E3S Web of Conferences 16, 13004 (2017) DOI: 10.1051/ e3sconf/20171613004 ESPC 2016 POWER SUBSYSTEM APPROACH FOR THE EUROPA MISSION Antonio Ulloa-Severino(1), Gregory A. Carr(1), Douglas J. Clark(1), Sonny M. Orellana(1), Roxanne Arellano(1), Marshall C. Smart(1), Ratnakumar V. Bugga(1), Andreea Boca(1), and Stephen F. Dawson(1) (1)Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 USA, Email: [email protected] ABSTRACT implementation of the spacecraft and instrument hardware. The mission design team is expending great NASA is planning to launch a spacecraft on a mission to efforts to minimize the amount of total dose radiation the Jovian moon Europa, in order to conduct a detailed the flight system would accumulate and maximize reconnaissance and investigation of its habitability. The science data return. spacecraft would orbit Jupiter and perform a detailed science investigation of Europa, utilizing a number of The NASA-selected instruments intend to characterize science instruments including an ice-penetrating radar to Europa by producing high-resolution images and determine the icy shell thickness and presence of surface composition maps. An ice-penetrating radar subsurface oceans. The spacecraft would be exposed to intends to determine the crust thickness, and a harsh radiation and extreme temperature environments. magnetometer would measure the magnetic field To meet mission objectives, the spacecraft power strength and direction to further help scientists subsystem is being architected and designed to operate determine ocean depth and salinity. A thermal efficiently, and with a high degree of reliability. instrument would be used to identify warmer water that could have erupted through the crust, while other 1. EUROPA MISSION OVERVIEW instruments would help detect water and particles in the The planned Europa mission’s main focus is to execute moon’s atmosphere. an in-depth science investigation of Jupiter’s icy moon Europa shown in Fig. 1 [1]. The moon has shown The nine NASA-selected instruments that would be evidence of a liquid water ocean underneath the used for the Europa mission are the following [2]: unknown thickness of icy surface. The ocean could provide an environment suitable for life due its 1) Plasma Instrument for Magnetic Sounding interaction with a volcanic seafloor. (PIMS) 2) Interior Characterization of Europa using Magnetometry (ICEMAG) 3) Mapping Imaging Spectrometer for Europa (MISE) 4) Europa Imaging System (EIS) 5) Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) 6) Europa Thermal Emission Imaging System (E- THEMIS) 7) Mass Spectrometer for Planetary Exploration/Europa (MASPEX) 8) Ultraviolet Spectrograph/Europa (UVS) 9) Surface Dust Mass Analyser (SUDA) 2. POWER SUBSYSTEM ARCHITECTURE The power subsystem architecture design approach involves implementing a robust single-fault tolerant Figure 1: Cutaway diagram of Europa's interior. design with small fault containment regions, based on a Artwork credit: Michael Carroll similar approach used for the Cassini mission to Saturn. The design has focused on three main areas: the baseline The combination of harsh radiation, low temperature solar arrays, power management electronics, and (30 K) environments, and distance from the sun (5 AU batteries as shown in Fig 2. to 5.5 AU) creates challenges on design and © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). E3S Web of Conferences 16, 13004 (2017) DOI: 10.1051/ e3sconf/20171613004 ESPC 2016 Power Control and Distribution Assembly controlled via commands. Power control uses an N+K Power Bus architecture for the number of power stages, where N is Solar Array Safe/Arm number required for the mission and K is the number of Plug Battery Prop Bus A & B additional stages provided for redundancy, as shown in Plug Array & Battery Interface Slice Battery Deployment Fig. 3. The command and telemetry interface to the Bus A & B power management electronics is block redundant. Umbilical Switched loads Engineering Bus A/B Pulsed Regulation Power Bus RSB A RSB A/B Propulsion Module loads Stage FCR 1 SA Pwr Controller A (Incl. xRTI) Electronics A/B Voltage Control N Local Ctrl Power Bus Power Bus RSB B Power Bus Volt Controller B Control FCR A Ctrl Volt A Regulation Ctrl Volt B Stage 1 Power Switch Switched power Engineering Slice 1 Power Bus Communication Regulated Power Switch Control A Slice 2 Bus A Regulation Power Bus Stage FCR N SA Pwr Power Bus Power Switch Slice 3 Power Bus Voltage Control N Local Ctrl Power Bus Power Power Switch Control FCR B Volt Slice 4 Ctrl Volt A Regulation Distribution Stage N Function Power Switch Engineering Ctrl Volt B Slice 5 Power Bus Communication Control B Power Switch Bus B Regulation Slice 6 Stage FCR N+1 SA Pwr Voltage Control 1 Switched power Local Ctrl Power Bus Volt Regulation Figure 2. Baseline Power Subsystem Architecture. Legend: Ctrl Volt A Stage Ctrl voltage CMD Ctrl Volt B N+1 Power Voltage setpoint Functional FCR 2.1 Power Source Region Figure 3. Power Regulation