343 in gas chromatography using capillary columns does not exceed 50 ml/min. However the other  specifications are quite harsh: the valve has to  withstand the gas tank pressure of 50 bar, and a  temperature of 130°C (temperature of the sterilization process) should not modify its behavior.   A further purpose of this work is to propose a smal ler and more rugged alternative to the existing valves  that provide hermetic sealing at pressures in the range of 50 bar. Such valves can have a wide range of applications. Indeed, it can be used in the space indu stry for deployment of inflatable structures, for  opening of a gas tank for an electrostatic ion thruster or other space experiments with gas. On the ground, it can be used for opening a CO 2 tank for active fire protection. A gas tank of an underwater robot  could also be opened to bring it at the surface.    Existing Sealing Valves 

Having an overview of existing valves was a very helpful step of this work. The focus was placed on  single-shot valves, for the reason t hat it seemed the only reliable way to keep a tank hermetically sealed  years after filling. The first encountered single-shot valve was the solenoid valve from the company Industria, France. This valve was already on board the Aerosol Colle ctor Pyrolyzer (a GC-MS) for  Huygens, and isolates the gas tank of the COSAC instrument, part of the ESA cometary mission Rosetta.  It consists of a quite bulky solenoid actuator and of a thin membrane that obstructs the gas flow and can  withstand pressures up to 50 bar. When it is electr ically powered, the solenoid displaces a pin that  pierces the membrane.  Very original other means to pierce a metallic foil to open a tank were found for space. In the Thermal Evolved Gas Analyzer on-board with the Phoenix Ma rs mission, the gas tanks are opened with a foil  punctured by a wax actuator [1]. Here the device is not suited for the application because typical fusion  temperatures of wax are below 130°C. Often used in  the space industry is the Pyrovalve, manufactured  by Conax, Florida [2]. It has been used for instance  in the Titan and Delta II NASA missions. Here, the  gas flow is blocked by a ram that is moved downwa rds by an explosive initiator placed on a tube above.  The data sheet claims that the valve does not c ontaminate the gas and that the operating pressure  reaches 647 bar. The reasons why it is not suitable for the application is the maximum storage temperature of only 60°C and its bulky construction.  The Space Science and Technology Department, Rutherford Appleton Laboratory has designed a one-time opening valve for the carrier gas tank of the Ptolemy instrument, a gas chromatograph coupled with a ion trap mass spectrometer built for the Rosetta mission [3]. Using this valve, the tank is normally sealed by a hollow and frangible pillar. A shape memo ry alloy (SMA, here Nitinol) actuator, having the  ability to bend when heated over a certain temperature, is  placed near this pillar. To release the gas, a  heater outside of the housing elevates the temperat ure of the nitinol actuator, which bends and breaks  the seal. The pressure of the Ptolemy tank is simila r to the one of MOMA, and the valve is effective more  than 15 years after filling. But also here the low actuation temperature of 76°C prohibits its use for MOMA.  Interesting is also the concept of  an alternative to the Pyrovalve [4 ] which utilizes three burst discs  (diaphragms specially designed to burst at a precise pressure difference) to form two chambers. The left chamber is pressurized at the tank initial pressure, while the right chamber is pr essurized at the external  pressure. A heater is able to elevate the temperatur e and thus the pressure inside the left chamber. This  causes the discs to burst in the succession middle-left-right. This system has apparently not yet been realized.  A further project, launched by NASA aims at finding an al ternative to the Pyrovalve. The principle is close  to the one of the Ptolemy valve: the actuator is a SMA tube that can be heated from the outside. As the SMA is heated, it recovers its expanded shape, and the evolving force breaks a titanium tube responsible for the gas-tightness. Here the actuation temperature is  linked to the used alloy nitinol, and is therefore  approximately the same as for the Ptolemy valve.  For this reason it cannot be used in MOMA. 

NASA/CP-2010-216272