Spacecraft power distribution unit test system re-use: Advantages, pitfalls and challenges

The Solar Probe Plus (SPP) mission, part of NASA's Living With a Star program, is set to launch in July of 2018 on a trip to travel through the Sun's corona. The first component that will be integrated to the spacecraft is the Power Distribution Unit (PDU). The SPP PDU was based on the PDU design utilized for the Van Allen Probes (formerly Radiation Belt Storm Probes) mission, but with some very significant differences. Due to the fact that the SPP spacecraft is a much more complex vehicle, it requires nearly twice as many power services as the Van Allen mission which resulted in a PDU twice the size. Also, the Van Allen PDU utilized a single non-redundant Inter-Integrated Circuit (I2C) bus and command/telemetry interface (since the PDU redundancy was considered to be built into the dual spacecraft design) and the SPP PDU utilizes a dual redundant E2C bus along with 2 separate command and telemetry interfaces. Additionally, the command and telemetry Interface (I/F) paradigm between the PDU and the spacecraft was significantly modified. Although the Van Allen mission ultimately used L3 Telemetry West's In Control as their operational Telemetry and Command (T&C) Commercial off the Shelf (COTS) system, it was not ready for use during the original PDU development effort, so a legacy COTS system (EPOCH 2000) was utilized for Van Allen PDU testing. This required the conversion of the entire set of test scripts (tens of 1000's of lines) from one scripting language to another. These PDU design changes and test system changes presented a challenge to the test team since they were required to re-use the original Van Allen test systems as a cost savings measure. This paper will discuss the advantages of the re-use of the PDU test systems and how the test team overcame the various roadblocks, problems, and challenges that the team encountered in order to deliver the test sets along with the fully qualified PDU in a timely manner. The SPP PDU was successfully integrated to the spacecraft in August of 2016. It continues to operate as designed, providing power services for each and every component that has and will be added. The Integration and Test phase is set to be completed in the spring of 2017 after which the Solar Probe Plus spacecraft will be then readied for launch in July of 2018 from Cape Canaveral, Florida.