Large System (JNSQ) Retrograde Stability on air braking

[flart]

installed only the 

JNSQ (Kopernicus, KopernicusExpansion, ModularFlightIntegrator, RationalResources)
MissingHistory
Squad
SquadExpansion
ModuleManager

 

It's starts ok

Spoiler

but after some time it deviates from the retrograde, while It looks like it should be pretty stable

Spoiler

 

Landing was successful, but Jeb didn't feel safe.

 

The same is happening with the stock Mk2 Pod

Spoiler

 

 

UPD.
I have added Stabilizer, and find out that the problem is responsive to Physic warp, even while the clock is green:
https://www.dropbox.com/s/lgaexkcjhuimmze/20191118-154645.331.mp4

UPD2.
I have removed the Stabilizer, and similar Warp-relation has place there:
https://www.dropbox.com/s/015kw9ddzgxxulo/20191118-155807.287.mp4?dl=0

but I have made another run without touching the warp, and the problem still appears:
https://www.dropbox.com/s/wb8gxqs2m3806jk/20191118-160255.725.mp4?dl=0

Edited November 18, 2019 by flart

[James Kerman]

Thread moved to gameplay questions.

[Streetwind]

Can confirm, physics warp during reentry tends to cause my craft to veer off retrograde.

However, in my experience, it happens only to craft which need SAS Hold Retrograde to stay pointed the right way - in other words, craft which are not passively stable. In such a case, the higher the G-loading becomes, the less physics timewarp is safe. Past 2G, even x2 warp tends to screw things up for me.

Meanwhile, with a passively stable reentry module, I can leave x4 warp on for the entire descent, and it typically stays stable.

I say "typically" because I haven't specifically tested this. I'm just reporting my personal anecdotal experience.

[flart]

47 minutes ago, Streetwind said:

passively stable

That means just Center of Lift behind Center of Gravity? 
Since pods or crew cabins usually doesn't have any lift, and Heat Shield has, almost any airbraking stuff aren't stable (have CoL before CG), including Mk1pod + heatshield
Also I was testing the vessel with an empennage (last image), and behavior was the same. But that was before I discovered the warp correlation

    

Edited November 18, 2019 by flart

[Streetwind]

Center of "lift" doesn't really apply to objects on a ballistic trajectory through the atmosphere, which is why the CoL indicator the editor shows you is not useful for checking passive stability. For example, the mk1 pod plus heatshield is perfectly stable, no matter what the CoL marker says. Throw one at the planet from orbit, leave SAS off, and you'll see that it orients itself correctly blunt end forwards.

Could you try that exact test with your crew cabin reentry vehicle? No timewarp, no SAS, see what it does?

[flart]

13 minutes ago, Streetwind said:

No timewarp, no SAS, see what it does?

it's oscillate hardly, and then the cabins are exploded

https://www.dropbox.com/s/m45s9dw0pziv5c2/20191118-180014.010.mp4?dl=0

Edited November 18, 2019 by flart

[Streetwind]

Nice video, that really shows it well.

You can see the craft is not passively stable. It wants to drift off retrograde by itself. Then, as the crew cabins swing out of the shielded area into the airstream, they start generating drag, and that pushes them back inside. Until they slip out again, because the craft is still not happy to point retrograde on its own. Thus, you get this constant back-and-forth where the crew cabins swing out into the airstream and then back into the shielded zone over and over.

At the end, where overheating kills the middle crew cabin and leaves only the pod itself, you can see that the remaining craft immediately becomes passively stable, sticking to retrograde. It still wobbles back and forth slightly, but that is only because it was already in motion, and that motion needs to dampen out first before it can hold perfectly still.

You can also see why adding fins didn't help. The crew cabins themselves could already generate plenty of drag to stabilize the orientation... IF they were in the airstream. And they aren't, until the point where the craft is already sideways. The tiny fins you added were, similarly, not in the airstream until the craft was already sideways.

You would need something that sticks out past the radius of the heatshield at all times, so that it can generate drag at all times... or, you need to shift more of your mass closer to the heatshield, in order to achieve passive stability.

 

Edited November 18, 2019 by Streetwind

[CBase]

45 minutes ago, Streetwind said:

IF they were in the airstream. And they aren't, until the point where the craft is already sideways.

Actually in stock aerodynamics radial attached fins are calculated as if they are in the airstream even if there are bigger parts below.

In reality there is some airstream as well, just at lower pressure as air expands behind the shielding.

[flart]

Exactly the Basic Fin has max temp 934°K so they are exploded quickly, I was using 0.5 scale basic AV-T1 Winglet, but it wasn't working.
The unscaled winglets are too large for that design, A.I.R.B.R.A.K.E.S extended max temp is 1200°K, so they also exploded.
Using 4 heatshields for moving CoM makes it better, but some oscillation still there... playing further 

[Gordon Fecyk]

This happened with a contraption I built for Alien Space Programs and homeworld Eve. Similar design to this, only topped with a Mk1 pod. Stock Eve having a similar re-entry speed to JNSQ Kerbin, I kept flipping at a certain point. The only solution I had was making the heat shield-end of the craft heavier, which involved some Mk1 pod and 1.25m service bay clipping.

[flart]

Yes, 4 heatshields:

Spoiler

but the winner is an elevon on the hinge

[Snark]

Another thing you can do-- crude, but very simple-- is to just increase the ablator load on the heat shield to full capacity.  That will make the heat shield heavier, and move the CoM closer to that end of the craft.  Even a small move on the CoM can make a significant difference to stability.

