The Phoebus team has already proven it is possible, overcoming extremes in material science to contain liquid oxygen and liquid hydrogen with carbon fibre. Previous articles have highlighted progress on the tanks; this article focuses on the surrounding structures, exploring both the challenges and advantages of carbon-fibre reinforced plastic.

Opening the Tank

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 Phoebus team has worked wonders from an initial sketch of a concept to produce a manufactured full-scale part that works,” says ESA propulsion engineer Kate Underhill, “performing some really cool (pun intended) and complicated cryogenic tests on the way.”  

MT Aerospace in Augsburg, Germany, started the manufacture in July 2025 of the 2 covers that will be installed on a full-scale oxygen tank later this year.

Shaping up for more

With the tanks and the covers sorted, project Phoebus is making significant progress in demonstrating that carbon fibre-reinforced plastic is suitable for rocket tanks. These tanks are integral to the rocket stage, connecting to the engine and adding structural stability to endure powerful launch forces.

The Ariane 6 rocket upper stage’s engine is attached to the oxygen tank via a “thrust frame” with pipes running down it from the tanks to the engine. ArianeGroup has come up with an innovation that allows the fuel pipes to be a part of the thrust frame structure – 2 functions in one part equal a lighter launcher. This innovative thrust frame started production in December 2024, at various suppliers in Germany, with the assembly to be performed at ArianeGroup in Bremen, Germany in 2026. This innovative thrust frame is now in production, using the best techniques available worldwide such as including additive manufacturing for the central hub. A full-scale demonstrator with integrated pipes will be ready next year for integration onto the Phoebus liquid oxygen tank.