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 Chart 210 th International Planetary Probe Workshop, San Jose State University, June 2013 Introduction: Status of Rosetta/Philae Objectives for the Retesting Campaign Excursion: Understanding Philae‘s Landing Gear Heritage Tests using a Pendulum The Landing & Mobility Test Facility Test Cases for the new Campaign Selected Results from the Preliminary Data Analysis Conclusions & Next Steps

Things ahead… Exit of hibernation mode: 01/2014 Entry into orbit of CG in 05/2014 and begin of remote sensing phase Philae landing: 11/2014 … so why investigations on Philae touchdown dynamics at this stage? Status of Rosetta/Philae Things behind… Philae development & qualification: 1996 – 2002 Launch: 03/2004 targeting Churyumov- Gerasimenko (CG) Encounter with asteroids Steins (09/2008) and Lutetia (07/2010) Rosetta/Philae entered hibernation: 06/ Chart 3 Today, Rosetta/Philae are about here Image credit: ESOC 10 th International Planetary Probe Workshop, San Jose State University, June 2013

Objectives for Retesting the Touchdown 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: 1.Address primarily asymmetric load cases T/D conditions (which were out of capability of the pendulum test facility used during D & Q), 2.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), 3.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). Governed by these objectives, the re-test series has been executed from in 12/2012 to 02/ Chart 410 th International Planetary Probe Workshop, San Jose State University, June 2013

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. Understanding Philae‘s Landing Gear (1/2) Chart 510 th International Planetary Probe Workshop, San Jose State University, June 2013

Understanding Philae‘s Landing Gear (2/2) A cardanic joint between the tripod and the central damper unit allows the LG to adapt to the local terrain (+/-30°). Chart 6 This range was reduced to +/-3° by installation of the tilt limiter (late design change). Further elements (not shown): 2 anchoring harpoons, Active Descend System (ADS) 10 th International Planetary Probe Workshop, San Jose State University, June 2013

Landing Gear Tests during D & Q Phase Test principle: mounting landing system as pendulum. Advantage: simple set-up, large reduction of apparent gravity Disadvantage: severly constrained motion, no granular surfaces Chart 710 th International Planetary Probe Workshop, San Jose State University, June 2013

The LAMA Test Facility and Integration of Philae Chart 8 Test principle: active weight off-loading. Advantage: 3D motion and tests on granular soil possible. Disadvantage: active robotic system in the loop. 10 th International Planetary Probe Workshop, San Jose State University, June 2013

Test Cases Chart 9 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 th International Planetary Probe Workshop, San Jose State University, June 2013

Test Results: Comparison to Pendulum Tests Chart 10 … allows a spread-sheet based quick check by relating the initial touchdown velocities v 0 to the resultant damper stroke s end. D = 601 a mass of Philae of 98kg. 10 th International Planetary Probe Workshop, San Jose State University, June 2013

Test Results: Comparison of Hard vs. Granular Surface Touchdown Chart 11 FFT of LG accelerometer data Set-up identical as Base_1 tests, except the surface. Example Base_3d : MSS-D soil simulant 10 th International Planetary Probe Workshop, San Jose State University, June 2013

Test Results: A Fully Asymmetric T/D Condition Example: Spec_2a, Vv=0.8m/s, Vh=0.2m/s, r/p/y=17/0/90°, surface: wood Chart 12 Cardan angle data Robot hand position IRU data 10 th 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 Chart 1310 th International Planetary Probe Workshop, San Jose State University, June 2013