Sample Fetching Rover - Lightweight Rover Concepts for Mars Sample Return Elie Allouis, Elie.Allouis@astrium.eads.net T.Jorden, N.Patel, A.Ratcliffe ASTRA 2011 ESTEC 14 April 2011
Contents Scope Introduction The SFR mission Concept SFR and the MSR Mission Architecture SFR Mission Operational Baseline Rover System Design Drivers Rover Design Philosophies Selected System concepts: Mobility Locomotion System GNC Baseline Down-Selection Conclusion & Wrap-up -2
Scope Introduce the SFR mission concept, design drivers and mass reduction philosophies Concentrate on Mobility issues and first preliminary concepts Introduction Study objectives To carry out an assessment study of a lightweight rover for fetching cached samples on Mars and return them to a Mars Ascent Vehicle (MAV). Challenges Very stringent mass constraints: target 60 kg, (ExoMars is ~ 300kg) Compact 1 x 1 x 0.7m stowed envelope Must traverse 15 km in 180 sols mission Design Highlight interleaved aspects of environmental and operational requirements Identify and thoroughly understand of ripple effects through the subsystems design. Identify enabling technologies and future developments -3
Introduction - The Team -4
MSR Mission Architecture NASA/ESA Joint Mars Exploration Programme The caching rover will collect and deposit a sample cache onto the Martian surface to be collected by SFR. 2018 2022-2024 -5
MSR Mission Architecture Baseline Architecture : MAV and SFR deployed together Rover begins operations in September 2025 Ls 133 NASA concept allocates 150 kg to a single purpose fetch rover The rover shall fit in a 1 x 1 x 0.7 m envelope to transfer to Mars Need to investigate alternative deployment concepts in case of MAV mass growth ( ESA Mars Precision Lander) NASA -6
Mission Operational Baseline Mission Nominal Operational Scenario : Navigate and traverse to the location of a sample cache deposited by a previous rover mission Retrieve the sample cache Deliver it to the MSR Ascent Vehicle and cooperate in transferring the sample cache to the MSR lander A compressed mission timeline : 180 sol mission Accounting for post landing operations, checkouts and dust storm contingencies Only 125 sols remaining for egress Minimum traverse of 120m/sol for a 6-month mission If mission is to be done before the dust season ~170m/sol MAX-C End MSR Lander Cache MAX-C Start Traverse Distance = 14 km straight line -7
Mission Environment Environment Similar to ExoMars/Caching Rover Same location, however Mission operation from Ls 133 to 212 shortly before Dust season Different illumination conditions and thermal environment Terrain Conditions At worst, SFR should be compatible with ExoMars terrain, At best, the cache is deposited in a more benign location (fewer/smaller rocks and slopes, better characterised terrain, etc) -8
SFR Rover - Rover System Design Drivers - -9
Rover System Design Drivers The SFR design presents significant challenges in the areas of Mass : Target mass for the rover platform is 60 kg - 1/5 Exomars The four major contributors to mass (90%) are locomotion, structure, power and harness. Data Handling Communication Power Harness Structure Performance : Accumulated ground track of at least 15km Nominal 180 sol mission baseline 125 sol traverse SFR highly dependent on robust mobility system. Thermal Control Deployable Mast Navigation Locomotion Risk : Overall MSR mission architecture high degree of risk. No Rover return No samples -10
Rover System Design Drivers SFR Design Drivers, Dependencies and Ultimate Impact on Mass -11
Approaches to Design Evolution Solution Type Heritage Modified Alternative Radical Displacement Approach Optimise existing Altered heritage solution New development Same functions Thinking out of the box Remove the need for a solution Comments / issues Optimisation of heritage Low risk / high confidence Good retention of TRL Based on heritage item medium uncertainty / risk Same functions but re-development from scratch Design needs validation and qualification Novel architecture & technology Innovative & ground breaking Full Validation and qualification reqd usually high risk Remove the need for the solution from the design Not usually practical severe loss of functionality -12
Proposed Rover Design Philosophies Stripped out approach Removes any mass that is not essential for the achievement of the major goals of the mission. Each element of the rover is pared down to its minimum required functionality/performance. Sub-systems can be de-scoped and performance traded to achieve a lower mass solution, but component redundancy must be maintained. Ready-To-Go Seeks to remove any deployments or mechanisms that are not essential i.e. utilisation of fixed solar arrays, fixed mast, no wheel deployment etc. This approach removes rover complexity reducing risk during the commissioning phase. The gain of removing mechanisms must be carefully traded against the impact on performance and the reciprocal effects on the system design. E.g. a rover with a fixed solar panel may have a small total array size due to the constraints imposed by the lander. -13
Proposed Rover Design Philosophies Cold Skeleton The rover is stripped down to it fundamental structures and uses minimum thermal control to save structure mass and packaging Involves the use of cold electronics and mechanisms Limited by battery temperature requirements Locomotion Optimisation The design is driven by the optimisation of the locomotion system. Potential to alleviate GNC load Rover is designed around the Locomotion Sub-System ensuring mobility is not compromised at any stage in the design. -14
