Why does this spacecraft always flips during reentry?

[montferrer]

I'm devoloping an Eve launcher from sea level, and while testing it on a Kerbin reentry, it always flips at 15,000 m aprox, and gets destroyed. It has 5 heat shields (1 big, 4 medium) and 4 AV-T1 Winglets. Also has 4 Advanced Inline Stabilizers. And it's perfectly symmetric. What I'm doing wrong?

Here are two pics, one before the flip and other after: http://imgur.com/a/r0kJI

Thank you very much for your answers! 

[Streetwind]

Because your vessel is not aerodynamically stable.

Any object falling through an atmosphere will prefer to do so with its heaviest part facing forward, and its draggiest part trailing behind. Your cluster of boosters has its center of mass pretty much dead in the middle, because the boosters' mass dominates everything. So that means, in a first order approximation, the vessel cannot fall with its heaviest part forward, because the CoM won't shift when it rotates. Therefore, the vessel's orientation is defined entirely by its drag characteristics.

And it turns out that a massive solid wall of heatshields creates a little bit more drag than a few small fins...

Edited June 17, 2016 by Streetwind

[montferrer]

59 minutes ago, Streetwind said:

Because your vessel is not aerodynamically stable.

Any object falling through an atmosphere will prefer to do so with its heaviest part facing forward, and its draggiest part trailing behind. Your cluster of boosters has its center of mass pretty much dead in the middle, because the boosters' mass dominates everything. So that means, in a first order approximation, the vessel cannot fall with its heaviest part forward, because the CoM won't shift when it rotates. Therefore, the vessel's orientation is defined entirely by its drag characteristics.

And it turns out that a massive solid wall of heatshields creates a little bit more drag than a few small fins...

Thank you! So if I want to land this thing, I have to lower the center of mass? Can I do that with this design?

Or maybe I could use a fairing to make it aerodynamically stable? Or... is this design useless?

Edited June 17, 2016 by montferrer

[Padishar]

You might be able to add an inflatable heat shield (or more than one) to the top of the vessel to make that end draggier.

[Streetwind]

50 minutes ago, montferrer said:

Thank you! So if I want to land this thing, I have to lower the center of mass? Can I do that with this design?

Well, since it looks to be coming down headfirst, you'd have to raise the CoM of that section in the VAB...

And, I dunno, can you? It's your design!

[montferrer]

9 hours ago, Padishar said:

You might be able to add an inflatable heat shield (or more than one) to the top of the vessel to make that end draggier.

Ok, I'll try! Thank you

8 hours ago, Streetwind said:

Well, since it looks to be coming down headfirst, you'd have to raise the CoM of that section in the VAB...

And, I dunno, can you? It's your design!

I can rise it! hahaha, although I don't understand why you say I have to raise it when before you said it has to be as lower as possible...  the problem of rising it is that there must be something (a pod) at the base of the rocket because it's an Eve rescue mission, and that pod is what lowers the CoM... and I don't want to use ladders (more mass, drag, explosions...); maybe I should haha. Thank you!

Edited June 17, 2016 by montferrer

[magnemoe]

Liked the large upside down inflateable, 
two other ways is
lots of airbrakes, make sure they are inside the shadow of the heatshield. 
lots of reaction wheels, 
At lower speeds you use parachutes. 
An flipover in low speed and high attitude is an nice way to get rid of the heatshield before landing. 


This is my Eve science lander, launch clamps is for hyperedit testing.
I spammed both reaction wheels and airbrakes as center of mass is high and its not aerodynamic balanced because of the rover.

Then i get down to vertical decent on Eve around 25 km up I activates parachutes on heatshield and deorbit module turn off airbrakes and flip, 
eject deorbit module, then rover who has higher drag and lower mass than the accent module, switch to rover and activate its parachutes. 
Switch back to accent module, I use airbrakes to turn this bottom first then let it fall to 4 km above ground before using parachutes. 
Land accent module, switch to rover, do science while flying and land it, rockets on rover is to slow down to land and to do tiny jumps for science while flying in other biomes

Accent module is small as kerbal is put in an small airlock module not an real pod.