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.'"

Comments

Summary is a bit off

[evanbd]

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

[evanbd]

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.

Re:Summary is a bit off

[jake-in-a-box]

I think you answered your own question. Copper is the best conductor of the materials you mentioned. The engines are cooled by circulating the fuel around them prior to its introduction into the combustion chamber. Heat conduction is analogous to electrical conduction, so copper is probably the best combination of inherent strength and heat conduction available. Still probably alloyed because pure copper is soft.

Re:Summary is a bit off

[evanbd]

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.

"Strange wave phenomenon" = resonance

[Anonymous Coward]

The summary makes it sound like this is a new and mysterious phenomenon. It isn't.

Resonant frequencies inside the fuel pumps and associated plumbing is one of the major problems of getting a real rocket engine run in a stable cycle. This is a 50-year-old problem. There are also 50-year-old solutions, mostly involving re-shaping the plumbing so that resonances are dampened.

See also Pogo oscillation and the famous case of Apollo 13.

Re:"Strange wave phenomenon" = resonance

[evanbd]

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.

Re:Didn't Nikola Tesla talk about something like t

[GuNgA-DiN]

From the New York World-Telegram -- July 11, 1935:
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."

Re:Pogo Oscillations

[evanbd]

Similar, but different. Both are oscillations, but pogo is characterized by low frequency variations in chamber pressure coupling through the thrust structure and into the propellant feed system (and from there back to the chamber pressure). These are high frequency (kHz, no tens of Hz) acoustic modes, contained entirely within the chamber. They're much harder simulate and much harder to get rid of, and much less well understood. They couple from local chamber pressure to the injectors, and operate much like an organ pipe.

Somthing Wrong Here. was Re:Nothing new here

[FlyingGuy]

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.

Re:Turn it inside out.

[icebrain]

It does depict them, you just aren't looking hard enough. On a traditonal rocket engine, the chamber is a bulbed or cylindrical chamber above the nozzle. It narrows down, then expands into a bell shape to allow the combusted hot gasses to expand and accelerate.

http://en.wikipedia.org/wiki/Image:Aerospikeprinciplediagram.gif

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 out) are combustion chambers. You just have a bunch of small ones instead of one large one. Compare that to the image on the left (a standard bell nozzle)--notice that you have a chamber at the top, which narrows down to a throat, and then opens back up. Basically, an aerospike does is cut off at the throat and turn the nozzle inside out. You then have an inner wall to expand against, with the outside atmosphere providing the outer wal.

Read this site for more: http://www.aerospaceweb.org/design/aerospike/main.shtml

Re:Combustion instability is an old problem

[randyjparker]

I worked on the SR-71 engine (the J-58, aka JT-11D) at Pratt & Whitney. The inlet unstart problem was not related to combustion instability, but to the difficulty of sealing the inlet spike shock to the nacelle lip as atmospheric conditions changed. You run into meteorological changes quickly when you're flying faster than a .50 caliber bullet. When the inlet shock did not meet the lip, some of the pressure behind the shock would "spill" out of the compressor and cause an engine flame-out. http://mobiledyne.com/about/j58

Re:Defense System?

[evanbd]

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 tricky; that part of why they're often added as part of the injector, since the injectors are cooled by the propellant flowing through them.