Todd J. Barber, Cassini lead propulsion engineer
Hello and Happy New Year from the Cassini flight team at JPL and NASA! Through the seemingly endless deluge of science results from Saturn, it’s a rare treat when engineering takes center stage. I’m extremely pleased to report a successful fuel-side repressurization a few weeks ago, a critical propulsion event that should enable Cassini science for years to come. When I’m not penning this column, my day job is lead propulsion engineer on this mission, and if you’ll allow this foray into the realm of engineering, I’ll try to explain why this event was momentous and important.
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.” Engine chugging is an oscillating instability sometimes seen in large rocket engines, and the chugging frequency for these large engines is roughly 20 Hertz (cycles per second). Hence, the name “chugging” is appropriate (imagine something going “glug-glug” twenty times per second and you’re basically there). The Cassini engine is much smaller, so its chugging frequency is vastly higher, around 260 Hz (that’s middle C to you music buffs). Chugging of the Cassini R4D was tested on the ground and would likely not be a problem for our engine or even the spacecraft, but the project has taken a wisely conservative position of avoiding the chugging region of the engine. Our recent fuel-side repressurization did exactly that, and in fact was the last required flow of helium through our pressurization system for the remainder of the mission, no matter how long it may last!
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.