Hybrid Direct Drive PPU for Deep Space CubeSat Propulsion System

Busek proposes to develop an innovative, hybrid direct-drive (HDD) Power Processing Unit (PPU) for CubeSat electric propulsion (EP) systems. The technological approach is an adaptation of a Busek patented technology, US patent number 8,550,405, entitled "Solar Powered Spacecraft Power System for a Hall Effect Thruster." This technology will immediately benefit the deep space 6U CubeSat currently being developed under NASA's "LunarCube" program, which is equipped with Busek's groundbreaking, iodine-fueled RF ion thruster BIT-3. The HDD concept will take advantage of the solar array's raw voltage, which is much higher than the conditioned voltages typically supplied by the spacecraft's electrical power system (EPS), and directly feeds it into the propulsion system's PPU to minimize efficiency losses during high-voltage conversion. The end result is increased available power for thruster utilization, which in turn can help maximize the vehicle's thrust, specific impulse (Isp) and delta-V maneuverability during the continuous-thrusting phase of the mission. Operating the ion thruster at higher power will ultimately help reduce time spent on trajectory transfer, thus saving mission operation costs and lessening burden on electronics in high radiation environments. In typical Cube-Sat systems the bus voltage is quite low. This requires additional power conditioning using step up DC/DC converters when high voltages are needed (such as the grids on the BIT-3 RF ion thruster). In the proposed approach the HDD unit conditions the high voltage directly from the solar array and passes it to the PPU, thus eliminating redundant power processing of step up converters and improving system efficiency. This topology also offers the novel feature of thruster dependent peak power tracking, providing direct control of the power flow from the arrays to the PPU by throttling the thruster.

The application of the Hybrid Direct Drive PPU is best realized when coupled to Busek's miniaturized RF ion propulsion systems. Exploring our solar system with low-cost robotic/scout vehicles as precursors for human missions or science missions will benefit from these technologies. For small vehicle platforms such as CubeSats, maintaining high thruster power (hence high Isp and delta-V performance) through management of PPU efficiency will be paramount to the mission's success. Although the initial application of the Direct Drive PPU targets the LunarCube's BIT-3 RF ion thruster, this concept can be easily scaled up to adapt to other EP systems of different sizes and power levels. Potential NASA applications of the Direct Drive enabled EP system include missions to the moon (such as the upcoming EM-1 mission), inner planets, dwarf plants on the asteroid belt and near-Earth asteroids. Additionally, small ion propulsion system like the BIT-3 is ideal for drag make-up applications for earth observation (EO) missions from low flying platforms, down to altitudes of ~200km. Altitude reduction is essential for high resolution EO from small, low-cost satellites that are by definition unsuitable for large optical or RF apertures and thus lower altitude is the only option for higher image resolution.

Potential non-NASA customers of the Hybrid Direct Drive enabled EP systems include commercial human exploration and presence in space, commercial asteroid missions, DoD and commercial EO missions. NRO may also have interest in such technologies for long-duration LEO missions. The high-efficiency power processing topology may also find markets in other satellite subsystems or UAV related fields. This technology could also provide a compact, universal energy harvester for solar blankets for infantry soldiers. Soldiers are increasingly dependent on batteries and power sources to power the wide array of electronic gadgets they are equipped with. This Direct Drive unit could potentially be adapted to interface with a solar blanket or vehicle battery to provide a regulated source of power to charge/power soldier devices. Programs such as the Army's Nett Warrior PEO Soldier could potentially benefit from this technology.

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