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. IntroductionDuring 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 openings had been injected with grit from the high-speed spray [Cour-Palais 1972]. This treatment is not acceptable for functional spacecraft.

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The scaling laws for the simulation of noise from subsonic and ideally expanded supersonic jets are examined with regard to their applicability to deduce full scale conditions from small-scale model testing. Important parameters of scale model testing for the simulation of jet noise are identified, and the methods of estimating fullscale noise levels from simulated scale model data are addressed. The limitations of cold-jet data in estimating high-temperature supersonic jet noise levels are discussed. It is shown that the jet Mach number (jet exit velocitytsound speed at jet exit) is a more general and convenient parameter for noise scaling purposes than the ratio of jet exit velocity to ambient speed of sound. A similarity spectrum is also proposed, which accounts for jet Mach number, angle to the jet axis, and jet density ratio. The proposed spectrum reduces nearly to the well-known similarity spectra proposed by Tam' for the large-scale and the fine-scale turbulence noise in the appropriate limit.