Todd J. Barber, Cassini lead propulsion engineer
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.