Hybrid Chemical-Electric Propulsion Systems for CubeSats

Abstract

As CubeSats have proved their benefits for missions ranging from Earth observation, communication, navigation, or science, miniaturized propulsion systems have been actively developed and demonstrated in-flight to support these applications. Propulsion systems are primarily divided into two main categories. Chemical propulsion systems benefit from high thrust to perform impulsive maneuvers but have a low specific impulse. On the other hand, electric propulsion systems have a much lower thrust but a high specific impulse, thus resulting in large delta-v budgets. While common for larger spacecraft to include both types of propulsion on-board, the stringent size, weight, and power constraints on CubeSat have mainly limited CubeSats to only one type of propulsion.

However, the research and development in propulsion systems miniaturization over the last couple of years provides an opportunity to reconsider and evaluate the current and future feasibility of hybrid chemical-electric systems. Hybrid propulsion systems combine two or more propulsion technologies into a spacecraft without any shared hardware and could unlock ambitious missions. One such example is ReCon (Reconfigurable Constellations), a concept developed to enable remote sensing constellations to image specific areas of interest with an increased spatial and temporal resolution, on-demand, and without increasing constellation sizes. These constellations require significant maneuvering capabilities, including responsive impulsive transfers when time-sensitive observation needs arise -- for instance, during extreme weather events or conflicts.

This thesis will evaluate the performance and feasibility of hybrid chemical-electric propulsion systems on CubeSats as an alternative to chemical-only systems for missions requiring high-thrust capabilities and large delta-v budgets. Feasible architectures relying on commercial off-the-shelf (COTS) systems are identified, and their performance is compared to single-mode systems according to different constraints and performance requirements. 2U was identified as the minimum volume required for COTS hybrid chemical-electric architectures to be advantageous over single-mode systems. In a 2U volume, more than 20 hybrid chemical-electric architectures can provide a delta-v for impulsive maneuvers above 70 m/s with a delta-v for low-thrust maneuvers superior to 220 m/s while satisfying power constraints, while the optimal chemical system can provide only up to 245 m/s of delta-v.

Improved hybrid architectures can be generated by concurrent design optimization of the chemical and electric systems. The design space of hybrid architectures is explored through the parametric modeling of a cold gas thruster, a green monopropellant, and an ion thruster. The optimality gap with previously generated designs is then quantified. Hybrid designs with a volume as small as 1.5U can then exceed the performance of single-mode systems. This approach demonstrates that custom-designed hybrid payloads can meet and exceed mission requirements better than COTS hybrid payloads; however, it comes with an increased cost due to additional research and development needs, resulting in necessary tradeoffs.