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What exactly is the coolant used in the Soyuz external thermal loop, the one that just leaked out of a Soyuz docked to the ISS?

I checked the Soyuz Crew Operations Manual but it just calls it the "agent". Other searches turned up conflicting or obviously wrong info.

(system schematic from the manual - arrow indicates the radiator)

enter image description here

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    $\begingroup$ @blobbymcblobby - I'm sure your deleted answer is correct, it's isooctane: nplus1.ru/material/2022/12/19/cto-proteklo-v-soyuze The site nplus1.ru is indepedent and rather reliable pop-sci source, with qualified authors and editors. I read it regularly. And their review of the leak is the best I could find currently in Russian or English. $\endgroup$
    – Heopps
    Commented Dec 20, 2022 at 8:24
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    $\begingroup$ @Heopps thanks, that was a good article. Hopefully blobby will undelete their answer. $\endgroup$ Commented Dec 20, 2022 at 13:24
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    $\begingroup$ @Heopps and Organic Marble, thank you for undeleting the answer and for endorsing the article - I was away and unable to do any more researching. The article was a great read yet I just wanted to see a little confirmation elsewhere. Definitely a go to site in the future, thanks! $\endgroup$ Commented Dec 21, 2022 at 1:39

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Partial as I have insufficient time to check:

TL;DR:

External radiator that was punctured is said to have leaked isooctane according to this article.

https://nplus1.ru/material/2022/12/19/cto-proteklo-v-soyuze

The impact of a hitherto unknown object hit the external radiator of the Soyuz's thermoregulation system.

The liquid that flew from the Soyuz is isooctane, which is used on Earth as a standard for assessing the quality of motor gasoline, since its octane number is 100. On a Russian ship, 33 liters of isooctane play the role of a coolant in one of the two circuits of the thermal control system - hinged radiator circuit, which collects excess heat from the on-board computer and other equipment in the instrument-aggregate compartment.

enter image description here

Having lost the coolant, the entire circuit of external radiators cannot work - so that excess heat from the equipment remains in the instrument-aggregate compartment. Even worse, this disrupted the work of the Soyuz's second thermoregulation circuit - the circuit of the living compartments.

It, like the first one, is filled with a coolant - 21 liters of “triol” liquid circulates in it - this is a non-toxic aqueous solution of glycerin with anti-corrosion additives that does not freeze up to 18 degrees below zero (but the system still provides for the possibility of electric heating of the liquid just in case) .

This circuit maintains the temperature in the descent vehicle and the utility compartment of the ship in the range from 18 to 25 degrees, and also provides thermal conditions for the approaching and orientation engines in the instrument-aggregate compartment. To collect heat, each of them has a refrigeration-drying unit. In fact, these are ordinary air conditioners: fans, radiator, condenser. The air gives off heat and excess moisture to the thermoregulation system and returns to the compartment.

enter image description here

All the collected heat is transferred from the residential compartment circuit to the external radiator circuit through a heat exchanger. The liquids in it do not mix and do not contact in any way, heat is transferred through the walls of the pipes - from triol to isooctane.

The contour of the living compartments was probably not damaged. But it has become useless, because it can only give off heat to the external radiator circuit, in which there is no more coolant, and there is no one to receive this heat.

All the collected heat is transferred from the residential compartment circuit to the external radiator circuit through a heat exchanger. The liquids in it do not mix and do not contact in any way, heat is transferred through the walls of the pipes - from triol to isooctane.

https://twitter.com/katlinegrey/status/1603319799903551488

what heat transfer agent the external cooling system of Soyuz contains. People on NK forum say, it is Isooctan LZ-TK-2 (mostly). Here’s an article about it (in Russian):

https://cyberleninka.ru/article/n/teplonositeli-dlya-teplovyh-trub-i-naruzhnyh-gidravlicheskih-konturov-sistem-termoregulirovaniya-avtomaticheskih-i-pilotiruemyh/viewer

(link in top right for pdf version)

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    $\begingroup$ Is the answer "isooctane"? $\endgroup$ Commented Dec 20, 2022 at 2:55
  • $\begingroup$ at the moment, yes - but its based on twitter and forums and those articles... Nothing else to hand atm.. Due to my uncertainty I will delete this for now until I see something else $\endgroup$ Commented Dec 20, 2022 at 3:06
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    $\begingroup$ The pdf at your last link has a lot of good information on LZ-TK-2 coolant, aka isooctane. $\endgroup$ Commented Dec 21, 2022 at 0:31

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