Todd J. Barber, Cassini lead propulsion engineer
Have you checked your gas gauge lately? One of my main jobs as the lead Cassini propulsion engineer is to constantly keep track of the amount of propellant remaining on board the spacecraft. My kingdom for the equivalent of an automobile fuel-remaining display, or even a “low-gas” warning light! Truth be told, if we had such a light for bipropellant, it would already be shining brightly, since our oxidizer (nitrogen tetroxide) and fuel (monomethylhydrazine) tanks in the bipropellant system are already over 90 percent empty. Fortunately, Cassini is very miserly when it comes to expending bipropellant, and we anticipate we will have ample oxidizer and fuel to sustain us through even a doubly extended mission, through 2017!
Cassini’s other propulsion system, the monopropellant hydrazine system, is actually a wee bit above half full. We have two methods of figuring out how much hydrazine is left (or bipropellant, for that matter), and I call these the “bean-counting” method and the “PV=mRT” method. The former model is used to determine remaining hydrazine by trying to account for each molecule of hydrazine that is used by the thrusters, so it is more properly called a “consumption” model. The latter model may look familiar to those of you that survived high school chemistry—it’s the ideal gas law. On Cassini, we use a slightly more complex model, but essentially we determine the remaining hydrazine in the tank from telemetry data (from 900 million miles away, mind you) of tank pressure and temperature. Back on Earth, we know the tank volume and gas constant very well, so this allows us to solve for the hydrazine mass in this so-called “tank” model.
Early in the mission, our consumption and tank models for the quantity of hydrazine consumed from the tank disagreed by about 20 percent, a typical number for these kinds of missions. Through some excellent work by two summer students in propulsion, we were able to sharpen the pencil and improve the consumption model, cutting the discrepancy between the two models to only 8-10 percent. A final tweak to the consumption model (to force it to agree with the likely more-accurate tank model) years ago was implemented as well, using defensible technical rationale. Even though this adjustment was only made once, I’m thrilled to report that the modified consumption model and the tank model continue tracking each other very closely, year after year. This gives me more confidence when my coworkers and managers ask for the latest news from the Cassini gas gauges.
Even with this knowledge of the current hydrazine and bipropellant masses, it’s another matter entirely to make sure we have enough propellant to pull off extended missions, especially given the amount of unusable and reserve propellant that we must hold back. That sounds like a fine topic for a future column. Until then, given a lack of interplanetary filling stations, and with apologies to Tom Bodett and Motel 6, “We’ll leave the gas light on for ya….”