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Tests prove carbon-fibre fuel tank for Phoebus upper stage

(20 January 2021 - ESA) Recent tests show that lightweight carbon-fibre reinforced plastic is strong enough to replace metal used in upper-stage rocket structures.

This is an important milestone in Europe for the development of a prototype of a highly-optimised ‘black’ upper stage, Phoebus, a joint initiative by MT Aerospace and ArianeGroup, funded by ESA.

The key goal of the Phoebus project is to increase launch vehicle payload performance by over two tonnes by reducing the mass of the upper stage through new design and lighter materials. At the same time, Phoebus shall also reduce production costs.

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Test of carbon-composite oxidiser tank (courtesy: MT Aerospace)

Carbon-fibre reinforced plastic (CFRP) materials, or carbon composites, allow new architectures and combinations of functions otherwise not possible using metallic materials. CFRP is lightweight and dark in appearance and will be used for the cryogenic propellant tanks as well as primary and secondary structures of Phoebus, hence the name ‘black’ upper stage.

Furthermore, their manufacturing process allows for an integrated layout that results in fewer parts compared to a comparable metallic configuration, thereby reducing production and assembly costs.

“The technology challenges include developing the machine capability that allows high-precision placement of the carbon composite materials and identification of the optimal subsequent curing steps to set the composite. The carbon fibre must withstand the extremely low temperatures of liquid oxygen and liquid hydrogen propellants whilst ensuring no leaks,” explained Kate Underhill, ESA propulsion engineer.

“CFRP material can be chemically very reactive with oxygen, therefore the proper selection of an appropriate material system of fibres and resin is an especially demanding task. Mastering this compatibility is a crucial milestone, which has now been achieved within the Phoebus project.”

During experiments by MT Aerospace on a testing site managed by Rheinmetall in Unterlüß, Germany, a subscale CFRP tank was tested with liquid oxygen. During these tests, the tank was filled and drained multiple times, pressurised beyond operational limits and shock tested to ensure no ignition event of the oxygen tank.

The test tank was equipped with a variety of sensors to monitor pressure, temperature, strain or a possible leakage. The analysis of the results and the overall good structural integrity of the liquid oxygen tank prove the technology.

Test of carbon-composite oxidiser tank (courtesy: MT Aerospace)

This achievement clears the way for further activities and indicates that the Phoebus demonstrator is on track. The next steps are the application of the CFRP material to a leak-tight liquid hydrogen tank design, and finally, a proper upscaling to and ground testing of the near full-scale Phoebus upper stage structural demonstrator in 2023.

These activities are being carried out within the Future Launchers Preparatory Programme of ESA’s Directorate of Space Transportation.

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