Aerospace Sealing Solutions for Space Missions
By Kingston Vickers
Cryogenic fluid management is the foundation for current and future space programs. The availability of these propulsive fluids at launch, in orbital depots, and on the lunar surface is essential for future human exploration missions to Mars. Cryogenic fluids reside within pressurized fuel tanks and propellant subsystems. Maintaining safe operational pressures of these fuels is critical and an overpressure event could result in the catastrophic loss of a spacecraft. Another concern regarding the management of cryogenic fuel is a process called “boil-off.” Boil-off is the term given to the product losses from cryogenic propellant storage systems during long-duration space mission applications.
Within commodity burst disc designs, the diaphragm ruptures when exposed to pressures exceeding its limit. This “frangible” element has a profound impact on the capability of these types of burst discs. Shock loads, vibrations at launch, abort procedures, operational boil-off cycling, and the requirement for long-term service make the reliance on a thin singular diaphragm as an over-pressure solution somewhat disconcerting at best.
Whether working on terrestrial, in-atmosphere, or in-space applications, the impact of temperature must be factored into overpressure protection. Complications occur when specifying the specific rupture temperature for conventional frangible burst disc pressure.
Frangible disc rupture pressure is solely dependent on the physical strength of the thin diaphragm. These commodity discs risk high variance in specified burst pressure; lower temperature risks higher than rated burst pressure, higher temperature risks lower than rated burst pressure.
Obscuring this critical specification further, designers must make their best judgment as to the conditions that most likely risk an overpressure event.
This can be extremely difficult to predict in cases where runaway chemical reactions or unexpected component failures occur. (e.g. valves, pumps, switches, etc.)
Frangible discs limit system safety and limit the choice as to where the disc can be physically placed within the system. Due to their thermal sensitivity, frangible designs must be placed very close to where the expected upset conditions occur. Any locational offset can result in heat transfer and negatively impact frangible burst pressure.
There are reports that a 100° F difference in temperature yields a 20% offset in rupture pressure for conventional frangible designs.
In contrast, advanced SAFE-SHEAR™ burst disc designs are immune to many critical application parameters that destroy conventional/frangible rupture discs. These advanced burst discs can withstand millions of cycles of shock, vibration, thermal cycling, and sloshing up to 95% of rupture pressure. Our burst disc capabilities allow cryogenic fluid designers to optimize cylinder designs for high pressures, thereby ensuring adequate propellant and oxidizer are available to activate pressure-dependent systems. Our advanced burst disc accomplishes this while preventing product loss. Our leakage rates are less than 10-8 standard cubic centimeters per second of GHe. (sccs)
The enhanced safety feature and leakage protection are made possible through its method of operation. When internal pressure from the liquid or gas reaches the predetermined (and calibrated) differential pressure, a precisely tuned Belleville spring reverses, forcing the diaphragm into a serrated punch, thereby releasing the system pressure. This mechanism of operation enables advanced burst discs to offer remarkable rupture precision. Our SAFE-SHEAR™ burst disc can be set at ±1% of rupture pressures from 1 to 10,000 psi (0.007 to 690 bar) and offer tolerances at temperatures ranging from –457 to 1,000°F (– 270 to 540°C).
Once the punch pierces the diaphragm, the burst disc rapidly releases all pressure. To prevent losses for applications with minimum pressure operational requirements or where the system is remote and cannot be recharged, we can include pressure-relief valves integral with the burst disc. Our relief valves prevent loss of all product pressure and therefore allow pressure-dependent components to continue operation. Our burst disc/relief valve components offer up to 80% weight savings compared to other redundant systems.
NASA considers our type of advanced disc equivalent to two relief valves, letting a single one meet NASA’s two-fault-tolerant requirement (Ref. NASA payload safety requirement NHB 1700.7; NASA-JSC NSTS/ISS 18798; JCS Letter TA-88-074). – “The preferred burst disk design for payloads is one which employs a reversing membrane against a cutting edge to assure rupture. Historical use and experience indicate that a burst disk of this type can be certified as a highly reliable pressure relief device.”
Space-based applications of burst discs require material solutions designed to withstand aggressive rocket fuels, oxidizers, and other harsh fluids. Materials of construction can include aluminum, stainless steel, superalloys, and PTFE.
Aside from our Human Spaceflight applications, including the International Space Station, additional spacecraft, satellites and aircraft that utilize our SAFE-SHEAR™ technology include:
- RRM3 Orbital Refueling Mission
- The Cassini Spacecraft
- The Hubble Space Telescope
- The Spitzer Telescope
- Wide-Field Infrared Survey Explorer
- Mars Global Surveyor
- Gravity Probe-B
- V-22 Osprey / KC-390
