The European Phoebus project, jointly developed by the European Space Agency (ESA), ArianeGroup, and MT Aerospace, is entering a new phase where composite technology moves from the laboratory to large, concrete demonstrators. The goal is ambitious: to replace the classic metal tanks on the upper stage of the Ariane 6 rocket with tanks made of carbon-fibre reinforced plastic (CFRP), thereby saving several tons of mass, enabling greater performance, and reducing costs per launch. Behind the term "black upper stage" lies a multi-year development that connects materials, cryogenic engineering, safety systems, and industrial logistics from Augsburg and Bremen to Trauen and Lampoldshausen.

Why Composites on the Upper Stage at All?

Every kilogram of mass saved on the rocket's upper stage multiplies in the final payload. CFRP solutions, with the right layer architecture, matrix, and production control, allow for a delicate balance between strength, stiffness, and resistance to extreme temperatures, all with a significantly lower mass than aluminium or cryogenically optimized metal alloys. In rocket systems today, we already use composites for fairings and large solid motors (like the P120C), but using composites as the primary structure for cryogenic tanks is a much harder task. This is precisely the field where Phoebus is raising the technological bar and readiness level (TRL) so that the future "Black Upper Stage" can truly be commercialized as the successor to today's Ariane 6 upper stage.

The Extremes of Liquid Hydrogen: −253 °C and a Molecule That Finds Every Microcrack

Hydrogen is the smallest and most lively molecule in the universe. In the Ariane 6 rocket, it is used as a cryogenic fuel and is kept in a liquid state at around −253 °C, just twenty degrees above absolute zero. At these temperatures, most polymers and composites become more brittle, so engineers face phenomena like microcracks in the matrix, layer separation (delamination), or changes in permeability. Additionally, hydrogen easily diffuses through microscopic pathways, so even minimal defects can become channels for leakage. The designers of Phoebus must therefore design a tank that can simultaneously withstand extreme temperature gradients, cyclic pressure loads, and mechanical stresses from launch vibrations.

From a "Bottle" to a 2-Meter Diameter Tank: A Step-by-Step Journey

The first experiments within the Phoebus project used small demonstrators of about 60 litres, so-called "bottles," to prove that a CFRP structure, with the appropriate laminate architecture and post-processing, could reliably hold liquid hydrogen without leakage and without unwanted reactions. These demonstrators confirmed fundamental material models, calibrated parameters for low temperatures, and developed testing methodologies that until then had mostly existed only for metal tanks.

After the "bottles" comes a significant step forward: the creation of a 2-meter diameter tank with a volume of nearly 2600 litres. At this scale, a whole new set of challenges arises, from controlling tolerances during automated fibre placement (AFP) and the precision of joints, to managing residual stresses after curing and integrating connections, valves, and sensors that must remain reliable at −253 °C.

Production in Augsburg, Tests in Germany: Who Does What

MT Aerospace in Augsburg carries out the key steps of manufacturing the inner pressure vessel and its associated composite elements. The company has built capacities for producing composite structures and tanks up to 3.5 meters in diameter, with advanced automated machines for tape and tow placement and native quality control systems. ArianeGroup takes responsibility for testing the tanks, including designing and building a new test facility, as well as defining test procedures, safety protocols, and measurement architecture. The teams also include partners specializing in cryogenic engineering and analytics, while some system capability verifications are entrusted to state institutes.

Development Timeline: Autumn 2025, Reviews by Year-End, and Tests in Spring 2026

In September 2025, the first set of production operations on the inner chamber of the 2-meter diameter liquid hydrogen tank was completed. The production phase is scheduled to close by December, which includes the final composite wraps, connecting flanges, and preparation for instrument integration. The critical design review (CDR) is planned by the end of 2025 to confirm all key design assumptions and give the "green light" for civil works at the test site and equipment installation. The liquid hydrogen testing campaign is planned for April 2026 and will be conducted at ArianeGroup's test site in Trauen, Germany.

Trauen as a Cryogenic Stage: What This Means Logistically

Trauen is part of a German network of facilities and plants specializing in hydrogen: alongside ArianeGroup's primary role in Europe, the location serves as a hub for the development, qualification, and testing of subsystems, equipment, and demonstrators that work with liquid cryogenic media. For Phoebus, this means that systems for filling, a society of tanks for inert gases, systems for venting and gas recovery, fire-resistant infrastructure, and safety perimeter zones will be integrated in one place. Given that liquid hydrogen, despite being at −253 °C, very easily forms flammable mixtures, every filling and testing operation is conducted with rigorous procedures and multi-layered safety barriers.