Proposed Rover Design Philosophies 24/7 Rover Not Possible A perpetual power supply the rover Achieved by a power system not constrained by solar flux such as RTG and SRG But Very low TRL and energy density of current radioisotopes too low for small rovers The SFR Mission currently baselines photovoltaics Day Rover Not Possible The rover only operates during daylight and only from the power of the solar arrays No battery Some mass saving and lower thermal requirements as per Cold skeleton But Mission requires a night communication window -15
SFR Rover - Locomotion Sub-System - -16
Locomotion Overview Terrain topography and its physical properties play a critical role in the design and performance of the LSS The soil properties - rover slope traverse capability, power requirements, grousers size, number of wheels. Size of the rocks - design of the suspension system, the wheel size, power requirements. Rock distribution - mean free path of the rover which influences GNC. Slope - minimum gradeability, the static stability of the rover and power requirements. -17
Locomotion System Options Extensive locomotion system concepts review 20 configurations 4,5,6,8 wheels Preliminary trade-off: Mechanical and actuation complexity, Ground clearance Redundancy, Risk and TRL, Stowage and deployment,. Wheel Types and Constructions Rigid Semi-rigid Flexible NASA AMSTL NASA -18
Locomotion System Candidates Heritage and redundancy 6WD 3 bogies Simpler, lower mass 4WD +diff Lowest mass, Lowest TRL 4WD -19
SFR Rover Concepts and Trade-Offs - Guidance Navigation and Control - -20
GNC Sub-System Functional specification Key to rapid traverse (15km-120m/sol) However Unlike past missions a wealth of local terrain data will be potentially available to the platform Key design drivers -21
GNC Sub-System Navigation Options: Stop-Go Stop to image the way ahead and process the data to derive a safe path Used on MER and ExoMars Requires heavy processing, Provide careful path planning and dead-reckoning Holds up the progress of the traverse (Processing power limitations) SLAM and Obstacle Avoidance Continuous Drive Set off in the target direction and avoid obstacles along the way Can be time (and therefore power) inefficient as the navigation seeks a path continuously through obstacles -22
GNC Sub-System An alternative Navigation Concept: Hybrid Architecture Current missions are relying on direct-drive with some operational autonomy for obstacle avoidance In the timeframe of SFR Wealth of data gathered of the ExoMars and MSR landing sites Unprecedented opportunity to perform a preliminary route mapping to the cache Obital imagery, altimetry and shape from shading may identify obstacles - HiRISE already provides 0.3m pixel sizes on the ground It is possible to envisage that high resolution digital terrain models (DTM) will be available -23
GNC Sub-System GNC architecture Using High res DTMs Localisation: Visual feature matching Navigation and Path planning: - Identification of all the main obstacles in the path large scale nav map (uploaded in manageable chunks. However, specific terrain data such as the soil condition will be missing. On-board replanning required to find altrenative path. Control: The DTM could be used in conjunction of the Structure from Motion techniques to improve the control and accuracy of the rover along the prescribed path. -24
SFR Rover - Baseline Downselection - -25
Baseline Candidates - Discussion Candidates mass comparison The concepts range from ~72kg (4WD_diff) to ~79kg (Exomars based, flexi wheels) Mainly relates to mass savings in: Locomotion System OBDH mass reduction scheme However: Not as large a difference as initially anticipated Limited by Power sub-system and Solar Array Size and deployment Structure Mass 80 SFR Baseline Candidates Mass 78 Mass [kg] 76 74 72 70 68 ExoLight ExoEvo 4WD_diff 4WD_XLW_300 4WD_XLW_400-26
Preliminary Baseline Based on Trade-off exercise, a 6WD 3 bogies configuration selected Higher redundancy than 4WD concepts Heritage in a smaller package evolution of the ExoMars configuration (not necessarily implementation) 1.4m2 array (configuration TBC) Large area in the front for Cache Acquisition System Stowed envelope fits into the allowable volume Footprint deployed 1500x1000mm ~75kg (with 300mm spoke/mesh wheels) However: Mass > target of ~60kg, but will be optimised in next phase Alternative locomotion formula and their performance will be investigated i.e. from 6x6x6 down to 6x4x4 Wheel construction and dimensions TBC Rigid Vs flexible, 250-300+mm Innovative lightweight solutions for collateral systems to be investigated e.g. OBDH, Comms, Power -27
SFR Rover Concepts and Trade-Offs - Conclusion & Wrap-up - -28
Conclusion This activity is looking at a wide range of architecture, system and sub-system options for the Sample Fetch Rover Mobility subsystem critical to mission success Based on the current mission constraints and following this preliminary review and trades: None of the concepts proposed currently fit into the 60kg target mass envelope (~75kg). Preliminary analysis showed that it is difficult to drastically reduce the mass of the main mass drivers further, but other system may be optimised Rover must be compatible with ExoMars environment drives locomotion and power systems The preliminary rover concept draws on heritage, but leaves open a number of options for actual implementation The next phase Will see further definition and mass optimisation of the rover subsystems Careful evaluation of GNC scheme and Locomotion system sizing. Preliminary design of the rover concept -29
Sample Fetching Rover - Lightweight Rover Concepts for Mars Sample Return Elie Allouis, Elie.Allouis@astrium.eads.net T. Jorden, N.Patel, A. Ratcliffe ASTRA 2011 ESTEC 14 April 2011