SpaceX Texas launch site Discussion and Updates – Thread 10 – NASASpaceflight.com

This is why the water is sprayed into the air as it needs to be in the air to be able to move from the sound traveling through the air. Once it's on the ground it's not really doing anything.

Water deluge is needed near the ground because the sound reflects off surfaces and back on to the rocket effectively creating a variety of different length resonance paths for standing waves to form (different length means different frequency). So you spray water between the rocket exhaust and any non-rocket surfaces to absorb the energy. Edit: This paragraph has a bit of speculation on the idea that standing waves are formed, and it seems to me this is a good hypothesis, but I couldn't find any sources talking about it.

One talks about the Shuttle systems, and it's not so much about resonance and standing waves (which I thought was plausible too), but sheer reflection of energy onto the craft and the structures holding it up. Plus dealing with some hot exhaust, if anybody cared. Apparently it's largely about the compression of bubbles within droplets, as associated with sonar detection management:

https://interestingengineering.com/nasa-sound-suppression-system-prevents-rocket-from-exploding

It's a bit clumsy in referencing both bubbles and molecules at the end. Another neat source of info is from a provider of software for modelling fluid flows:

https://www.aft.com/blog/entry/2017/12/10/shhhh-suppressing-sound-waves

with two neat case-studies referenced therein, albeit more about water-delivery to strict flow-rate requirements, less about the physics of sound suppression, but hey, enjoy . One is a 'hypothetical launch scenario', with a phenomenal number of rainbirds and complex valve timing, the other is a variation on a real KSC pad design. Reflecting others in this thread, the second study there also notes the potential use of an additional N2-pressurised water supply system, running at 200psi (~13.8 bar) to augment a pump/gravity-fed deluge. So, quite possible.

A(nother) surprise to me - permanently mounted enclosed water bags as sound baffles... https://www.vinyltechnology.com/Portfolio/NASA-sound-suppression-system.aspx

For those wanting a bit more physics:

A much geekier paper is "Sixty years of launch vehicle acoustics" https://asa.scitation.org/doi/pdf/10.1121/2.0000704 - that's the Acoustical Society of America. And turns out these guys at NASA did some work on it too 🙂 https://www.nas.nasa.gov/assets/pdf/ams/2014/AMS_20140408_Panda.pdf

And for a bit of additional academic gravitas, the NASA slides referenced some papers by Dr Fukuda et al at JAXA, and this one looks like a great read: "Examination of Sound Suppression by Water Injection at Lift-off of Launch Vehicles"https://arc.aiaa.org/doi/abs/10.2514/6.2011-2814

Turns out, there are multiple effects between noise and water, with different efficiencies at different frequencies. Absorption by droplets for the (main) win, in this situation. I can't access the full paper right now, but will see if I can get to it later.

There'll be a quiz for you on all this in the morning

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SpaceX Texas launch site Discussion and Updates - Thread 10 - NASASpaceflight.com