Working Fluid
Thermal Transport Working Fluid Inventory
The thermal transport working fluid inventory is the single-phase liquid, typically anhydrous ammonia, that circulates through pumped piping loops to transport 20 to 30 kWt of waste heat from acquisition interfaces to radiator rejection surfaces. Ammonia provides high specific heat (~4700 J/kg-K) and thermal conductivity (~0.5 W/m-K), but its -78 °C freezing point requires active trace heating and radiator bypass circuits to prevent loop failure during the extreme cold of the lunar night. Over a 100-year settlement lifespan, the fluid inventory requires continuous filtration to remove chemical decomposition products, alongside sealed containment to isolate the toxic medium from habitable volumes.
The single-phase ammonia or alternative fluid inventory serving as the thermal energy transport medium in all pumped fluid loops.
Purpose
Provide the heat transfer medium that absorbs thermal energy at heat acquisition interfaces, transports it through the piping network, and releases it at radiator heat rejection surfaces. Working fluid selection drives the entire loop design including material compatibility, operating temperature range, pump sizing, and safety provisions. Must support transport of 20-30 kWt waste heat.
Context
Anhydrous ammonia (NH3) is the baseline working fluid for main thermal transport loops following ISS ETCS heritage. Ammonia offers excellent thermal properties (high specific heat ~4700 J/kg-K, high thermal conductivity ~0.5 W/m-K) but freezes at -78C requiring trace heating or radiator bypass during deep cold. Alternative fluids such as HFE-7100 (freezes -135C) or silicone oils may serve loops in extremely cold zones. Fluid inventory must be maintained over 100 years with leak detection, top-off, and purity management.
Principles
- ▸Working fluid thermal properties (Cp, k, mu, rho) directly determine heat transport capacity and required flow rates
- ▸Single-phase operation keeps fluid liquid throughout loop with heat transferred via sensible temperature rise
- ▸Fluid freezing point sets minimum allowable loop temperature; subcooling below this destroys loop function
- ▸Fluid decomposition and contamination degrade heat transfer over time; filtration and chemistry management required
- ▸Fluid charge quantity must account for thermal expansion, accumulator volume, and leak margin
Typical implementations
- ▸ISS ETCS: anhydrous ammonia, ~150 kg charge per loop, single-phase at -40C to +50C
- ▸ISS ITCS: deionized water with biocide, single-phase internal cabin loops
- ▸Orion MPCV: 50/50 propylene glycol/water, freeze protection to -50C
- ▸3M Novec HFE-7100: fluorinated fluid, dielectric, freezes at -135C
Lunar considerations
- ▸Ammonia freezing at -78C requires trace heating of exposed lines and radiator bypass during lunar night
- ▸HFE-7100 tolerates colder conditions but has ~50% lower heat capacity than ammonia
- ▸100-year fluid chemistry management requires decomposition product removal via filtration and ion exchange
- ▸Ammonia toxicity requires sealed construction and leak detection with no habitable-volume exposure
- ▸Fluid inventory top-off over 100 years requires spare fluid storage or in-situ production capability
Specifications
Functional
| primary function | Serve as thermal energy transport medium in pumped fluid loops |
| inputs | Thermal energy from heat sources via cold plates and heat exchangers, Makeup fluid from storage for leak compensation |
| outputs | Thermal energy delivered to heat sinks at radiators via fluid cooling |
| specific heat j per kg k | >4000 (ammonia ~4700 at 20C) |
| thermal conductivity w per mk | >0.4 (ammonia ~0.5 at 20C) |
| freeze point c | <-75 (ammonia -78C) |
| chemical stability years | 100 years with filtration and chemistry management |
Physical
| materials | Anhydrous ammonia NH3 (primary baseline), HFE-7100 (alternative for cold-zone loops), Syltherm XLT silicone oil (alternative for ultra-long-life loops) |
| operating temperature range c | -60, 80 |
| operating pressure range kpa | 150-500 |
| phase | Single-phase liquid throughout operating range |
Operational
| power consumption w | 0 |
| thermal range c | -78, 80 |
| lifetime years | 100 |
Interfaces
Provides
- Thermal transport medium filling all loop piping, components, and heat exchangers
Requires
- Filtration and ion exchange to maintain fluid purity and remove decomposition products
- Accumulator volume to accommodate fluid thermal expansion across temperature range
Cite this entry
Lunar Ark Codex. "Working Fluid" (L3-TCS-FLOP-WKFL). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-TCS-FLOP-WKFL
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.