PHOEBUS project makes progress on CFRP liquid oxygen, hydrogen upper stage tanks

Progress is being made with PHOEBUS, a European Space Agency (ESA, Paris, France) project developed together with ArianeGroup (Paris) and MT Aerospace (Augsburg, Germany). The carbon fiber-reinforced polymer (CFRP) tanks being developed in lieu of metallic tanks — one tank developed for liquid hydrogen, and one developed for liquid oxygen — will offer potential future application on the Ariane 6 rocket’s upper stage and spin-off industrial applications.

PHOEBUS aims to assess the feasibility and benefits of replacing metal tanks. While this CFRP 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 the ESA’s Ariane 6 rocket runs on liquid oxygen and liquid hydrogen, which are two very different molecules, so PHOEBUS project partners need to develop and produce two types of tanks able to withstand these extremes.

Liquid hydrogen tank milestones

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° above absolute zero. Generally, carbon fiber composites, and thus carbon fiber tanks, do not like it that cold — when filled with cool propellants under pressure, small cracks can form.

With these extreme conditions, the PHOEBUS project team has already overcome many technical hurdles — not only on development and tank concepts but also on measurement, as there are no off-the-shelf devices that accurately measure minuscule leak rates at cryogenic temperatures as low as -253°C. This has led to the successful development of small 60-liter demonstration “bottle” tanks, which have shown that CFRP is capable of holding hydrogen in liquid form without leaking, even without a protective liner.

Now, the team is working on a larger version with an updated design that will hold almost 2,600 liters. This 2-meter-diameter hydrogen tank will be tested by filling it with liquid hydrogen in 2026.

Manufacture of the inner tank pressure vessel began in September 2025 at MT Aerospace’s Augsburg facility and is expected to be finished in December. A test campaign is planned for April 2026, of which ArianeGroup will be responsible.

Multiple tests will be run to try and learn as much as possible about how the tank behaves as it would on the launch pad and during liftoff to understand how and where the first cracks could appear. Sensors on and in the tank will provide pressure, temperature and strain data to monitor during testing and evaluated in-depth between each test.

Liquid oxygen tank milestones

In comparison, liquid oxygen is cooled and stored to -180°C, a highly reactive and possibly corrosive propellant. Similar to its work with liquid hydrogen, PHOEBUS has proven the use of CFRP tanks via small demonstration “bottle” tanks without a protective liner. In addition, following a 2019 demonstration of holding liquid oxygen in a 2-meter tank, further tests were successful 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-meter 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. The experience from the PHOEBUS project has enabled a very smooth production of this first full-scale tank element, which will be finished in a few months.

PHOEBUS is part of the ESA’s Future Launchers Preparatory Programme (FLPP).