Todd J. Barber, Cassini lead propulsion engineer
I often get asked what “powers” the Cassini spacecraft, and when I start feebly trying to explain (with my limited knowledge) radioisotope thermoelectric generators, shunt regulators, and 30-volt buses, the blank gazes begin in earnest. Very often, the follow-up question is something like, “No, what makes it go?” So then I realize “power” and “propulsion” have been confused, a common mistake. For a simple example, in your car, the “power” system includes the battery, starter, spark plugs, etc., while the “propulsion” system is the internal combustion engine, drive shaft, and so forth. After receiving this common second question, I’m only too happy to dive into a prolonged discourse about the Cassini propulsion system, only feeling a tad guilty that it is actually gravity that does the yeoman’s share of the work in making Cassini “go.” On the rare occasion when someone asking about Cassini’s “power” system actually means the electrical power system, I do my best to explain the subject—but the blank stares often persist. Therefore, I decided to interview Cassini Lead Power Engineer, Andrew Ging, for this month’s column. I learned quite a bit and perhaps you might too!
Andrew Ging started at JPL about five years ago as an intern from Cal Poly Pomona, nearly finished with his degree in aerospace engineering. His first job was operating and maintaining the Environmental Test Lab here, but right after graduation he joined the Cassini team as a power analyst. The power lead at that time was Chuck Keith, but the allure of retirement beckoned Chuck a few short months later. Andrew confessed to me he was “apprehensive” about Chuck retiring and being in charge at such a young age, but this seems eons ago. He quickly assumed responsibility for the power system (by himself), wrote two AIAA conference papers, and now it seems as if he has been here since launch (as a few of us have).
Andrew grew up in Wrightwood, Calif., with a fondness for military jets and childhood dreams of being a baseball player or scientist. He always loved space, though—including reading every book about planets in the Wrightwood Elementary School Library! I think I was most intrigued to learn he was home-schooled for one year in sixth grade, only then to be put into the lower-level mathematics class in seventh grade because of one tiny oversight. When he took the seventh grade math placement exam, he didn’t know the “dot” symbol meant multiplication, so this erroneously forced him into the basic math class! He recovered, moving on to upper-level math classes and his degree in engineering. Like me, though, he only had one “circuits” class in college, so his entry into the discipline of power engineering is somewhat random (but fortunate for us).
The basic job of Cassini’s power system is to provide 30 volts DC (direct current) for science instruments, heaters, communication equipment, computers, valves, reaction wheels and many other electrical “pieces” of Cassini. This is accomplished primarily by utilizing three nuclear "batteries" on the spacecraft called Radioisotope Thermoelectric Generators (RTGs). Cassini has three RTGs, one of which was a spare from the Galileo mission to Jupiter. RTGs use the heat of plutonium (Pu-238) alpha decay to create DC electricity using 572 thermocouples per RTG, each with a beginning-of-life efficiency of about 6.5 percent. Pu-238 has a half-life of about 87.7 years, but the effective half-life of the RTGs is less than this, due to aging of the thermocouples. Even so, they are amazing devices, offering stable, repeatable, slowly decaying DC power for decades, with no moving parts! The total power output from the three RTGs was about 880 watts at launch. It is currently about 670 watts, only decaying to about 605 watts by the anticipated end of the Cassini Solstice Mission in September 2017. If you used a hairdryer today, I’d wager you required far more power than Cassini requires for the entire spacecraft! Most hairdryers use between 1,200 and 1,800 watts. This is even more amazing when you realize Cassini is the size of a small school bus!
Other portions of the power system include the power control boards, with 192 solid-state power switches (SSPSs), a radiator to “shunt” excess, unused power as heat to space, and a shunt regulator assembly, which (1) tells the radiator how much power to shunt and (2) maintains the spacecraft bus voltage at 30 volts. Actually, these 30 volts are divvied up into two lines, one at +15 volts and one at -15 volts, with the chassis of the spacecraft as the electrical ground (0 volts) and separated from the powered lines by 2,000 ohms of resistance (anyone remember V = iR? That’s about all I remember!). Andrew’s first paper dealt with shifts in the +15 volt/-15 volt bus imbalance; these have had no impact on the mission. On a happy note, Andrew was able to “find” an extra 20 watts of power recently. The Cassini team thought the shunt regulator circuitry was taking up these 20 watts of power, but it turned out to be a calibration issue with spacecraft power telemetry. This freed up 20 watts of power to enable more science activities, and this is covered in detail in Mr. Ging’s second AIAA paper. Another thing that keeps Andrew busy is keeping track of SSPS “trips,” presumably caused by galactic cosmic rays (GCRs). He was even able to somewhat correlate trips with the extrasolar neutron flux, venturing into the world of particle physics! Cassini has experienced 32 SSPS trips since launch nearly thirteen years ago; the expected rate is about two per year, so we are close to this rate. This year has been busier than usual, with five trips to date, but we don’t have any reason to think this is anything other than the normal statistical variation. Trips on some switches that power heaters could have had some serious consequences before we would have had time to see the trip and respond from the ground, so in these cases we modified the fault protection response to turn heaters back on autonomously.
I closed out our meeting asking Andrew about challenges for the power system during the final mission extension. Andrew told me the Solstice Mission operational modes (opmodes) were just developed and now they have to be implemented. Essentially, with RTG power levels decaying year to year, it will become impossible to do an endless amount of concurrent science as the mission progresses. Opmodes offer an easy way to “reign in” voracious scientific appetites (as they should be), given the reality of less electrical power available. If you’ve seen “Apollo 13” where the engineers and crew are pushing to save every precious watt to enable their return trip to Earth, the situation is completely analogous. The important thing is to avoid (“at all costs,” according to Andrew) an undervoltage trip, where there is a higher load on the power bus than it can handle and the system responds by switching off all but a small set of necessary components and calling fault protection. (An undervoltage trip, for example, could be caused if we turned on too many things at the same time.) In fact, we have a flight rule that states we must not let the power margin dip below 20 watts (actually, we’ve never been below 60 watts in flight). I think Mr. Ging summed it up best when he said his job was to “get the most out of the power we have while avoiding an undervoltage trip.” As he poignantly opined, “Like all of life, it is all about balance.” Andrew, I couldn’t agree more. Thank you for your time and a glimpse into the inner workings of the Cassini power subsystem (and, if you’ll pardon the pun—more power to you!).