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

10th International Planetary Probe Workshop, San Jose State University, June 2013

Overview

www.DLR.de • Chart 2 10th 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: 06/2011

www.DLR.de • Chart 3

Today, Rosetta/Philae are about here

Image credit: ESOC


Objectives for Retesting the Touchdown DynamicsOverarching: 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 02/2013


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)

; kD..cable tension stiffness, IR..moment of inertia of rotating parts,

del..brake momentum, σ..spindle thread pitch

Transfer function:

(Quasi-)stationary transfer behaviour:


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°).


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)


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


The LAMA Test Facility and Integration of Philae


Test principle: active weight off-loading.

Advantage: 3D motion and tests on granular soil possible.

Disadvantage: active robotic system in the loop.


Test Cases


Identifier Objective Vvertical [m/s]

Vhorizontal [m/s]

Pitch / Yaw [°] (*)

Fly Wheel Status

Surface Cond. Remark

Base_1a 0.2 0.0 0.0 / 0.0 off wood none

Base_1b 0.5 0.0 0.0 / 0.0 off wood none

Base_1c 0.8 0.0 0.0 / 0.0 off wood none

Base_1d 1.1 0.0 0.0 / 0.0 off wood none


Base_2b 1.1 0.0 17.0 / 0.0 on (low) wood cancelled

Base_2c 1.1 0.0 17.0 / 0.0 on (high) wood none

Base_2d n/a n/a n/a n/a n/a unused identifier

Base_2e 0.5 0.0 17.0 / 0.0 off steel/oil none

Base_2f 0.8 0.0 17.0 / 0.0 off steel/oil none

Base_3a 0.5 0.0 0.0 / 0.0 off soft soil (Wf34) none

Base_3b 1.1 0.0 0.0 / 0.0 off soft soil (Wf34) none

Base_3c 0.5 0.0 0.0 / 0.0 off soft soil (MSS-D) none

Base_3d 0.8 0.0 0.0 / 0.0 off soft soil (MSS-D) none

Base_3e 0.8 0.0 0.0 / 0.0 off soft soil (Wf34) none


Base_4b 0.8 0.21 17.0 / 0.0 off wood none

Base_4c 0.8 0.0 0.0 / 0.0 off Slope / mixed surf. none

Base_4d 1.1 0.0 0.0 / 0.0 off Slope / mixed surf. none

Identifier Objective Vvertical [m/s]

Vhorizontal [m/s]

Pitch / Yaw [°] (*)

Fly Wheel Status

Surface Cond. Remark

Spec_1a n/a n/a n/a on n/a none

Spec_1b n/a n/a n/a on n/a included in Spec_1a

Spec_2a 0.8 0.21 17.0 / 90.0 off wood none

Spec_2b 0.8 0.21 17.0 / 0.0 off wood none

Spec_3a 0.1 0.0 0.0 / 0.0 off concrete none

Spec_3b 0.2 - 1.1 0.0 0.0 / 0.0 off soft soil (MSS-D) none

Spec_3c 0.5, 1.1 0.0 0.0 / 0.0 off soft soil (Wf34) none

CASSE

Damping / Stiffness

Characterization

Fly Wheel / Tilt Limiter Effects

Characterization

Terrain andSoft Soil Effects

Characterization

Landing Stabil ity Characterization

Fly Wheel Effects during Descend

Phase

Advanced Landing Stabil ity

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.


Test Results: Comparison to Pendulum 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 by relating the initial touchdown velocities v0 to the resultant damper stroke send.

D = 601 [Ns/m] @ a mass of Philae of 98kg.


Test Results: Comparison of Hard vs. Granular Surface Touchdown


FFT of LG accelerometer data

Set-up identical as Base_1 tests, except the surface. Example Base_3d : MSS-D soil simulant


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


Cardan angle data

Robot hand position

IRU data


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


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

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