Results for “thrusters” — Page 49

NASA NTRS2018

3D Simulations of Ion Thruster Accelerator Grid Erosion Accounting for Charge Exchange Ion Space Charge

Accelerator (accel) grid sputtering by ions formed through charge-exchange (CEX) reactions between beam ions and residual neutral gas is a critical life-limiting mechanism for gridded ion thrusters. The three-dimensional ion optics code CEX3D is designed to simulate this grid erosion for a single beamlet, with a particular emphasis on non-axisymmetric features such as the "pits and grooves" erosion commonly observed on the accel grid downstream face in two-grid thrusters. The treatment of CEX ions in the code was recently upgraded with a new particle-in-cell (PIC) module to account for the influence of these ions' space charge on the electrostatic potential downstream of the grids. In order to achieve reasonable computation times while resolving the Debye length near the grids and avoiding gross violations of the Courant-Friedrichs-Lewy (CFL) condition, macroparticle velocities in the PIC calculation are limited through a rescaling procedure that preserves ion trajectories and space charge density. The code accounts for beam divergence, finite momentum transfer in CEX collisions, and radial losses of CEX ions from the beam; these effects are important for determining the CEX ion flux to the accel grid because the calculated potential downstream of the grids can become very flat. The upgraded code has been used to simulate operation of NASA's Evolutionary Xenon Thruster (NEXT) during the 51 kHr Long Duration Test - a selection of results is presented and compared with experimental data.

Chaplin, Vernon H.↗

NASA NTRS2018

Ion Velocity in the Discharge Channel and Near-Field of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system, which is baselined in a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Development Unit (EDU). A new laser-induced fluorescence diagnostic that is compatible with the testing of engineering hardware was developed to obtain data for thruster model validation in the lowest background pressure achievable. Prior to performing the test on the EDU, the team performed a functional checkout test of this new diagnostic using the TDU. In addition to providing a checkout of the diagnostic, this test provided data that can be correlated to electron mobility for comparison to the EDU at a later date. A number of technical challenges related to large test facilities and interfacing with engineering hardware were overcome while implementing the new laser diagnostic system. The initial data set was in good agreement with prior optical and non-optical diagnostics data collected on the TDU thrusters. This data set also revealed the spatial origin of high angle ions that have been of concern for spacecraft integration.

Huang, Wensheng↗

NASA NTRS2018

Ion Velocity in the Discharge Channel and Near-Field of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system, which is baselined in a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Development Unit (EDU). A new laser-induced fluorescence diagnostic that is compatible with the testing of engineering hardware was developed to obtain data for thruster model validation in the lowest background pressure achievable. Prior to performing the test on the EDU, the team performed a functional checkout test of this new diagnostic using the TDU. In addition to providing a checkout of the diagnostic, this test provided data that can be correlated to electron mobility for comparison to the EDU at a later date. A number of technical challenges related to large test facilities and interfacing with engineering hardware were overcome while implementing the new laser diagnostic system. The initial data set was in good agreement with prior optical and non-optical diagnostics data collected on the TDU thrusters. This data set also revealed the spatial origin of high angle ions that have been of concern for spacecraft integration.

Huang, Wensheng↗

NASA NTRS2018

An Innovative Control Allocation Scheme to Address Reaction Thruster Interactions on a 3U CubeSat

A wealth of literature exists on control allocation algorithms for over-actuated air vehicles, launch vehicles, and spacecraft's. Most of these algorithms focus primarily on minimizing some objective function such as command tracking error and/or control effector usage. Linear allocators (pseudo inverses) are usually the conventional choice due to their simplicity and the ability to achieve a significant portion of the theoretical moment/impulse space. Generally, it is assumed that there exists minimal interaction effects between control effectors. In fact, very few studies address the problem of control effector interactions in the context of control allocation, especially for small spacecraft's with a reaction control system (RCS). This paper presents a CubeSat RCS design with a four thruster tetrahedral layout such that when two or more thrusters re, the resultant impulse differs noticeably compared to the sum of the contributions from individual thruster rings. This undesirable effect is caused by the design of the propellant tank and regulator. To mitigate this issue, an innovative modified pseudo inverse (MPI) control allocation algorithm was developed that adjusts the pseudo inverse solution based on test data. The algorithm is iteration-free and superior to the standard pseudo inverse in minimizing the command tracking error.

