Todd J. Barber, Cassini lead propulsion engineer
To start, Cassini is a three-axis-stabilized spacecraft instead of a spinner, but to help control spacecraft pointing, there is a set of four Reaction Wheel Assemblies (RWAs), RWA-1 through RWA-4. Only three RWAs are needed for spacecraft control, with one spare ready to step in, if needed. In fact, we started the mission on RWAs 1-3 with RWA-4 as the back-up, but due to some performance concerns on RWA-3, we put RWA-4 in the game and benched RWA-3 in July of 2003. Biasing the RWAs is accomplished by firing the RCS thrusters while changing the RWA rotation speeds, all while the spacecraft attitude (pointing) remains fixed. RWA biases are done quite frequently and are relatively heavy users of hydrazine, but they are absolutely necessary to keep the wheel speeds within safe ranges.
I’ve talked before about our swap to the B-branch thrusters, and one unanimous recommendation from our internal team and an external review board was to try to “spread the pain” between Cassini’s Z-thrusters and Y-thrusters more evenly for the B-branch. On the A-branch thrusters, which were used from launch until March of 2009, we saw issues on two of four Z-thrusters but no discernible degradation on Y-branch thrusters. The A-branch Z-thrusters had more than twice the hydrazine throughput of the Y-thrusters because they were used for RCS maneuvers in addition to the wheel biasing and attitude control functions that both Y and Z-thrusters perform. Even so, the A-branch Z-thrusters were well below their specification limits on throughput and cycles, so there is still a good propulsion mystery to be tackled. Given this premature degradation on the A-branch, we set off trying to see how we could use the Y-thrusters more (and thus the Z-thrusters less) on the B-branch. Given geometric and thruster orientation constraints, the only way to substantially decrease the Z-thruster burden was to execute as many RWA biases as possible on Y-thrusters, knowing full well this would preclude real-time visibility (since they could no longer be done on Earth-point) and, in fact, would likely raise hydrazine consumption, given the Y-thrusters have shorter moment arms than the Z-thrusters. Undaunted, we made our propulsion-centric recommendation and then the systems trades began.
To our great surprise and joy, we found out that Y-thruster biases actually use LESS hydrazine than the current mostly Z-thruster RWA biases! This was verified in ground models and then even confirmed on the spacecraft back in August. Essentially, there is much less “fighting” between the thrusters to control the RWA spin rates in this optimized mode of using the Y-thrusters. The savings is typically on the order of 35 percent, a very large number that will greatly help our bottom line for hydrazine projections. While this all was transpiring, our Mission Planning engineers finished analyzing their projections for the Solstice mission through 2017—and initial results were not encouraging. There was precious little hydrazine left, if any, by the end of mission—but that was before Y-biases were included. I’m happy to say that when conservative estimates of Y-bias placement are included, our propellant margin shoots back up into more comfortable territory. We now only need to make sure our ground software processes can handle this new way of doing business, but, once again, the future looks bright. Stay tuned for many Y-biases in the years to come!