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Liquid Propellant Mass Measurement in Microgravity

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Liquid Propellant Mass Measurement in Microgravity Research Note • DOI: 10.2478/gsr-2021-0004 GSR • 9 • 2021 • 50–61 Gravitational and Space Research Liquid Propellant Mass Measurement in Microgravity Kevin M. Crosby1, Rudy J. Werlink2, Eric A. Hurlbert3 1Department of Physics and Astronomy, Carthage College, 2001 Alford Park Drive, Kenosha, WI 53140; 2NASA Kennedy Space Center; 3NASA Johnson Space Center Abstract The Modal Propellant Gauging (MPG) experiment has demonstrated sub-1% gauging accuracy under laboratory conditions on both flight hardware and subscale tanks. Recently, MPG was adapted for flight on Blue Origin’s New Shepard vehicle and has flown twice, achieving equilibrated, zero-g surface configurations of propellant simulant at three different fill fractions. Flight data from MPG missions on New Shepard P7 and P9 show agreement between known and measured propellant levels of 0.3% for the fill fractions investigated in the present study. Two approaches for estimating zero-g propellant mass are described here. Both approaches rely on measuring shifts in modal frequencies of a tank excited by acoustic surface waves and subject to fluid mass loading by the propellant. In the first approach, shifts in the lowest mode frequency (LMF) are measured and associated with liquid fill-level changes. In the second approach, 1-g modal spectra at a range of known fill levels are used in a cross-correlation calculation to predict fill levels associated with a zero-g modal spectrum. Flight data for both approaches are consistent with finite element predictions using a simple fluid–structure interaction model. In both settled and unsettled microgravity environments, MPG meets or exceeds NASA Roadmap goals for in-space propellant mass gauging. Keywords Artemis Program • Propellant Gauging • Microgravity Liquids • Propellant Depot • Lunar Gateway • Modal Analysis INTRODUCTION NASA’s proposed Lunar Gateway (LG) represents a was one of the first Gateway elements to be contracted. The compromise between the limited delta-V achievable by risk reduction and economic potential of deep space exploration the Orion Multipurpose Crew Vehicle (MPCV) and the depend on the existence of high-TRL on-orbit propellant transfer programmatic goal of a sustainable human presence on (Notardonato et al., 2012). and near the lunar surface. The MPCV comprises the Crew In-space liquid propellant transfer, however, has been Capsule (CC) and its European Service Module (ESM). achieved only in small-scale demonstration tests of limited The propellant load and mass of the CC/ESM architecture, scope. End-to-end propellant transfer experiments on flight designed as part of the Constellation program, preclude the hardware have not yet been conducted. For this reason, in-space spacecraft from reaching and returning from a low lunar orbit propellant transfer remains a “critical path” objective for fuel (Whitley and Martinez, 2016). Instead, the MPCV will dock at depot capability (Casanova et al., 2017). The primary challenge the LG, located in a halo orbit near the Earth-Moon Lagrange of in-space liquid propellant transfer is low-gravity mass gauging Point 2 (EML-2). The LG will provide staging facilities for lunar of liquids and liquid distribution knowledge inside propellant landers and ascent vehicles. tanks. Despite over 50 years of work in this area, accurate, The first element of the proposed LG architecture is the low-g liquid mass gauging remains an unsolved problem as well power and propulsion element (PPE). The PPE provides as a problem of significance to a variety of potential customers station-keeping and orbit-raising functions as well as chemical beyond the human spaceflight community (Holmes et al., 2019). and electric (xenon) propellant refueling. Sustainable human The critical nature of low-g gauging is captured in the 2020 presence on the surface of the moon and, ultimately, human NASA Technology Taxonomy (NASA, 2020). missions to Mars, depend critically on the ability to provide The Modal Propellant Gauging (MPG) experiment is one in-space propellant transfer and propellant depot functions to of several emerging approaches to low-gravity liquid propellant visiting spacecraft. The essential importance of providing in- gauging. MPG provides real-time, noninvasive (no tank space propellant transfer is one of the reasons that the PPE penetrations), gauging of both cryogenic and Earth-storable † Corresponding author: Kevin M. Crosby E-mail: [email protected] 50 Gravitational and Space Research propellants by measuring the “added-mass” effect of contained estimates of gauging accuracy near EOL ranging from 3.5% liquid on the resonant mode frequencies of thin-walled tanks to 10% or more. subject to acoustic