Cryogenic boiloff management at L4/L5 thermal environment

Background

The resolution of rq-0-14 (propellant production scope) established that in-situ propellant production should be designed into the Material Processing Station from Day 1 and deployed at 18-24 months. However, the resolution explicitly identified cryogenic boiloff management as the highest-uncertainty technical element remaining. Liquid hydrogen and liquid oxygen storage at the L4/L5 Lagrange points presents a severe thermal challenge: full solar thermal loading at 1 AU with no planetary shadow for passive cooling.

At projected production rates of 70-130 tonnes of propellant per year, the station must store significant quantities of cryogenic fluids while maintaining acceptable boiloff rates. Active cooling systems compete for the station's limited 2.5-3.25 MW power budget, and excessive boiloff directly reduces the economic benefit of in-situ production.

Why This Matters

Boiloff rates determine whether cryogenic propellant storage is economically viable at L4/L5. If boiloff exceeds production rates during storage periods, the station effectively cannot accumulate propellant reserves. This would force either:

1. Just-in-time propellant production synchronized with vehicle arrival schedules
2. Adoption of storable propellants instead of high-performance LH2/LOX
3. Massive investment in active cooling infrastructure

Each alternative has cascading effects on station power budget, vehicle design, and mission planning flexibility.

Key Considerations

• Solar thermal loading at 1 AU is approximately 1,360 W/m^2 — storage tanks receive continuous heating
• Liquid hydrogen boils at 20.3 K; liquid oxygen at 90.2 K — very large temperature differentials to maintain
• Multi-layer insulation (MLI) alone may be insufficient for large-scale storage
• Active cooling (cryocoolers) at this scale consumes 100-500 kW depending on storage volume
• Zero-boiloff storage has been demonstrated at small scales but not at 50-100+ tonne capacity
• Propellant depot concepts from NASA and commercial studies provide baseline designs

Research Directions

1. Thermal modeling of large-scale cryogenic storage at L4/L5: Model boiloff rates for 50, 100, and 200 tonne LH2/LOX tanks with MLI and sunshield configurations specific to L4/L5 thermal environment.

2. Active cooling power trade study: Quantify cryocooler power requirements for zero-boiloff storage at various tank sizes, comparing against the station's power budget allocation.

3. Hybrid storage architecture: Evaluate designs combining passive thermal control (sunshields, shadow-casting structures) with active cooling to minimize power draw.

4. Subcooled storage concepts: Assess whether subcooling propellants below boiling point during production provides sufficient thermal margin for storage periods between vehicle visits.

5. Comparison with storable alternatives: Quantify the performance penalty of switching to storable propellants if cryogenic storage proves impractical, informing the fallback decision point.