ExoLance: Preliminary Modeling for Penetration Effectiveness
Aerojet Rocketdyne has been performing modeling of the Mars penetrators under internal funding as in-kind support for the ExoLance project. This work uses an industry standard simulation known as hydrocode modeling. In the hydrocode model, a grid is generated to represent the material that the penetrator is impacting and the model is run to determine the depth of penetration for various parameters such as penetrator mass, shape, initial velocity at impact and angle of entry. In order to use the hydrocode model as a design tool, it is first necessary to calibrate the model against a known test result. Since the DS-2 project did extensive testing in the 1990’s on simulated Martian soils, we elected to use it as our calibration case.
Figure 1 shows the DS-2 penetrator as configured for the tests.
Figure 2 shows the test set-up including the gas gun used to accelerate the penetrator to a velocity of 200 m/s at impact.
The test results for a 90 degree impact (normal to the surface) from test firing shot number 9 were as follows:
- Shot #9 Target: clay
- Forebody penetration (m) 0.38
- Aftbody penetration (m) 0.2
- Impact speed (m/s) 172
- Incidence (deg) 90
The first movie shows the results of the preliminary hydrocode modeling. After selecting a soil composition that matched the clay type soil used in the test, the model was run with a DS-2 penetrator design and the grid mesh was adjusted until the results shown below were achieved.
This result shows the penetrator entering and generating a small spray of ejecta as it burrows down past 10 cm. At approximately 0.9 milliseconds, as the penetrator is passing through 10 cm, the ejecta pattern resembles a classic cratering process with a small cone forming on the surface. Of course, in the actual impact, the aftbody will be interacting with this region. One millisecond later the tunnel has reached 20 cm depth and the tunnel is now showing signs of tapering on the walls. The penetrator comes to rest at approximately 39 cm depth in approximately 10 milliseconds. This compares very favorably with the measured DS-2 results shown above.
A second case was run to document the sensitivity to off-nominal entrance angles. In this case, the penetrator enters at a 70 degree angle relative to the surface. As you can see from the movie, the penetration depth is reduced to approximately 25 cm. As is evident from the simulation, the penetrator body “slides” a little sideways into the soil, causing the tunnel to be wider and also reducing the ultimate depth.
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