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ISO 28702:2021 – Space Systems — Pressure Vessels and Pressurized Structures — Design and Verification
Design, manufacturing, testing, and verification requirements for pressure vessels and pressurized structures used in space systems
Scope and Classification of Pressure Vessels
ISO 28702:2021 establishes comprehensive requirements for pressure vessels and pressurized structures used in space systems, including propellant tanks, pressurant spheres, composite overwrapped pressure vessels (COPVs), and metallic liners. The standard recognizes three vessel classifications: Class A (metallic vessels), Class B (composite overwrapped with metallic liner), and Class C (all-composite or polymeric liner with composite overwrap). Each class carries specific design verification requirements reflecting the different failure mechanisms — ductile rupture in metals, fiber breakage and matrix cracking in composites, and liner collapse in COPV designs.
COPVs offer 30-50% mass savings compared to equivalent all-metallic pressure vessels, making them the preferred choice for weight-critical space applications. However, their complex failure modes require specialized verification approaches beyond traditional metallic vessel design.
Pressure vessels in space systems operate under extreme conditions that terrestrial codes do not fully address: cryogenic temperatures (20-90 K for propellants), vacuum exposure, radiation, micrometeoroid impacts, and highly cyclic pressure loading from multiple mission phases. The standard’s design philosophy centers on two fundamental principles: first, that the proof pressure test demonstrates margin over maximum expected operating pressure (MEOP), and second, that the burst pressure provides ultimate margin against the limit load. For crewed systems, additional leak-before-burst demonstration is required to provide early warning of incipient failure.
Design Requirements and Safety Factors
ISO 28702 mandates minimum design safety factors that vary by vessel class, application criticality, and whether the vessel is located in crewed or uncrewed systems. For metallic vessels (Class A), the ultimate factor of safety relative to burst pressure is 1.5 for uncrewed and 2.0 for crewed applications. For COPVs (Class B and C), minimum burst-to-MEOP ratios range from 1.5 to 2.5 depending on the failure consequence classification. Fatigue life requirements specify a minimum of four times the expected number of operational pressure cycles including ground testing, flight operations, and a safety factor of 2 on cyclic life.
| Vessel Class | Material Type | Burst Factor (Uncrewed) | Burst Factor (Crewed) | Proof Factor | Fatigue Life Factor |
|---|---|---|---|---|---|
| Class A | Metallic (Al, Ti, Inconel) | 1.5 | 2.0 | 1.2 | 4× service life |
| Class B | COPV with metallic liner | 1.75 | 2.5 | 1.1 | 4× service life |
| Class C | All-composite / polymeric liner | 2.0 | 2.5 | 1.05 | 4× service life |
The transition from metallic to COPV propellant tanks in geostationary satellite platforms has enabled dry mass reductions of 25-35 kg per spacecraft — translating to either increased payload capacity or reduced launch costs exceeding $500,000 per kg of saved mass on commercial launch vehicles.
Material Selection and Environmental Compatibility
The standard sets stringent requirements for material compatibility with stored media. For composite vessels containing oxidizing propellants (nitrogen tetroxide, nitric acid), the resin matrix must demonstrate chemical stability through immersion testing at maximum service temperature for a minimum of 1,000 hours with less than 15% strength degradation. Metallic liners in COPVs must be evaluated for stress corrosion cracking susceptibility using appropriate test methods. For cryogenic service, materials must maintain fracture toughness at minimum design temperature with KIC or KQ values verified by testing at the actual operating temperature, not extrapolated from ambient data.
Hydrogen embrittlement is a critical concern for high-strength steel and titanium pressure vessels in space systems. ISO 28702 requires that materials with tensile strength exceeding 1,200 MPa undergo hydrogen embrittlement testing per ASTM F1624 if exposed to any hydrogen-producing environment during manufacturing, testing, or service.
Verification Testing and In-Service Monitoring
ISO 28702 mandates a qualification test program consisting of a minimum of five production-representative vessels subjected to the full range of environmental and pressure loads. Proof pressure testing at 1.1 to 1.5 times MEOP (depending on class) is required for flight vessels, with permanent set measurements to detect yielding. Leak testing using helium mass spectrometry with a maximum acceptable leak rate of 1×10⁻⁶ std·cm³/s for general service and 1×10⁻⁸ for crewed applications. The standard also requires acoustic emission monitoring during proof testing of COPVs to detect fiber breakage and matrix cracking. For long-duration missions exceeding 5 years, periodic in-service inspection intervals must be established, and for crewed systems, continuous leak monitoring is required.
Never exceed the rated burst pressure of a pressure vessel in any ground test without a blast containment structure. The energy released by a catastrophic COPV burst at 600 bar is equivalent to approximately 2.5 kg of TNT — sufficient to cause lethal fragmentation hazards within a 50-meter radius.
FAQ
Q: Can existing ASME BPVC Section VIII vessels be used in space applications?
A: Only with extensive supplementary verification. ASME codes do not address vacuum environment, micrometeoroid impact, zero-g fluid dynamics, or the full range of launch vibration loads. ISO 28702 requirements must be applied in addition to ASME design for space-qualified vessels.
A: Only with extensive supplementary verification. ASME codes do not address vacuum environment, micrometeoroid impact, zero-g fluid dynamics, or the full range of launch vibration loads. ISO 28702 requirements must be applied in addition to ASME design for space-qualified vessels.
Q: What is the typical service life of a COPV in orbit?
A> Space-qualified COPVs have demonstrated service lives exceeding 20 years on GEO communications satellites. The primary life-limiting factor is liner fatigue, not composite degradation, provided the vessel is not exposed to temperatures exceeding the resin glass transition temperature (typically 150-180°C).
A> Space-qualified COPVs have demonstrated service lives exceeding 20 years on GEO communications satellites. The primary life-limiting factor is liner fatigue, not composite degradation, provided the vessel is not exposed to temperatures exceeding the resin glass transition temperature (typically 150-180°C).
Q: How is leak-before-burst verified for COPVs?
A: Leak-before-burst is demonstrated through a combination of fracture mechanics analysis and subscale testing showing that a through-wall crack in the metallic liner produces a detectable leak rate before the composite overwrap reaches its critical failure strain. Full-scale validation testing on a minimum of two vessels is required.
A: Leak-before-burst is demonstrated through a combination of fracture mechanics analysis and subscale testing showing that a through-wall crack in the metallic liner produces a detectable leak rate before the composite overwrap reaches its critical failure strain. Full-scale validation testing on a minimum of two vessels is required.
Q: Are in-flight pressure vessel repairs permitted?
A: Generally no. Pressure vessels are classified as single-point failures in most space systems, and in-flight repair is not feasible. However, pressure relief and isolation capabilities are required to manage overpressure events and localize damage from micrometeoroid punctures.
A: Generally no. Pressure vessels are classified as single-point failures in most space systems, and in-flight repair is not feasible. However, pressure relief and isolation capabilities are required to manage overpressure events and localize damage from micrometeoroid punctures.
