STRUCTURAL PART STIFFNESS TEST IN COMPARISON TO THE FE-PREDICTION. A TEST COMBINING CONTINUOUS STRUCTURE WITH COMPLEX INTERFACES

Kurzfassung

ABSTRACT In JAXA’s (Japan Aerospace Exploration Agency) MMX (Martian Moons eXploration) mission [1] the two Martian moons, Phobos and Deimos, and the Martian environment shall be studied. In this regard DLR (German Aerospace Center) and CNES (Centre National d’Etudes Spatiales) developed the MMX rover IDEFIX that shall explore the surface of Phobos and will be launched onboard the Japanese MMX spacecraft in 2026 [2]. The MMX rover will be delivered to the surface of Phobos by the MMX spacecraft, where it is mounted on the MECSS (Mechanical, Electrical and Communication Support System) structure. This structure interconnects the rover with the spacecraft and provides interfaces to the HDRM (Hold-Down and Release Mechanism) and to a dedicated push-off mechanism, which are used to separate and eject the rover from the spacecraft. This paper addresses mechanical tests in order to determine the MECSS in-plane stiffness in comparison to its predicted stiffness from a finite element (FE) simulation. Knowing this stiffness is important, because it highly influences the thermoelastic stresses and interface loads of the MECSS and its adjacent structures, mainly the MMX spacecraft aluminum sandwich panel. The MECSS structure is a sandwich panel consisting of CFRP (Carbon-fiber-reinforced polymers) face sheets with aluminum honeycomb core and four rather large aluminum inserts in its corners, cf Fig. 1. The inserts at the corner provide the interface to the spacecraft and to the release mechanisms of the rover. While the prediction of the stiffness and thermoelastic properties of the continuous CFRP face sheets is a simple analytical task, the prediction of the MECSS’s corner inserts interface stiffness is more complicated due to different parallel load paths. The inserts are manufactured via ALM (Additive Layer Manufacturing). Due to the very demanding thermal environment in combination with the high CTE (Coefficient of Thermal Expansion) mismatch between the materials used, a flexible glue (cf. Fig. 1, red markings) is used to interconnect the corner inserts to the facesheets. Locally the three screws that connect the release mechanisms cup interface to the corner inserts also clamp the facesheets to the inserts. This double load path in a complex setup makes the prediction of the actual (“effective”) stiffness difficult, as different assumptions can be taken on how the interface will behave. Therefor a load test was conducted to measure the overall stiffness of the MECSS. In the test, the deformation under load is measured with the help of the optical 3D measuring device GOM ARAMIS, which uses digital image correlation. As a result, the deformation of the visible side of the MECSS is given in 3D coordinates. This 3D deformation is calculated back to the in-plane deformation of the central plane of the MECSS and compared to the FE simulation. It is evaluated which assumption predicts the actual deformation best.