A paper just came out analyzing rocket exhaust blowing lunar soil. It is important for at least 2 reasons. 1/n
Reference: Chinnapan et al., "Modeling of dusty gas flows due to plume impingement on a lunar surface," Physics of Fluids 33, 053307 (2021) aip.scitation.org/doi/full/10.10….
2/n First, there is great uncertainty in how *fast* the lunar dust goes. It is hard to model rocket exhaust physics on the Moon because fluid flow equations break down as the gas spreads into vacuum. The relevant equation is the Navier-Stokes equation. (screenshot from Wikipedia)
3/n In that equation, the constant μ is gas viscosity. It is not really a fundamental thing in nature. It was invented by averaging lots of molecules bouncing in a small volume of space. It tells us how much the momentum from one volume diffuses into nearby volumes.
4/n The averaging process that invents "viscosity" assumes the molecules will actually collide within those volumes. But when the gas spreads too far into lunar vacuum, the "mean free path length" (the average distance between collisions) is too big. So viscosity "breaks down."
5/n Lots of work has been done to create new physics models of gas blowing into vacuum, but the models have flaws. They do not have good turbulence models. They do not handle boundary conditions well enough. Etc. The predictions have some uncertainty, which is normally OK. But...
6/n ...it turns out that these gaps in our physics models produce a lot of uncertainty in how fast the dust particles get lifted off the surface of the Moon and how fast they end up going. Different models predict between 200 m/s up to 3000 m/s. A huge difference!
7/n We can't tell how fast the dust particles are going by looking at the Apollo or Chang'e landing videos because it all looks like a continuous cloud.
8/n This new paper is significant in part because it uses a good method that handles dust in lunar vacuum fairly well (Direct Simulation Monte Carlo) using realistic parameters, and it predicts the dust goes close to lunar escape velocity -- about 2.38 km/s.
9/n That velocity happens when the lander is still 15 m above the lunar surface, and the dust goes even faster as the lander descends. So we expect dust will be blown completely off the Moon. This validates earlier work that had used simpler approximations.
10/n A 2nd reason this new paper is cool is because it tests the sensitivity of "2-way coupling" between the gas and dust in the physics model. It is vastly easier to do "1-way coupling": to model just the gas flow by itself and then use those results to calculate the dust speed.
11/n In order to do 2-way coupling your model has to use increments of time that are correct for the gas molecules (very fast) and the dust particles (very slow). It is hard to put both into the same model. Most of our past work used 1-way coupling, assuming it was good enough.
12/n We used various tricks to validate the 1-way coupling was good enough, but questions remained. Some folks claimed this made our models predict velocities that are too high. The new work tests the difference between 1-way & 2-way coupling and they showed almost no difference!
13/n So I am excited over this new paper in the Physics of Fluids journal. It validates that we have a pretty good grasp of how rocket exhaust blows soil during lunar landings. This improves requirements for lunar landing pads. (This pic is for a particular case.)
14/14 However, there is another entire problem that has not been solved. That is when rocket exhaust is able to dig a deep hole in the soil. We haven't solved how to model that part of the physics, yet. @mastenspace is working on that problem right now.
A little background. The earlier version of this mission was the Resource Prospector Mission. When Jim Bridenstine was appointed NASA Administrator, NASA cancelled it without his permission just hours before he was sworn in. I can’t confirm this, but rumors say he was livid! /1
2/ Mr. Bridenstine was appointed by Pres. Trump, and the Trump Transition Team had people assigned to plan space policy. They were calling people for input. I got one such call and the person told me they not only WEREN’T going to cancel Resource Prospector, but instead…
3/ …they were thinking about having MANY Resource Prospector missions. We talked about what would be the scientific, engineering, and economic value of building multiple copies of the mission. There was strong interest in the lunar ice to support building a sustainable program.
1/ You need enough surface area around the base of the rocket for the gas to flow out, or the engines will choke. Imagine a cylinder extended below the rocket to the ground. The exterior of that cylinder must exceed the exit area of all the rocket nozzles that are firing.
2/ With more engines firing you would need longer legs to keep that area large enough. If not, then the flow will choke meaning it goes subsonic and super high temperature and pressure, comparable to inside the combustion chamber, which can destroy the nozzles or engines.
2/ Here is what they look like on the inside. They are something like 98% empty space, and the rest is a glass fiber. The fibers touch each other along small contacts, so thermal conductivity is very low. (The scale bar is 100 microns, or 0.1 millimeter.)
3/ This is an extreme case of a “granular material” where the grains are long fibers. I did research on shuttle tiles when I worked in a physics lab at NASA, and I did research on thermal conductivity through granular materials, so I can report something interesting about this.
This was the same reaction the science team had during the Apollo program — surprise that bone-dry soil could have so much cohesion! See the clods in the footpad image, especially. Short 🧵 1/N
2/ Closeup image of the clods. These are likely very porous, low density clods — very fluffy material — that will easily fall apart between your fingers. Yet they are in blocky shapes somehow held together as the footpad impacted and disrupted the ground.
3/ The first hint of this came from the famous boot print made by @TheRealBuzz. Scientists’ jaws dropped when they saw the clean, vertical sidewalls of this print in such dry, fluffy material! How could the sidewalls stand straight without any moisture?!
Untrue. This does touch on something related that actually happened, which people have apparently distorted and used to prop up the dumb conspiracy theory. I will explain… 1/N
2/ First I’ll tell you what I know about the videos, then the telemetry.
When I analyzed the plume effects of the lunar landings, starting in the late 1990s and early 2000s, I tracked down the original data. One of the guys on my team worked with Houston to get the videos.
3/ The originals had been converted to digital and this was more convenient for us to use, since we wouldn’t need reel-to-reel NTSC video equipment, so this is what we got. I had high resolution copies of all the landing videos. There was no lost video. It all exists.