NASA's Evolutionary Xenon Thruster Research Articles

Modeling grid erosion in the NEXT ion thruster using the CEX2D and CEX3D codes

NASA’s Evolutionary Xenon Thruster (NEXT) is a candidate for future deep space missions that offers high efficiency and specific impulse over a large power throttling range. One of the key life-limiting components is the ion accelerator system, which is subject to sputter erosion by low energy discharge plasma ions incident on the upstream screen grid and higher energy charge exchange ions that impact the downstream accelerator grid. The grid erosion codes CEX2D and CEX3D were validated with data from tests of NEXT as well as the NSTAR ion thruster and then used to assess the time to failure in space due to screen grid erosion and electron backstreaming caused by accelerator grid aperture erosion. Screen grid erosion was found to be important only at the lowest throttle levels, and was conservatively estimated to lead to failure after processing over 900 kg of xenon. The first failure mode at high power levels was found to be electron backstreaming due to accelerator grid hole wall erosion, which would occur after processing over 700 kg of propellant.

Indirect low-thrust trajectory optimization with gridded ion thruster model

The work deals with indirect optimization of minimum-time and minimum-fuel interplanetary trajectories when gridded ion thruster models are considered. Using an accurate model of solar electric propulsion is beneficial in preliminary mission design, and allows including operational constraints. The maximum thrust and the specific impulse are expressed as a function of thruster input power, which is achieved by means of point-fitting lines that match the performance capabilities of the thrusters. Minimum-time and minimum-fuel problems are formulated to be solved by indirect optimization. In order to increase the accuracy and robustness of the shooting procedure, analytic Jacobians are derived, and a hybrid switching detection technique is used to improve the integration accuracy for minimum-fuel problems. Two examples of Earth-to-Mars transfer and Near-Earth rendezvous mission using the realistic NASA’s Evolutionary Xenon Thruster (NEXT) are given to substantiate the feasibility and effectiveness of the proposed method.

A new spacecraft mission concept combining the first exploration of the Centaurs and an astrophysical space telescope for the outer solar system

We present a concept study for a distinctive robotic spacecraft mission that combines both the exploration of a never-before-visited class of planetary bodies and cutting-edge astrophysical investigations. The planetary targets are a class of objects called Centaurs that have relatively recently escaped from the Kuiper Belt and currently orbit closer to the Sun among the giant planets. We developed a trajectory that would visit several Centaurs, including the second largest known Centaur, 2060 Chiron, which displays enigmatic coma activity at large heliocentric distances and orbiting ring or dust structures. This concept takes advantage of the cruise times between planetary encounters to conduct nearly continuous astrophysical observations at wavelengths that are not accessible by ground-based facilities. Additionally, ride-along cubesats included aboard can be deployed at different points in the mission to perform various experiments or observations. This mission concept achieves its objectives with solar power (MegaFlex arrays), no new technology development, and within the approximate budget of a NASA New Frontiers class mission. The mission design accomplishes its objectives using solar electric propulsion and the recently developed NASA Evolutionary Xenon Thruster (NEXT) ion engines.

Optimization of variable-specific-impulse gravity-assist trajectories via optimality-preserving transformation

This paper concerns optimization of fuel-optimal multi-gravity-assist trajectories based on the solutions of single-leg transfers that utilize the practical solar electric propulsion with variable specific impulse. This paper studies the optimization of fuel-optimal trajectories with multiple gravity assists and variable specific impulse. The multi-gravity-assist trajectory is composed of multiple legs each of which is solved individually and combined to provide an initial guess for the multi-leg problem. The specific impulse is taken as a control variable involved in solving the optimal control problems. To reduce the strong nonlinearity of the multi-point boundary-value problem caused by multiple gravity assists, an affine transformation on the costate variables based on single-leg transfers that preserves the fuel optimality is stated and termed optimality-preserving transformation. Secondly, the optimality-preserving transformation is applied to reformulate the first-order necessary conditions for the multi-leg transfer. As a result, the solutions of the single-leg transfers can be directly used as the initial guesses for the shooting function of the multi-leg transfer. The stated optimality-preserving transformation is readily suitable to parallel computing. Additionally, the modified logarithmic homotopy method is specifically applied in variable-specific-impulse cases. The strategy combined with these two methods is efficient in optimizing the fuel-optimal multi-gravity-assist trajectories. The realistic variable-specific-impulse NASA's Evolutionary Xenon Thruster is employed in numerical simulations considering one, two, and three gravity assists cases. As a result, this proposed strategy is demonstrated to result in significant improvement of the calculation efficiency.

