Ariane 5 attitude control system passivation: theoretical and experimental determination of the explosion threshold pressure

The Attitude Control System (ACS) of the future European heavy launcher Ariane 5 is a Hydrazine system including two bladder tanks pressurised by Nitrogen ; depending on the mission, a residual pressure of some 10 to 20 bars may still remain at the end of the mission. In order to avoid any debris generation, the explosion threshold pressure has been determined theoretically, then demonstrated experimentally. This pressure is defined by the behaviour of the tank under the impact of a micrometeoroid or a space debris : for pressures above it, the tank explodes ; otherwise, it is simply punctured. A theoretical analysis based on fracture mechanics was led first, showing that the crack propagation would become divergent for pressures in the range of 20 to 25 bars. A series of 2D simulations was then performed in two steps : first the local effect of the impacting particle on the tank wall, which enables the computation of the stress fields in the material ; second, the global propagation of such initial conditions in the complete tank. Several cases were computed, with different particles, impacting at different velocities on tanks with different pressures. The worst case was determined and a test prevision was led. The test was performed with a tank pressurized at 18 bar; the impacting particle was 1 mm diameter Aluminium sphere, launched at a velocity of 9 km/s. The tank did not explode, and the resulting hole was close to the one predicted (3.2 mm in diameter, vs. 3 to 3.5 predicted). The resulting maximal residual pressure specified to the ACS is 15 bar, a 20% margin being selected. With this new requirement, we are confident that no explosion can occur in orbit and that the general debris mitigation rules are fully applied.