How a Tank is "Listened To": Sensors, Measurements, and Models

To understand the behaviour of the composite tank during filling, tight containment, and emptying at −253 °C and under increased pressures, the structure is "sprinkled" with sensors. On and in the laminate, there are strain gauges, fibre optic networks, temperature probes, pressure sensors, and highly sensitive leak analysers. A particular challenge is measuring microscopic leaks under cryogenic conditions: ready-made industrial solutions are almost non-existent, so the team has developed its own test configurations with calibrated leaks, inert media (helium), and algorithms that separate instrumental noise from real signals. Data is recorded at different load "steps" to map the locations where microcracks appear earliest and to compare the results with numerical layer-by-layer models.

How Far Will We Go in Testing: To the Point Before Fracture

The plan is to "push" the tank through multiple phases, up to the borderline point where controlled cracks begin to appear—but to stop the test before complete fracture. This provides crucial information about strength reserves, damage progression, and safety margins relative to the real loads during launch preparation and in the first few minutes of flight. Each filling and draining cycle is monitored by telemetry, and between steps, a detailed data analysis and structural inspection are performed using non-destructive methods (e.g., ultrasound, thermography, acoustic emission).

A Parallel Front: Large-Diameter Oxygen Tanks

While the hydrogen tank progresses along its development path, demonstrators for liquid oxygen are also advancing within the same program. There, the emphasis is on a lineless (without an inner liner) approach and confirming that CFRP can contain LOX without undesirable reactions and without leakage. In the past production cycle, the first full-scale tank with a diameter of approximately 3.5 meters was made and completed, which is a strong signal that composite solutions in the cryogenic field are maturing even at larger diameters. In parallel, automated layer placement processes and inline quality control systems that recognize anomalies and defects during layer placement are being improved.

From Phoebus to ICARUS: What "Black Upper Stage" Means

Phoebus is conceived as a technological demonstrator that lays the groundwork for the next generation of the upper stage, often mentioned under the working title ICARUS (Innovative Carbon ARiane Upper Stage). The success of the hydrogen and oxygen demonstrators would open the door to an integrated upper stage with composite tanks, cryogenic systems, and a compatible structure that keeps the total mass minimal and functionality (autonomy, re-ignition, deep discharge) maximal. Additionally, the implications for the architecture of supply lines, filling systems on the launch pad, and maintenance standards during the operational life cycle are being considered.

Safety First: Managing Risk with Liquid Hydrogen

Although hydrogen is kept at an extremely low temperature in tests, its ability to ignite in contact with an oxidizer or a spark demands extreme discipline. Test campaigns are conducted with strict no-access zones, redundant hydrogen detection systems, automatic inerting with nitrogen or helium, rapid pressure relief valves, and fire protection systems adapted for cryogenic conditions. Each procedure has pre-prepared scenarios and abort points, and the teams go through multiple dry runs with non-cryogenic media before the first LH₂ filling.

The Broader Industrial Picture: From Materials to Jobs

The development of Phoebus is not just a technological story: it is also an industrial program that strengthens European autonomy in the field of composite cryogenic tanks. Investments in equipment, people, and processes in Augsburg, Bremen, and at German test sites strengthen supply chains and create the prerequisites for Europe to design, produce, test, and qualify key elements of rocket upper stages itself. In the long term, the knowledge gained in the space sector spills over into aviation (initiatives around LH₂ aircraft), energy (tanks and pipelines for hydrogen), and mobility (ground tanks, logistics).

A Closer Look at the Technology: What Makes a Good CFRP Tank for LH₂

• Laminate Architecture: precise stacking of fibre orientations to manage anisotropy and prevent stress "swinging" around openings and connections.


• Matrix and Compatibility: a resin system that retains toughness and adhesion to fibres at −253 °C, with a minimal microcrack network after curing.


• Production Methods: automated fibre placement (AFP/ATL) with inline quality monitoring, controlled curing cycles, and post-process heat treatments.


• Connections and Transitions: metal-composite hybrids that mitigate stress concentrations and ensure sealing under cyclic loading.


• Barrier Layers and Permeability: optimizations that reduce hydrogen diffusion through the matrix without a significant increase in mass.

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NDT Inspections:

ultrasound, thermography, and optical fibres embedded in the structure to monitor damage in real time.

What to Expect from the Test Campaign in April 2026

The planned series of tests includes fillings and reliefs at defined pressure levels, thermal cycles, tests under vibrational loading, and finally, a test approaching limit states. Special attention is paid to monitoring the "first cracks"—these are microscopic phenomena that indicate where stresses are locally accumulating. Such observations directly feed into design iterations and production instructions, shortening the path to a tank that can withstand real flight conditions with greater margins.

Impact on Ariane 6 Operations and Market Positioning

A successful transition to a lighter upper stage brings a dual benefit: a larger payload to the target orbit and a potentially lower cost per kilogram. This gives Ariane 6 additional flexibility for constellations, interplanetary missions with more complex injection profiles, and missions that require re-ignition and precise management of orbital energy. A "black upper stage" would also be a strong message to the market that Europe possesses its own innovative composite technologies at a level that solves the cryogenic challenges of both liquid hydrogen and oxygen.