Optimization of Variable-Specific-Impulse Gravity-Assist Trajectories

This paper presents a series of optimization methods for finding fuel-optimal gravity-assist trajectories that use the practical solar electric propulsion with variable specific impulse. The fuel-optimal low-thrust gravity-assist trajectories are first solved using specific impulse as a control variable. An optimization method combining the particle swarm optimization and the indirect method is proposed to optimize the variable-specific-impulse gravity-assist low-thrust trajectories. Additionally, the modified logarithmic homotopy function is applied to serve as a gateway in variable-specific-impulse case. The algorithm for optimizing multiple-gravity-assist trajectories is also presented in this paper. Moreover, the experimental data of NASA’s Evolutionary Xenon Thruster (NEXT) are employed in numerical simulations. Main-belt asteroids missions are given to substantiate the feasibility and efficiency of the proposed methods.

To Ceres and beyond [ion drives

A low-fuel engine that accelerates a spacecraft slowly over many years could open up new possibilities for solar system exploration. Nasa's Evolutionary Xenon Thruster (NEXT) project powered a 7kW ion thruster for five and a half years, demonstrating a capability for longduration deep-space missions.

Thrust Stand Characterization of the NASA Evolutionary Xenon Thruster

Thrust Stand Characterization of the NASA Evolutionary Xenon Thruster

Ion Thruster Grid Design Using an Evolutionary Algorithm

An evolutionary algorithm was used to optimize the geometry and accelerator grid voltage of an ion thruster grid set with regard to maximizing impulse per unit area, essentially equivalent to maximizing propellant throughput capability per unit area. Grid operating conditions, including a net accelerating voltage of 1800 V and current density of 4.0 mA/cm 2 , corresponded to high-power operation of NASA's evolutionary xenon thruster. The evolutionary-algorithm-derived grid set had a predicted lifetime nearly twice that of the NASA's evolutionary xenon thruster grid set, primarily the result of a lower accelerator grid voltage magnitude.

Environmental Testing of NASA's Evolutionary Xenon Thruster Prototype Model 1 Reworked Ion Engine

NASA’s Evolutionary Xenon Thruster propulsion system is an advanced ion propulsion system that is oriented toward robotic exploration of the solar system using solar electric power. The subsystem includes an ion engine, a power processing unit, feed system components, and a thruster gimbal. The prototype model engine, PM1, was subjected to qualification-level environmental testing to demonstrate compatibility with environments representative of anticipated mission requirements. Although the testing was largely successful, several issues were identified, including the fragmentation of potting cement on the discharge and neutralizer cathode heater terminations during vibration, which led to abbreviated thermal testing, and the generation of particulate contamination from manufacturing processes and engine materials. The engine was reworked to address most of these findings and renamed PM1R, and the environmental test sequence was repeated. Thruster functional testing was performed before and after the vibration and thermal-vacuum tests. Random vibration testing, conducted with the thruster mated to the breadboard gimbal, was executed at 10:0 grms for 2min in each of the three axes. Thermalvacuum testing included three thermal cycles from 120 to 215 C with hot engine restarts. Thruster performance was nominal throughout the test program, with minor variations in a few engine operating parameters likely caused by facility effects. There were no significant changes in engine performance as characterized by the engine operating parameters, ion optics performance measurements, and beam current density measurements, indicating no significant changes to the hardware as a result of the environmental testing. The redesigned cathode heater terminations successfully survived the vibration environments. NASA’s EvolutionaryXenonThruster PM1Rengine and the breadboard gimbal were found to be well designed tomeet environmental requirements based on the results reported herein.

