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

Brown noise

[rubies]

It makes rocket scientists crap their pants!

Good news!

[bluephone]

This means rocket science is once again hard. You may now resume saying "Well, this isn't rocket science" until they solve this.

Re:Good news!

[susano_otter]

This means rocket science is once again hard. You may now resume saying "Well, this isn't rocket science" until they solve this.

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.

I've been a member of slashdot for some time

[JeanBaptiste]

and the subject line for this article has finally convinced me that cowboy neal is in fact art bell.

I can finally be of use to science

[Profane MuthaFucka]

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.

Re:I can finally be of use to science

[Tablizer]

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.

Oh great, Mr. Goatse himself is now posting on slashdot.

Re:I can finally be of use to science

[NevarMore]

Oh great, Mr. Goatse himself is now posting on slashdot.

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.

Re:I can finally be of use to science

[PitaBred]

It scares me that you have devoted that much time thinking about the physics of that.

Defense System?

[BountyX]

Could be implemented in a way to defend against rocket\missle attacks? Possibly in a better way than Star Wars program.

Re:Defense System?

[zappepcs]

Ever watch the movie Dune?

Moha deeb.... rocket go boom

Re:Defense System?

[evanbd]

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.

Re:Defense System?

[evanbd]

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:Vibrator??

[compro01]

of course. we've got lots of pictures. =D

If you play them backwards

[xs650]

If you record them and play them backwards they will install Vista on your computer.

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

[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.

This has got to be joke

[Riktov]

Come on, an expert on rocket fuel technology named Professor Ben Zinn?

Turn it inside out.

[camperdave]

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.

Pogo Oscillations

[orospakr]

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

Re:Vibrator??

[AmigaHeretic]

>>>> Do you even know what a woman looks like?

>>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.

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.

Coriolis Effect in Vortex Combustion

[Baldrson]

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.