Abstract
Electric propulsion (EP) is a proven solution for many space applications; its low thrust and high specific impulse allows for delicate, repeatable manoeuvres and long-distance travel. However, its capabilities are limited by the available power, resulting in the continued need for chemical propulsion (CP) to perform high thrust, rapid manoeuvres. Combined chemical-electric propulsion provides an attractive solution, but the need for multiple propellants and feed systems results in increased mass, cost, and complexity of the system. In addition, established EP technologies face significant challenges with the most common propellant, xenon, due to its increasingly high cost and limited availability. This research investigates two propellants identified as “green” alternatives to hydrazine for CP in a novel application to EP, to address the need for alternative propellants to alleviate the reliance of EP on xenon, while enabling hybrid chemical-electric propulsion systems to operate on a single propellant with the potential of delivering the benefits of both CP and EP from the same propulsion architecture. The first candidate, triethylamine (TEA), was demonstrated in the 1960’s as a feasible alternative to hydrazine in CP. It presents similar mass and ionisation potential to xenon; however, its molecular nature may lead to fragmentation in EP and, thus, reduced performance. The second candidate, tripropylamine (TPA), is chemically similar to TEA and has higher mass, which may counteract any potential fragmentation.
The propellant candidates are investigated and demonstrated for the first time with EP technology. Performance of these tertiary amines is directly measured using a pendulum-type thrust balance and characterised with a low-power Cylindrical Hall Thruster over a range of operating conditions and mass flow rates. Both propellants show overall reduced performance compared to xenon, with a decrease in thrust of ~32% (TEA) and ~22% (TPA) observed at ~100 W and 0.2 mg/s, and further reductions at higher mass flow rates. The existence of molecular fragmentation is observed within the thruster’s exhaust plume using optical emission spectroscopy and confirmed through analysis of deposits generated by the amine plasmas. The fragmentation processes occurring within the plasmas are investigated in a dedicated experimental setup, using mass spectrometry, indicating the most probable ion masses to be expected are 86 amu (TEA) and 114 amu (TPA).