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Now a days every rocket launch (except Russian or cliff hanging launch) happens using water acoustic suppression system.

How does the water based acoustic suppression works?

What is the water requirement per kg propellant burn?

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    $\begingroup$ @Hobbes I think now it is fine! :D $\endgroup$ – Amar Jul 28 '18 at 10:02
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The mechanism seems to be related to the bubbles in water rather than the water itself:

Bubbles are excellent at absorbing the sound. They absorb the acoustic energy and as a consequence of which get heated up. NASA exploited this and sprayed water molecules in the air surrounding the Mobile Launcher Platform. This reduced the sound from the firing of the rockets by almost a half! (3 dB)

Further reading in this study:

There are two primary types of water-based acoustic suppression system; a below-deck system where water is injected into the exhaust plume with the aim of reducing far-field noise by more rapid dispersion of the rocket exhaust [12] or an above-deck system where water is injected around the pad.

Norum [16] carried out an extensive investigation into the suppression of dominant noise sources in both subsonic and supersonic jets, and the results were able to be extrapolated to full-scale engines. This work showed that within the jet plume, water injection clearly disrupts the shock noise sources, leading to a reduction in SAN.

Water injection into the flame trench:

For the water injection, critical parameters were the injection angle, the injection water flow rate and the location of the water injection point. Their results indicated that to achieve any significant noise reduction, the quantity of water injected must be at least three times the jet flow rate.

and more reading.

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    $\begingroup$ The information in that first link's "underlying principle" is .... questionable. "Sprayed water molecules in the air..."?? Who would say that. $\endgroup$ – Organic Marble Jul 28 '18 at 20:08
  • $\begingroup$ I noticed the odd English in the first link, but I'd assumed it was written by a non-native speaker. $\endgroup$ – Hobbes Jul 29 '18 at 8:13
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This paper "Space Shuttle Noise Suppression Concepts for the Eastern Test Range" doesn't mention "tiny bubbles". Rather, it states the rationale for flooding the launch pad with water is that "Adding mass is a means of attempting to reduce the turbulent flow velocities in a constant momentum flow system" (This question Why are rocket launches so loud? tells us that rocket noise is due to turbulent eddies in the plume)

This paper is prior to the first shuttle launch, fixes had to be made due to inadequate performance. (Basically adding more water).

Here's a picture I took on the deck of the Mobile Launcher while it was in the VAB. The rainbirds are big!

enter image description here

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