Dr. Phil Metzger Profile picture
Oct 6, 2023 9 tweets 3 min read Read on X
I got word today that our research into the Starship launch pad anomaly is being forwarded uphill to NASA HQ. They are focusing on what we learned about launch/landing pad failure modes and how we can make lunar landings safer. /1
2/ What we found is that the pressure that built up beneath the launch pad was comparable to a volcanic eruption when the buildup of hot gas that evolves from the magma busts apart the caprock and expels it. (Pic: Fagents & Wilson, Geophys J Int 113(2): 359.) Image
3/ The theory on these volcanic eruptions predicts a range of velocities that matches what was measured for the ejected chunks of concrete from the Starship launch pad — about 90 m/s. Image
4/ But to explain the mass of gas needed to expel at this velocity, we had to conclude that groundwater under the pad was vaporizing. We estimated the water based on crater volume and permeability of the sand under the pad. It is in the range that agrees with theory. Image
5/ This raises the question about ice in lunar polar soil under a launch pad. If the pad cracks and hot gas is pushed through (like we think happened for Starship) then vaporization of the ice may create a similar situation. Ice can be as much as 5%wt (actually higher), which…
6/…predicts that ejection velocity of a lunar launch pad could be even faster than what happened for Starship. 5 wt % vs 1 wt % on this plot. Image
7/ So we need to develop methods to prevent this. It shouldn’t be hard to do. Examples: make the pad thicker. Measure the ice before construction. Put vents under the pad. In fact… Image
8/…we are already collaborating with Cislune on a project to develop these technologies. “Deflector cone and vented launch pad” — read more here: spaceref.com/science-and-ex…
9/9 So that’s how the Starship launch pad anomaly, while not *desired* ofc, turned out to produce insights for engineering lunar landing technology. This is the message that got attention and is being sent up to NASA HQ.

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More from @DrPhiltill

Dec 14
Great question. I don’t think the Moon can have solid sheets or lenses of ice like the Earth and Mars have. The absence of an atmosphere means everything on the lunar surface gets pummeled by meteorites and broken into granular material to 10s of meters depth. /1
2/ We saw evidence of this at the NASA LCROSS mission’s impact into Cabeus crater in 2009. The target soil was so soft that the spacecraft apparently buried into the soil 2-3 meters deep before meeting much resistance. This caused three observable:
3/ First, the visible flash that we expected to result from the impact was entirely suppressed. I remember watching it in realtime. It was a big disappointment because the satellite images in the visible spectrum showed absolutely nothing at impact. Image
Read 14 tweets
Jul 18
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.
Read 17 tweets
Jun 22
Ok, here’s a little thread of some of the recent, awesome fluid dynamics content on here.

1. Checkout the computer modeling of airflow over an aircraft!

1/N
2. Vortices made visible by water vapor

/2
3. Just awesome! Look how the rings pull each other toward the end.
Read 14 tweets
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

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