19
$\begingroup$

Rocket launches are notoriously loud. So much so that measures are taken to prevent the noise/vibrations from damaging the rocket, the pad, or basically anything else within a few hundred meters.

How much actual energy is carried away in that noise? (for the sake of specificity, in a Falcon 9 launch, but I'd also be interested in other rockets). How much extra delta-V would be achieved if that energy could be used for propulsion? Is there any way, even in theory, to do that?

$\endgroup$
1
  • 1
    $\begingroup$ Unless I'm missing something, 100% of the noise quickly becomes heat. So it should be possible to apply the laws of thermodynamics to see what's possible and what's not. $\endgroup$ – Eric Duminil Sep 5 '20 at 9:42
21
$\begingroup$

The maximum percentage wasted to noise is approximately 1%.

The acoustic efficiency, defined as the ratio of the sound power to the rocket exhaust's mechanical power, for the majority of these data range between 0.2 and 1 percent...with 0.5% as the most probable value.

enter image description here

NASA SP-8072 ACOUSTIC LOADS GENERATED BY THE PROPULSION SYSTEM

Using the equation for power given in this answer

$$P={g I_{sp}T\over 2}$$

If that 1% power loss could be utilized instead of thrown away, it would be a directly proportional gain to the product of specific impulse and thrust.

But as explained in this answer the noise is a direct result of the aerodynamics of the plume. There is no way apparent to me that would convert these losses into useful power. There are techniques to mitigate the noise (page 8 in link) but they generally work by disrupting the noise sources, not by returning the power to the engine.

$\endgroup$
2
  • 2
    $\begingroup$ This is very cool and good to know! I think there may be other questions on the site that this might also answer, but I'm not sure. Luckily the question doesn't ask about the acoustic power of rockets underwater! How much power and energy is (actually) in a 230 dB “click” from a whale? $\endgroup$ – uhoh Sep 4 '20 at 17:41
  • 3
    $\begingroup$ @uhoh Underwater acoustic power is one of the things I've often worried about in regards to the Sea Dragon concept, which I'm otherwise enamored with. I can't imagine it would be good for sea life for dozens of miles. $\endgroup$ – Erin Anne Sep 4 '20 at 23:59
8
$\begingroup$

Of the the 0.2% to 1% of energy that is generated as sound, most of the sound is generated well after the exhaust gases have fulfilled their purpose of thrust:

https://en.m.wikipedia.org/wiki/Jet_noise

As a crude analogy, imagine being on roller skates, then taking a bowling ball and throwing it. You will roll backwards on the skates. This is thrust.

Next, a moment later, the bowling ball slams into the ground, generating lots of sound waves. Those sound waves won’t have taken away from the thrust generated earlier.

Similarly, exhaust gases slamming into the atmospheric gases will generate lots of sound, albeit by different mechanisms (outlined in the jet noise Wikipedia article linked above).

Now I do know that there are also tremendous sound waves that are generated inside the bodies of rockets that can damage the rocket and cause it to explode, and they must be mitigated (using fins, baffling, etc.). These internal sound waves are waste energy, but they are a relatively small fraction of the total sound generated.

NASA's launch pad sound suppression system uses water jets to convert sound energy into heat energy:

https://craffic.co.in/why-doesnt-the-sound-of-rocket-launch-kill-you/

“This sound energy is sufficient to kill a person or damage the surroundings. Thus NASA’s launch pads consists of large water tanks. As the rocket is launched these water tanks dumps one million lbs or water onto the  launch pad in just forty seconds. When the sound waves meet water, the bubbles of air absorb them. Further these bubbles contract and heat up, hence converting the sound energy into heat energy.”

EDIT Geez this question is extremely complex. Sorry I didn’t save links to these sources.

  • one source mentions that it’s not possible to distinguish chaotic “turbulent flow” from “acoustics”. Instead, they give up and instead they just measure sound on the outside of the jet flow.
  • be wary of different measurement units sound dbA, dbW, or the familiar db.
  • SPL vs SL
  • The only example I could piece together is that Saturn V consumed up to 200 gigawatts of thermal power. Another source mentions that Saturn V rocket measured 204 dB, which is a supersonic shock wave above the 195dB “sound” limit. More on this later, but this calculation works out to a (temporary) 2 gigawatts of acoustic power, presumably during a detonation event. In other words, 2 GW / 200GW = 1% of the power.
  • conflagration vs detonation
$\endgroup$
6
  • $\begingroup$ Thanks for the nice words, but since you refer to my answer a couple of times, I should let future readers know that I don't agree with your interpretation of it. The numbers quoted in my answer refer to noise generated by the exhaust plume, not anything internal. $\endgroup$ – Organic Marble Sep 5 '20 at 2:18
  • $\begingroup$ As far as I can tell, even the most efficient loudspeakers are only about 1% efficient at producing sound waves. So my original hunch that more than 1% of rocket energy produces sound waves appears to be wrong. I’ve edited my answer to reflect this. $\endgroup$ – Keith Knauber Sep 6 '20 at 0:18
  • $\begingroup$ I believe the bowling ball analogy is at least lacking, if not flawed. Noise is already generated while you accelerate the bowling ball (by turbulent flow). $\endgroup$ – Jens Sep 6 '20 at 7:53
  • $\begingroup$ I took the liberty of editing my username out of this answer since the edit made it even worse. $\endgroup$ – Organic Marble Sep 6 '20 at 12:01
  • $\begingroup$ @uhoh here is a paper which discusses sound generation from high velocity gas shear layer itself. math.fsu.edu/~hju/cht2.pdf. Here is the wikipedia article discussing jet noise en.m.wikipedia.org/wiki/Jet_noise $\endgroup$ – Keith Knauber Sep 6 '20 at 21:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.