For complex, long-duration orbital maneuvers, what operational priority often supersedes maximizing the performance of the propellant combination?

Storability and reliability

While maximizing Specific Impulse ($ ext{I}_{ ext{sp}}$) is crucial for initial launch performance or reaching deep space velocity targets, mission profiles involving long-duration station-keeping or complex orbital adjustments often prioritize reliability and storability. For missions where an engine might need to fire only once after sitting dormant for years in the harsh vacuum of space, engineers often select storable propellants like hypergolics. These systems, though potentially less efficient than cryogenics, guarantee that the seals have not degraded and the fuel has not boiled off, ensuring the critical maneuver can be executed successfully when required, making operational safety and guaranteed ignition paramount over peak energy efficiency.