Takeaway: Rubicon’s test series totaled >12,000 thruster firings with ~1.6 hours of total firing time and propellant throughput of 550 grams. Moves Sprite towards space flight readiness.

Story: Rubicon Space recently performed a series of hot fire tests of our new propulsion system - Sprite. The tests are part of our path to qualification, simulating a series of thruster firings and a range of conditions in a flight-like configuration. Sprite is a self-contained propulsion module that features our 0.1N ASCENT thruster with flight heritage. It weighs less than 2 kilograms and is designed to deliver over 1200 Newton-seconds of total impulse. Up to eight (8) Sprite modules can be installed and work collectively on a spacecraft.

This was the first system level hot fire test (HFT) of a propulsion system designed by Rubicon. Additionally, Sprite applies lessons learned from previous missions with the ASCENT propellant, additively manufactured components. The test unit was nicknamed Pinky; after one of the ghosts from Pac-Man and for the pinkish color of the thruster when firing. Pinky was modified to integrate within our hot fire test stand.

While the performance characteristics of the 0.1N thruster and other components are well understood, our team built Pinky to demonstrate system level integration and operation in a flight-like way. The team designed a test campaign that approximated how Sprite would be operated in space. Our objectives were to demonstrate the lessons learned from NASA’s Lunar Flashlight mission, exercise Sprite’s full concept of operations (ConOps), and validate the hardware and firmware under flight-like thruster firing operation and conditions for the duration of mission life.

Sprite is designed to operate as a simple blowdown system. It will be pressurized to an initial high pressure on the ground. We call this “super nominal feed pressure” because the thruster will be operating above the pressure required to create 0.1 Newtons of thrust. Then, through the mission and as propellant is expelled, the pressure falls, first reaching the “nominal feed pressure”, the point where thrust equals 0.1 Newtons.  Continued operation enters the “sub-nominal feed pressure” range, where thrust is less than 0.1 Newton. Eventually, there is not enough pressure for the thruster to operate.

The hot fire test program simulated the full range of feed pressures from the beginning of mission (BoM) super nominal feed pressure down to the end of mission (EoM) and lowest sub-nominal feed pressure.  At super nominal pressures, more propellant is forced into the thruster generating more thermal energy than at nominal feed pressure. Pinky included a suite of additional sensors to provide greater insight into its thermal state. During the HFT program, we mapped those thermal responses. That data was useful to better anchor our thermal models and to assess any necessary operational limitations on thruster firing. The test series totaled more than 12,000 thruster firings with nearly 1.6 hours of total firing time and propellant throughput of 550 grams.

The Sprite propulsion system’s flight firmware was developed using JPL’s FPrime framework, ensuring that it is robust, modular, and adaptable. The spacecraft bus interface exposes some of this functionality in a way that is easy to implement for a variety of flight systems. 10-byte command packets and 86-byte telemetry packets are transmitted via the RS-422 hardware standard, using a simple, well-defined packet structure with flight heritage. Throughout testing Pinky, the flight firmware and the user interface software enabled easy monitoring of the sensor and actuator states. The team also took the opportunity to debug and tune the system’s command response and operational parameters.

The test article was outfitted with sensors to collect data on system-level temperature profiles, pressures, and thruster performance. In flight, Sprite will have only a few of these sensors for operators on the ground to use to determine how the system is functioning. We call these two sets of sensors development and flight sensors, respectively. Flight sensor data for the test campaign was recorded through Sprite’s telemetry readback, just like it will be during a mission; however, development sensor data was recorded through our test cell interface.

A challenge with data reduction was how to synchronize the telemetry data from the propulsion system and the data gathered from the test facility. While performance data was measured by test facility instrumentation, the information needed to reduce the performance characteristics was embedded within the Pinky’s telemetry. The two data sets, while not recording at the same rate or starting at the same time, had to be lined up together. This challenge helped us develop new tools to data reduction that we will write more about in a future publication.

With the successful completion of the hot fire testing, Rubicon is one step closure to qualifying the Sprite propulsion system. The test campaign increased our knowledge for building and testing an entire propulsion system, and informed us about Sprite’s operational envelope when firing the thruster at super nominal feed pressures to preserve the overall life of the system. Now we are ready to move ahead with assembly of our first flight unit.

About Rubicon Space Systems: 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 are in-space propulsion, collision avoidance, de-orbit, and multi-mode. Our team leads the world in the development and infusion of ASCENT-based technology. ASCENT (Advanced Spacecraft Energetic Non-Toxic) propellant is an advanced monopropellant formulation developed by the Air Force Research Laboratory (AFRL), formerly referred to as AF-M315E.