Phoebus 2m-scale hydrogen tank pressure vessel on an Automatic Fibre Placement machine at MT Aerospace in Augsburg, Germany.

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.

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.

Hydrogen is the smallest molecule in the Universe, and when used as fuel on the Ariane 6 rocket it has to be cooled to −253 °C, just 20 degrees above absolute zero, the coldest temperature in the Universe.

Generally, carbon fibre composites do not like it that cold, like your skin in winter. With the cold your skin gets dry and brittle such that when you move it can crack. This is the same for carbon fibre tanks: when filled with cool propellants under pressure, small cracks can form which is not what you want on a rocket tank.

Phoebus has already proved it is possible: small 60-l demonstration ‘bottle’ tanks have shown that carbon-fibre reinforced-plastic can hold hydrogen in liquid form – without leaking.

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 corrosion and the cold temperatures and figured out how to lay the carbon fibre so that they can bear the extreme conditions without cracking.

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.