Dr. Phil Metzger Profile picture
Apr 21, 2021 9 tweets 4 min read Read on X
From the talk I gave at the ASCE Earth & Space conference today. When you land on the Moon, your rocket exhaust is faster than lunar escape velocity and there is no atmosphere to slow down the dust you blow. We need to worry about damaging things in orbit.

Short thread... /1
We've done a lot of experimental work to understand how much lunar soil will blow because of the rocket exhaust during a landing. The work included reduced gravity flights measuring soil erosion in lunar gravity. /2
As you would expect, erosion rate is faster when gravity is lower. That part of the physics is easily understood, at least. Erosion of soil on another planet scales as 1-over-gravity. /3
The equation shows that erosion is proportional to the shear stress that the rocket exhaust causes upon the lunar surface, and inversely proportional to the energy that it takes to lift a lunar sand grain. Seems pretty obvious, now, but it took years of work to prove this. /4
However, there are huge uncertainties, like what happens when the gas expands into vacuum so the viscosity breaks down and it stops acting like a normal gas? What happens to turbulence (which is crucial to soil erosion)? Etc. Lots of unsolved physics, here! /5
But doing the best we can, this shows how much soil will blow as a function of the mass of the lunar lander. The Apollo LM was 5-7 tons. Future landers will blow a LOT more soil. (This graph assumes the engines UNDER the vehicle. I know...not all landers are like that👍🙂) /6
Here is what happens after the soil is blown off the Moon. The smallest dust goes 5X the speed of a bullet, so it looks like a straight line over this distance. An orbiting spacecraft might fly through this ejecta as it is blown off the Moon. /7
The number of dust particles that hit the spacecraft is 256 million per square meter. It is so many because they are so small. Most of them are smaller than a micron. But it adds up to a LOT of damage. One exposure could chip of 4% of a piece of glass, like a camera lens. /8
As a result, we need to do collision avoidance -- timing the lunar landings so that no spacecraft will happen to fly through their ejecta sheet. We will need landing pads when lunar traffic becomes very high. We need international agreements to manage these effects. /END

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More from @DrPhiltill

Jun 12
Four other problems with landing on a flat pad, even if it is a steel with water deluge.

(I’m assuming the larger size of the Super Heavy booster is why they can’t use flat concrete like ordinary booster landings.)

The four problems: … /1
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. Image
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.
Read 9 tweets
Jun 10
If I had to guess it would be this: same exact material as the existing tiles but just a wee bit thicker. Here is why…

1/N
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.)
Image
Image
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. Image
Read 14 tweets
Jun 4
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. Image
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?! Image
Read 18 tweets
Apr 28
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.
Read 15 tweets
Apr 18
NASA now building a flight-ready lunar excavator for a resource utilization pilot plant (not a demonstration — the actual pilot plant) on the Moon.
Discussing the challenges of reoeatably setting up the correct lunar soil conditions (compaction, rocks) for testing the lunar excavator on Earth. Image
3/ The robot will not be joystick-operated from Earth due to time delay and bandwidth limits. It will have software for autonomous mining & roving.
Read 8 tweets
Apr 10
I'm tired of reading in the news people proclaim that starting a city on another planet is economically ridiculous when clearly they are just guessing. So I'm finally starting to write a paper on the analysis I did a few years ago that found (to my surprise) it is quite feasible.
The main thing ppl don't seem to grasp is that the cost of the extra stuff for Mars, like building a dome, recycling air, using mass for radiation shielding, washing perchlorates from dirt, etc., are utterly trivial compared to the cost of frivolous things we do in our economy.
The 2nd main point that ppl don't seem to grasp is that you don't need any particular advantage from being on Mars to make it economically viable. Mars doesn't need special minerals or anything. Any location becomes economically viable simply by there being enough humans there.
Read 4 tweets

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