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A flame trench is a massive expensive thing that needs constant repair. After each launch, concrete and steel suffer cracks, chips, melting etc., and require expensive, time consuming repair.

Would it be feasible to instead use a huge pool of seawater? The idea is for it to hold enough that the water itself, and not the walls of the container absorb and carry away the energy of launch. It would be sized so that it would not be blown dry, there would always be water between the concrete and the rocket blast. Yes it would still be big and expensive to build, but the idea is to avoid the maintenance. After each launch, you simply open a valve and refill it.

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    $\begingroup$ Reliability experts for rockets would not like the idea of so much salty seawater just below the rocket. $\endgroup$ – Uwe Nov 24 '20 at 0:17
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    $\begingroup$ How to transport the rocket to the launch pad if there is a huge pool of seawater below and around it? $\endgroup$ – Uwe Nov 24 '20 at 0:20
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    $\begingroup$ I would strongly urge you to refocus your question to fresh water instead of seawater. With seawater, you're going to get a trivial answer like this: With some care, you can easily build a rocket to withstand a little bit of nice cold sea spray for a couple of weeks. However, a cloud of hot steam carrying boiling brine droplets is the single worst thing imaginable when it comes to corrosion of anything made of any metal (most stainless steels included). You could just as well use a pool of sulphuric acid. $\endgroup$ – TooTea Nov 24 '20 at 8:21
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    $\begingroup$ Even with fresh water, I'm not sure I'd want my rocket surrounded by a cloud of superheated steam. All in all, I'll stick with a solid surface redirecting the exhaust flow $\endgroup$ – Carl Witthoft Nov 24 '20 at 12:24
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    $\begingroup$ now I want to ask the @TooTea suggestion, "could a pool of sulphuric acid be used instead of a flame trench" $\endgroup$ – tedder42 Nov 25 '20 at 23:03
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A flame trench is used to keep the exhaust of the rocket engines (the flames) away from the rocket just before lift off and during the short time when the rocket is still close to the launch pad.

If the rocket is launched from a flat horizontal surface without a trench, the hot gas is reflected by the surface and goes up to the rocket where it should not be.

If you replace the solid surface by a horizontal liquid surface you got no trench to keep the exhaust away. Exhaust still goes up to the rocket mixed with a lot of hot steam.

So you need a trench to guide the flames away from the rocket. To protect the trench against the hot flames you may flood the trench surfaces with a lot of water just like it is done in the last decades.

You may improve the cooling of the trench surfaces by using film cooling, but you need a lot of spray nozzles and pipes all over the surfaces to get a closed film of water of adequate thickness. Of course you need large water tanks and powerful pumps to generate and maintain the water film. But the flame trench should not be flooded with hot water, so you need an outflow too.

It may be cheaper to inspect and repair the flame trench after each launch.

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