Drew:
Dynamic stability can cover a lot of sin but with regard to sounding
rockets only the first rule of thumb is that the length should never exceed
20 times the maximum diameter. Ben Brocket has previously pointed out that
a lot of minimum diameter rockets are really very long.
One can find examples of professional sounding rockets that violate this
rule but one will discover that they have very strong interstage
connections, are fully balanced around the long axis, and are spun at just
the right rate to maintain axial stability.
In my experience, the increasing sophistication of payloads for hobby
rockets is leading to a lack of axisymmetric balance at the forward end due
to the boards and batteries being placed without regard to balance. This
leads to the coning that is commonly seen in higher performance hobby
rockets that are not spin balanced. Making certain that the payload weight
distribution is at least axially statically balanced will go a very long
way to ending this problem.
If all that is cared for, then spinning the rocket at just the correct rate
(typically 6-9 hertz) will tend to prevent the coupling that can occur at
lower spin rates. The exact spin rate required is a function of the
inertial balance of the hardware and has to be estimated, typically using
the USAF dat.com tools.
Bill
On Sun, Nov 14, 2021 at 11:51 PM Plugger Lockett <plugger.lockett@xxxxxxxxx>
wrote:
Hello aRocket,
I've been having some conversations with people in the high power rocketry
community as of late regarding minimum diameter rockets encountering issues
with stability during boost when they reach the Mach 3-4 range. The
conventional wisdom in these conversations is that the failures are related
to a lack of dynamic stability. Of course unlike static stability, there's
not an easily passed on guideline for ensuring dynamic stability of a given
vehicle.
So I was wondering if anyone here is willing to chime in and shed some
light on how to improve the dynamic stability of a rocket that will see
Mach 3+ during boost?
Any advice is appreciated and thank you for your time.
Kind regards,
Drew Hamilton