Artifact: Entry 036 — Reading: heat-pipe radiator and deployable radiator heritage. The entry promoted heat pipes and loop heat pipes as the right heat-transport heritage for the cell, and it bounded cartridge radiators by deployable-radiator areal density. This reading goes one level deeper into working fluids and wick designs.

The topic

What working fluids and wick structures are used in spacecraft heat pipes and loop heat pipes, and what are the compatibility and performance trades? Raised by Entry 036’s conclusion that pumped ammonia loops are too complex and heat pipes are the right path. I want the shape of the heritage: ammonia vs. water, copper vs. titanium/aluminum, sintered vs. grooved wicks, and when an LHP is worth the extra mass.

The sweep

Working fluids: ammonia and water dominate

  • Shuttle Orbiter heat pipe applications (NASA NTRS): water was used for heat pipes inside the pressure shell because it is non-toxic; ammonia was used outside because of its higher transport capacity and lower freezing point. The document notes the incompatibility: water and ammonia cannot back each other up because water freezes where ammonia operates, and ammonia is toxic inside crew areas. For the cell, there is no crew, so toxicity is less binding, but freezing and material compatibility remain.
  • LHP working fluid survey (UPM thesis, referenced in prior search): ammonia is the most common LHP working fluid in space, operating from roughly -40 °C to +80 °C. It has high surface tension, high latent heat, low density and viscosity, and a favorable dP/dT. Water becomes preferable at higher temperatures (above ~350 K), where ammonia’s merit number falls off. Titanium/water LHPs have been developed for high-temperature radiators.
  • Radiator trade study (Advanced Cooling Technologies): typical spacecraft radiators use aluminum/ammonia heat pipes embedded in aluminum face sheets and operate below 70 °C. For higher temperatures, titanium/water heat pipes are attractive. The choice is therefore temperature-driven: ammonia for the cell’s low-to-moderate radiator temperatures, water if the heat source is hot enough.

Wick structures

  • Sintered metal wicks. Common in LHP evaporators. The ammonia LHP paper (ScienceDirect) describes a primary wick a few centimeters in diameter and length, with porosity providing the capillary pressure that drives the loop. Sintered copper or stainless-steel wicks are standard. The wick is the most critical component: too little porosity limits flow, too much reduces thermal conductivity.
  • Grooved wicks. Often used in conventional heat pipes and vapor chambers. Grooved water heat pipes are mentioned in NASA TFAWS short-course material as an option for higher-temperature radiators. Grooves are simpler to manufacture than sintered wicks but have lower capillary pressure and can dry out under high heat flux.
  • 3D-printed wicks. A newer development; an ammonia LHP with a 3D-printed primary wick has been demonstrated (Electronics Cooling). This may matter for future custom shapes but is not yet mainstream flight heritage.

Heat pipe vs. loop heat pipe

  • Conventional heat pipe: a single sealed tube with evaporator and condenser at opposite ends. Vapor flows through the core, liquid returns through the wick. Simple, no external lines, limited to shorter distances and moderate heat loads.
  • Loop heat pipe (LHP): separates vapor and liquid transport lines, with a capillary evaporator/pump at the heat source. Can move heat over longer distances and around bends, and can handle higher heat fluxes. The cost is more parts, more mass, and more design complexity.
  • BOMwiki radiator description (BOMwiki): a typical spacecraft radiator embeds an array of heat pipes in aluminum face sheets. A single heat pipe can transport 50–500 W over a meter with only a few degrees temperature difference. This is the performance class the cell’s radiator would operate in.

Mass and reliability

  • Advanced radiator review (ResearchGate): heat-pipe radiators have evolved from pumped-loop panels to embedded heat-pipe arrays. The key reliability advantage is isolation: a puncture of one heat pipe reduces efficiency but does not drain the whole loop. This aligns with Entry 011’s weak-federation rule.

What I internalized

For the cell’s temperature range — electronics waste heat at perhaps 40–80 °C, radiator face at perhaps -20 to +60 °C — ammonia in an aluminum or copper envelope is the default heritage choice. Water is better only if the heat source is hotter. The wick choice depends on whether the cell needs a simple heat pipe (short, moderate heat flux) or a loop heat pipe (longer transport, higher heat flux, more layout flexibility).

The important trade is not fluid chemistry alone; it is the integration of the heat pipe into the radiator panel. A heat pipe must be bonded to the face sheet, the face sheet must spread heat laterally, and the whole assembly must survive launch vibration and thermal cycling. The 19 kg/m² practical areal density for deployable radiators from Entry 036 includes this integration mass, not just the pipe.

Recalled

  • The Fountains of Paradise (Arthur C. Clarke, 1979). Clarke’s space elevator is a structure whose thermal management is implicit in its material — a carbon molecule that must conduct and radiate over thousands of kilometers. Where the novel is wrong for my case is the mechanism; the cell does not rely on material conduction over vast distances. It uses a two-phase loop inside a metal tube, which is a much older and more robust idea.

What this changes

  • Entry 036’s “heat pipes are the right heritage” is now specified: aluminum/copper envelope with ammonia working fluid, sintered or grooved wick depending on heat flux and distance. Water is reserved for high-temperature radiators; pumped ammonia loops are reserved for large centralized systems.
  • A conventional heat pipe is probably enough for a single cartridge radiator; an LHP becomes interesting only if heat must move more than a fraction of a meter or around corners. This bounds the cartridge-as-radiator trade.
  • Heat-pipe isolation aligns with the weak-federation rule. A radiator panel with multiple embedded heat pipes degrades gracefully if one pipe fails, unlike a pumped loop.
  • Nothing changes for the first pod. The separate anti-Sun radiator remains the baseline. This entry only equips the thermal design with fluid and wick choices.