<< And for all you folks at home, don’t try demonstrating this concept with a
garden hose :P >>
Great analogy. You can actually feel the damping with a high flow rate garden
hose. Like turning/maneuvering a rotating bicycle wheel, you can swing/rotate
back and forth a high flow rate garden hose with and without water flow and
feel the difference, feel the damping.
I'm assuming this actually becomes a big issue with high flow rate fire hoses.
I'm sure firemen are familiar with it.
Charles E. (Chuck) Rogers
-----Original Message-----
From: Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx; anthony@xxxxxxxxxxx
Sent: Wed, Nov 17, 2021 2:41 pm
Subject: [AR] Re: Dynamic stability in supersonic rockets
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{}#yiv2781837886 div.yiv2781837886WordSection1 {}-->And for all you folks at
home, don’t try demonstrating this concept with a garden hose :P Troy From:
arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of
crogers168 ("crogers168")
Sent: Thursday, 18 November 2021 4:06 AM
To: anthony@xxxxxxxxxxx; arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Dynamic stability in supersonic rockets Anthony: Jet
damping is included in the RASAero II software. For others jet damping is the
mdot V term where the fact the motor is thrusting damps the oscillation of the
rocket, the higher the thrust the higher the damping. So there is not just the
Cmq aerodynamic pitch damping coefficient, there is the pitch damping from the
thrust of the rocket motor (kind of the thrust analog for Cmq). The original
source is "The Exterior Ballistics of Rockets" published in the 1950's. The
MIT Topics in Advanced Model Rocketry book has the equations, based on this
source. You can actually have a rocket be unstable at launch, but after it
clears the launch rail if the jet damping is high enough it slows the turning
of the rocket enough that if the rocket is burning propellant fast enough the
rocket can have time to get stable and then successfully fly to apogee. Note
that on some of these Mach 3 "wobbles" and break-ups if the motor has high
thrust there is high jet damping. But once the thrust tail-off has begun, the
jet damping goes down proportionally with the thrust. The loss in jet damping,
combined with everything else going on, may be the one last thing which adds to
the failure of the rocket. Using the RASAero II predicted CP and keeping the
static stability margin at least 2.0 calibers for all supersonic Mach numbers,
you basically aerodynamically "brute force" your way through all of these
complex dynamics and complex dynamic stability issues. The best example of jet
damping I've ever seen was a DC-X scale model, a pretty big one (3-4 ft tall?),
with no additional fins, on a high thrust L motor. I wasn't involved in the
rocket; I just saw it fly. It turns out the rocket was probably unstable
(statically unstable). But as long as that high thrust L motor was thrusting,
it was flying straight and was having a great flight. What was actually
happening was the rocket was statically unstable, and was slowly rotating, but
the rotation was very, very slow because the jet damping was incredibly high,
the jet damping was keeping the rocket pointy end forward. As soon as that
motor went into tail-off, and it was a pretty short tall-off, the rocket
immediately swapped ends. The jet damping contribution to the dynamic
stability allowed a statically unstable rocket to successfully fly, at least
until the motor shut down. Charles E. (Chuck) Rogers
-----Original Message-----
From: Anthony Cesaroni <anthony@xxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <arocket@xxxxxxxxxxxxx>
Sent: Tue, Nov 16, 2021 3:27 pm
Subject: [AR] Re: Dynamic stability in supersonic rocketsHi Chuck, Did you
publish discussion of augmented dynamic stability during motor burn or was it a
citation? Thanks. Anthony J. CesaroniPresident/CEOCesaroni Technology/Cesaroni
Aerospacehttp://www.cesaronitech.com/(941) 360-3100 x1004 Sarasota(905)
887-2370 x222 Toronto From: arocket-bounce@xxxxxxxxxxxxx
<arocket-bounce@xxxxxxxxxxxxx> On Behalf Of crogers168
Sent: Tuesday, November 16, 2021 3:40 PM
To: plugger.lockett@xxxxxxxxx; arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Dynamic stability in supersonic rockets << 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 rocketsHi 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