In-depth

Phoebus is a European Space Agency (ESA) project together with ArianeGroup and MT Aerospace. It aims to assess the feasibility and benefits of replacing the metallic tanks on ESA’s Ariane 6 upper stage with carbon-fibre reinforced-plastic tanks. While this lightweight material offers the possibility of saving several tonnes of mass, such an approach has never been implemented before and presents significant technical challenges.

The central core of ESA’s Ariane 6 rocket runs on liquid oxygen and liquid hydrogen, two very different molecules, so the Phoebus project is developing and producing two versions of the same carbon fibre tank concept. This article focusses on progress made on the liquid oxygen tanks, more news on the Phoebus project is linked at the end of this article.

Liquid oxygen is cooled and stored to –180 °C, a highly reactive and possibly corrosive propellant. Phoebus has already proved it is possible to use carbon fibre tanks for liquid oxygen storage: small demonstration ‘bottle’ tanks have shown that even without a protective liner, carbon-fibre reinforced-plastic can hold oxygen in liquid form  without leaking nor reacting.

Not all carbon fibre is created equal

Although carbon fibre reinforced plastic materials are found in many places and often abbreviated to carbon fibre or just carbon, not all carbon fibre is created equal. Carbon fibre used in racing cars is not the same grade as used in a tennis racket, for example.

Carbon fibre reinforced plastics are a mix of two materials: the fibres and a plastic resin infused together. You can think about it like clothing: most clothes are made of a mix of cotton and synthetic fibres, but a cheap t-shirt is not the same as technical garment. The difference is in which threads are used, their composition and how they are weaved together.

The Phoebus team has found the right reinforcing plastic resin to resist both oxygen corrosion and the cold temperatures and figured out how to lay the carbon fibre so that they can bear the extreme conditions without cracking.

Going big, not going bang

After the 2019 demonstration of holding liquid oxygen in a 2-metre tank, further tests were a great success and did not end up in any unplanned disassembly of the tank, as could be feared if the material reacted with oxygen.

An improved tank design has now been expanded to a full-scale 3.5-metre tank concept, which is in the last phases of production: the manufacturing tools are being removed and quality control inspections performed at MT Aerospace in Augsburg, Germany. The experience from the Phoebus project has allowed for a very smooth production of this first full-scale tank element, which will be finished in a few months.

These parts are made using automatic fibre placement technology. The part’s design is programmed into a machine, which lays down the carbon fibre tape, similar to how a sewing machine uses thread. The machine is also being improved to automatically check the quality of the parts which saves time and reduces inspection costs. Future upgrades may include using automation and artificial intelligence to further improve quality control.

Phoebus is part of ESA’s Future Launchers Preparatory Programme (FLPP), that helps develop the technology for future for space transportation systems. By conceiving, designing and investing in technology that doesn’t exist yet, this programme is reducing the risk entailed in developing untried and unproven projects for space.

Source: ESA