OFJ Field Journal from Todd Barber - 10/10/95
WORK ON PROPULSION SYSTEM MINIMIZES POTENTIAL LEAKWork in the propulsion area of Galileo mission operations continues to go very well, although it's perhaps a bit TOO exciting for my taste! As of this writing, we are only 71 days from the primary use of the Galileo main rocket engine for the Jupiter Orbit Insertion (JOI). Along with our myriad of normal duties as propulsion analysts, we are devoting much time and energy into understanding some surprises in our data from the first use of the RetroPropulsion Module's (RPM's) main engine in July. Perhaps you have seen articles in the Los Angeles or New York Times, or USA Today concerning a leaking valve in the Galileo propulsion system. The existence of such a leak was postulated following the reconstruction of the rocket performance during the first firing in July. Of course, with the spacecraft nearly half a billion miles from Earth, diagnosing a "sick" valve is a difficult medical proposition!
By analyzing RPM propellant tank pressure and temperature measurements that are sent to ground (telemetry), we have determined that it is possible that a valve (specifically, the oxidizer check valve) may be stuck in the open position. This is one possible explanation for the discrepancies in the data; another possibility is an electronic parts drift of two pressure measurement devices (transducers) that monitor oxidizer tank pressures. The oxidizer check valve is a one-way valve that allows high-pressure helium to flow from upstream pressurant tanks in order to "recharge" the propellant tanks to keep engine performance "respectable" and consistent throughout the mission. The check valve is "one-way," meaning that another of its duties is to prevent oxidizer vapors from moving over to the fuel side of the RPM.
To understand why it's so important to keep the fuel and oxidizer separate, I should mention here that the RPM is what is known as a "bipropellant" system, utilizing nitrogen tetroxide as the oxidizer and monomethylhydrazine as the fuel. This is in contrast with, say, a jet engine which only requires fuel to be provided on-board the aircraft. This is because the oxidizer (which is needed in order to burn the fuel) in this case is oxygen, available from Earth's atmosphere).
Nitrogen tetroxide and monomethylhydrazine are hypergolic--meaning, they ignite on upon physical contact--so keeping them from mixing except in the rocket engines (for which they were designed to mix in a controlled manner) is an important safety consideration for the whole spacecraft! In the last two months or so, I have been primarily concerned with keeping the spacecraft safe by minimizing propellant tank temperature excursions in case this oxidizer check valve really is stuck open. This is of concern because an increase in propellant tank temperature will cause the oxidizer tank pressure to increase more than the fuel tank pressure (due to higher vapor pressure of nitrogen tetroxide--forgive the foray into chemistry!), which could transport oxidizer vapor over to the fuel check valve or even into the fuel propellant lines and propellant tanks. We are confident that we can successfully execute JOI and the remainder of the mission even if the valve is failed open, but it will require even harder work from the already busy flight team. Also, I have been spending much of my time performing "what if" calculations to determine how much oxidizer and fuel could react through various phases of the mission. This is a large team effort, because the effect of the amounts that I calculate is not able to be interpreted without help from experts (in propellant chemistry, for example).
We are still finishing the design for the computer sequence to be sent up to the spacecraft to perform the Jupiter Orbit Insertion and to obtain the science data associated with our first close pass by the solar system's giant among planets. What a tremendous day December 7 will be! First we fly by the Jovian satellite Europa at a distance much closer than the Voyager or Pioneer spacecraft closest approach distances, allowing more detailed images of this intriguing, icy body. Then it is on to Io, perhaps the most interesting moon in the solar system, with its teeming sulfurous volcanoes. We will by flying only 600 miles above the surface! The eventual science return should be phenomenal, but this closest approach to Io actually is dictated by the requirement for a "gravity assist," whereby Io will actually slow us down and reduce our propulsive requirements to get into orbit about Jupiter. You might ask yourself (especially if you have had physics) how we can get "something for nothing" from this gravity assist. The answer is that energy is conserved, and Io will actually speed up imperceptibly from the flyby! This technique was used by Galileo once at Venus and twice at the Earth just to enable Galileo to get to Jupiter. Indeed, without gravity assist, it would have been impossible to get the massive Galileo spacecraft to Jupiter!
Following the Io flyby, a few hours later the orbiter should lock on to the signal from the atmospheric entry probe, just beginning its grandiose plunge into the cloud tops of Jupiter. A full 75 minutes of data on the pressure, temperature, and composition of the Jupiter cloud layers should be gleaned from this spectacular event. Then just an hour or so later, the main rocket will fire for about 48 minutes to place Galileo in orbit around Jupiter for an exciting two-year tour of the Jovian system--its magnetosphere, collection of satellites, and of course the gas giant Jupiter itself.