Ann M. Delleur

Load-following power timeline analyses for the International Space Station

Spacecraft are typically complex assemblies of interconnected systems and components that have highly time-varying thermal, communications and power requirements. It is essential that power systems designers be able to assess the capability of the spacecraft to meet these requirements which should represent a realistic projection of demand for these resources once the vehicle is on-orbit. To accomplish the assessment from the power standpoint, a computer code called ECAPS has been developed at NASA Lewis Research Center that performs a load-driven analysis of a spacecraft power system given time-varying distributed loading and other mission data. This program is uniquely capable of synthesizing all of the changing spacecraft conditions into a single, seamless analysis for a complete mission. This paper presents example power load timelines with which numerous data are integrated to provide a realistic assessment of the load-following capabilities of the stations PV power system. Results of analyses show how well the power system can meet the time-varying power resource demand.

Electrical performance of the international space station US photovoltaic array during bifacial illumination

With the first United States (US) photovoltaic array (PVA) activated on international space station (ISS) in December 2000, on-orbit data can now be compared to analytical predictions. Due to ISS operational constraints, it is not always possible to point the front side of the arrays at the Sun. Thus, in many cases, sunlight directly illuminates the backside of the PVA as well as albedo illumination on either the front or the back. During this time, appreciable power is produced since the solar cells are mounted on a thin, solar transparent substrate. It is important to present accurate predictions for both front and backside power generation for mission planning, certification of flight readiness for a given mission, and on-orbit mission support. To provide a more detailed assessment of the ISS power production capability, the authors developed a PVA electrical performance model applicable to generalized bifacial illumination conditions. On-orbit PVA performance data were also collected and analyzed. This work describes the ISS PVA performance model, and the methods used to reduce orbital performance data. Analyses were performed using SPACE, a NASA-GRC developed computer code for the ISS program office. Results showed a excellent comparison of on-orbit performance data and analytical results.

Analysis of Direct Solar Illumination on the Backside of Space Station Solar Cells

Ann M. Delleur and Thomas W. KerslakeGlenn Research Center, Cleveland, OhioDavid A. ScheimanOhio Aerospace Institute, Cleveland, OhioPrepared for the34th Intersociety Energy Conversion Engineering Conferencesponsored by the Society of Automotive EngineersVancouver, British Columbia, Canada, August 1-5, 1999National Aeronautics andSpace AdministrationGlenn Research Center

Electrical performance from bifacial illumination International Space Station photovoltaic array

With the first U.S. photovoltaic array activated on the International Space Station in December 2000, on-orbit data can be compared to analytical predictions. Because of space station operational constraints, it is not always possible to point the front side of the arrays at the sun. Thus, in many cases, sunlight directly illuminates the backside of the array as well as albedo illumination on either the front or the back. During this time, appreciable power is produced because the solar cells are mounted on a thin, solar transparent substrate. It is important to present accurate predictions for both front and backside power generation for mission planning, certification of flight readiness, and on-orbit mission support. To provide a more detailed assessment of the power production capability, the authors developed a photovoltaic array electrical performance model applicable to generalized bifacial illumination conditions. We describe the space station photovoltaic array performance model and the methods used to reduce orbital performance data. Analyses were performed using SPACE, a NASA Glenn Research Center-developed computer code for the space station. Results showed an excellent comparison of on-orbit performance data and analytical results.

The SPACE Computer Code for Analyzing the International Space Station Electrical Power System: Past, Present, and Future [STUB]

The System Power Analysis for Capability Evaluation (SPACE) computer code was initially developed by NASA in 1988 to assess the Space Station Freedom electric power system and later adapted to support contractor electrical power system capability analyses for the International Space Station (ISS). Over time, the code has supported many efforts such as ISS redesign activities in the early 1990s, assessment of time-phased loads against power system operating limits for future ISS assembly flights (including Certification of Flight Readiness reviews by the ISS program office), and determining the optimum solar array gimbal positions while respecting keep-out zones which minimize both solar array contamination and structural loads. The code has been validated by comparisons with ISS onorbit data in multiple validation episodes. Recent updates to the code include the incorporation of a Lithium-Ion battery model in addition to the nickel-hydrogen battery model and modifications to the solar array degradation model to better match on-orbit test results. SPACE has also been extended beyond the ISS to include modeling of the Orion MultiPurpose Crew Vehicle electrical power system (SPACE-MPCV) and Mars Surface Electrical Power Systems (MSEPS). Portions of SPACE were integrated with a trajectory code to form a Solar Electric Propulsion Simulation (SEPSim), which can be used for analyzing solar electric propulsion missions. In addition, SPACE methods and subroutines have been adapted to a multitude of other projects37. This paper summarizes the initial code development and subsequent code utilization in the context of the overall ISS program development and onorbit operations. Recent updates and results from the code are discussed, including preliminary analyses for the Orion power system.

