Software Controlled Thermal and Power Management on New Horizons
- Paper number
IAC-06-D1.4.10
- Author
Mr. Edward Birrane, The John Hopkins University Applied Physics Laboratory, United States
- Coauthor
Mr. Steve Williams, The John Hopkins University Applied Physics Laboratory, United States
- Coauthor
Mr. Doug Mehoke, The John Hopkins University Applied Physics Laboratory, United States
- Year
2006
- Abstract
New Horizons is a NASA New Frontiers mission to study dwarf planets, most notably the binary planet Pluto-Charon. Scheduled for launch in January 2006, the observatory will spend eight years of its ten-year travel time in a hibernating interplanetary cruise. The primary encounter, Pluto-Charon, is scheduled for July 2015 with the potential for extended Kuiper Belt observations through at least 2020. The observatory uses a radioisotope thermoelectric generator (RTG) to provide power due to its extended voyage away from the sun. At the time of primary encounter the RTG is expected to provide approximately 200 watts of power. The spacecraft thermal design is driven by the limited RTG power generation. This thermal design uses a “thermos-bottle” approach that balances internal dissipation with naturally occurring leaks through both the thermal blankets that surround the spacecraft body and through the instrument and component apertures. Some waste heat from the RTG is used and louvers support high power modes. All spacecraft components and instruments are tied to the common bus temperature. This approach makes the bulk temperature of the spacecraft sensitive to its internal power dissipation. The nine-hour round trip communication time precludes ground assurance of a thermally safe internal environment. This design necessitates a thermal control function through which the thermal dissipation inside the spacecraft is autonomously kept within a narrow range based on power, not temperature. Flight software within the spacecraft Command and Data Handling System maintains this thermal dissipation through control of internal heaters. This software calculates the thermal contribution of each electrical load on the spacecraft and adjusts heater settings as needed for thermal stabilization. Additionally, this software adjusts heaters as new, significant loads are powered to minimize the chance of an overload on the RTG and a subsequent low voltage condition. To provide a framework for discussion, the paper gives an overall view of the hardware configuration of the New Horizons observatory and the design and configuration of the thermal measurement algorithms. This is followed by a discussion of the interaction between the auto-generated heater control commands and the overall command control software architecture, including changes to the architecture to provide transaction, priority inversion detection and priority inheritance capabilities. The remainder of the paper addresses impacts to the autonomous fault recovery system, caveats for mission operations and the validation of this approach through thermal vacuum testing.
- Abstract document