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Science Technology

Mysterious Sound Waves Can Destroy Rockets 193

Posted by Soulskill
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.'"
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Mysterious Sound Waves Can Destroy Rockets

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  • Weapon (Score:2, Interesting)

    by MrGHemp (189288) on Thursday April 10, 2008 @11:42PM (#23032612) Homepage
    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.
  • by Nefarious Wheel (628136) on Thursday April 10, 2008 @11:55PM (#23032680) Journal
    A problem with dealing with acoustic resonance is just how to manufacture the rocket nozzle to avoid the buildup of these dangerous resonances. Modelling them is the first step, but how can you build the nozzle with sufficient strength while building in structure to interrupt the phase of the wave repeats? Experimentation is a lot cheaper when you can simulate the results. I could imagine moving to non-round shapes might solve the problem, or heterogenious structures - possibly by introduction of dissimilar materials in the bell, perhaps a strapped interspersion of titanium and stainless? I wonder how difficult that could be to model.

    Or I could be full of crap, which is also a distinct possibility.

  • Turn it inside out. (Score:5, Interesting)

    by camperdave (969942) on Friday April 11, 2008 @12:08AM (#23032748) Journal
    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 [google.com] suffers from the same problem.
  • Pogo Oscillations (Score:4, Interesting)

    by orospakr (715849) on Friday April 11, 2008 @12:19AM (#23032808) Homepage
    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 [wikipedia.org]
  • Re:Defense System? (Score:5, Interesting)

    by evanbd (210358) on Friday April 11, 2008 @12:50AM (#23032918)

    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.

  • by evanbd (210358) on Friday April 11, 2008 @01:06AM (#23032980)
    Aerospike engines still have an enclosed chamber; it's only the nozzle that's been changed. The chamber is where the problems occur, not the nozzle. The odd chamber shapes certainly make things complicated, but I have no idea whether they hurt or help overall. The usual technique to get rid of these involves various ways to de-tune the engine -- for example, some of the SSME injectors protrude deeper into the chamber to interrupt the otherwise flat injector face.
  • by Animats (122034) on Friday April 11, 2008 @02:07AM (#23033238) Homepage

    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.

  • by FlyingGuy (989135) <<flyingguy> <at> <gmail.com>> on Friday April 11, 2008 @02:46AM (#23033422)

    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:

    1. A source
    2. A medium to transmit is through
    3. A detector

    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,

  • by Baldrson (78598) * on Friday April 11, 2008 @04:06AM (#23033714) Homepage Journal
    This racetrack instability is actually a well known problem with annular combustion chambers such as those used with the toroidal aerospike engine [wikipedia.org]. One of the main virtues of vortex engines, like Orbital Technologies [california...hority.org] or the ultracentrifugal one [geocities.com] invented by Roger Gregory [wikipedia.org] and myself, is that the coriolis effect [wikipedia.org] 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.
  • by MickLinux (579158) on Friday April 11, 2008 @07:12AM (#23034486) Journal
    I seem to remember back in 1987/8, working at VA Tech on a project where they were considering scalloped semicircles all around the edge of the cross section. It seems to me that the purpose was to get a more even burn (read, reduce some of the acoustic/shock wave artifacts of the combustion).

    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.

  • Re:Harmonics (Score:4, Interesting)

    by IndustrialComplex (975015) on Friday April 11, 2008 @10:29AM (#23036120)
    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.

We warn the reader in advance that the proof presented here depends on a clever but highly unmotivated trick. -- Howard Anton, "Elementary Linear Algebra"

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