Problems with spin stabilization

[cami]

On 18.2.2017 at 0:43 PM, John FX said:

And here we in fact have extra mass at one end a little way out from the axis of rotation.

[...]

If you have an image of your craft it would help greatly in diagnosis.

There actually is an image further up in the thread. The fins are only used for spinning up, and are attached to a lower stage. Only after the final stage separation, when there is no attitude control at all, and the craft is already spinning at several 100 to 1000 times per minute, it loses orientation. In the image above, there actually are slightly off-axis parts at one end, the separation motors. There is another craft without separation motors however, whose final stage is basically this <(__()<  (to scale, it's just an aerobee with a tank 150% of its diameter in length, and a 30° apex cone), and it does exactly the same thing.

[John FX]

45 minutes ago, cami said:

There actually is an image further up in the thread. The fins are only used for spinning up, and are attached to a lower stage. Only after the final stage separation, when there is no attitude control at all, and the craft is already spinning at several 100 to 1000 times per minute, it loses orientation. In the image above, there actually are slightly off-axis parts at one end, the separation motors. There is another craft without separation motors however, whose final stage is basically this <(__()<  (to scale, it's just an aerobee with a tank 150% of its diameter in length, and a 30° apex cone), and it does exactly the same thing.

If you mean this one

then I have had trouble with multiple aerobees on a single stage where one engine runs out of burn time before the others, leading to instability.

Are you watching the TestFlight window during your burn to see if any engines fail early?

[Urses]

On 13.2.2017 at 10:23 PM, Snark said:

/shipped the Wall.. .

As a Good example for @Snark 's Repository may be a Crossbow Bolt without feathers

https://en.m.wikipedia.org/wiki/Crossbow

Some Models use spin stabilisation and Show the Tendency to stabilise the Projektil without following the trajektorie.

[cami]

5 hours ago, John FX said:

then I have had trouble with multiple aerobees on a single stage where one engine runs out of burn time before the others, leading to instability.

Are you watching the TestFlight window during your burn to see if any engines fail early?

It's unrelated to part failures. There is just one aerobee, and it's on the spin axis. The other two motors are SF separation motors.

[John FX]

On 29/03/2017 at 3:46 PM, cami said:

It's unrelated to part failures. There is just one aerobee, and it's on the spin axis. The other two motors are SF separation motors.

Ah, I see. Not that then. If there is not a way for the engines to create unstable spin, the spin is stable to begin with and then becomes unstable, then it might be some sort of weird effect caused by fuel drain changing COM.

[Red Iron Crown]

@cami When playing with spin stabilized sounding rockets in RO I found that having a ring of tanks around some central part worked better than a single large tank. Something is a bit snaky with how KSP/Unity models mass distribution of parts, single parts have less polar moment of inertia than they should. Example, this probe did not work well with a single tank instead of four.

[cami]

@Red Iron Crown thank you for the feedback. I guess my suspect is at least correct to some extent, then. I'll keep that in mind.