Pei, Jing↗

NASA NTRS2016

Mechanisms for Pole Piece Erosion in a 6-kW Magnetically-Shielded Hall Thruster

The mechanisms contributing to the erosion of the inner magnetic pole of a 6-kW magnetically shielded Hall thruster are experimentally and numerically investigated. Sputtering from ion bombardment is believed to be the cause of this wear, but the nature and source of the responsible ions remains unknown. Laser induced fluorescence and translating and surface-mounted probes are employed to characterize the flux and energy distribution of ions incident at the inner pole. It is found that the bulk population is comprised of ions originating from the cathode plume and the near field immediately adjacent to the inner pole. These ions have a low average kinetic energy, < 5 eV kinetic, but a high thermal spread in velocity, Ti = 10 eV. Calculations for the erosion at the pole from experimental measurements indicate that the bulk population only accounts for 10-25% of the observed erosion. It is postulated that the remainder of the erosion results from a separate population of ions with sufficiently high energy (> 150 V) to cause high sputtering but too low density to be detected experimentally by the plasma diagnostics in this region. The existence for this high energy population is shown to be a consequence of the high plasma potentials downstream of the thruster exit plane that are characteristic of a magnetically-shielded topography. A simulation validated by experimental measurements of the thruster provides further evidence that a high energy ion populations exists with sufficient density to explain the observed erosion.

Jorns, Benjamin A.↗

NASA NTRS2017

High-Power Performance of a 100-kW Class Nested Hall Thruster

The performance of a three-channel, 100-kW class nested Hall thruster was evaluated on xenon propellant for total powers up to 102 kW at NASA Glenn Research Center. The thruster demonstrated stable operation in all seven available channel combinations at discharge voltages from 300 V to 500 V and three different current densities. The resulting test matrix contained forty-six unique conditions ranging from 5 to 102 kW total power and 16 to 247 A discharge current. At each operating condition, thruster performance was measured, and from these measurements specific impulse and efficiency were calculated. All seven channel combinations showed similar performance at a given discharge voltage and current density. The largest thrust recorded was 5.4 N ± 0.1 N at 99 kW, 400 V discharge voltage. Total efficiency and specific impulse ranged from 0.54 to 0.67 ± 0.03 and 1800 seconds to 2650 seconds ± 60 seconds, respectively. It was found that the thrust of the three channels firing together was not larger than the sum of each channel firing individually. Discharge current oscillations were also characterized with peak-to-peak and root-mean-square values and with power spectral density analysis. The implications of these results are discussed in the context of operation beyond 100 kW, as well as the general viability of NHT technology for future mission applications.

Hall, Scott J.↗

NASA NTRS2019

Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation.

Huang, Wensheng↗

NASA NTRS2019

Impulse Measurements of Electric Solid Propellant in an Electrothermal Ablation-Fed Pulsed Plasma Thruster

Electric solid propellants are advanced solid chemical rocket propellants that can be controlled (ignited, throttled and extinguished) through the application and removal of an electric current. These propellants are also being considered for use in the ablative pulsed plasma thruster. In this paper, the performance of an electric solid propellant operating in an electrothermal ablation-fed pulsed plasma thruster was investigated using an inverted pendulum micro-Newton thrust stand. The impulse bit and specific impulse of the device using the electric solid propellant were measured for short-duration test runs of 100 pulses and longer-duration runs to end-of-life, at energy levels of 5, 10, 15 and 20 J. Also, the device was operated using the current state-of-the-art ablation-fed pulsed plasma thruster propellant, polytetrafluoroethylene or PTFE. Impulse bit measurements for PTFE indicate 100±20 μN-s at an initial energy level of 5 J, which increases linearly by ~30 μN-s/J with increased initial energy. Measurements of the impulse bit for the electric solid propellant are on average lower than PTFE by 10% or less. Specific impulse for when operating on PTFE is calculated to be about 450 s compared to 225 s for the electric solid propellant. The 50% reduction in specific impulse is due to increased mass ablated during operation with the electric solid propellant relative to PTFE.

Glascock, Matthew S.↗

NASA NTRS2019

Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation.

