A NASA technical bulletin, published on 23 September 2026, reviews the risks of freeze–thaw cycles in hydrazine monopropellant systems and details general recommendations for mitigating damage, operational hazards and loss of system reliability. The bulletin says hydrazine freezes near 1.6°C, and that phase change is known to introduce hazards to flight hardware.

The bulletin cites earlier NASA programs, including Space Shuttle rules that permitted no more than two freeze–thaw cycles before a system was considered degraded or failed, and Voyager thermal modeling aimed at preventing line blockage and thruster malfunction. Listed risks include superpacking, in which contraction during freezing draws extra propellant into confined lines and thawing can rupture them; valve, seal and diaphragm damage; slushy hydrazine restricting flow; hazardous leaks; and reduced system redundancy and life span.

Recommended prevention includes keeping all hydrazine systems above freezing and avoiding uncontrolled cooling during ground operations. If freezing cannot be ruled out, the bulletin suggests suspending system use, treating each cycle as life limiting, checking integrity and warming hardware gradually and evenly. It also advises thermal stabilization before commanding flow and relying on detailed line thermal modeling rather than bulk plate or tank temperatures alone. All freeze exposures should be recorded as reportable anomalies, with engineering disposition and tracked hardware life reduction.