I need help identifying my vehicle's aerodynamic instability.

[PTNLemay]

I built a fairly large ship and assumed that the 2nd stage wouldn't need ailerons, because it would be pretty high up.  Sure enough, I staged the part in question at around 24 km in altitude.  And yet, the thing flipped instantly.  I turned on the aerodynamic vector indicators, and it does look like something near the tip is creating a huge amount of drag (note the really long red arrow).

And idea which part it could be?  I've got a science lab, the 1-seat lander can, the 2-seat passenger tube.  That 1.25 to 2.5 m adapter.  Aside for that it's just a lot of nicknacks that shouldn't be affecting drag coefficient that much... I'm really stumped.  Any help would be really appreciated.

Oh, and in case it's relevant, I'm playing on a 2.5x scaled Kerbin.  Which doesn't scale up the atmosphere in the same way.  It actually goes up to 94-ish kilometers.

 

Edited December 19, 2017 by PTNLemay

[Geonovast]

Do you have fairings unlocked yet?  They help a lot.  I would put all the riggamaroo at the top inside a fairing.

[PTNLemay]

Yeah, that's what I'm doing for the second attempt (in addition to the mini-2nd-stage-winlgets).  Still... I can usually omit all that stability stuff when I reach high enough in the atmo.  I guess 24 km isn't all that high, all things considered.  But I would love to know what's causing that drag spike.

 

[Harry Rhodan]

8 hours ago, PTNLemay said:

But I would love to know what's causing that drag spike.

There's a drag spike the moment you reach Mach 1. But in your case the problem is that you accelerate extremely hard and then stage away the only things that added a sizable amount of drag to  the bottom of your rocket.

[Streetwind]

9 hours ago, PTNLemay said:

And idea which part it could be?

Using the highly scientific method of holding a piece of paper up against the screenshot you posted as a makeshift ruler, it would appear that the extremely long red spike originates in the center of the science lab. Ergo, the science lab would be the part that creates so much drag... unless you have something hidden clipped into it, perhaps even accidentally.

I don't have much experience launching science labs; the last time I've done it was 2-3 versions ago. Back then, my rocket had no trouble keeping straight while ascending.

[Zeiss Ikon]

The angles of those drag vectors suggests another problem: your heading is too far from your prograde vector.  Are you still following the old-atmosphere launch profile, boost vertically to near 15 km, then turn east to be at 45 degrees when you hit 20 km?  The atmosphere changes from 1.0 forward make it more efficient (and safer) to start a little turn -- 5 to 10 degrees -- around 100 m/s, then follow the prograde marker all the way to MECO (which you do when your apoapsis is high enough, then set up your circularizing maneuver).  The small, early turn has you building up the horizontal velocity you need for orbit right from the start, and following prograde while you do it can let you keep even a pretty unstable vessel headed where it needs to be.