Slashdot Log In
Mysterious Sound Waves Can Destroy Rockets
Posted by
Soulskill
on Thu Apr 10, 2008 10:12 PM
from the scientists-get-the-best-toys dept.
from the scientists-get-the-best-toys dept.
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.'"
Related Stories
This discussion has been archived.
No new comments can be posted.
The Fine Print: The following comments are owned by whoever posted them. We are not responsible for them in any way.
Full
Abbreviated
Hidden
Loading... please wait.
Brown noise (Score:5, Funny)
Re:Brown noise (Score:4, Funny)
Parent
Re:Brown noise (Score:4, Funny)
Parent
Re: (Score:2)
Re:Harmonics (Score:4, Interesting)
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.
Parent
Good news! (Score:5, Funny)
Re:Good news! (Score:4, Funny)
Parent
Re:Good news! (Score:5, Insightful)
But shockwave instability in rocket propulsion systems has been a known problem since the very beginning of rocketry. They've been solving it repeatedly for decades. Heck, the Saturn V's F1 engine had it bad in early designs. Solving the F1's shockwave problem required significant innovations in testing methods and tools, and in fuel injection techniques, but solved it was.
The only thing going on today is the same thing that's been going recently in a lot of fields from building architecture to aerodynamics: the replacement of empirical trial-and-error problem-solving methods with highly complex mathematically-driven computer simulation methods.
Indeed, this advancement of the state of the art will make rocket science easier, since it allows researchers to model different designs in much greater detail, without having to physically build them.
Parent
I've been a member of slashdot for some time (Score:5, Funny)
Vibrator?? (Score:2, Funny)
Re:Vibrator?? (Score:5, Funny)
Parent
Re: (Score:2)
What the gp asked and what the parent referred to. [uncyclopedia.org]
Re:Vibrator?? (Score:4, Funny)
>>of course. we've got lots of pictures. =D
Yeah, and I know all about womens emotions and stuff. Like, chicks HATE it when you call them broads.
Parent
I can finally be of use to science (Score:5, Funny)
Re:I can finally be of use to science (Score:5, Funny)
Oh great, Mr. Goatse himself is now posting on slashdot.
Parent
Re:I can finally be of use to science (Score:5, Funny)
Thats impossible. In order to get a good sound out of that it'd have to be a bit tighter and able to make a good PHHHHHHBBBBBTTT sound. The way Mr. Goatse is now it would just kind of go phooooo. Even then I doubt if there is enough of a seal left to keep it from leaking out long enough to build up sufficient volume and pressure to do anything noticable.
Parent
Re:I can finally be of use to science (Score:5, Funny)
Parent
What TFA leaves out (Score:3, Funny)
Re: (Score:2)
Defense System? (Score:4, Insightful)
Re:Defense System? (Score:5, Funny)
Moha deeb.... rocket go boom
Parent
Re:Defense System? (Score:5, Insightful)
No. We're talking about pressure waves inside the engine, at pressures measured in tens or hundreds of psi, that resonate with the injector to build power -- think about blowing across the top of a beer bottle. The small power input from your breath induces a higher power oscillation. Same effect, where the bottle is replaced by the combustion chamber and your breath by the injectors. Except the power involved is a hundred million times higher (maybe more, I didn't do the math very carefully).
These waves can't be set up unless the engine will support them, and if it will then they'll happen on their own. If you could deliver that much energy to the engine remotely, you could just as easily destroy the rocket. It's the *resonance* that's the problem, not the fact that there's a crapload of sound energy available.
Parent
Re:Defense System? (Score:5, Interesting)
The laser beam is way more feasible, even if you ignore considerations of range. Not to mention that when a liquid-fueled military rocket is operating, it's going to be either over the horizon or in vacuum -- we're not talking about small tactical missiles here.
We're talking about loud sounds here -- and not just a little bit loud. 1 pascal of pressure wave is 94dB SPL -- a fairly loud sound. 1 psi is 6894 pascals; we're talking about many psi of pressure variation. A 10 psi wave would be 190dB. That's not just loud enough to cause hearing damage; that's well past loud enough to knock over buildings. Overpressure from large bombs is less than that at the edge of the blast radius.
It should be obvious why that's destructive when it happens inside a rocket chamber, especially since oscillations like that tend to start small, grow *rapidly*, and not stop growing until something breaks. It should also be obvious why you won't be able to create such a wave via external influence unless the chamber can already resonate in that mode. When developing the F1 (Saturn V main engine) NASA had trouble with instability; in order to see whether the engine was barely stable or had plenty of margin, they had to find techniques to induce these waves. What they developed, and still use today, is a set of techniques for putting an explosive charge *inside* the engine, bringing the engine up to normal operating conditions (making the charge survive this is nontrivial), and *then* detonating it to see how the engine responded.
