Water ice expands when it freezes , but it ' s very unusual in that respect most substances do not expand when they freeze . I believe the proposed mechanism for the COPV rupture involved frozen oxygen forming in the spaces in the wraps while the pressure in the helium tank was relatively low , and then when more helium was added and the pressure of the tank increased , that increased tank pressure caused the inner liner to press harder against the wraps . Liquid oxygen would have flowed out of the wraps into the main area of the oxygen tank , but the the solid oxygen was not able to flow , and thus experienced increased pressure from the increasing pressure of the helium in the COPV tank , not from frozen oxygen expanding .