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Radiators are used for thermoregulation of spacecraft. https://www.academia.edu/934756/Thermal_Control_Handbook . Their effectiveness requires their surface be exposed to deep space, not an IR radiating body (like the Earth) or the sun. Closed loop fluid circulation is sometimes used to transport heat from inside the spacecraft to the radiator.

In LEO, it is difficult to shade radiators from both the sun and the Earth

The inside of a vacuum nozzle would seem to be an ideal radiator. It already has fluid passages. The surrounding spaceship skin shades it from one hemisphere. A final few seconds of fuel rich-rich burn would likely coat it with high-e coke.

Are nozzles suitable as radiators?

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  • $\begingroup$ What is a "high-e coke" coating? $\endgroup$
    – Ng Ph
    Oct 29, 2021 at 20:06
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    $\begingroup$ Coke is black, like soot. It likely has a higher emissivity than coke-free metal. I can't find data on IR emissivity of coke. $\endgroup$
    – Woody
    Oct 29, 2021 at 23:20

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It's impossible to say such a thing just can't be done, but:

  • It would require designing the cooling channels to function with a different heat transfer fluid, and for the system to operate without radiators for as long as the engines are needed for propulsion. Things get even more complicated if it has to actually switch back and forth between modes, purging coolant before starting engine chilldown, etc.
  • It would need the emissivity of the sooting and coking deposits to be very predictable and consistent, or for the system to tolerate a lot of variation.
  • It ties thermal control and propulsion together in a way that makes them difficult to test separately, and complicates design changes.
  • It would require additional plumbing and valves. If the nozzle doesn't have enough effective radiating area to do the job alone, you'll need dedicated radiators, and this plumbing may mass more than just making those radiators bigger.
  • It would involve adding potential failure modes, like high-pressure fuel being injected into the cooling loop or coolant getting into the nozzle early and freezing there, or just leaks forming or the valves failing to operate.
  • Big regeneratively cooled nozzles are generally located on upper stages that deploy the payloads and go their separate ways. Or on a multi-mission vehicle like Starship, they're at the opposite end of the vehicle from the bits that might need cooling.

In all, it seems unlikely to be a useful approach.

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  • $\begingroup$ Liquid methane is circulated through the nozzle wall channels as coolant before it is burned in the methane preburner of the Raptor engine. If methane was used as the heat transfer fluid, it would solve the purging and contamination issues, as well as simplify plumbing design. Good point about it being at the wrong end for cooling payload. But it is at the right end for cooling propellant to prevent boil-off on longer missions. Failure modes are a worry since they would involve propulsion as well as thermal control. $\endgroup$
    – Woody
    Oct 29, 2021 at 23:34
  • $\begingroup$ The LOX header is in the nose, actually cooling it will be a problem with methane's freezing point and the long insulated pipe runs, and that's not really very much area for a radiator operating at cryogenic temperatures. The header tanks (and coolant lines) would have to be very thermally isolated from their surroundings, and that would require the LOX header to be pointed at the sun. And in the specific example of Starship, the Raptor engines are very tightly integrated...I doubt you could isolate the cooling channels without harming their performance as rocket engines. $\endgroup$ Oct 30, 2021 at 1:16
  • $\begingroup$ The layout of the header tanks in Starship is an artifact of center-of-gravity manipulation for a prototype sub-orbital spacecraft with no payload. A “working” spacecraft with payload mass would likely have propellants at the same end as the engines. Cooling a LOX tank in the nose is only a problem if the LOX tank is in the nose… an unlikely design in anything other than a sub-orbital prototype. $\endgroup$
    – Woody
    Oct 30, 2021 at 7:07
  • $\begingroup$ The Methane/Oxygen propellant promoted by SpaceX is likely to become the propellant currency of the near future. Prolonged storage of the propellants in structural proximity (as in shared bulkheads) requires them to be at nearly the same temperature. The triple point of Methane and the vapor pressure of LOX confine the temperature of propellants to -183C to-170C, if they are both at the same temperature. Cooling of one propellant could be adequate to provide stable temperature for both. $\endgroup$
    – Woody
    Oct 30, 2021 at 7:07
  • $\begingroup$ Cryogenic propellants, which are significantly below “ambient” temperatures in the inner solar system, require refrigeration to prevent boil-off in storage. Maintaining these temperatures requires radiators with sky access to deep space. $\endgroup$
    – Woody
    Oct 30, 2021 at 7:07

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