Sub-newton monopropellant thrusters for small spacecraft propulsion

Abstract

This thesis documents the design and testing of a sub-newton High Test Peroxide (HTP) monopropellant thruster architecture, suitable for micro and nanosatellite applications. The main goals were to demonstrate that a thruster targeting a 0.1 N thrust level could achieve a high performance, and to investigate the fundamental processes governing its operation. The thruster design exhibited high thermal performance using 87.5 %wt. HTP, with chamber temperatures in excess of 600 °C, corresponding to characteristic velocity efficiencies approaching 0.963 ± 0.004 (1𝜎). These values are similar to equivalent larger-thrust systems. The thruster was also tested with 98.0 %wt. HTP, with temperatures approaching 800 °C. These results represent some of the highest performance of an HTP monopropellant system, and is a first at sub-newton thrust levels. The experimental results were collected by direct measurement of the internal fluid conditions, something not previously achieved on a thruster of this scale. This was made possible by the novel application of conventional metal additive manufacturing techniques, permitting the inclusion of integrated instrumentation standpipes and other complex ancillary geometry. The manufacturing process also enabled the production of a large number of different catalyst bed designs for use in one of the most extensive experimental studies on the impact of the catalyst bed geometry. The geometry was parametrically defined by the catalyst bed loading and aspect ratio, with respective values ranging from 1.48 kg.m^-2.s^-1 to 64.8 kg.m^-2.s^-1 and 0.33 to 6. The baseline thruster design, with a nominal bed loading of 10 kg.m^-2. s^-1 and an aspect ratio of 2, had the highest demonstrated performance. However, data analysis suggested that lower catalyst bed loadings and aspect ratios, i.e. shorter and wider beds, should be more optimal over a blowdown range. The study provided data towards validating a simplified pseudo-physical front model of the catalyst bed. This model describes the phase transition between the cool liquid/multiphase and the hot gas regimes. The phase change front was found to be axially close to the complete decomposition front, representing the maximum temperature in the bed. The catalyst bed flooding condition was used to relate the fundamental reactivity of the catalyst bed to the pseudo-physical front model. It was proposed that the reactivity can be used to size a bed with minimal experimental testing, through the Damköhler number: the estimated liquid-phase decomposition rate was greater than the rate of forced propellant diffusion for nominal catalyst bed operation. The pressure roughness phenomenon was also investigated using spectral analysis. These results were used to justify the proposed local oscillatory diffusion process in the catalyst bed. This is responsible for the pressure roughness, as well as the localised choking in the upstream liquid/multiphase regime that causes high pressure drop over this region. An extensive study on microinjectors was also conducted. This is a field of limited published research. The study characterised the performance of Poiseuille-type microbore tube and Venturitype orifice plate injectors. Poiseuille injectors demonstrated stable performance while orifice injectors were challenging to manufacture and prone to blocking. ‘Chugging’ flow instabilities were also captured, and it is proposed that the onset of this condition is tied to the inertia of the propellant flow through the injector. This, along with the minimum critical flow rate for the Venturi cavitation phenomenon, suggests that the Poiseuille microinjectors are a more robust architecture and better suited to sub-newton monopropellant thrusters.

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ORCID for Ewan, Alexander Patrick Fonda-Marsland: orcid.org/0000-0002-1584-0067

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