Spacecraft Heat Pipe Capillary Calculator

Thermal control & two-phase transport: Calculate Constant Conductance Heat Pipe (CCHP) capillary pumping limit (Q·L_eff), axial groove geometry, and dryout margin.

Heat Pipe Wick & Fluid Configuration

Working Fluid:

Operating Temperature (°C): Vapor core saturation temperature

Vapor Core Diameter d_v (mm): Inner vapor channel bore

Wick Capillary Pore Radius r_p (µm): Axial groove width or mesh pore

Evaporator Length L_e (m): Heated equipment zone

Condenser Length L_c (m): Radiator heat rejection zone

Adiabatic Transport Length L_a (m): Insulated transport section

Operating Heat Load Q (W): Transferred thermal power

Capillary Limit & Transport Capacity

Capillary Power Limit (Q_max)

At effective transport length

Capillary Transport Metric (Q · L_eff)

Watt-meters (W·m) capacity

Maximum Capillary Head (ΔP_cap)

2 · σ / r_p meniscus pressure

Effective Heat Pipe Length (L_eff)

L_a + (L_e + L_c) / 2

Fluid Merit Number (N_f)

ρ · σ · λ / µ in 10¹⁰ W/m²

Dryout Safety Margin

Q_max / Q_actual

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Frequently Asked Questions

What is the capillary pumping limit in a heat pipe?

The capillary limit is the fundamental hydrodynamic ceiling for heat pipe operation. For continuous circulation, the maximum capillary pressure generated by the curved liquid menisci in the wick (ΔP_cap = 2σ/r_p) must equal or exceed the sum of the liquid frictional pressure drop and the vapor frictional pressure drop along the entire length.

Why is anhydrous ammonia (NH₃) the premier working fluid for satellite heat pipes?

Ammonia has an exceptionally high Figure of Merit (N_f = ρ·σ·λ/µ) over the typical spacecraft operating range (-40°C to +70°C). Its high surface tension provides powerful capillary pumping, while its high latent heat of vaporization (λ ≈ 1.2 MJ/kg) transports large quantities of heat with minimal mass flow rate.

Why are axially grooved wicks favored over sintered powder in aerospace CCHPs?

Axially grooved aluminum heat pipes have zero risk of wick compaction or pore shifting during high-g rocket launches, feature very low liquid flow resistance (enabling high W·m transport capacities over meters), and are manufactured with repeatable extrusion quality.