Parent
Re: (Score:3, Informative)
Sutton discusses that resonant mode (spinning tangential) briefly in Rocket Propulsion Elements (pg 353, 7th ed). There are plenty of more detailed discussions of combustion instability in general elsewhere in the literature. There are a variety of other resonant modes as well, this just happens to be the one the researchers looked at.
You can indeed add features to the chamber to de-tune it. It's anything but simple, though, and is mostly trial and error at present. Making the features not burn up is
If you play them backwards (Score:4, Funny)
Jimi at Berkeley (Score:2)
The Captain has known since 1986 that sound waves, particularly the very potent tones of Jimi at Berkeley, can destroy oncoming rockets.
Reference: Riders of the Storm [imdb.com]
And then... (Score:3, Funny)
Summary is a bit off (Score:5, Informative)
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.
Re:Summary is a bit off (Score:4, Interesting)
Or I could be full of crap, which is also a distinct possibility.
Parent
Re:Summary is a bit off (Score:5, Informative)
No, you're on the right track, but not quite there. Computational techniques are only barely able to simulate rocket chambers well; combustion dynamics are complex and not well understood. That's a large part of what makes this work interesting (the other part being the imaging techniques to actually photograph the waves).
The problem isn't actually the chamber or nozzle walls resonating, it's the acoustic cavity -- exactly analogous to an organ pipe. There are a variety of techniques used to de-tune the resonance modes. (It also happens in the chamber, not the nozzle -- gas in the expansion portion of the nozzle is locally supersonic, so sound can't propagate backwards, which means no resonance.) For example, the SSME has some of the injectors protruding further into the chamber than others, creating interruptions in the flat surface of the injector face. There exist other techniques, and some google searching will turn up some. Also, playing with the metals in the chamber wall is probably right out -- they're basically already decided by thermal considerations, and high performance engines almost universally use copper.
Historically, the design process has involved experienced engineers, rules of thumb, and lots of testing. Computer models will help, but they'll never really replace the "lots of testing" stage. At least for small engines (up to several thousand pounds of thrust), it's cheaper, easier, and more accurate to just build the thing.
Parent
Re: (Score:3, Informative)
Re:Summary is a bit off (Score:5, Informative)
Titanium may melt at 1900K, but rocket chambers operate in the realm of 2500-3500K. They have to be cooled, and copper is the material of choice for the same reason it makes good CPU heat sinks -- excellent thermal conductivity. Some older thrust chambers were made of steel (WAC Corporal, iirc), and it works at low chamber pressures (less heat flux), but it doesn't work as well and there are corrosion issues in storage. As performance increases and chamber pressures rise, metals other than copper look less and less appealing.
Some nozzles are uncooled in the aft portion (as the gas expands and accelerates, it cools down, so the environment gets easier to handle). The Kestrel engine used in the Falcon 1 upper stage, for example, has a radiatively cooled Niobium nozzle. Titanium has been used, but Niobium and a few others tend to perform better in that environment -- the combination of hot reactive gases is hard to handle.
Parent
This has got to be joke (Score:5, Funny)
Powerful sound waves? (Score:4, Funny)
Turn it inside out. (Score:5, Interesting)
Re:Turn it inside out. (Score:4, Interesting)
Parent
Re: (Score:3, Informative)
http://en.wikipedia.org/wiki/Image:Aerospikeprinciplediagram.gif [wikipedia.org]
In the linked illustration on the right, look along the top edge of the aerospike, where the flames are coming from. All of the little canisters along both edges (where the flames come
Re: (Score:3, Interesting)
Of course, this was long ago, and I was at the bottom level of the work, and only worked a very short time on that.
Pogo Oscillations (Score:4, Interesting)
http://en.wikipedia.org/wiki/Pogo_oscillations [wikipedia.org]
Re:Pogo Oscillations (Score:4, Informative)
Parent
Addicted to bass... (Score:3, Funny)
Sonic Tools (Score:3, Funny)
Combustion instability is an old problem (Score:4, Interesting)
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.
Coriolis Effect in Vortex Combustion (Score:5, Interesting)
Re: (Score:3, Insightful)
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 thes
Re:"Strange wave phenomenon" = resonance (Score:4, Informative)
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.
Parent
Somthing Wrong Here. was Re:Nothing new here (Score:5, Informative)
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.
Parent
Re: (Score:3, Insightful)
The wave argument is that the "rinse and repeat" frequency just happened to be the same as the resonant frequency [wikipedia.org] of the main bridge cables, this took the areodynamicly induced twisting motion and turned it into opposing waves of maximum amplitute along the two main cables. If yo
Re: (Score:3, Insightful)
Layne
Re: (Score:3, Interesting)
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 unti