Colloid Thruster Life Testing and Modeling

Colloid electrospray thruster technology development from TRL 4 to TRL 5 is proposed to enable the required exoplanet exploration mission pointing precision through the Strategic Astrophysics Technology (SAT) program. This development program includes an 8000 h life test for a 9-emitter colloid thruster head and scaled single emitter life testing with life model development and validation to mature the scalable thruster technology to TRL 5 in 3 years. Long-lifetime, high-precision colloid thrusters will enable exoplanet imaging and other fine measurements for the next generation of astronomy and astrophysics missions. The National Academy of Sciences’ 2021 Decadal Survey Report recommends that a Great Observatory (GO) capable of imaging exoplanets should be NASA’s highest priority for the next decade or more [Astro2020]. To image exoplanets, observatory and contrast stability is a key requirement and has been identified as a Tier 1 priority in the Exoplanet Exploration Program (ExEP) Technology Gap List. It states that contrast and observatory stability can be addressed with ultra-stable structures and disturbance isolation and reduction using flight-proven colloid thrusters. Busek colloid thrusters are the most developed electrospray thruster technology that meets GO requirements with long lifetime and characterized thrust and thrust vector noise and stability. They were demonstrated for 3,458 h during flight qualification to TRL 6 on the ground and >2,400 h on the LPF/ST7 mission in flight in 2017 for TRL 7. Since the LPF/ST7 mission, the thruster design and feed system have been improved for longer lifetimes for the LISA mission with support from the NASA LISA Study Office (NLSO), resetting the thruster TRL to 4 for mission requirements. The NLSO activities will conclude in 2023 with a brassboard thruster delivery built for long duration thruster testing and TRL 5 validation by a future program. The next critical step in maturing colloid thrusters for NASA science missions, including an exoplanet observatory mission, is a long duration thruster test and lifetime model validation over the full throttle range to enable credible lifetime prediction with quantified uncertainty for any mission thrust level requirement. Under the Strategic Astrophysics Technology (SAT) program, we propose a single-string colloid thruster long duration test for 8,000 hours at Busek. We will verify the scalability of the thruster design with number of emitters and capacity of electrodes by demonstrating that the mass accumulation rate in the extractor and accelerator, the primary life-limiting mechanism, in the 9-emitter thruster is 9x the rates measured in a 1 emitter assembly. We propose thruster lifetime model improvements working with the University of California, Irvine. We propose life model validation over the full range of thrust levels and transient modes expected to enable >7.5 yr lifetimes at JPL using a single emitter electrospray assembly and the results from the 8000 h thruster test. Success is defined by achieving thruster test results that agree with scaled single emitter results and model predictions that agree within 1-sigma quantified uncertainty with test results. We will also characterize spacecraft contamination potential with plume mass distribution measurements and thrust vector stability with single emitter tests. This combination of testing and modeling is required to validate the scalable thruster technology to TRL 5 with a predicted 7.5 year lifetime within the 3-year SAT program. This effort will produce a validated thruster lifetime model to be used to determine number of emitters per thruster and electrode capacity for any exoplanet observatory mission thruster thrust level and lifetime requirements. This effort will keep the momentum going from the LISA work moving forward to infusion in the next Great Exoplanet Observatory in time to be included in mission formulation.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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