Iac-13,a3,4,3 Philae Landing Test at the Landing and Mobility Test Facility (lama)

In the recent decades the number of spacecraft visiting asteroids and comets has risen. But only a few of them entered into an orbit of these small bodies and even only two had physical contact to the surface. The ESA mission Rosetta is on the way to meet the comet 67P/Churyumov-Gerasimenko in May 2014. Once in orbit the spacecraft will release the small lander Philae which is supposed to land softly on the cometary surface, anchor itself to the ground with harpoons and perform its scientific observations. This will be the first time in history a lander will touch down on a comet nucleus. The greatest challenges of the landing manoeuver are the unknown surface properties and the fact that the original target, comet 46P/Wirtanen, had to be re-designated into a much larger target with higher mass. Especially the second fact is a critical point, because the higher mass of the comet leads to a higher landing velocity and therefore a higher kinetic energy which has to be absorbed. This effect could not be compensated by a design change, because it was too late to change the design significantly, since the lander was ready at launch site at that time. For this reason a new test campaign in 2012/2013, led by a consortium of DLR Institutes and the Max-Planck-Institute of Solar System Research, has been set up at DLR's Landing & Mobility Test Facility (LAMA) where further touchdown conditions could be tested which have been out of capability of the pendulum test facility used for the original qualification of Philae. This paper gives an overview of the performed work and introduces the test facility concepts with its operation modes. The paper also presents and discusses the preliminary results from this recent campaign and gives an outlook to its further use in the upcoming landing preparations. I. MISSION & SPACECRAFT DESCRIPTION Rosetta is a 3000 kg space probe with dimensions of about 2.8 x 2.1 x 2.0 meters and additional two 14-meter solar panels. Philae is attached at one side to Rosetta. This three legged lander is a partial hexagonal cylinder, approximately 1 meter across and 80 cm high, with an open ”balcony” on one side. It is supported on a long squat tripod and consists of a baseplate, experiment platform and hood. Fig. 1: Rosetta Mission with Philae Lander [1] From launch in 2004 till 2009 Rosetta travelled through the inner solar system and performed a series of swing-by maneuvers at Earth and Mars to gather enough speed to rendezvous the comet ChuryumovGerasimenko in 2014. After an observation campaign the lander will be released from the spacecraft at an altitude of approx. 1 km and will fall down freely to the surface only stabilized by a flywheel. Directly after touchdown two harpoons will fire up and anchor Philae to the ground. Additionally, a cold gas engine will fire and press the lander to the ground. The landing strategy and involved mechanisms are described in further detail by Ulamec and Biele [2]. All Rosetta instruments, including the 10 PI instruments aboard Philae are described in detail in [3]. Fig. 2: Rosetta flight path [4] I.I The Landing Gear Subsystem Philae’s operation is supported by a Landing Gear (LG), which provides the mechanical interface between the comet and the main body. It consists of a foldable tripod with legs and feet and a central structure hosting several mechanisms to execute the various LG functions [5]. The main task of the Landing Gear is absorbing the kinetic energy at touch-down during the landing on the comet. In addition the LG provides a mechanical interface for the anchors harpoons, which are attached to the LG’s central structure, and the Sesame CASSE and PP sensors, which are located in the feet. So called “ice screws” in the feet provide additional anchoring to the surface and hinder gliding. An electronics system provides commanding and telemetry of the LG functions. II. OBJECTIVES The development of the Philae Lander started in the 90’s. The design and qualification tests of the Philae lander were done in the 1996 to 2002 timeframe. These primary tests made use of a pendulum facility, allowing the test object to swing against a vertical wall to separate the Earth gravity from the forces of inertia. However, a limitation of this concept is a severely constrained motion of the test object and the inability to touch down on loose granular material. These disadvantages have been overcome by using an active weightoffloading device which is provided for the recent new tests at DLR's Landing & Mobility Test Facility (LAMA). (see section IV). Since Rosetta is en route and Philae will land soon, the new tests can only serve to optimize the landing strategy (optimal ranges of velocities and angles at touchdown) and determine the landing gear performance envelope more precisely. Primary objectives for the new tests are:  Especially touchdown configurations constricted by the limited capabilities of the pendulum test facility are of particular interest and are reflected in the test objectives. This refers primarily to asymmetric load cases which become testable with DLR’s LAMA facility (operational since 2010).  To broaden the test data base on the influence of the landing gears tilt limiter.  To broaden the data base on the contact phenomenon on soft soil as (i) later missions (such as Deep Impact [6]) contributed to the comet surface property knowledge with relevance for Philae and (ii) touchdown test in granular media become possible with the LAMA facility as compared to the pendulum facility. III. TEST MODES The test matrix build-up reflects these objectives and groups the test cases into four basic (Base tests) and three special load case groups (Spec tests). Table 1 shows these load cases and the associated touchdown conditions. Base 1: The Base 1 tests, thus, shall ensure the consistency and seamless connectedness between the test data generated on the pendulum facility and the LAMA facility. This group falls in line with similar touchdown tests executed during the development and qualification phase of the landing gear. A side-effect is a quantification of the strength and weaknesses of both touchdown test facility concepts for small body landings. The Base 1 tests act as further reference for the subsequent Base and Spec tests. Base 2: the test cases in this group vary in comparison to the Base 1 group the lander pitch (around the body y-axis), the surface friction and the flywheel status. This group particularly addresses tilt limiter and flywheel effects on the touchdown dynamics. Base 3: this group is similar to the Base 1 group, however with the difference that the touchdown occurs on a granular, soft surface. 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. The objective is to gather data to verify the numerical simulations for the toppling stability boundary determination. The special load cases are used to address additional questions which are not directly related to the touchdown system performance, in particular: Spec 1: this test case is used to gather data on the lander precession motion induced by the flywheel and applied external torques during the descend phase. Spec 2: addresses further stability load cases and complements the Base 4 group. Spec 3: this group is basically a repetition of the Base 3 group, however with partly different touchdown velocities. During these tests the footpads were equipped with the scientific instrument CASSE [7], integrated into the foot soles. The focus of this test group is to assess whether this instrument is also able to utilize the touchdown loads to acquire scientifically meaningful data on the comets soil mechanical properties. Identifier Objective Vvertica