why this strange craft ​​​​​​​don't get into the gravity turn by itself?

[Spricigo]

I designed a basic rocket and it performed quite well. It even got into the gravity turn without need of steering input.

Then I decided to try to explore the limits o the design a bit and among other thing I wanted to look how it behaved  without the basic fins. To my surprise, it don't flip but also don't get into the gravity turn by itself.  

Despite being launched with more tilt than the original craft it went up. And is not that it didn't turn at all, rather it osculated slightly west-east, as if there was some sort of self-righting mechanism. Just I have no idea of what it might be.

There is the craft file if anyone is interested in trying it for himself: Strange

and a picture 

Spoiler

 

 

[VoidSquid]

No wobbling (I have Kerbal Joint Reinforcement Next), no tilting, it simply works. I'm surprised that the little reaction is wheel enough for steering, with a Reliant, not a Swivel.

Edited January 18, 2021 by VoidSquid

[Spricigo]

33 minutes ago, VoidSquid said:

it simply works.

Exactly what amuses me.

I was expecting it to become aerodynamic unstable without the fins. So it was a nice surprise.

However, the original craft (first link in the OP) fall nicely into a auto gravity turn without steering input and this craft don't do it.

 

PS:I don't have KJR but the craft have plenty of autostrust to prevent wobbling

Edited January 18, 2021 by Spricigo

[VoidSquid]

52 minutes ago, Spricigo said:

I was expecting it to become aerodynamic unstable without the fins. So it was a nice surprise.

Good mass distribution, I'd think  

52 minutes ago, Spricigo said:

and this craft don't do it

A bit of speculation, maybe the little drag of the fins did help, moving the rotational axis closer to the center of thrust?

Edited January 18, 2021 by VoidSquid

[Spricigo]

 

16 minutes ago, VoidSquid said:

A bit of speculation, maybe the little drag of the fins did help?

I think you almost  got it. Not the drag but lift. 

Does it make sense?

 

[VoidSquid]

11 minutes ago, Spricigo said:

but lift

Doubt that, lift from symmetrically placed fins should cancel each other out (provided the fins have plane symmetry themselves).

Edited January 18, 2021 by VoidSquid

[Daveroski]

The effect is caused by wind shear.

KSP doesn't have wind! I hear you cry!

If a completely symmetrical craft isn't pointing exactly prograde then drag caused by the wind hitting the side closest to prograde is the result.
The higher the airspeed the greater the shear, the greater the effect.

The fins on the first craft produced more drag on the prograde side than the retrograde side which helped to 'steer' it toward prograde.

Very close to prograde the difference may be small. It depends on how much surface area is exposed to the shear effect.
Stray too far from prograde and any craft will stall and flip.

This is the reason some craft tumble when going into orbit.

The upside is that if you make a craft close to the shape of a bullet without fins, it will be quite happy to keep flying toward prograde and will allow you to let it stray further from prograde than the same craft with fins would do at the same speed.

D.

 

 

Edited January 18, 2021 by Daveroski
clarity

[Spricigo]

Many thanks, @VoidSquidand @Daveroski 

 

[VoidSquid]

Always welcome  

[Daveroski]

You are welcome.

(Hmmm appreciation.. warm and fuzzy inside... hmmm)

;o)

 

D.