By: Kristin Uhmeyer
Principle Investigator: Dr. Greg Zimmerli
Background on RF Mass Gauging
The Constellation and Orion Projects to return humans to the Moon and eventually send humans to Mars have a number of development risk areas. Some of these have to do with propellant: long duration cryogenic storage, propellant acquisition, and propellant quality management within the distributed propellant feed system. When working in low gravity, it is difficult to accurately measure the mass of propellant in a tank, because the liquid does not remain at one end of the tank, as it does under the influence of gravity or some sort of acceleration. Bubbles can form within the tank as the fluid sticks to the sides of the tank, making a straightforward measurement of the mass difficult.
A technique being used to determine the amount of mass within the tank is radio frequencing gauging. Using a small antenna placed within the tank, the frequency of radio waves is swept through a range of frequencies. When the frequency hits a resonant mode of the system, the returned loss in dB will become very small. When the refractive index (η) is known, a fill level of the tank can be determined by looking at the resonant frequencies and the magnitude of the losses at those frequencies.
This technique has been shown to work with cylindrical tanks at one G using liquid oxygen and methane as propellants, with a degree of accuracy to within two percent. The challenge now will be to achieve similar results when working with a tank with more complex geometry and at low gravity. The tank will be tilted to different angles to simulate different configurations of the fluid that might be achieved in low gravity.
Also important will be to simulate the behavior of the cryogenic fuels within the proposed space vehicles. For the “1.5 Launch” option to get both astronauts and a payload to the moon, the Earth Departure Stage (EDS) and Lunar Surface Access Module (LSAM) Ascent and Descent modules propellant use and sloshing must be simulated.
My Role as a Research Associate
I will be working with Brianna Eller, a LERCIP intern, to carry out the RF testing for my Principal Investigator, Dr. Gregory Zimmerli. We will begin by looking at different types and sizes of antennas tested in an empty tank to get a clear Return Loss profile, with sharp peaks for different modes that are well-spaced to give accurate readings. We will look at both loop and monopole antennas, one located on the long axis of the tank and one off-axis, of different sizes to get the best profile. Then, we will fill the tank with expanded polystyrene pellets similar to those used in the micro-gravity drop test, dry out the tank using nitrogen, and measure the dielectric constant. The pellets simulate a fluid with a low dielectric constant. We will try different tilt angles and fill levels of the pellets, and then repeat the experiment using an actual liquid, Flourinert FC-77, which has a similar dielectric constant to the expected propellants of liquid oxygen and hydrogen, but can be used at room temperature. This data will be compared to the existing simulation software to determine the accuracy of the technique.
Secondly, Brianna and I will work to develop a simulation of propellant usage in the three vehicles. We plan to look at the fill-level of the tanks over time, the accelerations on the tanks over time (during burns, effects of centripetal force, aerodynamic drag and other environmental disturbances), To look at how those accelerations affect sloshing of liquid, we will use the Fluid Interface Tool (FIT) to find the fluid shapes that might result from sloshing. To do this, we will need to analyze the tank configurations that have been decided on and calculate how much propellant is used through the duration of the mission with the thrust and rocket equations.
Additionally, we may be working with Jim Wagner, Jeff Follo, and Mike Herlacher.
I have some experience working with radar systems in a previous internship, and Brianna is very interested in quantum mechanics, which is applicable to the analysis of the eigenmodes within the tank. I hope that by the end of July, we will have been able to carry out tests for both the polystyrene pellets and the Flourinert, and will be looking at how the next stage of testing may be carried out beyond our time here at GRC.
Through the documents that I have been reading about the planned architectures for the Orion missions, it is clear that this is a problem that must be solved for the success of the mission. I hope to make a contribution to the project by making a step forward on this part of the system.