Elapid
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over at Clay Bros.
Originally posted by cls
the video was discussed at length on several mailing lists. the consensus is the apparent flutter is (probably) mostly an MPEG encoding artifact; probably also a video scanning artifact (top to bottom); there was some flutter but not as bad as shown in the movie. (heck from the ground I can see my Calisto's 0.063 G10 fins fluttering on just an I161.)
it's a neat video though!!
bottom line: here is clear evidence of unsuitability of the material, for my anti-G10 campaign.
DynaSoar, are you on the aeropac mailing list? people did some of those things and it was pretty well determined that G10 can't flex so much (modulus too high - heck put some in a vice and try it!) so must be a visual artifact. someone with 10 years experience with MPEG encoders posted his explanation of the visual artifact and several other similar episodes he's seen.
Originally posted by cls
the video was discussed at length on several mailing lists. the consensus is the apparent flutter is (probably) mostly an MPEG encoding artifact; probably also a video scanning artifact (top to bottom); there was some flutter but not as bad as shown in the movie. (heck from the ground I can see my Calisto's 0.063 G10 fins fluttering on just an I161.)
Originally posted by bobkrech
Cliff and Dynasoar
I'm not on the list, but I was asked for my opinion on the video based on my professional experience which is posted on the webpage referenced.
The overwhelming evidence is that it is real fin-flutter and not an MPEG artifact. I wanted to believe that it was a transonic aerodynamic fluctuation in the air density causing a mirage effect, but the magnitude of the motion rules this out.
I looked at the flight data and observed that the flutter starts and stop abruptly at M=+0.90 and that the flight had a maximum M=1.04 velocity. It not possible to do quantative calculations without measuring the mechanical properties of the fins and the acoustic energy spectrum of the motor, but we can say qualitaively the flutter is a result of a number of thing, a coupling of the eddy turbulance in the transonic flow region, the flexible construction of the fins, enhanced aerodynamic coupling due to the non-zero angle of attack resulting fronm non-parallel installation of the fins as evidenced by the second stage rotation, and the coupling of the random vibration energy of combustion instabilities in the rocket motor with the natural vibrational frequency of the fins.
I showed the video to a co-worker who is an aerospace design year with 20 year of experience in spacecraft design and testing and he said that he had observed this kind of motion in fiberglass support structures within spacecraft undergoing randon vibration testing on a shake table. In fact he spent 1 1/2years analyzing the problem and finding an engineering solution to it. I can't go into details any futher.
The natural vibration frequency is proportional to the thickness of the fin, and the amplitude of the flutter is inversely proportional to the fourth power of the thickness for a given excitaton energy. A small increase in the thickness makes a huge difference in the flutter amplitude. A 50% increase results in a factor of 5 reduction in amplitude, and a 100% increase in thickness results in an astounding 16 fold reduction in flutter amplitude.
The fins on the rocket are extremely flexible. Look at the post flight video. I have a 1' x 2' x 1/8" sheet of G-10 and I can't get it to bend like those fins. Either they are thinner, or they were substantially weakened in the flutter incident.
The stock PML QL 3K has 1/16" G-10 fins which will flex easily. For the motor combination in the flight, PML recommends using 3/32" G-10 with a layer of cloth over it. That would make the thickness ~1/8". I think the thickness is actually 3/32" rather than the 3/16" mentioned on the webpage. This would be consistent with an epoxy rich 6 oz. glass layer over the standard 1/16" fin.
A consenus opinion is irrelevant in this instance. From what I have seen on the web, the HPR community has experienced a large number of shreds that are blamed on a number of possible reasons, but very few people have actually analyzed the failure modes from an engineering standpoint. Just because a majority of the rocket community has not observed this phenomon before doesn't mean it doesn't exit.
If Peter had not done such a good job glassing the fins to the airframe, the rocket would have shredded, and without an on-board video camera, the cause of a shred at 5000 ft would be pure speculation. Fin-flutter is real and probably accounts for a majority of HPR shreds not caused by a CATO.
Bob Krech
Originally posted by HeadHunter
From the AEROPAC list, I believe this was the prevailing theory. I understood the theory, but have no expertise in the field, so I must defer to the pros on this one.
...
The combination of low frame rates and MPEG compression could easily display
a movement three times as large as really happened.
Originally posted by bobkrech
Dynasoar
The flow behind a shock wave is not usually steady, nor is it supersonic. In many real situations, the boundary layer flow behind a shockwave is turbulent, and the shock heating increases the sound speed so that the flow velocity is subsonic at the elevated air temperature. Additionally, while you have a large pressure/temperature jump at the shock front, the heated air has to relax back to the ambient conditions, so there is always a gradient setup behind the shock wave of constantly changing pressure, temperature and velocity conditions. There isn't any clamping by the flow, it's just the opposite. The unsteadyness of the flow induces movement of the fin, and the fluttering of the fins can and does effect the airflow. It's a coupled problem that is difficult to solve analytically, and that's the reason why wind tunnel testing and test flights are conducted on aircraft and rockets.
There are some things you can learn the simple mechanical testing, and it can be useul. For example if you fins are flexible and bend easily, you can be sure that they will flutter at some velocity. The converse is not true. It your fins don't bend easily, it's not a guarantee that they won't flutter at some velocity, but it's less likely. Engineers design fins that don't flutter. If you download NACA TN 4197 you'll see the role that the mechanical tests such as those being done at rocketmaterials.org play in the design of fins that don't flutter. If you're motivated, you can get all the gory details from a college level aeronautical engineering textbook, or by surfing the net via google.
Bob Krech
Originally posted by KermieD
I guess, while the flutter is pretty intense, it brings back memories of another video that looks impossible:
https://www.enm.bris.ac.uk/research/nonlinear/tacoma/tacnarr.mpg
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