Wheelspins on Mars - Recreating Martian soil using silica flour
There are no tow trucks on Mars, so it is worth preventing extra terrestrial breakdown. To help robotic rovers out of potential jams, researchers at Cranfield University in Milton Keynes, UK, have created silica flour that simulates a Martian soil type.
Cranfield researcher Dr Terence Richards explains, ‘One of the major challenges to vehicles on the surface of the planet is avoiding getting stuck in fine soft sand.’ In 2010, Hertfordshire-based space company Astrium Ltd approached the university’s Centre for Automotive Technology to create a surrogate soil. The soil would be used to test the locomotion subsystems of the European Space Agency’s (ESA) rover vehicle ahead of its trip to the red planet in 2018.
To imitate Martian soil, the particle size, distribution, cohesion, angle of internal friction and bulk density of the unblended silica flour needed to be precise. (For example, more than 50% of the particles were required to be within a 30–70 micron range.) However, the strict material requirements did not end there.
‘The soil had to be made from terrestrial soil materials with proper constituents – not from manmade materials such as polymers and metals,’ adds Richards. ‘It also had to be available in bulk quantities [of] up to 30,000kg for use in large test environments. The chosen material had to be stable over time. Test vehicles could not noticeably alter the grain size distribution, and it couldn’t be hygroscopic, as absorbing atmospheric moisture could cause changes to its physical properties.’
The researchers used a translational shear box to measure cohesion and the angle of internal friction, a vibrational compactor to measure density and pressure sinkage tests to measure soil stiffness constants. All of the materials were tested in dry environments with less than 0.1% moisture content and relative humidity of 25–50%.
Despite successfully replicating the soil type, Richards notes that there is space for improvement. ‘Ideally, we would have liked to increase the cohesion value of the selected material … through doping with chemical salts. We did some doping experiments using silicon oil, iron oxide and kaolin, however, these dopants did not produce the desired results.’
Cranfield University will continue to work with Astrium in the development of the ExoMars Rover, and to test several wheel designs as part of ESA’s Sample Fetching Rover Study. Further work on the Mars soil project is contingent on the requirements of funding bodies such as ESA and NASA.
What do Martian rovers do when their wheels are stuck?
Dr Terence Richards says, ‘One way to regain mobility is to use a combination of different longitudinal slip ratios and vertical load control. These two parameters can be adjusted to modify the net thrust or reduce the rolling resistance of particular wheels – the wheels with most traction being used to recover the vehicle.’
The 2018 ExoMars mission will deliver a European roving vehicle and Russian surface platform to Mars’ surface. After its nine-month journey, the ExoMars Rover will collect underground samples and assess them using the vehicle’s analytical laboratory.