have no trouble at all, and a 30km periapsis from the Mun will allow you to aerobrake in one pass and won't burn through the shield.

If you're trying to bring back tanks and an engine you'll nosedive and burn up.

[Snark]

22 minutes ago, Zalx said:

the heavy MK1 cockpit command pod at the end

(FTFY)

Well, there's your problem right there.  Placement makes all the difference.

Seriously, post a screenshot-- it's a lot easier to point at a specific design with a specific problem and say "there's your issue right there" than it is to give general advice.  sal_vager's advice is great, and likely the answer to your problem, but it's hard to tell which bits are relevant without seeing what you're attempting.  It totally depends on what your reentering spacecraft looks like.  Likely the answer will boil down to some combination of "rearrange part X to be in this location instead of that one" and/or "get rid of part X, you don't need it".

Command pod with heatshield on the bottom:  no problem at all.

Command pod, sitting on top of a 2 ton LFO tank with a Terrier on the bottom:  also no problem for stability (just need to watch the reentry heat) so the engine doesn't go boom).

Command pod sitting on top of a stack of Science Jr's with maybe an empty fuel tank thrown into the mix:  recipe for sadness.

 

 

[Zalx]

27 minutes ago, Snark said:

Placement makes all the difference.

Seriously, post a screenshot

http://imgur.com/a/PFNRy

Here was what I've been trying to reenter with. All flip between 20-30K and the more weight on the drag side the higher the thing flips over. Also the single command pod with a heat shield really wants to point nose down anyway so I don't understand how that works by itself. The fact that reentering with just those explodes is what made me start building it more complex in the first place.

Edited January 24, 2016 by Zalx

[Vegetal]

Aaah that's better. Well, that configuration is always wanting to go nose first, the command pod is the heaviest part. And if you go nose first, your ship gets too aerodynamic to brake enough, and instead overheats. That's why my ships at that point in career ditch everything and only the pod reenters - it is made to be stable on retrograde.

At this level I don't bother much with the science junior, as it is so unwieldy early on, but if you have a level 2 crew building, you can EVA a scientist and get all the science from them before decoupling, and return only the capsule as I said.

[Zalx]

4 minutes ago, Vegetal said:

it is made to be stable on retrograde.

But It's not stable on retrograde for me?

Pod by itself just overheats and explodes, pod by itself with a heatshild flips and explodes at 23K. I've tried reentry from loads of different angles with just the pod testing it now and all have exploded. And I should have mentioned I'm playing science only mode as I'm relearning the game from like 0.2. I am entering in retrograde as that would obviously be stupid if I was pointing the heatshield away from the drag, The ship flips over no matter what. The third design in that imgur link has its weight most towards the heatshields yet it is even more unstable and flips anterograde the earliest! At roughly 40K.

[Evanitis]

Two things that helped me with reentry in 1.0.5 early careers:

Placing the Science Jr on top of the capsule. Yes it looks totally ridiculous, but I like that.

Uncontrollable spin. It looks even more ridiculous, but it shifts the parts so while one heats the rest can cool.

These should work for for suborbital / orbital trips, but I'm unsure if it's enough for returning from the Mun. For that task, I'd try unlocking batteries, so the pod has enough charge to keep SAS on, thus it can keep the heatshield on retrograde. Adding an OCTO probe core means that you don't need a pilot to use SAS, so Bob can go on the mission who can remove the data from the science modules, thus those don't need to reenter.

Edited January 24, 2016 by Evanitis

[Vegetal]

Well, if the simple mk1 pod is prograde-stable for you, then there's something wrong with your game. I don't see another explanation.

The way the game does this is by placing it's aerodynamic center quite high, near the tip of the capsule. Try pressing F12 during reentry and see what it's like, if possible post a screenshot.

There is a way to reset the physics parameters in the game, I've seen someone get a corrupted one once. I remember it involving deleting files from your install, I'm not gonna risk trying to teach you that

[Zalx]

44 minutes ago, Vegetal said:

Well, if the simple mk1 pod is prograde-stable for you, then there's something wrong with your game. I don't see another explanation.

