Slashdot Mirror
Mysterious Sound Waves Can Destroy Rockets
Ponca City, We love you writes "Scientists believe that powerful and unstable sound waves, created by energy supplied by the combustion process, were the cause of rocket failures in several US and Russian rockets. They have also observed these mysterious oscillations in other propulsion and power-generating systems such as missiles and gas turbines. Now, researchers at the Georgia Institute of Technology have developed a liquid rocket engine simulator and imaging techniques to help demystify the cause of these explosive sound waves and bring scientists a little closer to being able to understand and prevent them. The team was able to clearly demonstrate that the phenomenon manifests itself in the form of spinning acoustic waves that gain destructive power as they rotate around the rocket's combustion chamber at a rate of 5,000 revolutions per second. Researchers developed a low-pressure combustor to simulate larger rocket engines then used a very-high-speed camera with fiber optic probes to observe the formation and behavior of excited spinning sound waves within the engine. 'This is a very troublesome phenomenon in rockets,' said Professor Ben Zinn. 'These spinning acoustic oscillations destroy engines without anyone fully understanding how these waves are formed. Visualizing this phenomenon brings us a step closer to understanding it.'"
It makes rocket scientists crap their pants!
This means rocket science is once again hard. You may now resume saying "Well, this isn't rocket science" until they solve this.
jX [ Make everything as simple as possible, but no simpler. - Einstein ]
and the subject line for this article has finally convinced me that cowboy neal is in fact art bell.
Is that a rocket in your pocket? Finally I could satisfy a woman! :(
I wonder if they'd be interested in analyzing the smoking ruins of at least 5 toilet bowls I have personally destroyed with mysterious oscillating rocket powered sound waves.
Fascism trolls keeping me up every night. When I starts a preachin', he HITS ME WITH HIS REICH!
When analyzing the acoustic oscillations scientists discovered something quite striking. The sine wave was exactly identical to the master recording of Britney Spears' "Hit Me Baby One More Time".
Could be implemented in a way to defend against rocket\missle attacks? Possibly in a better way than Star Wars program.
Trying to install linux on my microwave, but keep getting a kernel panic...
If you record them and play them backwards they will install Vista on your computer.
Dr No will fish them out of the water and pass the rockets on to SMERSH....I don't like the sound of that!
Between the falling angel and the rising ape
The new result here isn't acoustic instabilities; those have been known for a long time. The interesting result is a new set of imaging techniques that give a better understanding of *why* they occur, rather than simply observing on pressure traces that they *do* occur. After a bit more research, this may turn into techniques to more reliably avoid them in the design stage, rather than having to go through various tweaks on the injector / combustion chamber to remove them should they appear.
This is very cool work. Of course, it's rocket science, not rocket engineering, so it's unlikely to impact new designs for several years yet.
Come on, an expert on rocket fuel technology named Professor Ben Zinn?
I can't help but wonder if understanding this won't lead to some powerful weapons... think about it a sonic cannon, that might make some interesting CNN coverage during war time.
*Blank stare* - "These go to eleven."
Light the blue touch-paper and retire immediately.
Heavy Metal can destroy even rockets now.
Rocket engines typically have a round cross section, which, if it doesn't aid the production of these circular waves, probably does little to dampen them. I wonder if the "inside out" design of a linear aerospike engine suffers from the same problem.
When our name is on the back of your car, we're behind you all the way!
This phenomenon sounds very similar to Pogo Oscillations, which incidentally caused the engine 5 shutdown on the Apollo 13 Saturn V.
http://en.wikipedia.org/wiki/Pogo_oscillations
pressure waves...acoustic waves... Hmm... I'm having a hard time seeing the difference. I think they use "acoustic" because of their frequency is probably between 20Hz-20KHz... otherwise known as "acoustic".
I'm wondering if the waves are just something related to how the energy goes out at such high pressures and it being a bit opposite in how the soda bottle "vortex generators" work. Pressure is high enough so that all the fluid flows out axially instead of rotating around the axis to some degree, but these rotating acoustic waves are just a form of conservation of (angular) momentum in the fluid flow that under less pressured circumstances would want to make a vortex?
Looks like an audio engineering issue. While not being a rocket engineer myself, I assume the combustion chamber is somewhat symmetrical. It is likely acting as a resonance chamber and increasing the amplitude of the soundwave to the point of physical damage. I shattered the rear window in my '96 Camaro twice with a 1200W Fosgate and a single 10" bazooka tube. Tell NASA to crack the window when they turn up the bass!
So THAT'S how the Doctor's screwdriver works...
Pogo, pump-related oscillations, and plumbing related oscillations are all low frequency (tens of Hz, sometimes less). These are acoustic modes internal to the chamber, in the kHz range. They're very distinct phenomenon, with distinct causes and distinct solutions. They're still a 50 year old problem with 50 year old techniques to solve them, but they're by no means understood in any meaningful sense -- the current technique mostly involves testing the engine and then tweaking it until they go away.
The new and interesting work here is the modelling, combined with the photography techniques. Seeing pressure waves at the injector face through the chamber full of flame is not trivial.
http://www.rastko.org.yu/rastko/delo/10896
[Nikola Tesla:] "I was experimenting with vibrations. I had one of my machines going and I wanted to see if I could get it in tune with the vibration of the building. I put it up notch after notch. There was a peculiar cracking sound.
"I asked my assistants where did the sound come from. They did not know. I put the machine up a few more notches. There was a louder cracking sound. I knew I was approaching the vibration of the steel building. I pushed the machine a little higher.
"Suddenly all the heavy machinery in the place was flying around. I grabbed a hammer and broke the machine. The building would have been down about our ears in another few minutes. Outside in the street there was pandemonium. The police and ambulances arrived. I told my assistants to say nothing. We told the police it must have been an earthquake. That's all they ever knew about it."
