Todd J. Barber, Cassini lead propulsion engineer
Cassini’s propulsion system is the most complex ever flown on a planetary spacecraft (which I guess translates to job security for yours truly). There are two separate systems for large maneuvers and small maneuvers, the bipropellant and monopropellant systems, respectively. Even though Cassini has depleted more than 90 percent of its bipropellant, we still plan to use the main engine for years to come. For over a decade, our fuel (monomethylhydrazine, or MMH) and oxidizer (nitrogen tetroxide, NTO) have become acquainted, explosively, in the chamber of our prime R4D rocket engine. These two propellants are hypergolic, which mean they ignite on contact (not unlike my ornery niece and nephew, unfortunately). There is a preferred amount of oxidizer vs. fuel to burn in this engine; we have been running the engine at a mixture ratio (oxidizer mass used divided by fuel mass used) of about 1.65. As it turns out, continuing to use the engine at this mixture ratio would likely deplete the oxidizer first, so our recent fuel-side repressurization was optimized to aim for running out of NTO and MMH at the same time, many years from now.
Even with this explanation, mixture ratio optimization and maximizing the propulsive capability of Cassini were not the prime drivers for our MMH-side repressurization. Rather, we were approaching a portion of our operating box (the allowed propellant flow rate and mixture ratio regime of the engine) called the “chugging boundary.”
Thanks for coming along on this narrative journey into the innards of engineering; I promise a return to Cassini’s ultimate purpose -- science -- in my next column.