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(How) would the water deluge system impact Starship superheavy captures, when they eventually manage to return to starbase?
Could it be turned on during landing attempts? Would that reduce lift (reduced ground effect perhaps) at the last second? Would it simply be turned off?
Is there anything we already know about this, or can we only speculate at this moment?
We don't know anything specific yet, but we can surely speculate.
First off, the landing thrust will be much lower than the launch thrust, as instead of whole stack filled to the brim with fuel, we'll have the booster and the starship landing separately and almost empty. Starship was landing just fine on its 3 atmospheric engines on concrete, the empty superheavy likely of similar mass (while it's bigger and heavier on launch, more of its mass is in fuel), and so the only worry might be some damage to the deluge system plates (which, without the water protecting them, are somewhat more fragile than a flat concrete landing pad) - plus potentially secondary damage to the booster/starship with torn off plates and debris if it goes very badly.
As result, I believe to offset this the deluge system will be activated but at much lower pressure/flow than during launch - much less heat and force to dissipate, no need for that much water, and reducing risk of problems of pumping the full blast of water into throttled and inactive engines, which could damage them.
At this point though, it's too early to say anything for sure - we'll certainly know more after the first successful capture of SuperHeavy.
In addition to the arguments in the other answers, there is an additional one: the landing will happen to the side of the OLM, not on top of it, so there is no water deluge system there. Since there is no water deluge system, it won't have an impact.
There should be little problem with catching Superheavy from a deluge perspective.
During landing Superheavy will only have three engines burning and these will be throttled to zero very shortly after its arrival. Perhaps more importantly the chop sticks used to catch Superheavy can be raised high enough to load Starship on top of Superheavy. If the chop sticks are raised to this level when catching Superheavy the base of Superheavy will be 40m or so above the deluge system so the exhaust plume will be considerably attenuated.
Time will tell if there is any need to operate the deluge system at all during landing.
