Jet-induced cratering of a granular surface with application to lunar spaceports

📝 Abstract

The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but previously unrecognized mechanism for erosion dynamics. It has also shown that slow regime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cratering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories cannot be entirely correct.

💡 Analysis

The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but previously unrecognized mechanism for erosion dynamics. It has also shown that slow regime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cratering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories cannot be entirely correct.

📄 Content

Jet-induced cratering of a granular surface with application to lunar spaceports

Philip T. Metzger1, Christopher D. Immer2, Carly M. Donahue3, Bruce M. Vu4, Robert C. Latta III5, Matthew Deyo-Svendsen6

1KSC Applied Physics Lab, NASA, Kennedy Space Center, Florida 32899, Philip.T.Metzger@nasa.gov 2ASRC Aerospace, Kennedy Space Center, Florida 32899 3Department of Physics, Astronomy and Geology, Berry College, 2277 Martha Berry Hwy. NW, Mount Berry, GA 30149 4Structures and Analysis, Engineering Development, NASA, Kennedy Space Center, Florida 32899 5 Department of Aerospace Engineering, Embry-Riddle Aeronautical University, 600 S. Clyde Morris Boulevard, Daytona Beach, FL 32114 6 Department of Mathematics and Computer Science, Stetson University, 421 N. Woodland Blvd. DeLand, Florida 32723

Abstract

The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but pre- viously unrecognized mechanism for erosion dynamics. It has also shown that slow re- gime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cra- tering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories can- not be entirely correct.

Introduction During the Apollo and Viking programs there was considerable research into the blast effects of launching and landing on planetary regoliths. That work ensured the success of those missions but also demonstrated that soil erosion or cratering will be a significant challenge for other mission scenarios. For example, the high-velocity spray of eroded soil will pose a serious challenge when we attempt to land multiple spacecraft within short distances of one another on the Moon. We have relevant experience because the Apollo 12 Lunar Module landed 155 meters away from the deactivated Surveyor 3 spacecraft. Portions of the Surveyor were returned by the Apollo astronauts to Earth for analysis. It was found that the surfaces had been sandblasted and pitted and that its open- ings had been injected with grit from the high-speed spray [Cour-Palais 1972]. This treatment is not acceptable for functional spacecraft.

A program has begun to develop plume/soil mitigation techniques, quantify their effec- tiveness, and provide the environmental design requirements for hardware near the launch and landing site. It is extremely expensive to perform the experiments using rea- listic lunar soil and a hypersonic engine plume while maintaining vacuum in the chamber, so we have adopted a strategy that relies heavily on numerical simulations of the physics.
These simulations will precede the more expensive, high fidelity tests that are expected to occur later in the program. Early testing is focused on understanding the physics of plume/soil interactions so that the numerical simulations may be coded properly. There has never been an adequate description of the physical processes or scalings that occur inside a jet-induced erosion event. Also, the prior lunar erosion theories based Shield’s parameter were incorrect in that they extrapolated terrestrial experience over many orders of magnitude for flow conditions where different aspects of the physics dominate. There- fore, the space program needs to better understand the basic physics of cratering and ero- sion processes before it can confidently develop the technology to control them.

The theory developed during the Apollo and Viking programs predicted three different cratering mechanisms, not all of which are applicable to the Moon. The first mechanism is viscous erosion, which occurs along the very top layer of sand grains as the dynamic pressure of the gas torques them up and over their neighbors and then pushes them away.
This theory as applied to lunar landings was developed primarily by Roberts [1963] and studied experimentally in relation to the Moon by Land and Clark [1965], Hutton [1968], and Clark [1970]. The second mechanism may be called bearing capacity failure, and it occurs when the stagnation pressure under the jet exceeds the bearing capacity of the soil and mechanically pushes it downward to form a rather narrow cup. It was studied by Al- exander, et al [1966] and has not occurred during lunar landings due to the high relative density and shear strength of the lunar soil. It is expected to be a very serious problem for martian landings and launches. The third mechanism is diffused gas eruption – large- ly an auxiliary effect rather than a primary cratering mechanism. It occurs when the dy- namic pressure of the jet drives gas into the pore spaces of the so

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