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Completed TRL 2 (started at 2, targeting 5)
Project Objective
Advance Platinum Group Metal (PGM)-based Additive Manufacturing (AM) TRL by developing processes to create PGM AM optimized isotropic ultrafine lattice catalysts for integration in green propulsion systems.
Project Description
Reaction control systems are usually powered by monopropellants like hydrazine and/or ASCENT green propellant and used ceramic/carbon-based Ir coated catalyst beds given the high temperature requirements. Traditional manufacturing of these catalysts is time consuming, expensive, and with only a few vendors, long lead times are the norm. Moreover, current ceramic/carbon-based, Ir-coated catalysts exhibit anisotropic properties leading to low performance and inferior mechanical properties with limited reproducibility. Due to the low volume, complexity and cost, additive manufacturing (AM) can be leveraged to create platinum group metal AM optimized isotropic catalyst for integration in refractory AM green propulsion chambers and tested under prototypic operating environments. The implementation of AM will reduce long lead times and enable fine tuning of catalyst design for specific needs. At the conclusions of the project, laser powder bed fusion of ultra-fine lattices/catalysts will be demonstrated using surrogate material as the minimum measure of success.
Project Results and Conclusions
Traditional manufacturing techniques of propulsion catalysts are time-consuming, and expensive. Typically, catalysts are made of ceramic or carbon foams with anisotropic pore distributions and coated with platinum metal group elements such as iridium (Ir). The anisotropic pore distribution causes deposition of the coating to be spatially non-uniform, resulting in performance variabilities and limited reproducibility. Moreover, foam and coating services are very limited and impact availability, reproducibility, and cost. From a material’s standpoint, the nature of the base material also makes the catalyst prone to fracture during integration into the thruster reaction chambers, leading to early failures and faulty performance. Additive manufacturing (AM) serves as a modern method to design and manufacture thruster catalysts that enable improved properties, availability, reproducibility, and cost, using AM ultra-fine catalysts.
The Laser Additive Technique Tailored for Ir Catalyst Engineering (LATTICE) project seeks to address the aforementioned problems using experimental and integrated computational materials engineering (ICME) tools. LATTICE’s objectives are to advance the Ir-based metal AM technology readiness level by developing methods and materials to generate Ir-based AM isotropic catalyst foams and to achieve design improvements and integration in refractory AM green propulsion chambers to test under prototypic operating environments.
To date, under the LATTICE project, lattices of surrogate materials (Mo and W) were created via L-PBF AM, saving our limited supply of PGM Ir-powder for printing and testing of optimized lattice design. Multiple lattice designs were evaluated from a printability perspective to obtain properties like those found in traditional catalysts from a porosity and average lattice strut thickness. A star lattice design was selected as the simplest structure, obtaining an average pore size of ~225 µm and lattice strut thickness of ~100 µm. The printed lattices were characterized using scanning electron microscopy (SEM) and 3D tomography was created using a RoboMET auto-polisher, expanding on techniques to evaluate complex printed structures.
Due to a small supply of PGM-Ir powder, L-PBF machine modifications had to be made, with some still underway. So far, the machine powder hopper and recoated mechanism component were redesigned and manufactured using AM. The build plate will also need to be changed to allow small batch component prints. Therefore, a risk mitigation step was taken to coat W-lattices with Ir for a hybrid AM/traditional approach that reduces cost/schedule and provides greater strength and control on lattice design. Next steps include: (1) pressure drop testing of the coated lattice, (2) testing in relevant environment to inform future iterations, (3) AM of Ir-PGM lattices and (4) AM of other PGM based on stakeholder needs.
Traditional manufacturing of green propulsion catalysts is time consuming and expensive. Current ceramic/carbon-based, Ir-coated catalysts exhibit anisotropic properties leading to low performance and inferior mechanical properties with limited reproducibility. AM of Ir- or W/Ir catalysts has the potential to revolutionize catalyst manufacturing processes for space applications. Just cost and schedule will benefit from >90% in cost and time save over traditional manufacturing techniques while improving design flexibility, flow properties and mechanical properties. Moreover, lattices can be designed such that porosity can be controlled as a function of axial length to maintain a constant propellant volumetric flowrate as the liquid propellant decomposes to a gaseous state. If the technology works, it is expected that all small catalyst beds for green propellant systems will be manufactured using AM, and more investment will be made to scale up the process.
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