Dewar Sealing System
Cryogenic Vacuum Seal and Pressure Boundary System
Indium wire seals, metal gaskets, and pressure relief systems maintaining the vacuum jacket integrity and inner vessel pressure boundary of all cryogenic dewar vessels
Purpose
Ensure hermetic sealing of both the vacuum annulus and the inner cryogenic volume, preventing vacuum degradation and cryogen loss while providing overpressure relief for safety
Context
The sealing system is the critical boundary that maintains dewar thermal performance. Vacuum jacket seals must hold <1e-5 torr for 100+ years. Inner vessel seals must contain LN2 at 1-3 bar. All penetrations (cold bus, fill lines, instrumentation feedthroughs, access ports) require individual sealing solutions. Indium wire seals are preferred for cryogenic flanges due to their excellent cold-flow properties and leak tightness at 77 K. Pressure relief valves on both inner and outer vessels prevent catastrophic failure.
Principles
- ▸Indium wire seals deform plastically to fill surface imperfections, providing excellent cryogenic leak tightness
- ▸Metal-to-metal seals (indium, copper, aluminum) are preferred over elastomers at cryogenic temperatures
- ▸Dual-seal configurations with inter-seal pumping/monitoring enable leak detection before total failure
- ▸Pressure relief valves must be sized for worst-case boil-off (sudden vacuum loss causing rapid LN2 evaporation)
- ▸All seal joints must accommodate differential thermal contraction between mating flanges
Typical implementations
- ▸Indium wire O-rings (0.5-2 mm diameter) on Conflat-style knife-edge flanges
- ▸Helicoflex spring-energized metal seals for high-reliability cryogenic joints
- ▸Burst discs (rupture discs) as passive backup overpressure protection
- ▸Instrumentation feedthrough seals using glass-to-metal or ceramic-to-metal brazed assemblies
Lunar considerations
- ▸Lunar vacuum eliminates atmospheric back-leak concern for vacuum jacket seals
- ▸Thermal cycling between PSR temperatures (~40 K) and operational (77 K) is minimal — beneficial for seal longevity
- ▸No moisture or atmospheric contaminants to degrade seal surfaces
- ▸Seal replacement by robotic systems requires accessible flange geometry and standardized tooling
- ▸Micrometeorite impact could create secondary seal loads — structural analysis required
Specifications
Functional
| primary function | Maintain hermetic vacuum and pressure boundaries for all dewar vessel flanges and penetrations |
| inputs | Flange clamping force from bolted connections, Thermal loads causing differential contraction at seal interfaces |
| outputs | Vacuum boundary integrity (<1e-5 torr in jacket space), Pressure boundary integrity (inner vessel rated to 3 bar), Overpressure relief (vent path for emergency boil-off) |
| vacuum leak rate mbar l per s | <1e-9 per seal joint |
| inner vessel design pressure bar | 3 |
| relief valve set pressure bar | 2.5 |
| burst disc rating bar | 5 |
| seal lifetime years | 100 |
| thermal cycles to failure | >1000 |
Physical
| dimensions | Various — seal geometry matches flange diameters |
| materials | Indium wire (primary cryogenic seals), OFHC copper gaskets (Conflat-style flanges), Stainless steel 316L (flange hardware, relief valve bodies), Inconel 718 (relief valve springs, burst disc holders), Glass-to-metal brazed assemblies (instrumentation feedthroughs) |
| temperature range k | 40-300 (seal surfaces span from cryogenic to ambient during assembly) |
| operating temperature k | 77 (inner seals) to 100 (outer jacket seals at PSR) |
| vacuum | Lunar ambient on exterior, <1e-5 torr in jacket |
Operational
| power consumption w | 0 |
| thermal range c | -196, 25 |
| lifetime years | 100 |
Interfaces
Provides
- Hermetic vacuum sealing for all dewar jacket flanges and penetrations
- Inner vessel pressure containment and overpressure relief for LN2 volume
- Resealable cryogenic seal for sample access port flange after each retrieval operation
Requires
- Precision-machined flange mating surfaces with specified surface finish for seal contact
- Residual gas analyzers and vacuum gauges for seal health monitoring
- Robotic capability for seal inspection and replacement during maintenance operations
Cite this entry
Lunar Ark Codex. "Dewar Sealing System" (L3-CRY-TANK-SEAL). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-CRY-TANK-SEAL
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.