A Portuguese company with experience in aerospace technology and energy systems developed a process for producing pressure vessel shells by cold forming of aluminium tube. This technology, developed in the context of ground segment vessels is being applied in vessels for Xe storage in satellites electric propulsion systems.
Possible application of this technology are Ionic and Chemical Propulsion, Cold Gas Thrusters or CNG Passenger Cars and Buses.
Helium Xenon has been gaining importance in the Space industry due to its use as propellant in electric propulsion thrusters which, due to their very high specific impulse, are often used in satellites. The storage of Xenon is complex due to the extremely high performance requirements, especially in mass, due to the fact that each additional kg of storage mass costs over €30,000 to put into orbit. Being a gas at ambient temperature and pressure its density is modest requiring substantial pressure application (150bar) to produce good storage performance. On the subject of gases stored at very high pressures ESA also needs to store Helium, usually at over 300bar, to act as a pressurant (for MMH (Mono methyl hydrazine) and N2O4 (Dinitrogen tetroxide)).
The Inovgas technology is the result of a long research process. This process was focused in technological development of the forming process which was initially focused on the process to produce small low cost aluminium spheres for a high
pressure gas storage system.
The spheres had small polar openings where interface valves and caps were crimped, and were tested successfully up to 120 bar. The production of cylinders brought about the introduction of cylindrical bosses on either end of the vessel, which allows to apply interface ports or caps by welding, in addition to crimping.
In the company was asked by a Space industry customer to produce a set of 120 mm aluminium pressure vessels (for N2 (Nitrogen gas), with 40 bar MEOP - maximum expected operating pressure) for a ground testing jig. Currently, the target of our research effort is the production of liners with over 100 L of volume; for xenon storage in electric propulsion systems of satellites. These liners will afterwards be overwrapped with composite fibres in order to increase their capability to withstand pressure without excessive weight penalties. The technology has been evolving further, from the smaller volume tanks to growing dimensions.
The InovGas technology process involves a specialized tool mounted on a hydraulic press and a recyclable mandrel made with a low melting point alloy. The forming tool uses specially shaped dies with the form of the desired pressure vessel head. By controlling the press stroke, the initial length and the thickness of the tube, it is possible to produce a range of different vessels to use for gas and fluid storage (monolithic tank application) or as liners for overwrapping. It is also possible to produce both spherical and cylindrical vessels using the same tooling.
The mandrel is an essential part of the process, since it provides the needed support to avoid buckling or wrinkling in the tube wall. The mandrel is fully removed as a part of the process and leaves no trace on the finished product. It is also recyclable, and can be used to form multiple vessels with negligible material losses on each turnaround.
This technology is an alternative solution to currently in‐use gas storage tanks which, through an ingenious concept for gas entrapment, can deliver the performance required by this demanding industry. The gas is stored in highly pressurized spheres, which are then placed in larger tanks (operating at intermediate pressures) whose shape constraints are much less stringent than those of currently existing tanks. The advantages are obvious: avoiding the Xenon pressure regulator, higher flux rates between tanks and thrusters and an easier and cheaper manufacturing.
Innovations & Advantages
There are some similar technologies to manufacture aluminium liners in the way they deform the base metal to achieve the dome shape, starting from a stock tube: spin forming without and with metal inner support. The latter uses a permanent inner mandrel, which means that each dome must be manufactured individually and then joined together. The former doesn’t require the inner mandrel support, however the base metal must be heated in order to allow the metal to flow during the spin forming operation; in this case, since there is no inner support, the material tends to wrinkle, creating a very irregular inner surface and it is more prone to develop cracks during the manufacturing process. These new tank use a low melting point alloy as the material for the inner mandrel, conferring the required support to avoid buckling, crack formation and minimizing the wrinkling effects that form during the forming process.
- Aluminium liners obtained by Cold forming process are free from weld seams on the cylindrical section. With this forming process the amount of wasted raw material is reduced in the dome fabrication and the concentricity of both ends guaranteed.
- This manufacturing technology not only ensures the alignment between both dome ends when compared to the traditional manufacturing methods, but also reduces the amount of warping introduced during the welding process, since only the tank ports are welded.
Current and Potential Domains of Application
Space applications: Ionic and Chemical Propulsion, Cold Gas Thrusters.
Terrestrial applications: CNG (compressed natura gas), Passenger Cars and Buses - Autogas (LPG) tanks.