Hydrazine Freezes Near 1.6°C, and the Thaw Can Burst a Line
Hydrazine freezes near 1.6°C, and that phase transition is known to introduce risks to flight hardware. The failure path NASA documents is a round trip: freeze-induced contraction followed by thaw-induced over-expansion, a sequence capable of bursting plumbing. It is the cycle, not the cold alone, that turns a propellant line into a liability.
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The Shuttle program counted cycles. Flight rules permitted no more than two freeze–thaw cycles before the system was considered degraded or failed.
Voyager shows how close hardware gets to that line. The bulletin notes mission-threatening scenarios as hydrazine temperatures approached the freezing and slush-formation range of 0.1–1.6°C, requiring detailed thermal modeling to prevent line freezing, blockage and thruster malfunction.