Plans Until the End of 2025: Critical Decisions and Construction Work

By December 31, 2025, a critical design review for the test infrastructure and the tank is planned, after which construction work and the installation of specialized equipment at the site will begin. The documentation must confirm that all safety criteria have been met, that the testing scenario is feasible, and that the margins are in line with the program's objectives. This opens the way for the spring schedule of liquid hydrogen tests in April 2026.

How This Story Fits into FLPP: The Program That Reduces Future Risk

Phoebus is part of ESA's FLPP (Future Launchers Preparatory Programme), whose role is to reduce technical and developmental risk before entering expensive phases of serial production and operations. Through FLPP, demonstrators are financed and coordinated, verifications are carried out, standards are set, and knowledge is transferred between industrial partners. A framework has been established in which new technologies—such as cryogenic CFRP tanks—can be proven in the field and then incorporated into real systems with clear market effects.

What We Learned from Small Demonstrators and Why It Matters

The 60-litre "bottle" is not just a symbolic step, but a crucial one for validating basic physical assumptions: how microcracks develop at cryogenic temperatures, how changes in production process parameters affect permeability, how much barrier layers help, and where the laminate needs to be reinforced around connections. These lessons form the core knowledge that is then scaled up to 2-meter diameter tanks and beyond, with constant alignment of computational models with test results.

Equipment That Makes a Difference: From AFP to Digital Quality Verification

Automation is key to producing large composite tanks. Automated fibre placement (AFP/ATL) machines work with great precision, but for cryogenic tanks, an additional "real-time eye" is needed. Built-in visual and thermal inspection systems during production allow defects to be recognized on the layer where they occurred, before they become hidden by deeper layers. In parallel, digital process recording—a digital thread—is carried out, which later facilitates the correlation between defects, processes, and the tank's behaviour during testing.

Integration with Rocket Systems: Not Just a Tank, But a Whole Ecosystem

A composite tank also changes other parts of the upper stage: from cryogenic lines and valves, through insulation and anti-geysering solutions, to how boil-off and pressure relaxation on the pad are managed. Subtle decisions come into play about where to place sensors, how to route cables and optical fibres to withstand vibrations and thermal stresses, and how to solve service connections that are compatible with existing ground filling equipment. All of this must fit within a mass and volume that makes sense for the mission's performance.

The Environmental Perspective: Hydrogen, Emissions, and Future Mobility

Although rocket emissions are specific and a small part of global statistics, the technologies developed for the safe handling of liquid hydrogen and lightweight cryogenic tanks will have a broader impact. Knowledge about permeability barriers, safety zones, inerting, and leak detection will spill over into aviation, which is exploring hydrogen as a fuel, as well as into land mobility and energy infrastructure, where it is crucial to reduce losses and risks throughout the entire chain.

What Follows After April 2026: The Path to Qualification

If the results show that the composite hydrogen tank meets the criteria for impermeability, strength, and durability with acceptable margins, the next steps will lead to extended testing campaigns, including long-duration liquid hydrogen hold tests, high-cycle thermomechanical tests, and integration checks at the upper stage system level. In parallel, production will be optimized, areas for further mass savings will be sought, and standards will be finalized to accompany the transition from a demonstrator to a flight-qualified configuration.

Key Points in One Place

• Development of composite liquid hydrogen and oxygen tanks for the Ariane 6 upper stage with the aim of significant mass savings.


• Production of the 2-meter diameter inner pressure vessel completed its first phases in September 2025; production completion planned by December.


• A critical design review by the end of 2025 will enable the start of construction work at the test site.


• The liquid hydrogen test campaign is planned for April 2026 in Trauen (Germany); the scenario includes a gradual approach to the point before fracture.


• In parallel, a full-scale (approx. 3.5 m diameter) lineless LOX tank is also progressing, confirming the maturity of CFRP solutions for cryogenic conditions.


• The project is part of ESA's FLPP and builds European autonomy in critical technologies for future launch systems.

Glossary of Terms for Quick Reference

• CFRP (Carbon-Fibre Reinforced Plastic) – plastic reinforced with carbon fibres; high strength-to-mass ratio.


• Cryogenic tank – a tank for ultra-cold liquids (LH₂, LOX) at temperatures below −150 °C.


• Lineless – a tank construction without an inner metal/plastic liner; the composite directly contains the cryogenic liquid.


• CDR (Critical Design Review) – a formal review that confirms the design is ready for the next phase.


• AFP/ATL – automated methods for placing composite fibres or tapes with a high degree of repeatability.