Gregory A. Zimmerli

Introducing SE-FIT: Surface Evolver - Fluid Interface Tool for Studying Capillary Surfaces

As an efficient program for studying capillary surfaces, Surface Evolver (SE) has a steep learning curve and is without a user-friendly graphical user interface. Here, we describe a new SE-based program, the Surface Evolver - Fluid Interface Tool (SE-FIT, pronounced ‘see-fit’). SE-FIT has been developed to dramatically broaden the application of SE and provide a user-friendly tool for studying the shape and stability of capillary surfaces. SE-FIT retains the core strengths of SE but adds a Windows-based Graphical User Interface (GUI) to interface with SE’s command-line functions. SE-FIT uses an intermediate text file layer to communicate with SE, automatically generating session logs while ensuring seamless communication between SE and the GUI. Groups of pre-defined geometric elements, such as spacecraft propellant tanks, have been created to enable rapid assembly of user-specified configurations. More generally, certain CAD/CAM drawing files generated from third-party software can be imported as well. For run-time processing, a general convergence algorithm has been developed to make the convergence computation a real routine. Features that enable batch process of parameter sweeps have been created. A real-time total energy variation graph and a histogram table are made available as convergence diagnostic tools. SE-FIT features utility tools including a parameter list table and a file explorer. A built-in text editor is included to facilitate editing and debugging of SE geometry and script files. Post-processing features include exporting data such as the container and interface geometry as an image file, and other file formats for further CFD simulations or rendering. A standalone SE data-file viewer enables real-time graphical viewing. These and other features make SE-FIT a unique software tool for rapidly calculating the shape and stability of capillary surfaces in complex geometric settings with an efficiency at least an order of magnitude greater than that of existing CFD programs.

Propellant Quantity Gauging Using the Radio Frequency Mass Gauge

The Radio Frequency Mass Gauge (RFMG) is a novel propellant quantity gauging technology being developed at NASA for gauging cryogenic propellant tanks in low-gravity. The RFMG operates by sensing the natural electromagnetic modes of a tank, comparing several mode frequencies with a database of calculated mode frequencies, and finding a best match at a particular fill ratio which is then reported as the gauged liquid quantity. Here, we summarize recent test results in liquid oxygen and liquid hydrogen, and present a sample of test data acquired during a low-gravity aircraft campaign at lunar and low-gravity flight levels. Liquid oxygen and liquid hydrogen testing was conducted in a normal, 1g settled liquid configuration to assess the gauging accuracy of the RFMG as compared to a reference weighing system. These cryogenic tests indicate an RFMG gauging uncertainty of approximately ±1% of full-scale. Testing of the RFMG on a low-g aircraft using an inert fluid shows very good performance at lunar-g flight levels, and approximately ±6% of fullscale gauging uncertainty at low-g flight levels. Changes in the tank RF spectra during the low-g parabolic flights indicate significant fluid motion inside the tank. Despite the higher uncertainty at low-g flight levels, averaging of the RFMG output data during the low-g portion of the aircraft flight shows very good agreement between the RFMG gauged fill level and the actual fill level.

Mastering Cryogenic Propellants

AbstractThe National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) began experimentation with cryogenic propellants in the early 1950s to understand the potential of these high-performance propellants for use in liquid propellant rocket engines. Supporting these tests required learning how to both design cryogenic systems and develop procedures to safely and reliably work with cryogenic fuels and oxidizers. This early work led to the development of a skill set that has been core to the center ever since. When NASA was formed and the exploration missions were defined, it became clear that the ability to use cryogenic propellants in the thermal and microgravity environment of space was critical to mission success, and the agency was tasked with enabling this capability. To support development of the Centaur upper stage and the Saturn S-IVB stage, GRC researchers and engineers initiated extensive technology development for the in-space application of cryogenic fluid management (CFM)...