Watch Out, Mr. Smith
Some shrewd reporter asked Dr. Tesla at this point what he would need to destroy the Empire State Building and the doctor replied: - "Five pounds of air pressure. If I attached the proper oscillating machine on a girder that is all the force I would need, five pounds. Vibration will do anything.- It would only be necessary to step up the vibrations of the machine to fit the natural vibration of the building and the building would come crashing down. That's why soldiers always break step crossing a bridge."
Combustion instability is an old problem with rocket engines. The Saturn V main engine had serious combustion instability problems, which were fixed by trial and error testing. The Apollo booster people had to resort to setting off small bombs inside engines on test stands to induce instability, then trying different patterns of holes in the plates the distributed fuel to find a stable configuration.
The SR-71 engine had serious combustion instability. That, too, was fixed with something of a hack, an automated "sympathetic unstart" system which, when one engine had a stall, would stall the other one, then restart both.
Better simulation tools in that area can't hurt. Not many big supersonic engines are designed any more. As Scott Crossfield pointed out just before he died a few years ago, every aircraft that went significantly over Mach 3 is now in a museum.
Ok, before parent gets any farther this has to be de-bunked. Sound waves did not destroy the bridge. A sound wave, in any medium consists of a compression and a rarifraction ., that is a leading pressure wave followed by a area of lower pressure that propagate in a known fashion. The intensity of a sound wave obeys the inverse square law.
What happened to the Tacoma Narrows Bride was caused be an error in aerodynamic calculations on the part of the design engineer. Air passing around the bridge deck acted exactly like air does when presented with a crude airfoil, it formed an area of low pressure leeward of the bridge deck and a low pressure area leeward and below the bridge deck. Th resulting high pressure and low pressure vectors imparted a twisting moment to the bridge deck.
The twisting moment was resisted by the torsional rigidity of the bridge deck. This caused the deck to twist to and build torsional tension. The twisting caused the aerodynamic profile of the bridge deck to change. The resulting change allowed the bridge deck to revert back to its original shape and aerodynamic profile, rinse and repeat. Thus the repeated twisting caused enough of the riveted and bolted joints to fail which led to a cascade failure as the remaining joints failed under the bridges weight and twisting motion.
This was not "low frequency sound waves" although the structures oscillations did cause some very low frequency sounds waves, it was destroyed by nothing more then bad aerodynamics.
Hey KID! Yeah you, get the fuck off my lawn!
And so the old question foes, "If a tree falls in the forest and there is no one there to hear it, does it make a noise?
Well by the definition of sound there are three components:
Hence the phrase, "In space no one can hear you scream.". Now that was a movie, but it is never the less true. We have all seen the experiment where you take an electric bell, place it in a vacuum chamber. As the air is pumped out the, softer the sound of the bell gets until it can no longer be heard.
No medium, no pressure wave, it's that simple. Now there is liquid fuel in pipes, that is being pushed hard into the combustion chamber by pumps. Ever seen what happens to a jet engine during a compressor stall? The high pressure exhaust comes out he front! YIKES, not a good thing at all. Now those pumps are pushing the liquid fuel and oxidizer into the combustion chamber against combustion pressure. The ONLY thing preventing the combusting fuel and oxidizer mix from going right back UP those pipes and making the whole damn thing blow up are the pumps. I would imagine that all sorts of pressure waves are transmitted back up into the inner working of the rocked via the medium of the fuel. Imagine if the pump "stalled" ie the pump vanes out paced the fuel supply? The pump impellers would effectively stall and pressure in the delivery lines to the combustion chamber would drop and allow back flow until the pump caught up and started pushing fuel again, I think this would definitely cause some pressure waves all over the place. It would also cause lots of vibration, perhaps enough to cause failure,
Hey KID! Yeah you, get the fuck off my lawn!
This racetrack instability is actually a well known problem with annular combustion chambers such as those used with the toroidal aerospike engine. One of the main virtues of vortex engines, like Orbital Technologies or the ultracentrifugal one invented by Roger Gregory and myself, is that the coriolis effect distorts the wave front sending it into the wall of the combustion chamber. In theory, at least, this should disrupt the resonance enough to prevent destructive standing waves. Experiments have not been conducted to test this theory yet to the best of my knowledge.
Seastead this.
If the waves reach the ear drum and are interpreted, would they become sound? Sound is vibration through a medium that is perceived by something. If I make waves in a jump rope at the right frequency, I'm pretty sure I can hear it. Just because the device (human ear) isn't sensitive enough to detect it doesn't mean it isn't sound.
Layne
Once the dB level of sound starts to approach that which is experienced in the engine of a rocket, it isn't even resonance anymore, it's just plain extreme force.
It generally takes about 110dB to shatter a wine glass via oscillation, but it isn't direct exposure to the pressure that causes that.
Depending on the quality, glass will begin to shatter above 160-165 dB, independant of its resonant frequency. Of course, if you are dealing with flexible glass, that value will increase.
It is quite possible to have a sound wave impact with enough force on a specific area in a rocket engine to cause enough fatigue which will result in a failure without actually resonating.
So why is this news? Because depending on the atmospheric pressure, once you get above 194 dB, the soundwave becomes distorted, it would be difficult enough to model a soundwave in a motionless, inactive engine, but I can't even begin to comprehend how complex the modeling must be of an engine that is generating sound waves in excess of 200dB in such an extreme and dynamic environment.
That they are able to model this is amazing.
Out of modpoints but really liked a post? 1BDkF6TtmmeZ3yqXbz9yhdYVqRYnwFoXDj