[AR] Re: Dynamic stability in supersonic rockets
- From: Plugger Lockett <plugger.lockett@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 10 Jan 2022 13:50:00 +0800
IMHO peoples intuition of how strong things need to be for supersonic
flight are just plain wrong.
Calculate the force on the fins with even TINY TINY -perturbations they
are huge.
If your seriously supersonic and can't set your truck on the fincan with no
measurable deflection its not strong enough.
There may be dynamic balance dragons at these speeds, but most failures I
see at FAR are just weak structure.
If there are dynamic dragons out there most don't live long enough to
challenge that dragon...
My fins were fine as they were very robust. I feel confident in saying this
because even though the NC/AV bay sheared off the vehicle the airframe and
fins + motor casing just kept on trucking. I'll be incredibly lucky if I
ever see that portion of the rocket again though.
It was my airframe that buckled. And I agree that the airframe structure
was too weak just above the forward closure as that's where the vehicle
failed. Would a stronger airframe have saved me on this flight? Maybe, but
I'm not sure. I'd like to have a bit of a better idea regarding my previous
failure though before I press the button on another flight that will
approach/exceed Mach 3. And the wobble that occurred before the buckling is
a bit of an anomaly in comparison to the previous Mach 2+ flights I've
successfully flown, hence my questions around dynamic stability.
IIRC Tony (HASH) recently flew a MD O3400 at FAR that appeared to suffer a
similar fate. a telltale wobble followed by a rapid unplanned disassembly.
His fins stayed on too for what it's worth.
https://www.rocketryforum.com/threads/o3400-min-diameter-l3.165546/page-14#post-2202634
On Mon, Jan 10, 2022 at 12:02 PM Paul Breed <paul@xxxxxxxxxx> wrote:
IMHO peoples intuition of how strong things need to be for supersonic
flight are just plain wrong.
Calculate the force on the fins with even TINY TINY -perturbations they
are huge.
If your seriously supersonic and can't set your truck on the fincan with
no measurable deflection its not strong enough.
There may be dynamic balance dragons at these speeds, but most failures I
see at FAR are just weak structure.
If there are dynamic dragons out there most don't live long enough to
challenge that dragon...
On Mon, Dec 27, 2021 at 2:47 AM Ben Brockert <wikkit@xxxxxxxxx> wrote:
Chuck,
Is it the case that an arbitrarily fine rocket will fly straight as
long as it has >2 calibers of stability? In general high fineness
rockets are easier to make stable and typical HPR implementations get
more stable with the CG shifting forward as the motor burns, yet these
are the ones that have exhibited "coning" and other bad behaviors. I
would have thought inertial and aero coupling comes for everyone,
whether you're aerodynamically stable or not, thus the existence of
maximum fineness guidelines.
The USAF "Flying Qualities Textbook" is an interesting read to put
more math behind this than I usually do. Making the "aerodynamic axis"
the same as the "inertial axis" is where balancing the rocket comes
in. https://archive.org/details/DTIC_ADA170960 discussion starts on
page 4.
Definitely one of the joys of guided liquids is that you put in some
good gimbal throw and hire Ian Garcia and give him the inertia matrix
and some CG data from the CAD, and then you don't care much after
that.
On Mon, Nov 15, 2021 at 8:49 PM crogers168 <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
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) Rogers
Rogers 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
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