Mysterious Sound Waves Can Destroy Rockets 193
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.'"
Weapon (Score:2, Interesting)
Re:Summary is a bit off (Score:4, Interesting)
Or I could be full of crap, which is also a distinct possibility.
Turn it inside out. (Score:5, Interesting)
Pogo Oscillations (Score:4, Interesting)
http://en.wikipedia.org/wiki/Pogo_oscillations [wikipedia.org]
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.
Re:Turn it inside out. (Score:4, Interesting)
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.
Re:What does sound have to do with it? (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 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,
Coriolis Effect in Vortex Combustion (Score:5, Interesting)
Re:Turn it inside out. (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.
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.