Cryo Depot Boil-Off vs ISRU Resupply

Propellant & Depot Setup

Propellant LOX

MLI insulation layers 20

Cryocooler power 120 W

ISRU production rate 80 kg/day

Time

Depot Status

Fill: 60% (1800 / 3000 kg)

Boil-off rate

0 kg/day

Net rate

0 kg/day

Heat leak

0 W

Mission day

0.0

How it works

A cryogenic propellant depot constantly leaks heat in from sunlight and the surrounding structure. That heat boils a small amount of liquid propellant into vapor, which must be vented (boil-off) — unless an on-site ISRU plant (electrolysing water ice into propellant) is producing new liquid faster than it's being lost.

Q_leak  = Q_base · shield_factor / (1 + 0.09·N_MLI)   [W]
m_boil  = max(0, Q_leak − P_cryo) / L_v               [kg/s]
m_net   = m_ISRU − m_boil                              [kg/day]
tank(t+dt) = clamp(tank(t) + m_net·dt, 0, capacity)

MLI layers — each extra multi-layer-insulation blanket layer adds a radiation barrier; effective heat leak falls with diminishing returns, per the empirical layer-density curves used on real cryogenic stages.
Sun shield — a deployable parasol that blocks ~85% of direct solar heating when facing the sun; stowing it (for maneuvering or launch) exposes the tank to full solar flux.
Cryocooler power — active refrigeration intercepts heat before it boils propellant; modeled here at unit efficiency for clarity (real cryocoolers have a coefficient of performance well below 1, so this is an optimistic simplification).
ISRU production rate — propellant manufactured locally (e.g. electrolysing lunar or asteroid water ice into LOX/LH2, or the Sabatier reaction for methane) instead of launched from Earth. This is the resource-utilization lever: a high enough production rate can outrun boil-off entirely, letting the depot fill even with a leaky tank.
L_v (latent heat of vaporization) differs by propellant — LH2 boils off fastest per watt of heat leak (lowest L_v), which is why hydrogen depots need the most aggressive insulation and cooling.

Real-world relevance: this is the core trade study behind every proposed cislunar or Mars propellant depot — NASA and commercial architectures all live or die on whether local resource production can keep ahead of unavoidable cryogenic boil-off.

Liquid propellant MLI blanket layers Sun shield Vented vapor

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