Development and characterisation of an additive manufactured high-temperature resistojet.

Abstract

The increasing capabilities of small satellites, and the transitioning of geostationary satellites to electric propulsion, are driving a demand for low-cost propulsion systems operating at low power and high thrust. Resistojets are an attractive technology to meet this demand, with high thrust-to-power ratio, simple construction, and compatibility with a wide range of propellants for integration into all-electric propulsion architectures. However, currently available resistojets operate at very low specific impulse, and their use is therefore limited. This can be improved by operating at higher temperatures, but there are many challenges associated with doing so. These include withstanding cyclic thermal stresses, counteracting increased inefficiency due to heat loss, and manufacturing difficulties associated with high-temperature materials. Advances in additive manufacturing technologies can help to overcome these challenges, enabling the production of complex heater geometries at low cost. The combination of industry demand and new manufacturing capabilities makes investigation of high-temperature resistojets timely.

In the first part of this thesis, a previously developed proof-of-concept additive manufactured resistojet was tested to determine its endurance to thermal cycling representative of its operational conditions. Two premature failure modes were discovered through this experimental campaign using electrical diagnostics during testing. X-ray computed tomography imaging was used to non-destructively inspect the prototypes after testing, and the resulting images were combined with the in-situ data to determine the failure mechanisms. 3D numerical models incorporating coupled electrical, thermal and mechanical physics were used to investigate the temperature and stress distributions in the prototypes. The experimental and numerical approaches provided complementary insights into the complex behaviour of the resistojet, which includes operation of materials close to their melting points, and highly non-linear phenomena such as radiative heat transfer. Using these approaches, the design was updated to eliminate the identified failure modes by reducing the buildup of thermal stresses from differential thermal expansion. The new design was manufactured in the nickel alloys Inconel 718 and Hastelloy X, as well as in the refractory metal tantalum. An experimental campaign on the new design, the first to be reported in candidate flight materials for the high-temperature resistojet, showed an order of magnitude improvement in endurance, but failed to meet the requirements. The updated design was demountable, allowing visual inspection of the failure sites after testing. Numerical modelling indicated that the approach to thermal stress reduction was promising, and provided directions for further improvement.

The second part of this thesis presented a second design iteration, manufactured using Inconel 625 and tantalum. Endurance tests showed for the first time that additive manufactured high-temperature resistojet heaters and engineering model thrusters could exceed the required cycle life for industrial application. Numerical models showed that this extended endurance was due to reduction of thermal stress, as a result of a design which matched the thermal expansion of different components. The performance of these thrusters was characterised by direct thrust measurements in vacuum. The Inconel 625 thruster achieved a maximum specific impulse with xenon of 61.6 +/- 0.6 s, at a thrust of 89 +/- 0.4 mN, using 3 bar supply pressure and 58 +/-$0.6 W input power. The thrust-to-power ratio in this test was 1.5 mN/W. Varying supply pressure from 2 to 4 bar at 58 W increased the thrust by a factor of 2 while maintaining >95 % of the maximum measured specific impulse. The tantalum thruster achieved a maximum specific impulse measured with argon of 120.6 +/- 0.9 s specific impulse at a thrust of 189.0 +/- 0.6 mN, using 7 bar supply pressure and 180 +/- 1.1 W input power. This performance implies a specific impulse of 66 s with xenon. These values exceed the specific impulse of commercial state-of-the-art resistojets by 28 % (Inconel 625) and 37 % (tantalum).

The work presented in this thesis advances the field of propulsion by demonstrating, for the first time, that engineering model additive manufactured resistojets made from high-temperature materials can withstand cyclic operation for mission-relevant timescales; by directly measuring significantly greater specific impulse than that provided by the state of the art; and by developing combined numerical and experimental methods for analysing the thrusters, which can be used in future design efforts. This work has raised the technology readiness level of high-temperature additive manufactured resistojets from 3 to 5. It has demonstrated the potential of resistojets to address the identified needs of the space industry, and identified paths to develop further to a viable propulsion system.

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Identifiers

ORCID for Matthew David Robinson: orcid.org/0000-0003-0709-1219

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