There is a lot to studying impactors and how they will perform. It is not just a case of seeing the capability of the metal spears, but also the strength of the soil or rocks that they will impact.
We will soon start our gas gun testing and that means firing a 6 kg probe into lots of soil that simulates what the probes may land in on Mars. To start we will use simple sand. The probes will impact the simulated material (SIM) at a speed of around 450 kph to 650 kph. Why the range? This is to allow the Gas Gun some ability to vary the charge used to propel the spear to the right speed. After all we cant expect them to have the perfect sized charge to hit the SIM at an exact speed. Our probes are a 6Kg mass because this is a best guess at a mass that is controllable on descent and hit Mars and stay close to upright. After all we do have to slow the device and that will not be easy if the mass is high. As we cannot take fuel with us, it will have to be air slowing – like a parachute. Too heavy and it just will not work. We may yet find that we can land 2 or 3 times that mass, but since the gas gun cannot handle that mass, we will have to extrapolate and do ballistic testing with rockets to finalise our probe design..
Both the impactor and soil will have its own stiffness and elasticity. Since the impactor will be way harder than the SIM we will ignore that at the moment. So what is the Young’s Modulus?
This from Wikipedia:
Young’s modulus, or the Young modulus, is a mechanical property that measures the stiffness of a solid material. It defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material in the linear elasticity regime of a uniaxial deformation.
Young’s modulus is named after the 19th-century British scientist Thomas Young; but the concept was developed in 1727 by Leonhard Euler, and the first experiments that used the concept of Young’s modulus in its current form were performed by the Italian scientist Giordano Riccati in 1782, pre-dating Young’s work by 25 years. The term modulus is derived from the Latin root term modus which means measure.
If that is not clear, what it means for us is that when we hit, the SIM with our probe travelling at around 500 kph, we may meet little resistance and plough right through the material and since we were to use a 44 gallon drum ( US 55 gallon) for the material, that means a distance of around 850 mm. It is likely that e will need to place two drums in series to slow the probe.
In a paper titled: Mechanical properties of seasonally frozen and permafrost soils at high strain rate published by: Zhaohui (Joey) Yang, Benjamin Still and Xiaoxuan Ge it was found that a great variety of stiffness existed in seasonally frozen and permafrost soils. On Mars it is clear that some soils warm enough to defrost the water immediately under the soil, but to what depth. These are often on slopes with a better tangential angle to the sun.
Our planetary scientist Jani Radebaugh also reminded us that the Mars InSight mole was meant to tell us the Mars thermal gradient. It got jammed in digging unexpectedly into a Calcium Cement. We still do not have anything to have a solid knowledge of the Mars situation so we just have to have an intelligent estimation based on the earth’s general gradient of around 20 degrees Celsius. Basically this means that in the top 6 feet of martian soil, the average temperature will be that of the average of the surface. That is a seasonal average and a daily average of day and night. There will be a variation of a small amount based on seasonal changes and a delay due to the insulation of the soil.
There is a difference on the earth between seasonal freezing and permafrost. The stiffness of the soil is different between the two. We will assume that most soil, especially away from the equatorial area, is more like permafrost. For the sake of the impact chamber of the gas gun, we will assume that there s little to no water and a seasonal freeze.
To make it easier for the gas gun impact we will assume a very lightly damp sand frozen by dry ice to about -50C (-58F)
Jani states: Yield strength ranges from 1.7 – 4 MPa, Young’s modulus ranges from 2000 – 6000 MPa, no data for bulk modulus, density (dry) ranges from 400-700 kg/m3.
Now all these great figures help us, but we still have no idea of the grain size of the sand and how it packs together. The compaction influences or many other variables. We will initially use a lightly packed sand of large grain size. It is something easily available from the local landscaping shop. It will give us the deepest impact.
As far as stiffness goes, we will also need to stiffen the barrels by wrapping them in steel straps to stop them bulging as the probe enters the barrel. Freezing the barrel and slightly damp soil will provide us with a stiffer environment keeping the barrel strength from adversely affecting the results.
The barrel will be wrapped in dry ice and left to freeze the entire barrel to the core. The ice will be topped up daily until a temperature sensor at the core of the barrel tells us that the temperature is satisfactory. Dry ice is CO2 at a temperature of around -60C (-76F). We will determine the number of days needed to freeze the barrel, noting that the wetter the soil, the more conductive it is to temperature variations.
The next step is to test fire the probe. That will happen soon.
Is this complicated? Yes
Is there guess work? Yes – lots of variables. We do our best to minimise the unknowns. Our planetary Scientist Jani Radebaugh is the best around.
Is the gas gun testing important? YES it is essential. too many unknowns and we have to minimise them. This will help us learn how it will all happen.
Young’s Modulus? Well this and other factors are so important. You probably still have only an vague idea now, but it and other factors are so important.
We will let you know about the testing soon enough. More on the probe soon.