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Comment by trevyn | Hacker News Reader

Wait, the water they spray on a land launch is only for sound suppression?

Yes. Without it large rockets like the space shuttle might create more than 200db at lift off, enough to kill someone or to damage the rocket

https://en.wikipedia.org/wiki/Sound_suppression_system

If rocket damage is a concern, why don’t such rockets destroy themselves right after leaving the launchpad? At that point the water suppression is no longer relevant but the sound waves are just as powerful. It can’t just be that the ground reflects sound waves, since that would only be a ≈3dB (2x) increase in sound energy. Rockets with >2x more thrust seem to use the same sound suppression, so rocket damage can’t be the main reason for installing such systems.

My guess is that water systems are needed to reduce damage to the launch pad.

You are proposing...a self destroying launch vehicle?

Would you ride it?

Larger rockets use water as part of the overall suppression system yes, but that doesn't make the whole system the same. The size, depth and shape of the flame pit, the rate of water delivery and the design of the spay system, etc will all make a difference.

The issue is that the sound waves bounce off the pad back towards the rocket. Once the rocket has left the pad, the sound waves proceed behind it and there's no risk of them hitting the body.

Mostly, yes. Some of it is used for protecting the structure (ablative armor, essentially). But that requires much less water than is actually used. This is why Sea Launch's launch platform also had a water deluge system. They're far enough out that the only people that can really hear it are the control ship, but they have the deluge to help keep the exhaust from scorching the launcher.

Similarly, the Saturn V MLP had "rainbirds" to wet it down, even though the vast majority of the water used was down the exhaust trench for sound suppression.

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