Together with MT Aerospace, Grasse Zur Composite Testing has developed a test stand for LOX ("liquid oxygen") testing. The main goal of the Phoebus project is to increase the payload performance of the launcher by more than two tons by reducing the mass of the upper stage through a new design and lighter materials. At the same time, Phoebus is also expected to reduce production costs.

Carbon fiber reinforced plastics (CFRP) enable new architectures and functional combinations that are not possible with metallic materials. Lightweight and dark-looking, CFRP is used for Phoebus' cryogenic fuel tanks and primary and secondary structures, hence the name "black" upper stage. In addition, its manufacturing process allows for an integrated layout that results in a smaller number of parts than a comparable metallic configuration, reducing production and assembly costs.

Using the test rig developed by Grasse Zur Composite Testing, the CFRP tanks are cooled to approximately -180 °C. Internal pressure is applied via a fully remote control system. Four independently switched impact pendulums realize the application of impacts on the upper and lower end cap of the tank (so-called "boss"). The tests, which were carried out at an open-air test site in Lower Saxony, show that the lightweight, carbon-fibre-reinforced plastic is strong enough to replace metal in the structures of the rocket upper stage. This is an important milestone in Europe for the development of a prototype of a highly optimized "black" upper stage, called Phoebus, a joint initiative of MT Aerospace and ArianeGroup funded by the Future Launchers Preparatory Programme of the European Space Agency - ESA 

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