Function FCRs. The solar array substrates, materials, and components are being tested under the relevant environmental The power distribution electronics are designed conditions to characterize their performance and with internal housekeeping power that provide reliability in the Jovian environment. The primary independent functional slices that can be utilized challenges for photovoltaic operation at Jupiter are the throughout the spacecraft. The power distribution extremely high radiation from both electrons and function incorporates a resettable circuit breaker power protons, extremely low temperatures (30 K), and low- switch to separate the fault containment region for each irradiance low-temperature (LILT) solar cell load interface. The power switch is part of the load fault performance. Also, mission design requirements dictate containment region as shown in Fig. 4 and is fail-safe that the solar array mass and size constraints are of off. Redundant functions on the spacecraft are powered critical importance. by another power switch in a separate fault containment region. Each power switch has a block redundant The solar array string design and layout has been command and telemetry interface to maintain the load- adjusted to match the capability of the radiation-hard level fault containment region. electronics. The number of cells in series was set by the Regulated PWR Bus topology of the power management electronics to Power Distribution CMD A Regulated Housekeeping Power Switch FCR deliver the most power at the end of the mission in the Power A PWR Bus most severe environment. The challenge was to obtain Power Power Switch S/C Load 1 Serial Switch Distribution Channel enough voltage for the power management electronics Bus A Drivers CMD A early in the mission, during the inner solar system Power Switch S/C Load 2 cruise, and then still produce efficient power at the end Power Distribution CMD B Channel Power of mission. The power management electronics was Serial Switch Distribution Power Bus B Drivers designed to deliver close to the maximum power CMD B Switch S/C Load X Channel available from the solar array at the end of mission in a Housekeeping Power B robust fault-tolerant architecture. The power management electronics has two primary functions, Regulated PWR Bus Legend: power control and power distribution. CMD Functional Regulated PWR Bus FCR Region Housekeeping PWR Driver 2.2 Power Electronics Figure 4. Power Distribution FCRs. The baseline power control design operates near the 2.3 Energy Storage maximum power point of the solar array throughout the Jovian tour based on solar cell telemetry and modelling. The preliminary battery architecture uses high specific Power control electronics can charge the battery after a energy small 18650-size lithium-ion cells, to capitalize flyby operating near the I-V (current-voltage) curve on their internal safety functions, high capacity, and maximum power point without collapsing the solar excellent cell-to-cell reproducibility. Utilizing such a array voltage. The design is single fault tolerant. Power format is advantageous, since the approach does not bus voltage and solar array voltage set points are 2 E3S Web of Conferences 16, 13004 (2017) DOI: 10.1051/ e3sconf/20171613004 ESPC 2016 require individual cell monitoring and balancing • determination of required power subsystem circuitry. Each string of battery cells is a separate capabilities, and electrical fault containment region. Each battery cell • design of interfaces, both within and external to the string power subsystem. 3. POWER SUBSYSTEM SYSTEM The set of power subsystem requirements is the formal ENGINEERING agreement between the power subsystem domain and 3.1 System Engineering Context, Requirements and the flight system domain. Requirements for the power Verification and Validation (V&V) subsystem are being developed and maintained in the Rational Dynamic Object Requirements System System engineering on the Europa Project is divided (DOORS) Next Generation web-based software tool. into the following domains: • Project system engineering, which is concerned Later, as the Project and subsystems proceed into with everything from securing funding to delivering implementation phases, the attention of power on the project’s stated science goals and subsystem engineers would shift into V&V activities. requirements. Verification involves the formal check that the delivered • Mission system engineering, which is concerned power subsystem products meet all their requirements. with the launched spacecraft, launch vehicle, flight- Validation is a check to ensure that the power subsystem to-ground communications networks, and the pre- would meet the project and mission objectives, when and post-launch operations of the Europa spacecraft operated in the context of the other subsystems, the and payload. flight system, and the mission system. • Flight system engineering, which is concerned with the Europa spacecraft and payload.