In a significant leap for launch-vehicle materials engineering, the European Space Agency's (ESA) Phoebus project has demonstrated that carbon-fibre-reinforced-plastic (CFRP) cryogenic tanks can safely contain liquid hydrogen, the smallest molecule in the universe, at temperatures near -253C without leaking.
The Phoebus project, led by ESA in collaboration with ArianeGroup and MT Aerospace, aims to evaluate the feasibility and advantages of replacing the metallic tanks on ESA's Ariane 6 upper stage with CFRP 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.
With a 2-metre-diameter, 2,600-litre composite hydrogen tank nearing production and testing scheduled for April 2026, the move away from metallic tanks on the Ariane 6 upper stage heralds substantial mass savings and a competitive edge in space-launch economics.
Let it Freeze
Hydrogen is the smallest molecule in the Universe, and when used as fuel on the Ariane 6 rocket it has to be cooled to -253C, just 20 degrees above absolute zero. Generally, carbon fibre composites do not like it that cold: when filled with cool propellants under pressure, small cracks can form, which is not what you want on a rocket tank.
With these extreme conditions, the Phoebus project team overcame many technical hurdles, not only on development and tank concepts but also on measurement, since there are no off-the-shelf devices that accurately measure miniscule leak rates at cryogenic temperatures as low as -253C. Phoebus has already proved it is possible: small 60-litre demonstration "bottle" tanks have shown that CFRP can hold hydrogen in liquid form without leaking.
Increasing the Ice-Cold Pressure to Cracking Point
Now the Phoebus team is working on a larger version with an updated design that will hold almost 2,600 litres. This 2-metre diameter hydrogen tank will be tested by filling it with hydrogen next year.
The tank completed the first manufacturing steps on its inner tank pressure vessel at MT Aerospace in Augsburg, Germany in September 2025, with production expected to be finished in December. ArianeGroup will be responsible for the testing, with engineers working on the design of the test facility.
A test campaign is planned for April next year. The testing will be conducted at an ArianeGroup test site in Trauen, Germany. The tank will be pushed step-by-step to its breaking point, creating the cracking effect but stopping before reaching complete failure. Work on the new test site kicked off in February 2025, with a preliminary design confirmed in June 2025, and a critical design review due by the end of this year.
Multiple tests will be run to 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 for evaluation between each test.
Phoebus is part of ESA's Future Launchers Preparatory Programme (FLPP), which helps develop technology for future space transportation systems, reducing the risk entailed in developing untried and unproven projects for space.
Source: www.esa.int
