Mole penetrator optimization using the numerical model and operational tests

An autonomic sub-surface penetration is currently a very promising technique for investigation of cometary, planetary and lunar surface layers properties. In many of the future missions penetration devices, called “mole,” are being considered (e.g. GEMS in US and Luna in Russia). Mole devices are low velocity, medium to high energy, self-driven penetrators, designed as a carrier of different sensors for in situ investigations of subsurface layers of planetary bodies.

The mole penetrator KRET (Polish for ‘mole’), a penetrator with high energetic strokes developed at the Space Research Centre PAS in the Laboratory of Space Mechatronics and Robotics, is a unique device intended for subsurface analyses of solar system bodies during i.a. future outer planet missions (e.g. to Titan). The depth achieved by this type of penetrators is limited to energy efficiency of the device and the penetrative resistance of the soil. The principle of operation of KRET bases on the interaction between three masses: the inserted cylindrical casing, the hammer, and the rest of the mass, acting as a support mass. Additionally, the driven spring acts on the hammer and the support, and the return spring should act on the support and the casing.

The laboratory has been granted the PECS contract from ESA to further the development of KRET penetrator. The project, which commenced in January 2009, has been concluded in June 2011.

The main purpose of the project was to build the appropriate numerical model of the mole penetrator dynamics coupled with mechanics of the granular matter, and to perform a series of tests of the mole penetrometers. Together they allow determining optimal parameters of the mole penetrometers.

In the frame of the project, two different numerical models of the mole in regolith were developed: first one (full 2D model) simulates in detail the regolith behavior based on the so-called “molecular dynamics” of the granular matter where the mole is simplified to two (tube and hammer) interacting bodies. In the second model, the interaction between mole and regolith is fully phenomenological one and its parameters were derived from the first model. This model is especially useful for mole design optimization.

A lot of resources in the project was dedicated to develop appropriate test-bed systems useful
in planetary exploration. Three test-bed systems were build: 5m one (in the background of the figure below) equipped with the system for remote mole detection and its inclination; 2m test-bed system which allows simulating various gravity conditions (various inclinations); and small test-bed system to test mole progress during one particular stroke. All of the systems can be filled with any materials; however, to perform a test in a relevant one, the Polish lunar regolith analogue (AGK2010) was developed. Particularly the mechanical properties of the AGK2010 material are similar to the properties of the lunar regolith.

The project was realized by two institutions: Space Research Centre PAS and Faculty of Drilling
of the Cracow University of Science and Technology

Penetrator KRET