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 −253 °C, just 20 degrees above absolute zero, the coldest temperature in the Universe.

Generally, carbon fibre composites do not like it that cold, like your skin in winter. With the cold your skin gets dry and brittle such that when you move it can crack. This is the same for carbon fibre tanks: 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 - there are no off-the-shelf devices that accurately measure miniscule leak rates at cryogenic temperatures as low as –253 °C.

Phoebus has already proved it is possible: small 60-l demonstration ‘bottle’ tanks have shown that carbon-fibre reinforced-plastic can hold hydrogen in liquid form – without leaking.