excitation. Thermal mass estimates rely on measurements of the MPG has been demonstrated in Earth gravity (1-g) on temperature change induced in a propellant tank by external a variety of subscale and flight hardware propellant tanks, heating. Thermal response is compared to simulation results including a hot-fire test of the Methane–LOX system onboard based on a tank-specific thermal capacitance model computed the Morpheus Prototype Lander, pressure and thermal cycle over a range of fill fractions. The uncertainty of propellant tests on a Space Shuttle OMS tank, the Orion ESM propellant mass estimates based on thermal propellant gauging (TPG) tank (qualification unit), and various composite tanks with methods depends sensitively on the sophistication and fidelity both cryogenic propellant simulants and water (Crosby et al., of the tank thermal model (Yendler, 2006). 2019). MPG has also been extensively tested on parabolic In addition to low-gravity methods, settled propellant flights, across three different flight campaigns (Crosby et al., levels may be estimated using capacitance probes and 2016). impedance sensors. In microgravity, capillary forces drive In this report, we share the results of two suborbital flight liquid propellant along the surfaces of such probes rendering tests of the MPG technology on the Blue Origin New Shepard their measurements useless. These electrical measurements (NS) crew capsule. The payload experiments were conducted are performed on settled liquids and are therefore used only aboard the NS P7 and NS P9 (payload) missions in January during thrusting maneuvers and are subject to error induced 2019 and December 2019, respectively. Both flights carried by residual adhered liquid on sensors. the same hardware and carried out essentially the same tests, Recent advances in gauging using radio frequency (RF) with the first flight considered a “shakeout” test to determine probes to correlate electromagnetic cavity modes with 1-g the functionality of the full system hardware and to record library data have demonstrated 1% gauging accuracies in environmental test data for use in the second flight, which the laboratory for settled propellants but require additional acquired the full set of MPG data reported here. hardware and invasive tank modifications (Zimmerli et al., 2011). A complete review of extant gauging technologies, their SURVEY OF LOW-GRAVITY PROPELLANT flight heritage, and their limitations is provided in the work of GAUGING TECHNOLOGIES Dodge (2008). Each family of methods has relatively reduced accuracy at low tank fill fractions, and, therefore, propellant Low-gravity propellant gauging technologies in wide use gauging is the primary limiting factor in the estimation include those that depend on bookkeeping, equations of of mission EOL. Error estimates on most of the gauging state, and thermal mass estimation. Bookkeeping methods methods range from 1% to 20% or more. For example, errors employs flow rate monitors or, often, just assumptions in the computed estimate of consumed propellant through about flow rates during thruster firings to estimate the total the bookkeeping methods are typically around ±3.0% (but amount of propellant consumed over time. Such methods much higher near EOL). Adding propellant mass to account accumulate error over time and therefore introduce growing for gauging errors can result in significant launch costs and/or uncertainties in the estimate of remaining propellant mass and reduced mission life. resulting estimation of end-of-life (EOL) (Dodge, 2008). Here, In MPG, the gauging errors in settled low-gravity propellant “uncertainty” in remaining propellant refers to the estimated are relatively well characterized by the material properties of percent of uncertainty between actual and indicated propellant. the tank wall and liquid propellant; modal frequencies depend Equation of state (PVT) methods rely on the inference only on tank geometry, material properties, and liquid density. of the liquid volume (V) fraction within a tank pressurized In the MPG approach, slosh affects instantaneous gauging by an external pressurizing gas (usually, helium) based on accuracy but can be “averaged over” for slosh dynamics measurements of pressure (P) and temperature (T) and that are roughly periodic. Computational overhead in MPG the application of an appropriate equation of state to the is minimal; a development path for radiation-hardened deep- ullage gas. In the PVT approach, the tank is considered space MPG avionics has been established by an avionics isothermal, and accuracy depends in part on the validity of team at the Johnson Space Center. this assumption. As in the case of bookkeeping methods, propellant mass estimates derived from applications of PVT OVERVIEW OF THE MPG APPROACH methods are increasingly inaccurate at lower fill
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