Over the past decade, the Propulsion Research Laboratory (PRL) at Utah State University has developed a promising “green” alternative to the current generation of space propulsion systems based on environmentally unsustainable, toxic, and highly sensitive propellants including nitrogen tetroxide, mixtures of nitrides (MON), MMH, UDMH, and mono-propellant hydrazine. Results and lessons learned from the USU green propellant development program will be presented. Although the current USU program has emphasized the development of green propulsion for small spacecraft systems, potential U.S. Army applications for insensitive munitions, reduced-hazard gas generation cycles, and “green” propulsion will be discussed.
The novel USU green propulsion system derives from the unusual electrical breakdown properties of certain 3-D printed plastics, most notably acrylonitrile butadiene styrene (ABS). This property was discovered serendipitously while investigating the use of printed ABS as an insulating material. The layered material of printed ABS possesses a high level of surface micro-structure. These structural features have the ability to concentrate charge at many discrete points on the material surface, even though ABS is technically a non-conductive material. This property allows a strong electrical arc to occur between layers at moderate voltage and low current input levels. Identical samples made from extruded/machined ABS do not exhibit these electrical arcing properties. The electric field generated by this inductive arc produces joule level heating, resulting in fuel pyrolysis along the conduction path. When an oxidizer is simultaneously introduced, the pyrolysis results in rapid and sustained combustion along the entire fuel port surface.
The discovery of the unique electrical breakdown characteristics inherent to 3-D printed ABS prompted the engineering of an ignition system that takes advantage of the previously described surface-arc-driven phenomenon. The arc-ignition method has been developed to a relatively high Technology Readiness Level (TRL 5). The power-efficient USU-system can be started and restarted with a high degree of reliability, and various ground-test units with thrust levels varying from 4.5 N to 900 N have been developed and tested. During the summers of 2016 and 2017, USU performed a series of vacuum tests on a small flight weight 25-N thruster system at the Marshall Flight Center in order to demonstrate the system restart capabilities and near-vacuum performance.
On March 25th, 2018 a flight experiment integrating two prototypes of this thruster system was launched aboard a two-stage Terrier-Improved Malemute sounding rocket from the NASA Wallops Flight Facility (WFF). The launch achieved apogee of 172 km, allowing more than 6 minutes in a true space environment above the Von- Karman line. During the mission the USU thruster was successfully fired 5. The payload section was successfully recovered by WFF flight support. Low-resolution telemetry data was successfully downlinked and delivered to USU for analysis. During this test flight, surface material deposition measurements were also gathered to assess potential issues associated with plume contamination of spacecraft optical sensors and solar panels. This test marks the first time that plume contamination measurements have been performed on a hybrid rocket system. The flight system designed to burn at equivalence rations below 1, burned extremely clean.
The “green” propulsion technologies developed at Utah State University are potentially market-disruptive. There exists the potential to reduce small spacecraft propulsion associated costs from the current level of over $100K to less than $10,000, more than order of magnitude cost reduction. Such a development would provide billions of dollars of mission value for government and commercial customers at dramatically reduced cost.
