Versatile Attitude Control Actuators for Sub-Milliarcsecond Precision Pointing

Growing scientific and practical needs exist for precision spacecraft pointing at milliarcsec (mas) levels. Present state-of-the-art reaction wheels, or similar, actuators introduce vibrational jitter and can ultimately drive pointing error. Busek electrospray thrusters aboard the ESA LISA Pathfinder mission have recently demonstrated nm scale position control. The proposed system will provide vibration-free sub-mas pointing control through integration of follow-on innovations and new deployable boom technologies. Recent thruster innovations have dramatically increased maximum thrust, while retaining control resolution, and reduced propellant loads through increased specific impulse. These gains will be applied in this work to replace reaction wheel actuators for both precision control and slew maneuvers. Accordingly, actuator induced vibrations are virtually eliminated while body pointing will be dramatically improved; resulting in major reductions in ACS SWaP and in the complexity of vibration compensating controllers/active sensors. With applications to, for example, observation and laser communication missions these benefits would both enable high-capability but reduced cost spacecraft and pave the way for new, presently, unobtainable levels of control authority on large spacecraft. Phase I will emphasize acquisition and analysis of precision thrust control measurements using the BET-1mN thruster-head which, uniquely, can be operated in either a high thrust, low Isp, or low thrust, high Isp configuration. The associated complex performance map will be analyzed in the context of highly quiescent ACS needs. These data will then be applied to define necessary control, circuitry and mechanical requirements needed to realize the full-attitude control potential of the technology. A conceptual system level design performed in collaboration with ROCCOR will outline strategies to overcome required developments and produce a complete system demonstration in Phase II.

The proposed technology has numerous applications to NASA missions improving both scientific and practical capabilities. Precision pointing applications include science objectives, such as exoplanet observations, laser communications and space situational awareness (SSA). In all cases, the greatly improved pointing over existing SoA (e.g reaction wheels) afforded by the proposed technology would both enhance pointing accuracy/stability and reduce the need for complex vibration and control accuracy mitigation strategies such as the use of active optics (e.g. fast steering mirrors). Laser communication benefits are particularly relevant with active NASA programs pursuing, for example, optical communication for Mars missions. SSA and identification of solar system objects would benefit from an ability to track commanded attitude profiles to <1 pixel. Long integration times can then be facilitated permitting resolution of very dim (small) objects. This system also applies to precise position control. Disturbance reduction has been demonstrated in space by larger versions of Busek's colloid thrusters on the NASA ST-7/ ESA LISA Pathfinder mission. Comparatively, the BET-1mN offers both higher maximum thrust and enhanced specific impulse; and therefore, lower propellant mass requirements. As an evolution of that work this program will enable other aspects of the NASA Disturbance Reduction System (DRS), such as controllers developed by GSFC to have new application routes.

Potential non-NASA customers include, international partners (such as ESA), the DoD and commercial EO missions. The readily configurable nature of the proposed technology would enable customized applications to simultaneously meet customer needs in precision pointing and disturbance compensation; therefore, maximizing the commercial applicability of the technology. Furthermore, the ability to scale both thrust output and boom moment arm, by virtue of Busek and ROCCORs respective scalable technologies, enables rapid customization of the system. Commercial EO applications may include optical communication pointing, supporting high bandwidth up/downlinks or precision pointing in earth orbit for earth observation organizations. With strong links to the now TRL-7 electrospray system Busek demonstrated on the LISA Pathfinder mission; the time is now right for other organizations to embrace this technology. The proposed effort will ensure such entities have access to a fully featured and versatile system.

This is early/mid-stage (TRL 4) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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