In this comment under an answer to Heating Perseverance components prior to deployment I wrote:

I think (but can not find a source yet) that there is even a loop of "hot water plumbing" that goes to the spacecraft itself, and those lines have to be disconnected before separation.

I'm pretty sure I've read that the heat exchange fluid loop on the rovers has another loop that warms the other spacecraft electronics and propulsion system during it's deep space trip to Mars. I even vaguely remember writing about it here but I can't find any trace of that now.

But I'm not sure if there was a physical heat exchanger without sharing of circulation, or if there was actual shared circulation and valves that shut permanently before separation.

And since there are several parts to the whole thing, I hesitate to specify which separation or what they might connect to. That will need to be part of the answer, if these exist.

From this answer to Where does MSL end and Curiosity begin?

Curiosity and Mars Science Laboratory

  • $\begingroup$ Might want to add "from the cruise stage" or something in the question title. I was rather confused until I read the rest of the question. $\endgroup$
    – Phiteros
    Commented Mar 7, 2021 at 1:09
  • $\begingroup$ @Phiteros Yes, my questions often need to be read before answering, this is true. But YIKES! the title is terribly confusing! I'll see what I can do. update: I've made a change, but please feel free to edit further, we wouldn't want another tennis racket scandal. :-) $\endgroup$
    – uhoh
    Commented Mar 7, 2021 at 2:10

2 Answers 2


There was a fluid loop that transferred heat from the RTG to cruise stage components, as shown in these diagrams.

enter image description here

enter image description here

Source: Thermal Control of MSL Rover 'Curiousity" Using an Active Fluid Loop

The separation of the thermal control system is described in this paper thusly:

The first major thermal event triggered by the EDL sequence was venting of the working fluid (Freon) from the cruise stage HRS. Before the CS could be separated away from the entry vehicle, the Freon needed to be evacuated from the CHRS in a controlled manner. Freon was vented through a valve at the top of the CS along a vector through the CS center of mass, so as to impart only a minimal nutation to the spinning vehicle. Venting of the Freon from the CHRS meant that all items on the CS, DS and Rover, with power dissipations that were thermally regulated by the CHRS in the cruise phase, were now free to transiently increase in temperature. The degree to which these items increased in temperature was limited by their power dissipations, local thermal environments and thermal masses. As shown in Figure 7, the CHRS controlled the Cruise Power & Analog Modules (CPAMs), Cruise Power Assembly (CPA) and Star Scanner Electronics (SSE) on the CS Avionics plate, the Descent Power & Analog Modules (DPAMs) & Descent Power Assembly (DPA) on the DS Avionics plate, the Traveling Wave Tube Amplifier (TWTA) & Small Deep Space Transponder (SDST) on the DS Telecom plate and the MMRTG on the Rover. There was also a link between the CHRS and RHRS at the Rover cold plates. However, this link was not really critical during the near-Mars, cruise cold condition since the Rover rejected most of its own heat through the Rover heat exchanger cold plates directly to the backshell. The CHRS system was vented at 04:57 UTC SCET on August 6, 2012. After CHRS venting, the electrical cables and HRS tubes across the CS-to-DS interface were severed using pyro-activated cable cutters.

(emphasis mine)

The referenced Figure 7

enter image description here


  • CHRS - Cruise HRS
  • CS - Cruise Stage
  • DS - Descent Stage
  • EDL - Entry, Descent and Landing
  • HRS - Heat Rejection System
  • MMRTG - Multi-Mission RTG
  • RAMP - Rover Avionics Mounting Panel
  • RTG - Radioisotope Thermoelectric Generator
  • SCET - Spacecraft Event Time

The linked paper is informative, and has a good bibliography too.

  • 1
    $\begingroup$ These are good, thanks. But I've seen a more detailed, complete and more complicated drawing somewhere years ago. I'll continue to look and ping again if I find anything. In the first drawing the "cruise HRS" black line passes over first the cold plate and then the MMRTG, and I don't see that kind of thing in the other drawings. I think the actual system is more complicated than what's shown here, and I still don't know yet but suspect that physical sections of the cruise HRS loop remain on the rover and so these fluid lines needed to break before separation, necessitating the venting. $\endgroup$
    – uhoh
    Commented Mar 7, 2021 at 3:31
  • 1
    $\begingroup$ In other words, it's not just the proximity of hot plates and cold plates, mounted on separate vehicles, pipes had to be cut or severed or disconnected somehow if my memory is correct. $\endgroup$
    – uhoh
    Commented Mar 7, 2021 at 3:38
  • 1
    $\begingroup$ @uhoh the highlighted line in the quote says the lines were severed by pyros. $\endgroup$ Commented Mar 7, 2021 at 3:54

See lower centre of pic, cylindrical thing is umbilical cutter, next to that is grey blocky thing being the point of attachment of support cable with cutter in block, next to that is a bunch of severed cables and pipes/tubes:

enter image description here

enter image description here original, click for larger

  • 3
    $\begingroup$ Sometime in October 2020 Nasa/JPL turned on the onboard microphone mid flight, the main whirring sound is the heat exchange pump circulating the fluid from the MMRTG $\endgroup$ Commented Mar 7, 2021 at 7:19
  • $\begingroup$ Cool pic! Is there anything to prove this is HRS related? $\endgroup$ Commented Mar 7, 2021 at 12:58
  • $\begingroup$ No proof, all conjecture on my part, but 3 severed tubes/pipes are visible and I cant imagine any other use for them but in distributing Trichlorofluoromethane to and from the MMRTG to the SC and other stuff during the cruise phase $\endgroup$ Commented Mar 7, 2021 at 21:56
  • $\begingroup$ I have very little pre-learned knowledge on this stuff, I just look at the amazing selfies from Perseverance and try to figure out what is what and why. It's a really enjoyable learning curve thanks, in part, to the knowledgeable space nerd types on here. $\endgroup$ Commented Mar 7, 2021 at 22:05
  • $\begingroup$ I think you are probably right! Was just hoping for a label. $\endgroup$ Commented Mar 7, 2021 at 22:06

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