OFJ97 Field Journal from Todd Barber - 2/6/97

Since the tour began, my two German colleagues (Fritz and Klaus-Peter) and I have had to adjust to challenge of performing a propulsive maneuver every 2-3 weeks, much more frequently than during interplanetary cruise. However, this is offset by the fact that these maneuvers have become quite routine (especially when contrasted with the uniqueness of each of the three main engine (what we refer to as the 400-Newton, or 400-N) firings!). A more mundane mission with respect to propulsion is certainly fine with Fritz, Klaus-Peter, and me, since the mission now is truly an odyssey of scientific discovery.

And, yet, intertwined with the routine there are always new challenges! Just today I sat in on a meeting looking at the problem of insufficient power near the end of the proposed Galileo Europa Mission (or GEM; this would keep Galileo active for another two years). As the spacecraft's radioactive main power sources continue to decay, there is less and less power available for the electrical and heating needs of the spacecraft. So, we need to examine ways to conserve power so that we can continue to run the spacecraft. We received some very interesting questions this morning, some of which follow:

"Can we turn off the 5 Watt heater on the 400-N engine to save power?"

This is a great way to save 5 Watts, but we have to ponder this carefully. The temperatures at the 400-N engine (which still contains fuel and oxidizer "upstream" from the valve seats) would plummet many tens of degrees in this scenario. In fact, propellant freezing near the 400-N engine is a certainty in this case; that's why we have a heater to begin with. Incidentally, the valve seats of which I speak are basically teflon (plastic) sealing devices that are opened and closed to control the flow of propellant to the main engine.

We have many questions about the "best" (read: safest) way to turn off the 400-N engine heater, should it prove necessary. Should we dump the propellant just upstream of the engine before freezing (by firing the 400-N engine ever-so-briefly with closed latch valves), or is this action more risky than just freezing the propellant already there? What will happen to the valve seats at the 400-N engine when their temperatures plunge below any previously tested limits?

It's interesting, isn't it? One seemingly innocuous question ("can we turn off the 5 Watt heater on the 400-N engine to save power?") has given rise to other, more provocative questions that are not easily answered. This is not unlike the challenges tackled by our science teams! Frequently, a new scientific discovery from Galileo answers one question (perhaps one posed twenty years ago after interpreting Voyager results), but raises many others! To me, this is one of the indescribable thrills of both science and engineering.

Another question from this morning, "Can the RPM [RetroPropulsion Module] transducers [sensors] be turned off towards the end of the GEM mission?" This action would save some 3.8 Watts, but again this inquiry raises further questions that Fritz, Klaus-Peter and I will have to try to answer. What are the chances that the transducers would never turn on again after being turned off? Could turning them off then on again affect their performance and calibration?

Without these transducers, we would not be able to monitor the pressures or temperatures for any of the RPM hardware. This is certainly a little discomforting, since we count on that data to check the RPM performance and any potential problems. However, this may be an acceptable risk near the end of GEM.

Other than asking these types of questions, I have kept busy looking at thruster performance during various types of spacecraft maneuvers (to be specific, spin corrections (S-thrusters), balanced turns (P-thrusters) and unbalanced turns (Z-thrusters)). This performance assessment nicely complements the work Fritz does in analyzing thruster performance during another type of spacecraft maneuver known as Orbit Trim Maneuvers (OTMs). Thruster performance continues to be excellent, generally with a small consistent overperformance of a few percent (vs. ground test levels) for each of the 12 small thrusters. We have to be concerned when the thrusters perform *too* well, in addition to when they don't perform up to snuff.

The three of us are also preparing a paper entitled "Final Galileo Propulsion System In-Flight Characterization" to be presented in Seattle this July. Disseminating information about the very interesting Galileo propulsion system performance to a knowledgeable audience in the beautiful "Emerald City" will be very rewarding and useful.

Thanks for your interest in the Galileo propulsion team! We'd like to take this opportunity to congratulate all science teams on Galileo as their ongoing discoveries continue to astound and baffle us. Truly Galileo is placing itself firmly and proudly in the rich history of planetary science.