Phoebus cover and tank bulkhead

Phoebus carbon-fibre reinforced-plastic cover and tank bulkhead.

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.

A propellant tank is more than a container. The liquids inside must be pumped in and sent to the rocket engines for liftoff, while sensors monitor fuel levels and other parameters. These “feed-through” elements pass through the top and bottom of the tank. These openings also make it possible to clean the tanks and install equipment.

Bolts secure the end covers, but the tanks are made to hold liquid oxygen and hydrogen at temperatures far below –100°C. At these extremely cold levels, metal bolts behave very differently from the carbon material. Because metal and carbon react differently to the cold, they push and pull against each other, which can quickly create a path for leaks even at the tiniest imperfection.

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. 

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.