<< Chuck, thanks so much for chiming in and providing that rough guideline
regarding how to attempt to mitigate dynamic stability issues with a simple
static stability rule to follow. That's pretty much EXACTLY what I was hoping
for out of this discussion and it coming from you definitely adds weight to the
guidance. It also meshes with the successful vs unsuccessful flights with
really aggressive reloads (eg M2245, N5800, O3400). >>
You're welcome! Glad to help you out with your flights.
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: Plugger Lockett <plugger.lockett@xxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: Mon, Nov 15, 2021 10:43 pm
Subject: [AR] Re: Dynamic stability in supersonic rockets
Hi all,
Firstly, thanks for all the thoughtful responses. I really appreciate it.
William, I remember Ben Brockert making that comment, I actually responded to
that thread. And I agree that most hobbyists don't spin balance their high
performance rockets and that could be useful. And I'm aware of spin
stabilization and the benefits it provides in this situation but to be honest I
think getting 6-9Hz of spin will be tricky, at least for my skills.
Chuck, thanks so much for chiming in and providing that rough guideline
regarding how to attempt to mitigate dynamic stability issues with a simple
static stability rule to follow. That's pretty much EXACTLY what I was hoping
for out of this discussion and it coming from you definitely adds weight to the
guidance. It also meshes with the successful vs unsuccessful flights with
really aggressive reloads (eg M2245, N5800, O3400).
Troy, thanks as well. I'm not sure, but I think your second point around
aerodynamic forces trying to pull the rocket apart is possibly what bit me with
my M2245 flight at the last THUNDA. With that flight my rocket folded just at
or after burnout at ~ Mach 3. Below is a screenshot of the video (video link
here https://www.youtube.com/watch?v=AaZ2lmrFbSo&t=172s ) showing a bit of
wobbling just before the Nose and AV bay separated from the airframe.
Luckily I recovered my AV Bay and was able to pull flight data from my Raven,
seen below. I've boxed the area where the photo above roughly matches the
graphed data.
And finally, the Raven flight summary.
I need to dig up my sim file but I thought I had ample static stability
(approaching 2) but I want to check that. And TBH I knew the section of
airframe where my rocket failed was by far the weakest point of the vehicle as
it was the place where I only had 4 wraps of 2x2 200gsm CF twill with no other
reinforcement. It was maybe a 2 inch section, above the motor casing and below
the internal FWFG AV Bay coupler tube. I did have the casing retained with an
aluminium bolt but that was more for positive retention than anything else. I
won't make this mistake again.
If anyone feels comfortable with sharing their thoughts on my flight failure
based on the above data I'm very keen on hearing that. Thanks for reading.
Kind regards,
Drew Hamilton
On Tue, Nov 16, 2021 at 5:35 AM Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
wrote:
There are 2 (aero)dynamic issues I would be focusing on for high performance
flight that can get you: (1) the forward shift in CP which Chuck has suggested
a good design allowance for. This is counterintuitive to aircraft designers as
the CP will generally shift backwards for typical high performance aircraft
within the supersonic regime, but is generally true for rockets of typical
sounding rocket geometry. (2) the aerodynamic forces trying to pull the rocket
apart. This will depend on the design of your rocket and how aggressive the
acceleration profile ie. going fast enough (velocity) quick enough (time, in
particular air density). This issue will spring up on you at (or just after)
burnout if it does catch you. It will depend on how draggy your lower section
is compared to the upper section but also internal pressure – ie. if you only
bleed your internal pressure out the side (A) there might not be enough time
for your internal volume to bleed enough pressure for burnout and (B) the
external pressure on the base will be lower than the ambient static pressure.
Again, this will be most apparent for aggressive acceleration profiles. Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Plugger Lockett
Sent: Monday, 15 November 2021 5:51 PM
To: arocket@xxxxxxxxxxxxx
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