Relationship between turning off cooling system and self destruct -- why was the cooling system involved?

Question

Ripley first turns off the cooling system and then turns it back on (when she decides she does not want the ship destroyed which I do not understand) and tells Mother this as she appeals to the computer to stop the self-destruct countdown.

If the cooling system being off is needed for self-destruct then her re-enabling the cooling system should stop the destruction; on the other hand, if the self-destruct proceeds even with the cooling system on, then why turn it off in the first place? To save energy?

Answer 1

The emergency destruction system aboard the USCSS Nostromo operated on a ten-minute delay, with an initial five-minute fail-safe window during which the process could be aborted. However, after these five minutes had elapsed, the meltdown of the ship's core was irreversible and could not be stopped.

https://avp.fandom.com/wiki/Emergency_destruction_system

Reading that, it takes 5 minutes for the coolant not operating to let the powerplant go into meltdown, and after 5 minutes, this is irreversible.

That is why the timer to cancel the self destruct is also 5 minutes - beyond that there is no stopping of what destruction comes with a powerplant meltdown.

....

In addition to what Valorum said (in the comments), it is generally accepted that anything generating power, generates heat, and needs cooling to not meltdown or explode. Space in itself can be very hot, or very cold, (Note the International Space Station has a very active cooling and heat dissipation system running) but in this context its probably that without cooling systems on, the ship is a very inhospitable place to work

Also, in the script, they make references to the life support requiring coolant to operate:

(transcript) Ripley:

We're gonna need coolant for
the air support system.

You two go down, get all
the coolant you can carry.

In light of the answer that Valorum linked to in the comments, it is more likely that the coolant was for power plant, and at 5 minutes to go, with coolant switched off, meltdown was irreversible.

For reference, some spaceships operated with explosive self-destruct devices at the time the movie was in production. This was separate to the FTS, range safety self destruct for launchers, which still operates today (Russians just turn off the engines for self-destruct during launch-phase, but the Soyuz operating from Kourou have FTS added).

https://scifi.stackexchange.com/a/268181/140676

Ripley first turns off the cooling system and then turns it back on (when she decides she does not want the ship destroyed which I do not understand)

Ripley decides she does not want the ship destroyed because originally she was going to leave in the escape craft, leaving the alien to get blown up along with the ship - but after setting the self-destruct and heading towards the escape craft, she found her route blocked by the alien, so she needed more time (than the self-destruct was going to give her) in order make a new plan to get to the escape craft.

Answer 2

The Nostramo is powered by a nuclear reactor

This is what happens when the cooling system of a nuclear reactor fails:

It is important to understand, that what you see here is not the aftermath of a nuclear explosion (there wouldn’t be anything to see after that) but the result of a steam explosion.

The basic process in nuclear power is there is a core undergoing a nuclear reaction (fission or fusion) which generates a huge amount of heat. That heat is transferred to a coolant (usually water) which becomes a superheated vapour and is taken somewhere else and used to do work, typically driving a turbine to generate electricity. The coolant then condenses and gets sent back to the reactor to do it again.

If you stop the coolant circulating, then you make more and more steam which, at some point, rips the pipes apart in a massive explosion. It doesn’t actually matter what the primary energy source was - coal-fired steam locomotives, gas-fired power stations can all fail in this way.

The self-destruct stops the coolant flowing

So, it starts to heat up. It will take 10 minutes of this before something, somewhere, ruptures with a big bang. Given that they can state the precise time, I guess there’s was a deliberate weak point with a known fail pressure built into the coolant system. Without that, when and where the failure will occur would be just a guess.

However, once the system has been off for 5 minutes, even if it is restarted, there is too much heat built up to be dissipated and the explosion is inevitable. Or, perhaps, the coolant is now so hot that it actually prevents the system from being restarted.

Answer 3

USCSS Nostromo is powered by a 271 MW Fusion Reactor (Tokamac Design) which occupies most of the aft space of the three decks. The cooling system operates using heat transfer from the Tokamac to external vents rearward of Engineering and uses liquid metal (Sodium?) for a medium.By shutting down the Cooling system, one creates a heat sink in the surrounding compartments leaving no place for the excessive heat to go. The Auto-Destruct sequence sets into motion the process of blast-venting the remaining coolant into space and throttling the Tokamac to maximum power setting to facilitate an overload and containment breach of the superheated energy plasma within the reactor core.