Relation between focused craft & physics? (Another rocket-flipping issue)

[Reusables]

Hi, I'm reusable TSTO rocket enthusiast. So when launching TSTO craft to orbit, the first stage sometimes tumbles and gets tons of drag to low its own apoapsis.

The strange thing is that the flipping depends on which craft you have focus on. The exactly same first stage doesn't flip back if it's focused, while it always ends up tumbling when I'm focusing on the second stage nearby.

Since I'm going for the first stage recovery before the second stage reaches apoapsis, this is quite important for me. So why is this happening? Is it a glitch with the physics engine?

EDIT: The first stage is the one which is flipping when not focused. It does not flip when it's focused.

EDIT: Here's the screenshot of the craft. It has fairing top to lessen the front node drag of the first stage, and added wings at the back. I'll try turning off prograde marker and see if something changes.

Edited November 15, 2017 by Reusables

[Zeiss Ikon]

I'm reading this as your second stage with payload  tumbling -- you might wish to edit the question to clarify if that's not the case.

If the stage is aerodynamically unstable after staging, it may be unable to hold heading if it's more than a couple degrees from prograde -- and your probe core will be switched to "stability assist" (i.e. "hold constant heading") when you switch focus away from the second stage, which inevitably happens while you recover the booster.  During this time, the stage tries to hold heading, but it's coasting, so its prograde vector will point more and more downward -- and if the heading gets too far off prograde for an unstable stage, it'll often will have enough aerodynamic destabilizing forces to overcome the reaction wheel(s) and start to tumble.

The dead simple solutions to this are:

1. Stage high enough that aerodynamics no longer matter.  For Kerbin, that's above about 40+ km.  I've got a vessel with a spaceplane orbiter, and the second asparagus staging event leaves this orbiter with only a single core engine/tanks assembly and a Swivel engine, but I have no trouble with it because by the time the I stage away the boosters with fins on them, there's not enough air left to matter.

2. Make the stage aerodynamically stable on its own.  Teeny bitty fins ("Basic Fins") can do this, if the COM is far enough forward.  Beware, however; any fins you put forward of the vessel's launch COM must be compensated by larger and /or steerable fins at the bottom/rear end of the vessel at launch (if the fins are really tiny and COM is far enough forward, you might be able to get away with just engine gimbaling, or by adding RCS, but usually you'll need fins for this).  My Taxicab family vessels, with the spaceplane orbiter, get along okay with just a pair of AV-8 steerable elevon surfaces at the rear and the same surfaces acting as canards for the orbiter at the nose, assisting three Swivels at launch and last-staged -- as long as the pilot doesn't let the heading get off the prograde vector above 100 m/s (I have to set up the gravity turn between 40 and 100 m/s and then go to prograde  hold, or the vessel will flip in pitch, unrecoverably).

[Reusables]

On 2017. 11. 13. at 6:36 AM, Zeiss Ikon said:

If the stage is aerodynamically unstable after staging, it may be unable to hold heading if it's more than a couple degrees from prograde -- and your probe core will be switched to "stability assist"

Thanks, I should check what happens with stability assist only.

Besides, I edited the question: it's the first stage that is flipping. It is designed to be aerodynamically stable, though.

I'm going to upload the screenshot.

Edited November 15, 2017 by Reusables
Edited useless bits

[Kryxal]

Is it still aerodynamically stable when EMPTY?  For that matter, body lift tends not to get factored in, and you're likely to have a big heavy engine on the bottom.