In a private message I was asked the following question, which I think might be interesting to some of you as well.
I have been wondering for several years now what you guys do to compensate in the test bed for .38 g on Mars.
I assume it's easy to lower the mass of the test rover, but what about the soil mechanics? Does the sand and soil not behave differently in that lower g environment? When you take an MI pan of the soil particle to reconstruct a similar mixture in Pasadena, how do you compensate for the soil particles' behavior in the lower gravity? Do you have a study of sands and their particle density characteristics that allows you to come up with an inventory of choices that behaves the same as the martian soil behaves in .38g? or is the gravity effect on the soil mechanics insignificant under these conditions? It's clear to me from old Apollo videos that lunar soil behaves far differently so there must be some threshold where you need to take the gravity into account.
This is an excellent question! We have two vehicles we can use for testing: SSTB1 a full size replica of the MER vehicles, minor some minor differences (no solar panels, some temperature probes are missing...) which of course has the same mass as MER and higher weight on Earth than MER has on Mars; and SSTB Lite, a stripped down vehicle with same wheel size, actuators and suspension system, same WEB size but major components like the IDD and others are missing. This vehicle has a weight on Earth that is similar to the weight of MER on Mars. For Purgatory and for this event we are going to use the SSTB1, not the SSTB Lite.
We are still in the process of soil simulant selection and we are following the same principle we followed during Purgatory: we try to replicate the vehicle performance during the embedding event. In Purgatory no matter what we tried, we could not get the SSTB Lite to embed in the soil. The soil we used was too heavy. Fortunately we were able to replicate the embedding event with the SSTB1, not perfectly, but close.
We really can't try to replicate the exact soil characteristics we measure with spectrometers and MIs but try to replicate how the vehicle behaves during the embedding event and hope it will be representative enough for testing the extrication maneuvers. We use some of the information we get on particle size, but ultimately the vehicle is the best instrument we have to select the soil simulant.