Radio Frequency Mass Gauging of Propellants

A combined experimental and computer simulation effort was conducted to measure radio frequency (RF) tank resonance modes in a dewar partially filled with liquid oxygen, and compare the measurements with numerical simulations. The goal of the effort was to demonstrate that computer simulations of a tanks electromagnetic eigenmodes can be used to accurately predict ground-based measurements, thereby providing a computational tool for predicting tank modes in a low-gravity environment. Matching the measured resonant frequencies of several tank modes with computer simulations can be used to gauge the amount of liquid in a tank, thus providing a possible method to gauge cryogenic propellant tanks in low-gravity. Using a handheld RF spectrum analyzer and a small antenna in a 46 liter capacity dewar for experimental measurements, we have verified that the four lowest transverse magnetic eigenmodes can be accurately predicted as a function of liquid oxygen fill level using computer simulations. The input to the computer simulations consisted of tank dimensions, and the dielectric constant of the fluid. Without using any adjustable parameters, the calculated and measured frequencies agree such that the liquid oxygen fill level was gauged to within 2 percent full scale uncertainty. These results demonstrate the utility of using electromagnetic simulations to form the basis of an RF mass gauging technology with the power to simulate tank resonance frequencies from arbitrary fluid configurations.

A Study of Fluid Interface Configurations in Exploration Vehicle Propellant Tanks

The equilibrium shape and location of fluid interfaces in spacecraft propellant tanks while in low-gravity is of interest to system designers, but can be challenging to predict. The propellant position can affect many aspects of the spacecraft such as the spacecraft center of mass, response to thruster firing due to sloshing, liquid acquisition, propellant mass gauging, and thermal control systems. We use Surface Evolver, a fluid interface energy minimizing algorithm, to investigate theoretical equilibrium liquid-vapor interfaces for spacecraft propellant tanks similar to those that have been considered for NASAs new class of Exploration vehicles. The choice of tank design parameters we consider are derived from the NASA Exploration Systems Architecture Study report. The local acceleration vector employed in the computations is determined by estimating low-Earth orbit (LEO) atmospheric drag effects and centrifugal forces due to a fixed spacecraft orientation with respect to the Earth or Moon, and rotisserie-type spacecraft rotation. Propellant/vapor interface positions are computed for the Earth Departure Stage and Altair lunar lander descent and ascent stage tanks for propellant loads applicable to LEO and low-lunar orbit. In some of the cases investigated the vapor ullage bubble is located at the drain end of the tank, where propellant management device hardware is often located.

An RF Sensor for Gauging Screen-Channel Liquid Acquisition Devices for Cryogenic Propellants

A key requirement of a low-gravity screen-channel liquid acquisition device (LAD) is the need to retain 100% liquid in the channel in response to propellant outflow and spacecraft maneuvers. The point at which a screen-channel LAD ingests vapor is known as breakdown, and can be measured several different ways such as: visual observation of bubbles in the LAD channel outflow; a sudden change in pressure drop between the propellant tank and LAD sump outlet; or, an indication by wet-dry sensors placed in the LAD channel or outflow stream. Here we describe a new type of sensor for gauging a screen-channel LAD, the Radio Frequency Mass Gauge (RFMG). The RFMG measures the natural electromagnetic modes of the screen-channel LAD, which is very similar to an RF waveguide, to determine the amount of propellant in the channel. By monitoring several of the RF modes, we show that the RFMG acts as a global sensor of the LAD channel propellant fill level, and enables detection of LAD breakdown even in the absence of outflow. This paper presents the theory behind the RFMG-LAD sensor, measurements and simulations of the RF modes of a LAD channel, and RFMG detection of LAD breakdown in a channel using a simulant fluid during inverted outflow and long-term stability tests.

Testing Microgravity Flight Hardware Concepts on the NASA KC-135

This paper provides an overview of utilizing the NASA KC-135 Reduced Gravity Aircraft for the Foam Optics and Mechanics (FOAM) microgravity flight project. The FOAM science requirements are summarized, and the KC-135 test-rig used to test hardware concepts designed to meet the requirements are described. Preliminary results regarding foam dispensing, foam/surface slip tests, and dynamic light scattering data are discussed in support of the flight hardware development for the FOAM experiment.