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Philae Lander Touchdown Dynamics Revisited – Tests For The Upcoming Landing Preparations –

Philae Lander Touchdown Dynamics Revisited – Tests For The Upcoming Landing Preparations –. L. Witte , S. Schröder, R. Roll, S. Ulamec, J. Biele, J. Block, T. van Zoest 10 th International Planetary Probe Workshop, San Jose State University, June 2013. Overview.

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Philae Lander Touchdown Dynamics Revisited – Tests For The Upcoming Landing Preparations –

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  1. Philae Lander Touchdown Dynamics Revisited– Tests For The Upcoming Landing Preparations – L. Witte, S. Schröder, R. Roll, S. Ulamec, J. Biele, J. Block, T. van Zoest 10th International Planetary Probe Workshop, San Jose State University, June 2013

  2. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Overview • Introduction: Status of Rosetta/Philae • ObjectivesfortheRetestingCampaign • Excursion: Understanding Philae‘sLandingGear • Heritage Tests using a Pendulum • The Landing & Mobility Test Facility • Test Cases forthenewCampaign • Selected ResultsfromthePreliminary Data Analysis • Conclusions & Next Steps

  3. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Status of Rosetta/Philae Things behind… • Philaedevelopment & qualification: 1996 – 2002 • Launch: 03/2004 targetingChuryumov-Gerasimenko (CG) • Encounter withasteroids Steins (09/2008) and Lutetia (07/2010) • Rosetta/Philaeenteredhibernation: 06/2011 Today, Rosetta/Philaeareabouthere Things ahead… • Exit of hibernationmode: 01/2014 • Entry intoorbit of CG in 05/2014 and begin of remote sensingphase • Philaelanding: 11/2014 … so whyinvestigations on Philaetouchdowndynamicsatthisstage? Image credit: ESOC

  4. 10th International Planetary Probe Workshop, San Jose State University, June 2013 ObjectivesforRetestingtheTouchdown Dynamics Overarching:As Rosetta is en route and Philae will land soon, the new tests can only serve to optimize the landing strategy and to determine the landing gear performance envelope more precisely. Thus the primary objectives for the new tests are: • Address primarily asymmetric load cases T/D conditions(which were out of capability of the pendulum test facility used during D & Q), • To broaden the test data base on the influence of the landing gears tilt limiter(a late design change due to the target comet re-designation from Wirtanen to CG as consequence from the launch delay after the A5 maiden flight failure), • To broaden the data base on the contact phenomenon on soft soils(Limitation: cometary soils cannot be emulated in these test facilities, but testing on granular media allows for getting a grip to plastic surface properties). Governedbytheseobjectives, there-test serieshasbeenexecutedfrom in 12/2012 to 02/2013

  5. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Understanding Philae‘sLandingGear (1/2) The landing gear consists of a foldable tripod and a central damping mechanism. Its damping behavior can be simplified as linear velocity dependent damping force. ; kD..cabletensionstiffness, IR..moment of inertia of rotatingparts, del..brakemomentum, σ..spindlethreadpitch Transfer function: (Quasi-)stationarytransferbehaviour:

  6. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Understanding Philae‘sLandingGear(2/2) A cardanic jointbetweenthetripod and thecentraldamperunitallowsthe LG toadapttothelocalterrain(+/-30°). This range was reducedto +/-3° byinstallation of thetiltlimiter (late design change). Further elements (not shown): • 2 anchoringharpoons, • ActiveDescend System (ADS)

  7. 10th International Planetary Probe Workshop, San Jose State University, June 2013 LandingGear Tests during D & Q Phase Test principle: mountinglandingsystemaspendulum. Advantage: simple set-up, large reduction of apparentgravity Disadvantage: severlyconstrainedmotion, no granular surfaces

  8. 10th International Planetary Probe Workshop, San Jose State University, June 2013 The LAMA Test Facility and Integration of Philae Test principle: activeweight off-loading. Advantage: 3D motion and tests on granular soilpossible. Disadvantage: activeroboticsystem in theloop.

  9. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Test Cases Base 1:shall ensure the consistency between pendulum facility and the LAMA facility test data, reference for subsequent cases. Base 2:This group particularly addresses tilt limiter and flywheel effects . Base 3:The objective is the quantification of soft soil contact mechanics and the ice screw operation. Base 4:these tests add lateral velocity and vary the terrain slope to excite destabilizing momentums. Spec 1: is used to gather data on flywheel effects the descend phase. Spec 2: addresses further stability load cases and complements the Base 4 group. Spec 3: similar to the Base 3 group, with partly different touchdown velocities. The footpads were equipped with the scientific instrument CASSE.

  10. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Test Results: ComparisontoPendulum Tests Example: Base_1c , Vv=0.8m/s, Vh=0.0m/s, r/p/y=0/0/0°, surface: wood Direct comparison in the time domain (below left) only for qualitative assessments.But using the simplified damper model: … allows a spread-sheet based quick check byrelatingtheinitialtouchdownvelocities v0tothe resultant damperstroke send. D = 601 [Ns/m] @ a mass of Philae of 98kg.

  11. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Test Results: Comparison of Hard vs. Granular SurfaceTouchdown Set-upidenticalas Base_1 tests, exceptthesurface. Example Base_3d : MSS-D soilsimulant FFT of LG accelerometerdata

  12. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Test Results: A FullyAsymmetric T/D Condition Example: Spec_2a , Vv=0.8m/s, Vh=0.2m/s, r/p/y=17/0/90°, surface: wood Robot handposition IRU data Cardan angle data

  13. 10th International Planetary Probe Workshop, San Jose State University, June 2013 Conclusions • A partial retesting of Philae’s touchdown dynamics has been done in spring 2013 • Test data has been acquired addressing test objectives with a focus on the upcoming landing preparation. • Besides their immediate relevance for the Philae lander, the test results and data allow a quantification of the strength and weaknesses of the different test facility pendulum and weight-offloading. • Touchdown data on a stiff (hard) surface as well as different granular media (soft) allow to understand soil mechanical effects. • The observation during the tests point out the effects and importance of the relative rotational degrees of freedom in the lander system. Its influence on stability needs to be further assessed. • A major next step is now to review and refine the numerical multibody simulations based on the findings and conclusions from this new test campaign. • The acquired data will be used for rigorous model verification. The verification needs to take into consideration the coupling and interference with test facility thus simulating it partially. • Finally concluding, the risk assessment for landing site selection will profit from the better understanding of the landing dynamics. More related information:  IPPW 10 paper: Philae-Lander Touchdown Dynamics Revisited – Tests for the Upcoming Landing Preparations  IPPW 10 poster: Analysis, Test and Simulation of Landing System Touchdown Dynamics

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