Heat pipe (passive heat transport)
Transports heat passively through a sealed wicked tube where a working fluid evaporates at the hot end and condenses at the cold end, returning by capillary action — no pump, no power, near-isothermal, and largely gravity-independent. It isothermalizes radiators, spreads heat from concentrated sources, and provides ultra-reliable thermal transport where a pumped loop's failure modes are unacceptable. Performance is bounded by capillary, boiling, and entrainment limits and by working-fluid temperature range.
Governing equations
The capillary limit: the wick's pumping head (surface tension / effective pore radius) must exceed the liquid, vapor, and gravity pressure drops, or the wick dries out. The defining heat-pipe constraint. [1]
Maximum heat transport = the capillary-limited fluid circulation rate × latent heat of vaporization. Heat pipes move heat as latent energy, achieving effective conductivities far above any solid metal. [1]
Effective thermal conductivity orders of magnitude above copper — the hot and cold ends sit within a few degrees, which is why heat pipes isothermalize radiator panels and spread hot spots. [1]
When capillary pumping dominates the gravity term, orientation barely matters — heat pipes work the same on Mars, the Moon, or in orbit, a major advantage over gravity-dependent thermosiphons. [2]
Key constants & quantities
| Symbol | Value | Units | Conditions | Description |
|---|---|---|---|---|
| Effective conductivity | 10000–100000 | W/m·K (effective) | — | Apparent thermal conductivity — 25-250× copper — enabling near-isothermal heat spreading and transport.[1] |
| Operating temperature (fluid-dependent) | -70–400 | °C | — | Range set by working fluid: ammonia/propylene for cold service, water for mid-range, alkali metals for high-temperature.[2] |
| End-to-end ΔT | 1–10 | K | — | Temperature drop from evaporator to condenser — small, the near-isothermal behavior that defines the device.[1] |
| Transport length (LHP) | 0.1–10 | m | — | Loop heat pipes extend transport to meters with flexible routing; classic heat pipes are shorter.[1] |
| Reliability | 1 | no moving parts (decades of passive life) | — | No pump, no power, sealed — heat pipes have multi-decade spacecraft service records, the basis of their reliability advantage.[2] |
Operating envelope
Mass balance
Basis: one radiator-isothermalizing heat pipe (functional unit)
Inputs
| Sealed tube + wick | 1 | unit | [1] |
| Working fluid charge | 1 | charge | [2] |
- Sealed tube + wick: Metal envelope (Al/steel), sintered or grooved wick — machine-tools fabricable.
- Working fluid charge: Ammonia/water/propylene per temperature range; small mass.
Outputs
| Passive isothermal heat transport | 1 | enabling | [1] |
- Passive isothermal heat transport: Moves heat / spreads it with no power; isothermalizes the radiator it serves.
A heat pipe consumes no energy at all — it moves heat for free, forever, with no moving parts. Its value is reliability and isothermalization, not efficiency; it complements the pumped thermal bus where passivity and zero-failure transport matter.
Variants & trade-offs
Constant-conductance heat pipe (CCHP)
[2]The classic sealed wicked tube — fixed performance, isothermalizing and transporting heat over short-to-moderate distances.
- Simplest, most reliable; decades of flight heritage
- Excellent radiator isothermalization and hot-spot spreading
- Fixed conductance; limited length; can't modulate heat flow
When preferred: Radiator isothermalization, electronics/component heat spreading.
Loop heat pipe (LHP)
[1]Separates vapor and liquid lines with the wick concentrated in a compact evaporator — flexible routing over meters, the modern spacecraft workhorse.
- Long, flexible, routable transport; high heat over meters
- Robust against gravity orientation
- More complex startup behavior; compensation-chamber control
When preferred: Transporting heat from a source to a distant radiator with flexible routing.
Variable-conductance heat pipe (VCHP)
[2]A non-condensable gas reservoir that throttles the active condenser area, self-regulating to hold the source near a setpoint as load/sink vary.
- Passive temperature regulation — holds setpoint through Mars's huge day/night swing without active control
- Protects sensitive equipment from thermal cycling
- Reservoir sizing; slightly lower peak transport
When preferred: Equipment needing a stable temperature despite the 80-100 K diurnal swing.
High-temperature (alkali-metal) heat pipe
[1]Sodium or potassium working fluid for 500-1000+ °C service — isothermalizing reactor, kiln, or concentrator receivers.
- Isothermalizes very-high-temperature sources (reactor, solar receiver)
- Extreme effective conductivity at high T
- Material compatibility and startup challenges; specialist fabrication
When preferred: High-temperature heat spreading — nuclear reactor and solar-concentrator receivers.
