October 2016 Astronautics

Unlocking the potential of annular ion engines

The problem with electric propulsion and advanced concepts is that the rate of implementation is so glacial it tends to stifle innovation. The perspective, ‘We won’t need that for at least XX years so why work on it?’

Artist’s concept showing NASA’s Dawn spacecraft thrusting with its centre ion engine high above the night side of Ceres.
Artist’s concept showing NASA’s Dawn spacecraft thrusting with its centre ion engine high above the night side of Ceres.
Michael J. Patterson Michael J. Patterson Senior Technologist, In-Space Propulsion

The problem with electric propulsion and advanced concepts is that the rate of implementation is so glacial it tends to stifle innovation. The perspective, ‘We won’t need that for at least XX years so why work on it?’ is erroneous on at least two levels. One, it implies that nothing discovered will have a derivative value in solving an immediate problem or need; and two, nothing can be discovered or learned which could affect the pace of implementation. This article summarises the technology development status of the AIE and potential forward-paths for further concept maturation.

The Annular Ion Engine (AIE) concept represents an evolutionary development in gridded ion thruster technology with the potential for delivering revolutionary capabilities. It has this potential because the AIE concept: (a) enables scaling of ion thruster technology to high power at specific impulse (Isp) values of interest for near-term mission applications, ?5000 sec; and (b) it enables an increase in both thrust density and thrust-topower (F/P) ratio exceeding conventional ion thrusters and other electric propulsion (EP) technology options, thereby yielding the highest performance over a broad range in Isp.

The AIE concept represents a natural progression of gridded ion thruster technology beyond the capabilities embodied by NASA’s Evolutionary Xenon Thruster (NEXT). The AIE would be appropriate for: (a) applications which require power levels exceeding NEXT’s capabilities (14 kW), with scalability potentially to 100s of kW; and/or (b) applications which require F/P conditions exceeding NEXT’s capabilities. The AIE concept consists of an annular discharge chamber with a set of annular ion optics, potentially configured with a centrally-mounted neutralizer cathode assembly.

The capabilities of SOA ion thruster technology, and the performance goals for the development of the AIE are enumerated in Table 2. SOA ion technology represents the performance capabilities of NASA’s high-fidelity NEXT ion thruster, which has superior demonstrated F/P, and efficiency, as compared to all other EP technology options above about 2600 seconds specific impulse (Isp).

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