Operational Workarounds for the Space Station Beta Gimbal Anomaly

The International Space Station (ISS) is the largest and most complex spacecraft ever assembled and operated in orbit. The first U.S. photovoltaic module, containing two solar arrays, was launched, installed, and activated in early December 2000. After the first week of continuously rotating the U.S. solar arrays, engineering personnel in the International Space Station Mission Evaluation Room observed higher than expected electrical currents on the drive motor in one of the beta gimbal assemblies, the mechanism used to maneuver a U.S. solar array. The magnitude of the motor currents continued to increase over time on both beta gimbal assemblies, creating concerns about the ability of the gimbals to continue pointing the solar arrays towards the sun, a function critical for continued assembly of the ISS. A number of engineering disciplines convened in May 2001 to address this on-orbit hardware anomaly. This paper reviews the International Space Station electrical power system analyses performed to develop viable operational workarounds that would minimize beta gimbal assembly use while maintaining sufficient solar-array power to continue assembly of the International Space Station. Additionally, electrical power system analyses performed in support of on-orbit beta gimbal assembly troubleshooting exercises are reviewed.

Managing ISS US Solar Array Electrical Hazards for SSU Replacement via EVA

The US solar array strings on the International Space Station (ISS) are connected to a sequential shunt unit (SSU). The job of the SSU is to shunt, or short, the excess current from the solar array, such that just enough current is provided downstream to maintain the 160V bus voltage while meeting the power load demand and recharging the batteries. Should a sequential shunt unit fail on-orbit, its removal and replacement with the on-orbit spare would be conducted via astronaut “space walk” or extra vehicular activity (EVA). However, removing an SSU during an orbit sun period with input solar array power connectors fully energized, could result in substantial hardware damage and/or safety risk to the EVA astronaut. The open circuit voltage of cold solar array strings can exceed 320V while warm array strings could feed a short circuit with a total current level exceeding 240A. Replacing the SSU during eclipse when the array is not in sunlight would seem optimal, except that even the maximum eclipse period is only 36 minutes. This does not provide sufficient time to remove and replace the SSU, while still allowing for contingencies that may arise. Several other options for the SSU remove and replace procedure are being assessed and include: (1) leave the failed SSU in place, (2) change-out the SSU during one eclipse period, (3) change-out the SSU during several eclipse periods, (4) retract the solar array then change-out the SSU during multiple orbit sun and eclipse periods, (5) design, build and install a temporary shunting plug on solar array string power connectors and change-out the SSU during several orbit eclipse periods, (6) design, build and install a temporary shunting plug on solar array string test port connectors and change-out the SSU during several orbit eclipse periods. To guide the assessment of these proposed options and ameliorate the EVA hazards, NASA Glenn Research Center used the bifacial solar array model incorporated in the “SPACE” electrical power system modeling code to analyze array string current and voltage capability during the various operating conditions. From the perspective of defining and managing solar array string electrical hazards, this paper discusses various SSU remove and replacement options and the associated analysis to develop a workable SSU replacement procedure via EVA.

Space Station Power Generation in Support of the Beta Gimbal Anomoly Resolution

ABSTRACT INTRODUCTION The International Space Station (ISS) is the largest and most complex spacecraft ever assembled and operated in orbit. The first U.S. photovoltaic (PV) module, containing two solar arrays, was launched, installed, and activated in early December 2000. After the first week of continuously rotating the U.S. solar arrays, engineering personnel in the ISS Mission Evaluation Room (MER) observed higher than expected electrical currents on the drive motor in one of the Beta Gimbal Assemblies (BGA), the mechanism used to maneuver a U.S. solar array. The magnitude of the motor currents continued to increase over time on both BGA’s, creating concerns about the ability of the gimbals to continue pointing the solar arrays towards the sun, a function critical for continued assembly of the ISS. A number of engineering disciplines convened in May 2001 to address this on-orbit hardware anomaly. This paper reviews the ISS electrical power system (EPS) analyses performed to develop viable operational workarounds that would minimize BGA use while maintaining sufficient solar array power to continue assembly of the ISS. Additionally, EPS analyses performed in support of on-orbit BGA troubleshooting exercises is reviewed. EPS capability analyses were performed using SPACE, a computer code developed by NASA Glenn Research Center (GRC) for the ISS program office. When fully assembled, the U.S. EPS will include four PV modules