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Over at FYFD is an interesting note on what those billowing clouds from rocket launches actually are :
If you've ever watched a rocket launch, you've probably noticed the billowing clouds around the launch pad during lift-off. What you're seeing is not actually the rocket's exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.
To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there's enough water to significantly affect the temperature and size of the rocket's jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier.
There's also more detail at Interesting Engineering and NASA.
[Ed Note: Due to the article being only 2 paragraphs, it has been reproduced here in its entirety.]
[Update: corrected broken links.]
(Score: 3, Interesting) by richtopia on Tuesday August 22 2017, @12:24PM (1 child)
I'm curious if everyone does this, or if there are alternative solutions to this sound suppression. The system looks to be integrated into the Mobile Launcher Platform. Perhaps it is only required on certain sizes of launches.
https://en.wikipedia.org/wiki/Mobile_Launcher_Platform [wikipedia.org]
(Score: 1) by khallow on Wednesday August 23 2017, @03:00AM
Size of launch vehicle is important. Small vehicles don't generate enough noise to cause this problem. Googling around I see that the Ariane lineage uses [nasaspaceflight.com] the same sort of water-based sound suppression system. I have heard of two other approaches and vehicles that exploit those approaches. First, I'm speaking of vertical launch vehicles that launch from ground level not rockets launched from airplanes or airports ("horizontal launch") which simply don't have this problem.
The two approaches are launch from higher up so that sound waves are diminished before hitting the launch structure (the MX missile used this approach with a steam catapult that would eject the missile prior to ignition of the rocket motors) and sea launch with partial or complete immersion in sea water (most sub launched missiles are launched from under water) which has the same sound absorbing properties as a water-based suppression system.