Huang, Wensheng↗

NASA NTRS2019

Completion of the Long Duration Wear Test of the NASA HERMeS Hall Thruster

The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5-kW Hall thruster has been the subject of extensive technology maturation by NASA GRC and JPL in preparation for development into a flight propulsion system. As part of this effort, a series of wear tests have been conducted to identify erosion phenomena and the accompanying failure modes as well as to validate service-life models for magnetically-shielded thrusters. This work presents a summary of the results obtained during the Long Duration Wear Test (LDWT), which was the third in this wear test series. The LDWT accumulated approximately 3,570 hours of operation and had the overall goal to identify and correct design or facility issues prior to the flight qualification campaign. Thruster performance, stability, and plume properties were invariant throughout the duration of the LDWT and consistent with measurements acquired during previous HERMeS performance and wear characterizations. Average erosion rates of a carbon-carbon composite pole cover were found to match those measured with graphite to within the empirical uncertainty while the previously observed time-dependence of pole cover erosion rates was linked to changes in pole cover roughness. Azimuthal variations in keeper wear rate were observed including deposition on one of the azimuthal-facing sides of the keeper mask. This strongly suggests the presence of an azimuthal component in the process driving keeper erosion.

Frieman, Jason D.↗

NASA NTRS2019

Completion of the Long Duration Wear Test of the NASA HERMeS Hall Thruster

The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5-kW Hall thruster has been the subject of extensive technology maturation by NASA GRC and JPL in preparation for development into a flight propulsion system. As part of this effort, a series of wear tests have been conducted to identify erosion phenomena and the accompanying failure modes as well as to validate service-life models for magnetically-shielded thrusters. This work presents a summary of the results obtained during the Long Duration Wear Test (LDWT), which was the third in this wear test series. The LDWT accumulated approximately 3,570 hours of operation and had the overall goal to identify and correct design or facility issues prior to the flight qualification campaign. Thruster performance, stability, and plume properties were invariant throughout the duration of the LDWT and consistent with measurements acquired during previous HERMeS performance and wear characterizations. Average erosion rates of a carbon-carbon composite pole cover were found to match those measured with graphite to within the empirical uncertainty while the previously observed time-dependence of pole cover erosion rates was linked to changes in pole cover roughness. Azimuthal variations in keeper wear rate were observed including deposition on one of the azimuthal-facing sides of the keeper mask. This strongly suggests the presence of an azimuthal component in the process driving keeper erosion.

Frieman, Jason D.↗

NASA NTRS2019

Uncertainty Quantification of Modeled Electron Backstreaming Failure for the NEXT Ion Thruster

Excessive electron back streaming is one of the primary life-limiting failure modes for gridded ion thrusters. Physics-based modeling of the optics grid erosion and electron back streaming margins is augmented with statistical uncertainty quantification techniques to generate a life expectancy distribution of this particular failure mechanism for the NEXT ion thruster. Generation of distributions instead of a single point estimate provide a more comprehensive picture of thruster failure probabilities and life expectation for various mission applications.

Yim, John T.↗

NASA NTRS2019

Completion of the Long Duration Wear Test of the NASA HERMeS Hall Thruster

The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5-kW Hall thruster has been the subject of extensive technology maturation by NASA GRC and JPL in preparation for development into a flight propulsion system. As part of this effort, a series of wear tests have been conducted to identify erosion phenomena and the accompanying failure modes as well as to validate service-life models for magnetically-shielded thrusters. This work presents a summary of the results obtained during the Long Duration Wear Test (LDWT), which was the third in this wear test series. The LDWT accumulated approximately 3,570 hours of operation and had the overall goal to identify and correct design or facility issues prior to the flight qualification campaign. Thruster performance, stability, and plume properties were invariant throughout the duration of the LDWT and consistent with measurements acquired during previous HERMeS performance and wear characterizations. Average erosion rates of a carbon-carbon composite pole cover were found to match those measured with graphite to within the empirical uncertainty while the previously observed time-dependence of pole cover erosion rates was linked to changes in pole cover roughness. Azimuthal variations in keeper wear rate were observed including deposition on one of the azimuthal-facing sides of the keeper mask. This strongly suggests the presence of an azimuthal component in the process driving keeper erosion.

Jason David Frieman↗

NASA NTRS2020

Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster

NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation.

