Adding to my post:
For supersonic Mach numbers up to Mach 4 and above the best way to avoid
dynamic stability issues (coning, pitch-roll coupling) is to have the static
stability margin never fall below 2.0 calibers for all supersonic Mach numbers.
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: crogers168 <dmarc-noreply@xxxxxxxxxxxxx>
To: plugger.lockett@xxxxxxxxx <plugger.lockett@xxxxxxxxx>;
arocket@xxxxxxxxxxxxx <arocket@xxxxxxxxxxxxx>
Sent: Mon, Nov 15, 2021 10:49 am
Subject: [AR] Re: Dynamic stability in supersonic rockets
The best solution is to run the RASAero II software, and never let the
stability margin fall below 2.0 calibers for all supersonic Mach numbers.
RASAero II also includes a warning message that if your stability margin for
supersonic Mach numbers falls below 2.0 calibers, the warning message appears,
so the user can then consider appropriate action to get the stability margin
back up to 2.0 calibers.
In RASAero II I recommend at least 2.0 calibers of stability margin at
Supersonic Mach numbers. Why is this greater than the 1.0 calibers of stability
margin typically used with the Barrowman Method for Subsonic? At Supersonic
Mach numbers, especially Mach 2-3 and Mach 3+, if the stability margin falls
below 1.0 calibers, there may be coning, and/or pitch-roll coupling, and other
undesirable effects. The additional 1.0 calibers is additional stability
margin, to make sure the total stability margin never falls below 1.0 calibers.
As an example, a rocket might have a stability margin at Mach 3 of 1.20
calibers, but there might be an error in the CP prediction of 0.25 calibers,
lowering the stability margin below 1.0 calibers and into the coning/pitch-roll
coupling potential danger area.
I've posted here and on The Rocketry Forum comparisons of RASAero II Supersonic
Center of Pressure (CP) predictions with wind tunnel data out to Mach 4.5.
RASAero II really does an excellent job predicting the Supersonic CP. But there
are some interesting variations in the wind tunnel CP data for one of the
configurations at Transonic Mach numbers. There can always be mispredictions in
aero data. Add the extra 1.0 calibers stability margin (for a total Supersonic
stability margin of 2.0 calibers) and don't risk it.
On RASAero II once you get the less than 2.0 calibers stability margin warning
message, you can pull up the CG and CP plot and look for where you're violating
the 2.0 calibers stability margin limit, and then decide how to address the
issue (larger fins, change the fin shape, add nose weight).
To my knowledge of all the Mach 2, Mach 3, Mach 3+ high power/amateur rockets
flown that were simulated preflight on RASAero II and had predicted stability
margins of at least 2.0 for all supersonic Mach numbers, none had failures due
to stability issues, with the exception of two rockets (one didn't fail but had
coning above 140,000 ft, the cause of the other failure hasn't been
analyzed/determined yet).
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: Plugger Lockett <plugger.lockett@xxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: Sun, Nov 14, 2021 10:51 pm
Subject: [AR] Dynamic stability in supersonic rockets
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