A recently closed GSTP activity, under the De-Risk framework, has shown that plastic may now be a better use for lining propulsion tanks and coating them to prevent leaks than the heavy metal currently used.
For many years engineers have been trying to make non-metallic tanks a viable option. Removing the metal linings and carbon coatings that makes current designs – such as the hydrogen tanks used in liquid propulsion seen above – impermeable and protect the helium, xenon or other material inside, would make them much lighter, easier to manufacture and cheaper overall.
But the liner and over wrap must be leak-proof, since often these tanks can be used for storage for up to fifteen years. In that timeframe, even a small leak can prove to be a big, and costly, problem.
Engineers have tried replacing the metal liners and overwrap with plastic polymers in the past, but the materials have not been able to match the impermeability of metal.
A de-risk activity was proposed to look into advances in the field, test for new state-of-the-art materials, shortlist which ones might provide alternatives to metal and assess the permeability and manufacturability of each.
The activity, conducted with Haydale Composite Solutions Ltd, a UK-based company, found that carbon fibre is the most preferable reinforcement as an overwrap, but potential resin matrixes could be made from epoxy, cyanate ester and polyurethane to line them.
Each material had pros and cons, where the cost and ease of manufacture sometimes outweighed the impermeability benefits.
“There wasn’t a single material that proved best, because there’s a trade off,” explains Christopher Hunter, the Technical Officer for the activity. “You find one material that has a great leech rate but it might be too expensive. Sometimes you have to let go of a prime characteristic to have a commercial product – at the end of day someone has to buy it.”
The project also had to take into account the material being stored. Helium, used for pressurising propulsion systems, will diffuse through anything non-metal, while xenon, which is used for electric propulsion can leech material from the tank lining that could end up poisoning the thruster.
This de-risk study is now progressing, with the results being taken forward in a follow up activity with another ESA programme. Going forward, the materials will be subjected to further tests of their properties, such as whether they can withstand vibration or acoustic tests, and their manufacturability.
GSTP activity G617-241Tag closed in November 2018.