The Effective Ground Pressure (EGP)
Technical specifications of the Mars Exploration Rovers (Spirit and Opportunity) and Curiosity wheels:
Curiosity (Mars Science Laboratory)
Mass = 960 kg
Rover Weight on Mars = 3572 N Average Weight per Wheel = 595.3 N
Wheel Center Diameter = 0.500 m (with cleats) Wheel Outer Diameter = 0.465 m (with cleats) Wheel Width = 0.400 m
Spirit and Opportunity
Mass = 176.5 kg
Rover Weight on Mars = 656.7 N Average Weight per Wheel = 109.45 N
Wheel Center Diameter = 0.262 m (with grouser bars) Wheel Outer Diameter = 0.232 m (with grouser bars) Wheel Width = 0.16 m
The cross-sectional area of each wheel’s contact patch with the Ground Plane are 0.0173 m2 for Spirit and Opportunity; and 0.0901 m2 for Curiosity.
The effective ground pressure (EGP) metric is defined as the average pressure under the average wheel. The average weight on a wheel is first found by dividing the total vehicle weight on a planetary body by the number of wheels. The EGP is found by dividing the average weight by the cross sectional area of the wheel’s contact patch on the ground plane, after the wheel has sunk into a terrain so as to have a contact patch length of one wheel radius. In the case of non-cylindrical tires, the largest radius, usually at the mid-plane is used.
When the wheel’s periphery has cleats, lugs, or grouser bars; a determination is made as to the amount of their height added to the tire diameter, based on the projected area ratio of these tractive elements.
The EGP defined as the average weight over the CS Area are similar in the MER and Curiosity:
EGP (Opportunity)= 109.45 N / 0.0173 m2 = 6335 N/m2= 6335 Pa (0.919 psi)
EGP (Curiosity) = 595.3 N / 0.0901 m2 = 6609 N/m2= 6609 Pa (0.959 psi)
Brine Corrosion tests
The following steps were followed: (i) the chamber was first vacuumed and filled with pure CO2 gas till it reached 6 mbar; (ii) water was injected to create water vapor in the atmosphere;(iii) the LN2 was allowed to flow through the cooling plate which reached a temperature of 260 K and the temperature slowly increases due to the thermal equilibrium with the ambient lab conditions; and (i) the experiment was stopped by opening the relief valve. The salt samples deliquesced as the salt in it absorbed water from the atmosphere and formed brines.
Simulation of martian near surface water cycle and thermal studies
The experiments for brine corrsion under simulated Martian enviroment were conducted in the SpaceQ chamber (figure 9), a cubical chamber with an internal volume of 27l. This chamber can simlaute temperatures ranging from 163 K to 423 K, pressures ranging from 10-5 mbar to ambient conditions and relative humidity from 0% to 100%. Here we inject traces of water to control the relative humidity using a stainless steel syringe connected to a ball valve in turn connected to the chamber wall with the means of a swagelok connector. So, the water can be injected several times during the experiment and as the ball valve opens the water is sucked into the chamber and vaporises 27.
We use sodium perchlorate salt (NaClO4) (ACS reagent, ≥98.0%, 410241) bought from Sigma Aldrich and the Mojave Martian Simulant (MMS-1) as Mars regolith simulant29. We use the “unsorted” grade bought from The Martian Garden-Austin Texas. The water used in the injection is from Sigma which is filtered at 0.1µm to guarantee sterility and cleanness (W4502). We have conducted tests in SpaceQ Martian environment for two sets of samples prepared (i) one with pure sodium perchlorate (NaClO4) and one with a mixture of (ii) MMS + 10% NaClO4 and exposed to Martian conditions. The samples were placed inside the SpaceQ and it was simulated for the near surface water cycle condition as it will experience on Mars30. The chamber was first vacuumed and filled with pure CO2 gas till it reached 6 mbar. Then, water was injected to create water vapor in the atmosphere till RH reached 40% then the liquid nitrogen (LN2) is allowed to flow through the cooling plate which reached a temperature of 260 K and then temperature it let slowly to increase to ambient lab conditions due to the thermal equilibrium. The experiment is stopped by opening the relief valve and the samples were weighed (shown in Table 4). The samples with pure NaClO4 were deliquesced and the samples with mixture with the simulant were in a hydrated state as it absorbed water from the atmosphere.
|
(NaClO4 salt)
|
(MMS soil + 10% NaClO4 salt)
|
11R51
|
SS4301
|
S235
|
11R51
|
SS4301
|
S235
|
Initial
|
1.55 g
|
1.59 g
|
1.53 g
|
1.69 g
|
1.77 g
|
1.74 g
|
Final
|
2.32 g
|
2.33 g
|
2.37 g
|
1.83 g
|
2.04 g
|
1.89 g
|
Increase in weight
|
0.77 g
|
0.74 g
|
0.84 g
|
0.14 g
|
0.27 g
|
0.15 g
|
Table 4: Summary of the weights for samples placed inside SpaceQ Martian environment
Tribocorrosion tests
The following materials were used:
Fluid medium:
-
“Water” (deionized water, H2O) The following methods have been used
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Tribological experiment: Optimol SRV, normally used for material development, (screening of surface-lubricant combinations)
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Applied Load 25 N. Contact pressure ~1
Selected based on reasonable assumption: Few kg load onto “pointy but not sharp”- rock…
Surface analysis
A Zygo NewView 7300 3D optical profilometer (Zygo Corporation, Middlefield, CT, USA) was employed for surface topography analysis. Measurements were conducted under both 10x magnification. The surface profilometry data was analysed using MountainsMap Premium 7.4 (Digital Surf, France).
Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray Spectroscopy (EDS) was employed for high magnification surface analysis including chemical (elemental) analysis: Magellan 400 FEG-SEM (FEI Company, Eindhoven, The Netherlands). EDS was performed using an X-Max 80 mm2 X-ray detector (Oxford Instruments, Abingdon, UK) operated at 10 kV and 50 pA.