Of course, that also means you're lugging more mass than you actually need-- ballast, in effect-- which normally is anathema to spacecraft.  So I totally understand if you'd prefer not to do that.    Just... it's a potential tool in the toolbox to be aware of.

9 hours ago, flart said:

That means just Center of Lift behind Center of Gravity?

No, it means center of dynamic pressure behind the center of mass/gravity.

Center of lift means where the cyan indicator in the VAB is.  Nice and easy to see where it is, but unfortunately is not what matters here.

Whereas what does matter-- the center of dynamic pressure (or, if you prefer, "center of drag" though that's not a technically accurate description)-- alas, does not have any visible indicator so you just have to eyeball it.

Overall, "move the CoM towards the end you want to be in front" is the main thing.  Fins and such can help, too, but the CoM really matters.

8 hours ago, Streetwind said:

You can also see why adding fins didn't help. The crew cabins themselves could already generate plenty of drag to stabilize the orientation... IF they were in the airstream. And they aren't, until the point where the craft is already sideways. The tiny fins you added were, similarly, not in the airstream until the craft was already sideways.

Are you sure this is true?  I was under the impression that KSP's simplistic aero model does not model occlusion in that regard, and that the "airstream" affects them even though they're behind another part.

Note that I'm talking about aero forces here.  Heating definitely does model occlusion.  But I believe all parts that aren't shielded in a fairing or service bay do in fact experience aero forces, or at least that was my impression.

My take on "why didn't the fins help" was that they simply weren't big enough, and/or far enough from the CoM, to overcome the big drag on the front end of the vessel.

[Streetwind]

@Snark I have no hard proof to show you that my interpretation is right; so who knows, I might have gotten it wrong. Thing is, he's not playing entirely stock; and he reported that adding a set of fins made no difference to the spacecraft's behavior during reentry. Considering that a set of the same basic fins can also take a 1.25m launch vehicle from "always flips at max-Q" to "holding prograde without SAS", it seems pretty hard to believe that the fins weren't somehow being kept from performing their function. The reentry vehicle isn't even flipping, it's just listing a bit to the side. Fins absolutely should make a visible difference if they are generating drag.

But yeah. Maybe I'm wrong. It has happened before...

[Laie]

On 11/18/2019 at 1:22 PM, flart said:

but after some time it deviates from the retrograde, while It looks like it should be pretty stable

I've had the same question some time ago. The answer we arrived at was that, as long as the vessel is pointed into the wind, drag only attacks at the bottom (or front, depending how you see it). The long tail will not experience much (if any) force while it's perfectly aligned with the airflow.

Put differently: in order to act as a stabilizer, the tail has to have a non-zero angle of attack.

[Cavscout74]

I have a similar design tourist pod and even in stock scale it either needs gridfins (Kerbal Re-useability Expansion) at the top or ballast at the bottom (or both) to remain stable without SAS.  Even with gridfins & SAS, it tries to go sideways under physics warp, it just doesn't go too far, and survives.  Airbrakes serve the same purpose - but as you noticed - have a nasty habit of exploding from overheating.

Edited November 19, 2019 by Cavscout74

[paul23]

On 11/18/2019 at 3:54 PM, Streetwind said:

Center of "lift" doesn't really apply to objects on a ballistic trajectory through the atmosphere, which is why the CoL indicator the editor shows you is not useful for checking passive stability. For example, the mk1 pod plus heatshield is perfectly stable, no matter what the CoL marker says. Throw one at the planet from orbit, leave SAS off, and you'll see that it orients itself correctly blunt end forwards.

Could you try that exact test with your crew cabin reentry vehicle? No timewarp, no SAS, see what it does?

This is only because ksp "makes it so", if you use something more realistic like FAR it will be highly unstable in that direction.

 

Of course center of lift behind center of gravity isn't telling the full story, at all. More important is the lifting moment, and the corresponding neutral point, the neutral point should be behind the center of gravity.

[Skorj]

Just in general, a cylinder is not aerodynamically stable end-on.  It wants to turn to the longest diagonal through the cylinder, more or less, to be forward.  You'll notice there's a sort of ring of stability at a fixed angle off prograde/retrograde.  Your craft then makes this less stable, because it drops abruptly from a wide cylinder to a narrow cylinder.  That puts even more drag near the front (well, the windward side as it re-enters). Note that in Kerbal physics, the drag due to the leading edge and the drag due to the trailing edge are basically the same, given the same shape on both ends.

What you want is more drag at the rear than at the front.  (Or fins as above, but it can sometimes be quite awkward to launch that way.)  That's quite tricky.   If you re-enter nosecone first with a flat rear, it can be stable depending on the cylinder, but then of course you burn up in the atmosphere because you don't have enough drag.

Note that the capsule shapes are very stable, despite what I said above.  In real life, this is just how blunt body physics in a supersonic airstream work.  It's very stable in Kerbal physics as well.  You can take a Mk1-3 Command Pod and stack a Mk1 Crew Cabin or two on top and it remains very stable, despite looking almost the same as above. 

Also note: if something is almost working, and just leaning out a little bit into the airstream to burn up, adding a reaction wheel or two can fix the problem.  The reason it work without time acceletation but fail if you warp is that for some reason reaction wheels and/or SAS just work less will during warp.  Sometimes quite a bit less well.  I struggle with that all the time, both on launch and re-entry, with craft that are fine at 1x but flip out at 3x or 4x.  Fingers crossed KSP2 won't have that particular  problem.