The way the game does this is by placing it's aerodynamic center quite high, near the tip of the capsule. Try pressing F12 during reentry and see what it's like, if possible post a screenshot.

There is a way to reset the physics parameters in the game, I've seen someone get a corrupted one once. I remember it involving deleting files from your install, I'm not gonna risk trying to teach you that

The thing is, it looks like what you're saying matches up to my game. I've uploaded this webm of my reentry with Aero Forces turned on, I've seen people reccommend turning SAS off for reentry but that makes the craft even less likely to point retrograde for me.

http://webm.land/w/IEj7/

You can see me wobbling trying to control the craft from pushing away towards anterograde but this just takes over at 23K.

I feel like it has to be something I'm doing wrong here because this is a fresh install of the game.

[Ketatrypt]

22 minutes ago, Zalx said:

The thing is, it looks like what you're saying matches up to my game. I've uploaded this webm of my reentry with Aero Forces turned on, I've seen people reccommend turning SAS off for reentry but that makes the craft even less likely to point retrograde for me.

http://webm.land/w/IEj7/

You can see me wobbling trying to control the craft from pushing away towards anterograde but this just takes over at 23K.

I feel like it has to be something I'm doing wrong here because this is a fresh install of the game.

 

I think I see the problem. I am 95% sure you have the heat shield on upside down. Just try flipping it around. See if it helps.

[Vegetal]

What the...? Did you just put the heat shield upside down? What the heck is that below the capsule?

Whatever it is, it is the reason for your problems. The aero center is *way* out in front of your ship. And as we are already on the topic of heatshields: A simple 1.25 command pod can reenter from low kerbin orbit (LKO - start getting used to the jargon here) without a heatshield just fine. From Mun and Minmus, a heatshield with no ablator is perfectly fine.

PS: speaking about jargon - it's retrograde, not anterograde

[Zalx]

5 minutes ago, Vegetal said:

What the...? Did you just put the heat shield upside down?

 

25 minutes ago, Ketatrypt said:

 

I think I see the problem. I am 95% sure you have the heat shield on upside down. Just try flipping it around. See if it helps.

Yep, as usual it is something incredibly stupid, hehe. I'm surprised I've been able to reenter at all! Thanks that seems to have fixed my drag problems.

7 minutes ago, Vegetal said:

PS: speaking about jargon - it's retrograde, not anterograde

Whoops! Meant prograde, mixing my psychology and physics terminology up. Seems fitting as I clearly have amnesia.

[Vegetal]

A pilot in a 747 once forgot to flip a simple navigation switch in an intercontinental flight. He ended up just above a soviet missile test during the cold war, and was promptly shot down. It's *always* a stupid mistake

[Waxing_Kibbous]

Re-entry can be tricky- a one star pilot helps in holding retrograde, but you need a bit more power onboard. Also try using the fine control mode, I generally switch to that when re-entering without a trained pilot, big moves can mean trouble.

 

[Zalx]

Thanks for everything guys. As usual best community for any game ever. KSP has to be one of the most rewarding games to learn.

[Evanitis]

20 minutes ago, Zalx said:

Meant prograde, mixing my psychology and physics terminology up.

Hahaha, I knew that word is familiar. Just wait until you diagnose someone with prograde amnesia. Or just being antinormal.

[Snark]

4 hours ago, Zalx said:

http://imgur.com/a/PFNRy

Here was what I've been trying to reenter with. All flip between 20-30K and the more weight on the drag side the higher the thing flips over. Also the single command pod with a heat shield really wants to point nose down anyway so I don't understand how that works by itself. The fact that reentering with just those explodes is what made me start building it more complex in the first place.

Ah.  There's your problem right there.  You've just built the poster child for Spaceship that Wants To Fly Nose-First.

Those two Science Jr's are what's killing you.  The Science Jr. is terrible for aero stability, if you want it to be in front.  It's huge and fluffy.  It's big and bulky while being very low mass.  And to make matters worse, you have two of the darn things.

