Roll Frequency For Stability

[Charles Barty]

Hi Team!

I am designing a rocket and need to calculate what roll frequency (Hz) my rocket should roll at in order to be stable. I have done some research into roll stability but cannot find any information on how to find the optimum roll frequency for your rocket. Obviously this frequency will change as size, inertia properties change but does anyone know a general formula or a method to figure this out?
Thanks for all of the help!

 

[Charles_McG]

How big and how high?

It’s my understanding that -most- sounding rockets don’t spin for stability in the sense of correcting forces proportional to off axis perturbations. Only a few have spun fast enough to have a gyroscope effect.

Must spin at a couple of Hz in order to reduce dispersion- the spread of possible landing area. The roll averages out perturbations.

That’s in atmosphere. Above the dense atmosphere, the big sounding rockets kill the roll because it’s likely to transfer into tumble.

Model model rockets don’t roll on purpose.
Unless they do - but I’m not aware of people planning a roll rate.

 

[UhClem]

Roll stability for rockets is largely a myth. Dealt with in the once again out of print Topics in Advnced Model Rocketry.

Go low or high but avoid the pitch natural frequency.

 

[jbsommerfeldt]

This is for bullets but I would think the same principle applies, Miller Twist Rule.
Are you trying to fly a rocket with no fins?
A rocket with fins spinning is wasting energy in addition to being unnecessary. Make those fins strong.
But, it does look cool.
Some rockets are known to spin slowly on their own by phenomenon. Im sure there is science behind it that can explain it.
Some by design, corkscrew, now a discontinued kit.

 

[manixFan]

The Loki Dart launch vehicle aimed for a spin rate of 8.5 RPS but in practice achieved just over 6 RPS. More data here:

https://apps.dtic.mil/dtic/tr/fulltext/u2/750796.pdf

Unfortunately it does not give any of the reasoning behind the rotational speed, but it does have some other interesting information.


Tony

 

[watheyak]

A few years back the Tripolo Class 3 committee was discussing requiring some of the large 2-stage flights to be spun. I was present for a conversation about spin rate and yo-yo de-spin math. Interesting stuff.

Unfortunately I don't remember the the people involved in the conversation, but the committee might be a good place to start.

 

[watheyak]

Here's a thread with some links to the math behind the spin and de-spin.

https://www.rocketryforum.com/threads/yo-yo-despin.136523/