Failure modes
| Mode | Cause | Detection | Mitigation |
|---|---|---|---|
| Capillary dryout[1] | Heat load exceeds the capillary limit; the wick can't return liquid fast enough, the evaporator dries, and temperature spikes. | Evaporator temperature excursion; conductance drop. | Operate below the capillary limit with margin, wick design for the duty, LHP for long transport; orientation help at 0.38 g. |
| Non-condensable gas generation[2] | Corrosion or material incompatibility generates non-condensable gas that blankets the condenser and degrades performance over time. | Gradual conductance decline; condenser-end cold spot. | Compatible envelope/fluid pairs, clean fabrication, proven material combinations (the classic heat-pipe compatibility tables). |
| Working-fluid freezing[3] | A heat pipe whose fluid freezes (e.g. water-charged pipe exposed to Mars cold) can't start and may be damaged by freeze expansion. | Failure to start; temperature monitoring. | Choose fluids with freeze points below the cold-end minimum (ammonia/propylene for cold service), startup heaters if needed. |
| Envelope breach[2] | Micrometeorite, corrosion, or mechanical damage punctures the sealed tube; the working fluid escapes and the pipe dies. | Performance loss; pressure/leak indication. | Robust/shielded envelopes, redundant pipes on critical paths, non-toxic fluids where breach reaches crew areas. |
| Startup difficulty (LHP / high-T)[1] | Loop and alkali-metal heat pipes can have finicky cold-start behavior (frozen/over-loaded startup). | Startup temperature behavior monitoring. | Startup heaters, proper charge and compensation-chamber sizing, validated startup procedures. |
Mars adjustments
Gravity-independence is proven and valuable[2]
Impact: Heat pipes work the same at 0.38 g as at 1 g (capillary-dominated), and their space heritage de-risks them completely — they are among the lowest-risk thermal technologies for Mars.
Mitigation: Use heat pipes wherever passive, orientation-free transport is wanted; lean on flight heritage.
The reliable passive backbone[1]
Impact: For decade-long unattended operation, a device with no moving parts and no power need is invaluable — heat pipes carry critical thermal paths (radiator isothermalization, reactor/receiver spreading) where a pump failure would be unacceptable.
Mitigation: Put heat pipes on the highest-reliability thermal paths; complement the active bus with passive redundancy.
Working-fluid choice for the cold[3]
Impact: Water-charged pipes freeze in Mars cold; cold-service paths need ammonia/propylene-class fluids with low freeze points, or startup heaters.
Mitigation: Select working fluid for the full temperature range including the cold-end minimum; VCHPs for setpoint stability.
Passive setpoint regulation through the diurnal swing[2]
Impact: Variable-conductance heat pipes hold equipment near a setpoint through Mars's 80-100 K day/night swing with no active control — a free thermal stabilizer for sensitive hardware.
Mitigation: Use VCHPs on temperature-sensitive equipment exposed to the diurnal cycle.
Locally fabricable, fluid is the import[1]
Impact: The metal envelope and wick are machine-tools products; the working-fluid charge and precision sealing are the demanding parts.
Mitigation: Local envelope/wick fabrication, imported/charged working fluids and seals, standardized pipe families.
Alternatives & substitutes
thermal-bus (pumped fluid loop)[2]
- Settlement-scale, flexible routing, variable distribution to many users
- Needs pumps and power; has active failure modes a heat pipe doesn't
When preferred: Large-scale variable heat distribution; heat pipes handle passive spot transport.
Solid conduction (metal straps)[4]
- Utterly simple and reliable for short distances
- Orders of magnitude lower effective conductivity; heavy for any real distance
When preferred: Very short, low-power thermal paths.
Pumped two-phase loop[1]
- Combines latent-heat transport with active pumping for long range + high flux
- Active components; more complex than a passive pipe
When preferred: High-flux sources needing long-range active two-phase transport.
Requires
Inputs
References
- (2016). Heat Pipe Science and Technology, 2nd Edition. Global Digital Press. ISBN 978-1-4939-2503-2. — Comprehensive heat-pipe engineering: capillary and loop heat pipes, working-fluid selection, capillary/boiling/entrainment limits, variable-conductance designs.
- (2002). Spacecraft Thermal Control Handbook, Vol. 1: Fundamental Technologies, 2nd Edition. The Aerospace Press / AIAA. ISBN 978-1-884989-11-7. — The definitive spacecraft thermal-control reference: thermal surfaces and coatings (α/ε), heat pipes, radiators, louvers, loops, and thermal-balance design.
- (2017). The Atmosphere and Climate of Mars. Cambridge University Press. ISBN 978-1-107-01618-7. — Reference handbook for Mars atmospheric pressure, temperature, dust climatology.
- (2017). Fundamentals of Heat and Mass Transfer, 8th Edition. John Wiley & Sons. ISBN 978-1-119-32042-5. — Standard undergraduate / engineering reference for heat transfer: Stefan-Boltzmann radiation, conduction, convection.