L2-PTC-HPIPE ESSENTIAL THERMAL Level 2 · hardware

Passive Heat Pipes

Capillary-Driven Passive Heat Pipe Network

A capillary-driven passive heat pipe network transports thermal energy from electronics and heat-generating components to radiator surfaces without pumps or external power, utilizing fluid evaporation and condensation within internal wick structures. Operating across a temperature range of -60 °C to 100 °C, individual pipes transport 10 to 500 W over distances up to 2 m, extending up to 10 m in loop heat pipe configurations. While lunar gravity at 1/6 g assists capillary fluid flow compared to microgravity environments, the subsystem must withstand 100 years of thermal cycling fatigue, and deep cold conditions risk freezing standard ammonia working fluid at its -77.7 °C phase boundary.

Network of passive heat pipes using capillary-driven working fluid to transport thermal energy from heat sources to radiator surfaces without pumps or external power.

Purpose

Moves waste heat from electronics and other heat-generating components to external radiator surfaces passively, bridging the gap between heat sources and heat sinks without active fluid loops.

Context

Complements the active pumped-loop thermal control (L1-TCS) by providing zero-power heat transport for steady-state loads, reducing reliance on active systems for baseline thermal management.

Principles

▸Capillary wick structure drives working fluid circulation without pumps
▸Evaporation at hot end and condensation at cold end transports latent heat
▸Heat pipe capacity limited by capillary, entrainment, boiling, and sonic limits
▸Variable-conductance heat pipes (VCHPs) provide self-regulating thermal control

Typical implementations

▸Constant-conductance heat pipes (CCHPs) with ammonia working fluid
▸Variable-conductance heat pipes with non-condensable gas reservoir
▸Loop heat pipes (LHPs) for longer transport distances
▸Sintered metal wick structures for reliable capillary pumping
▸Aluminum or copper pipe envelopes depending on working fluid compatibility

Lunar considerations

▸Lunar gravity (1/6 g) assists capillary operation compared to zero-g design
▸Working fluid must remain liquid across the operational temperature range
▸Ammonia freezes at -77.7C, requiring heaters or alternative fluids for deep cold survival
▸Heat pipe envelope must survive 100 years of thermal cycling fatigue
▸Micrometeorite impact on external heat pipe runs must be mitigated

Specifications

Functional

primary function	Passively transport thermal energy from heat-generating components to radiator surfaces using capillary-driven working fluid
inputs	Waste heat from electronics and subsystem components
outputs	Heat delivered to radiator surfaces for rejection to space
transport capacity w per pipe	10-500 depending on size
operating temperature range c	-60, 100
transport distance m	up to 2 for standard, up to 10 for loop heat pipes
power consumption w	0

Physical

materials	Aluminum alloy 6061 envelopes, Sintered nickel or stainless steel wicks, Ammonia or propylene working fluid
vacuum	True
radiation tolerant	True

Operational

power consumption w	0
thermal range c	-173, 127
lifetime years	100

Interfaces

Provides

→Passive Thermal ControlPassive heat transportessential
Transports waste heat from component mounting locations to radiator surfaces without power

Requires

←Primary StructureMounting and routingessential
Structural mounting for heat pipe runs and thermal interface to radiator panels
←Thermal CoatingsRadiator surface coatingsessential
High-emittance coatings on condenser/radiator end for effective heat rejection