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 <mailto:anthony@xxxxxxxxxxx> >
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
<arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx> >
Sent: Tue, Nov 16, 2021 3:27 pm
Subject: [AR] Re: Dynamic stability in supersonic rockets
Hi Chuck,
Did you publish discussion of augmented dynamic stability during motor burn or
was it a citation?
Thanks.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
<http://www.cesaronitech.com/> http://www.cesaronitech.com/
(941) 360-3100 x1004 Sarasota
(905) 887-2370 x222 Toronto
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
<arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx> > On Behalf
Of crogers168
Sent: Tuesday, November 16, 2021 3:40 PM
To: plugger.lockett@xxxxxxxxx <mailto:plugger.lockett@xxxxxxxxx> ;
arocket@xxxxxxxxxxxxx <mailto: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) Rogers
Rogers Aeroscience
-----Original Message-----
From: Plugger Lockett <plugger.lockett@xxxxxxxxx
<mailto:plugger.lockett@xxxxxxxxx> >
To: arocket@xxxxxxxxxxxxx <mailto: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
<https://www.youtube.com/watch?v=AaZ2lmrFbSo&t=172s> &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
<mailto: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>
[mailto:arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx> ] On
Behalf Of Plugger Lockett
Sent: Monday, 15 November 2021 5:51 PM
To: arocket@xxxxxxxxxxxxx <mailto: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