Huntsville, Alabama – February 3, 2023 - Rubicon Space Systems is a manufacturer of thrusters and propulsion systems that use the advanced space craft energetic non-toxic (ASCENT) propellant, AF-M315E. Applications of these products include in-space propulsion, collision avoidance, de-orbit capability, and multi-mode (chemical + electric) propulsion. Rubicon Space Systems, a division of Plasma Processes, has developed and will soon qualify a new 5N thruster that uses ASCENT, adding to its growing line of high-performance green prop thrusters. The redesigned 5N Low Throughput (LT), thruster better addresses manufacturing consideration, spacecraft interface and integration, and increased its performance. “The 5N LT ASCENT thruster is poised to play an important role in upcoming plans” says Daniel Cavender, the Director of Rubicon Space Systems. “We are excited to begin a new qualification program in the coming months.”

The recent history of the 5N LT thruster cannot be told without also talking about its designer. Fresh out of the University of Alabama in Huntsville with a degree in Aerospace Engineering, Allison Lentz joined Rubicon Space Systems in July of 2022.  As one of the first employees of the new division, Lentz was in a unique position to influence the culture and identity of the new division. She took on the role of Propulsion Systems Engineer, supporting both thruster and propulsion systems development, assembly, and test. Not long after, Lentz was assigned to take over as lead engineer of the 5N LT thruster program, continuing work that was previously performed under a NASA Small Business contract.

More about the 5N LT Thruster

A smallsat can only carry so much propellant; there is little benefit to having a thruster that is qualified and priced for a throughput that far exceeds that amount. The 5N LT thruster design enabled cheaper and faster manufacturability whilst still maintaining high performance, only sacrificing on total throughput. Throughput is the term used to describe how much propellant a thruster can put through it and still function well. The LT 5N thruster targets small satellite (smallsat) mission applications with limited propellant supply to perform functions such as orbit raising, circularization, collision avoidance, and de-orbiting. The objective is to qualify the 5N LT thruster for 5kg propellant throughput. The 5N LT thruster has already demonstrated a specific impulse of 264 seconds and an accumulated firing time of nearly 25 minutes. The thruster is designed for 24-28 VDC spacecraft bus and requires ~25W to preheat the catalyst. 

 The History of the 5N Development

Plasma Processes LLC., initially developed the 5N LT thruster design through a NASA STTR Phase II award managed at NASA Marshall Space Flight Center. The first 5N LT EDUs were hot fire tested in late 2021. That testing served to validate performance and material life. The first hot fire testing of the EDU design consisted of short single pulses at varying feed pressures. Thrusters in the hot fire test cell are monitored not only visually, but with various temperature, pressure, force, and flow sensors monitoring the thruster and system. The first EDU thruster reached a total throughput of 1.4 kg and nearly 11 minutes of firing time before noticeable performance degradation had occurred. Some minor tweaks were made in assembly of the second EDU thruster. That thruster reached a total throughput of over 3 kg and more than 24 minutes of firing time with the longest single firing being over six minutes. The test data was very consistent pulse-to-pulse, verifying that the thruster could maintain good performance across a range of duty cycles from 10% to 100%. The specific impulse (Isp) ranged from to 221 - 264 seconds across that range of duty cycles. Leveraging data collected in EDU hot fire testing, Rubicon engineers noted some needed improvements to bring the design to flight-grade. That is where Lentz stepped in.

The Reimagined 5N LT Thruster

Allison was challenged with familiarizing herself with the design state and history, completing a revision to the 5N LT thruster design, and holding a design review only two months away. “I was excited to step into the design process of the thruster” Allison said. “The design was in overall good shape, but the challenge was enhancing the design beyond thrust performance improvements.” A triangular mounting plate was added to support spacecraft integration. Two new components, a cable clamp and a junction box saddle, were added to the design to better secure the heater and thermocouple cables. The thermal standoff was redesigned to improve the thermal gradient and decrease vibrational loads to the thruster. The redesign happily went beyond just function; the hexagon is a recurring feature in Rubicon’s design style. Lentz used the opportunity to infuse some fashion into the standoffs as well.  “It was a fun idea and challenge to design this part this way” Allison recounted. “I think it’s a style that you’ll see getting infused in a lot of Rubicon’s designs in the future.” The design assessment of the 5N low throughput thruster was completed in October 2022 leading to finalizing and releasing the piece part drawing package. A final structural and thermal analysis cycle showed the new design was just as resilient as the earlier design, and Lentz moved forward with getting the new parts manufactured.

 What to Expect Next…

At the time of this story, nearly all parts for the new 5N LT thrusters have been received in house. Lentz and the rest of the Rubicon team are now starting assembly of the first five 5N LT thrusters, having just passed an assembly readiness assessment. According to Lentz, “By the end of March, we will be ready to start hot fire testing these thrusters. We will spend most of April performing the new qualification test campaign.” All five thrusters will undergo environmental testing including random vibration, thermal cycling, and hot fire testing. A subset of thrusters will then be selected to be qualification units.

Rubicon has now started to develop its 5N high throughput (HT) thruster, designed with a target throughput of more than 100 kg of ASCENT. Related to that activity is a development of a new long life ASCENT thruster valve that will work with both the 1N and 5N thruster lines. 

Rubicon and Plasma are also experimenting with new Ruthenium-Iridium alloy catalyst metal foams in an effort to further reduce the cost of the 5N LT thruster. Plasma Processes’ patented metal foam catalyst gives Rubicon’s thrusters a technological advantage over packed granular beds. “Our edge is our unique in-house manufacturing technology and expertise with ASCENT propellant,” Cavender says. “And the young engineers like Allison that are growing up with this technology are going to be the ones that will lead the green chemical propulsion revolution.”

Lentz says she is eager to complete assembly and participate in the qualification of the 5N LT thruster. It is obvious that she is passionate about thruster testing and excited to gain even more experience in participating and running hot fire tests. Lentz’s next goals include specialized training to take Rubicon’s thermal and structural analyses needs in-house, supporting Rubicon’s future thrusters and propulsion system design efforts. She plans to continue growing her knowledge with CAD and communicating design choices with others. The drive and ingenuity of Lentz and other young engineers like her will continue to enable exciting new capabilities, both at Rubicon and for the entire small satellite industry.