Wensheng Huang↗

NASA NTRS2013

Azimuthal spoke propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall Effect Thrusters (HETs) that travel in the E × B direction and have been observed in many different systems. The propagation of azimuthal spokes are investigated in a 6 kW HET known as the H6 using ultra-fast imaging and azimuthally spaced probes. A spoke surface is a 2-D plot of azimuthal light intensity evolution over time calculated from 87,500 frames/s videos. The spoke velocity has been determined using three methods with similar results: manual fitting of diagonal lines on the spoke surface, linear cross-correlation between azimuthal locations and an approximated dispersion relation. The spoke velocity for three discharge voltages (300, 400 and 450 V) and three anode mass flow rates (14.7, 19.5 and 25.2 mg/s) yielded spoke velocities between 1500 and 2200 m/s across a range of normalized magnetic field settings. The spoke velocity was inversely dependent on magnetic field strength for low B-field settings and asymptoted at B-field higher values. The velocities and frequencies are compared to standard drifts and plasma waves such as E × B drift, electrostatic ion cyclotron, magnetosonic and various drift waves. The empirically approximated dispersion relation yielded a characteristic velocity that matched the ion acoustic speed for 5 eV electrons that exist in the near-anode and near-field plume regions of the discharge channel based on internal measurements. Thruster performance has been linked to operating mode where thrust-to-power is maximized when azimuthal spokes are present so investigating the underlying mechanism of spokes will benefit thruster operation.

Polk, James E.↗

NASA NTRS2017

Development of a 50,000-s, Lithium-fueled, Gridded Ion Thruster

The ion propulsion system on NASA’s Dawn mission provided over 11 km/s delta-V to the spacecraft. There is potential interest in missions that have delta-V’s an order of magnitude greater than this, i.e., 100 km/s to 200 km/s. To perform such missions would require a thruster that can produce a specific impulse roughly an order of magnitude greater than the 3100 s of the Dawn ion propulsion system. A 50-kW gridded ion thruster is being developed for operation with lithium propellant to produce a specific impulse of 50,000 s. The resulting thruster design requires a net accelerating voltage of 9 kV and a beam current of 5.5 A. Discharge chamber modeling is used to design a 35-cm diameter ring-cusp discharge chamber with six magnet rings. The discharge chamber is masked down to produce an active grid area that is ~25 cm diameter. Modeling suggests that the unique ionization characteristics of lithium may enable discharge chamber operation at a propellant efficiency of 99%. Operation at such a high propellant efficiency could significantly reduce the production of charge-exchange ions and thereby significantly reduce erosion of the accelerator grid.

Goebel, Dan M.↗

NASA NTRS2017

Inner Front Pole Cover Erosion in the 12.5 kW HERMeS Hall Thruster Over a Range of Operating Conditions

Erosion characteristics on the cover of the inner front pole in a 12.5 kW Hall thruster were measured over a wide range of operating conditions in tests of 6 to 14 hours duration using an accelerated test method and a very sensitive radioactive tracer-based erosion diagnostic. The operating points included the nominal 300 - 600 V conditions on a constant 20.8 A throttle curve, but included additional conditions at other currents spanning the throttling envelope and measurements at varying magnetic eld strength, facility pressure, and discharge voltage oscillation amplitude. The results show that the 300 V condition produces the highest wear rates on the 20.8 A throttle curve, but that rates actually increase with decreasing current. The wear rate was insensitive to discharge voltage ripple, but increased monotonically with magnetic eld strength, particularly near the inner radius of the pole cover. The inner region was also sensitive to facility pressure, showing lower rates at a higher pressure level. Separate experiments in which the energy distributions of ions generated by the hollow cathode were measured suggest that the cathode plume may be a source of energetic ions responsible for some of the erosion trends, in addition to ions originating in the thruster plume. The Hall thruster simulation code Hall2De is able to reproduce the erosion characteristics observed at 600 V, 20.8 A, but cannot currently match the rates measured at lower voltages and currents.

Lopez Ortega, Alejandro↗

NASA NTRS2018

Life Qualiﬁcation of Hall Thrusters By Analysis and Test

High power Hall thrusters under development for deep space robotic and human exploration missions have demanding lifetime and operational requirements that make thruster life qualiﬁcation challenging. The required burn times make it infeasible to conduct multiple tests with durations exceeding the life requirement, and complex wearout phenomena and the need for power throttling during missions complicate analyses and testing. The traditional qualiﬁcation approach, a single life test that typically demonstrates 50 to 100% margin on the required lifetime, does not provide enough information to demonstrate low failure risk. It is a weak source of statistical information about the location of the peak in the failure probability distribution and provides no information on the width of the distribution. Testing does provide information that can be used to validate physics-based models of failure processes, however, and the combination can be used to assess mission risk. Validated, conservative, deterministic analysis can be used to demonstrate that most failure processes have such large margins that more detailed analysis is unnecessary. For a subset of the failure modes a more detailed probabilistic analysis is required. Pole erosion in a 12.5 kW Hall thruster is used as a detailed example of this qualiﬁcation process.

Brophy, John R.↗