OFJ Field Journal from Todd Barber - 10/17/95
BALANCING ELECTRICAL LOADS TO MINIMIZE LEAKING GASI have been continuing my work in trying to understand what the propulsion system data can tell us as we move ever closer to Jupiter orbit insertion. In my last journal entry, I mentioned that it is possible that an oxidizer check valve in the propulsion system has failed open, which could have large implications for the Jupiter orbital tour. The past few weeks have been devoted primarily to continuing to ask the "what if" questions, if the valve really is stuck open.
The possibility of a stuck-open oxidizer check valve has dictated very tight control over the temperature of the Galileo propellant tanks. Galileo is powered by Radioisotope Thermoelectric Generators (RTGs), rather than solar panels since it is traveling to a point five times further from the sun than the Earth (physics students might recognize here an application of the inverse square law; that is, the amount of solar energy reaching Galileo is not just 1/5 as much as reaching the Earth, but is 1/(5x5) = 1/25, or only 4%).
RTGs supply electrical power by turning the heat energy from the radio- active decay of plutonium into electricity. They are not nuclear reactors and were verified (by analysis) to be safe for Galileo's launch on the space shuttle Atlantis in October, 1989, even given a shuttle accident similar to the Challenger disaster. Even though proven not to be a health hazard, there were some concerned citizens who tried to block the launch of Galileo because of its radioactive power source. This is another example of some of the politics that can influence our missions.
Anyway, I digress from my point. The power supplied from the RTGs is used throughout the spacecraft for all spacecraft power needs; for example, radio transmitters, science instruments, heaters, etc. Now here is where propulsion comes in! The excess power not being used by the spacecraft (called power margin) is dissipated in heaters that are connected to the propellant tanks. So the propellant tank temperatures are really at the mercy of what electrical power loads are powered and unpowered on the spacecraft! This is a very interesting design, and, in the mission to date, has meant pretty large variations in the Galileo propellant tank temperatures.
It is precisely these temperature changes that we are now trying to avoid! We do know that if the oxidizer check valve is stuck open, large temperature changes in the propellant tanks can cause oxidizer vapor to migrate to the fuel plumbing, which could cause undesirable chemical reactions. So, given our uncertainty on whether or not the oxidizer check valve is open, we have to "play it safe" and try to keep the propellant tank temperatures constant as much as possible. This is a large effort between now and orbit insertion, but so far it has been handled expertly by my colleagues. You might imagine that every time you turn a science instrument on, for example, that you would have to turn an equivalently sized heater off to keep the power margin nearly constant. In fact, this is just precisely the kind of action that we have been taking.
One possible way to help alleviate this problem could be attempted next March or April. After we get into Jupiter orbit on December 7 of this year, we are on a very large orbit around Jupiter that takes over 200 days for us to come back into the vicinity of Jupiter again. About halfway through this first orbit, at the so-called apojove ("apo" = furthest point of the trajectory from and "jove" = Jupiter), there is to be another (the final!) use of the main rocket engine in March, 1996. This maneuver is called the perijove raise maneuver, and it does exactly what it says--it raises the perijove ("peri" = closest point of the trajectory to and "jove" = Jupiter). In other words, we are performing a rocket firing in order that we do not come so near Jupiter during our next close pass by the giant planet. This is necessary because repeated passes so close to the intense radiation belts near to Jupiter (like Earth's Van Allen belts, but MUCH stronger) could damage the spacecraft.
Anyway, that is a another digression. Basically, getting through the perijove raise maneuver will be much cause for celebration, since that is the last required use for the main rocket engine. Incidentally, after this point there is very little propellant left on-board the spacecraft (only about 135 kg out of a launch load of 925 kg), but this is ample to perform the 2-year, 10-orbit tour of Jupiter, its moons, and magnetosphere.
Back to the check valve issue. One way to make sure that no oxidizer could be transported to fuel plumbing would be to isolate the high pressure helium source after the perijove raise maneuver and then intentionally spill some oxidizer overboard through the main engine to get the oxidizer tank pressures down! This is a neat solution, but certainly might pose some risks to the spacecraft (for example, the oxidizer is nitrogen tetroxide, a very corrosive chemical that can attack metals and the Galileo thermal protection blankets!).
Very difficult decisions like this one are often made after putting together what's called a "trade study." This is basically a list of the pros and cons of performing a given action and it helps in the difficult decision process. A lot of our jobs here really entail the proper balancing of risk. That was difficult for me to comprehend coming out of school, since I was used to there being only one right answer. But this a very rewarding part of the job, too, using so-called "engineering judgment" to arrive at an acceptable solution.