Application of Solar Electric Propulsion to a Comet Surface Sample Return Mission

Current NSTAR (planned for the Discovery Mission: Dawn) and NASA's Evolutionary Xenon Thruster based propulsion systems were compared for a comet surface sample return mission to Tempe1 1. Mission and systems analyses were conducted over a range of array power for each propulsion system with an array of 12 kW EOL at 1 AU chosen for a baseline. Engine configurations investigated for NSTAR included 4 operational engines with 1 spare and 5 operational engines with 1 spare. The NEXT configuration investigated included 2 operational engines plus 1 spare, with performance estimated for high thrust and high Isp throttling modes. Figures of merit for this comparison include Solar Electric Propulsion dry mass, average engine throughput, and net non-propulsion payload returned to Earth flyby.

Modeling of Total Thruster Performance for NASA's Evolutionary Xenon Thruster Ion Optics

An axisymmetric ion-optics model is applied to NASA’s Evolutionary Xenon Thruster (NEXT) ion engine. The model is used to simulate the performance of the entire thruster by modeling several apertures at varying radii on the thruster face and integrating the results. The integrated results are compared to experimentally measured data for the NEXT thruster, showing good agreement in most areas. The primary area of discrepancy is in the accelerator grid current, although erosion results suggest that the measured current is unaccountably high. The model is also used to estimate the life of the thruster before the onset of electron backstreaming. Three separate methods are applied, and each predicts thruster failure after approximately 40,000 hours of operation or 845 kg of xenon throughput.

Wear Testing and Analysis of Ion Engine Discharge Cathode Keeper

Experimental and analytical investigations were conducted to predict the wear of the discharge cathode keeper orifice in the NASA Evolutionary Xenon Thruster. The ion current to the keeper was found to be highly dependent on the beam current, and the average keeper ion current density was nearly identical to that of the NASA Solar Electric Propulsion Technology and Applications Readiness (NSTAR) thruster for comparable beam current density. The ion current distribution was highly peaked toward the keeper orifice. The wear assessment predicted keeper orifice erosion to the same diameter as the cathode tube after processing 250 or 460 kg of xenon, depending on whether warm or cold ions, respectively, were assumed. Although the presented simple wear analysis does not predict failure, comparison with the NSTAR extended life test results suggests that the discharge cathode assembly will continue to operate beyond the qualification goal of processing 405 kg of propellant. Probabilistic wear analysis showed that the ion energy at the keeper surface and the sputter yield contributed most to the uncertainty in the wear assessment. It is recommended that fundamental experimental and modeling efforts focus on accurately describing the plasma potential, ion temperature, and the sputtering yield.

On the mechanical behavior of carbon-carbon optic grids determined using a bi-axial optical extensometer

Ion engines accelerate electrically charged plasma through two optic grids and emit the ions as exhaust. The process facilitates propulsion without use of chemical propellants. Braided carbon fiber reinforced composite (C(f)/C) optics are presently being considered for use as the accelerator and screen grids in ion propulsion engines. In this study the mechanical behavior of four candidate tow configurations proposed for the grids of NASA's Evolutionary Xenon Thruster (NEXT) were examined. A new bi-axial optical extensometer based on Digital Image Correlation (DIC) was developed and employed in determining the in-plane strain distribution resulting from uniaxial tension. The effective elastic modulus ranged from 4 GPa to 10 GPa at the onset of deformation. The stiffness either increased or decreased with further elongation as a result of bending of the axial tows and corresponding unit cell distortion. The transverse strain and Poisson's ratio of the panels were found to be a function of the tow dimensions and bonding between longitudinal and transverse tows.