So, first piece of advice:  If you have EVA available, then don't reenter with the Science Jr's.  After you get their science, while you're still in vacuum, go EVA and pull the science out of them for storage in the command pod.  Then jettison them, so that you're only reentering with a command pod with a heatshield right below it.

If that's not an option, then you need some other approach.  For example, reenter with an engine still attached; engines are massive, and also fairly heat-resistant, so you can let the engine be your heat shield on the way down.  If the weight of the engine makes your ship fall too fast on the parachute, then you could add a decoupler above the engine so you can blow it loose once you're past the hot part of reentry.

Edited January 24, 2016 by Snark

[Jim DiGriz]

33 minutes ago, Snark said:

So, first piece of advice:  If you have EVA available, then don't reenter with the Science Jr's.  After you get their science, while you're still in vacuum, go EVA and pull the science out of them for storage in the command pod.  Then jettison them, so that you're only reentering with a command pod with a heatshield right below it.

TIL:  you can extract science from an experiment with a scientist while on EVA.   Also looks like you can ferry science between command modules...  Wow, I've been doing it completely wrong...

[Snark]

4 hours ago, Ketatrypt said:

I think I see the problem. I am 95% sure you have the heat shield on upside down. Just try flipping it around. See if it helps.

The heat shield may very well be upside-down... but that's not the problem.  The problem is that there are two very big, very low-mass Science Jr.'s slung under the command pod.  The CoM is way towards the nose end, so it's going to want to go nose first.  Having the heatshield flipped one way or the other isn't going to make an appreciable difference to drag behavior.

1 hour ago, Jim DiGriz said:

TIL:  you can extract science from an experiment with a scientist while on EVA.   Also looks like you can ferry science between command modules...  Wow, I've been doing it completely wrong...

Don't even need a scientist.  Any EVA kerbal can pull science out of a science experiment and stash it in a command pod.

The benefit you get from a scientist is that they can reset a used goo unit or Science Jr.  For example, if that ship had a scientist on board, it would only need one Science Jr. instead of two (since it would be re-usable).

 

[Snark]

5 hours ago, Zalx said:

Pod by itself just overheats and explodes, pod by itself with a heatshild flips and explodes at 23K.

There's something wrong there.  I suspect you're entering far too steeply.

A pod by itself should enter just fine, even without a heatshield, if you're entering from LKO.  Not if you're coming back from the Mun or Minmus or something, since then you're going a lot faster.  But if you're just reentering the atmosphere from a circular orbit at 90 km or below, there should be no problem.

And a pod with a heatshield should be totally stable.

What kind of reentry trajectory are you following?  e.g. what's your periapsis, before you enter atmosphere?  It shouldn't be any lower than 30 km.  Even 35 km should be fine.

4 hours ago, Zalx said:

I've seen people reccommend turning SAS off for reentry but that makes the craft even less likely to point retrograde for me.

Nah, keep SAS turned on, it'll help keep you pointed the right way if you're marginally stable.

6 hours ago, Zalx said:

The fact that reentering with just those explodes is what made me start building it more complex in the first place.

This is exactly backwards.  Building it more complex is likely to make it more likely to flip around.  Doing just a command pod, or just a command pod with heatshield, should be fine.  If that's not fine, the problem is with the flying, not with the design.

[SSgt Baloo]

You could always mount the Science Juniors on the roof.

Examples:

ETA: I waited a while for it to tip over before I took the last image, but it seemed happy to sit there without tipping.

 

Edited January 25, 2016 by SSgt Baloo

[Wemb]

Like others have said, a cmd pod on it's own will re-enter at the correct attitude automatically.  The science Jnr pod can be enough to flip it arround due to the change in the CoM - but this is where the heat shield be your friend.  If you're coming in from a LKO and are't are at a shallow enough angle, you absolutely do not need the heat-shield to protect your ship from heat, but what it -is- useful for is as ballast - it's a relatively heavy part, and can help ensure you have enough weight on the bottom of your craft so you don't come in parachute first.

Wemb