Dr. Phil Metzger Profile picture
Oct 20, 2022 23 tweets 6 min read Read on X
This is article is a good overview of current thinking on SBSP. I will add a little about how I was changed from skepticism to cautious optimism to actual optimism. A short 🧵/1
2/ Maybe 20 years ago I was skeptical simply because I was always skeptical of every idea and especially those which come from enthusiasts, since enthusiasts are more likely to suffer cognitive bias. I was careful to *never* suggest that SBSP may be a benefit of space. But…
3/ …the skepticism of another skeptic shook me out of that funk because I could see he went too far. It was Pete Worden, former head of Reagan’s Star Wars program and later head of NASA Ames. Pete was never known for technological timidity, and yet he was a disbeliever in SBSP.
4/ But one time I heard him give a talk where he said that SBSP is too expensive by five orders of magnitude (100,000x more costly) compared to putting solar on the ground — e.g., by coating a desert with PV to supply national or regional energy needs. So I checked his numbers…
5/ It turns out he made a basic mistake, one which most skeptics of SBSP still routinely make. He was comparing the cost of SBSP, which is 24/7 100% market penetration baseload power, vs PV without storage or a smart grid to turn it into baseload, at ~30% market penetration.
6/ And he was using Space Shuttle or similar launch costs since newer heavy lifts were not available back then. And he was assuming no technological progress in SBSP. When you correct those, you get to within an order of magnitude of terrestrial renewables, where the latter…
7/ …are still at less than 100% market penetration — which is really hard and costly with renewables since they are mostly intermittent. (Exceptions like hydro are geographically limited and power-limited.) So if we are within 1-2 orders of magnitude, then maybe it is possible.
8/ So about 15 years ago I began to argue that SBSP is still not economically viable until after we have a full industrial supply chain operating outside Earth’s gravity well, but then it should become viable. That still pushes SBSP decades into the future. Image
9/ Then I was part of a very large proposal team for a gargantuan project (that was surprisingly *almost* funded) to directly work on that off-Earth supply chain with the goal of making SBSP viable. I worked with pro-SBSP John Mankins on that team, and got to hear his arguments.
10/ Mankins argues for hyper-modularity of the system so that large reductions in manufacturing cost can occur. If 90% of the mass consists of a small number of low-cost, easily manufactured modules, then we may get a 1-2 order of magnitude cost reduction, easily.
11/ About 4 years ago I was at a conference luncheon where Mankins was the speaker. He argued that SBSP is economically viable. Pete Worden was in the audience. After the talk, Worden immediately raised his hand and said, “I think you have convinced me.”
12/ Mankins was (and is) arguing for a version of SBSP where we don’t have to wait for off-Earth industry. The story only gets better as we add off-Earth industry. I gave a talk on this at @esa’s recent workshop.
13/ When we have lunar propellant manufacturing, the cost of boosting SBSP from LEO to GEO is cheaper. I know there are skeptics of that, too, but I have a paper that proves it currently in peer review. I will share that ASAP.
14/ Another early application of space resources for SBSP could be structural elements. That is only about 20% of the mass, so it may be a real business opportunity for metal production but by itself it is not a major reason to be optimistic or pessimistic of SBSP.
15/ In the long-run, I am very optimistic about SBSP and I think it will be crucial to health of our planet. Studies predict that energy use on Earth will continue to increase *despite* calls for sustainability. (image: thelancet.com/journals/lanon…) Image
16/ The 1-sigma estimate of published studies is that power demand may increase by a factor of 5 by 2100. That means the entire supply chain, making and then using the power, will increase by a factor of 5. Very bad for planet Earth! But we are a short-sighted & divisive species. Image
17/ (image in prior: education.nationalgeographic.org/resource/pollu…) A practical solution to solving Earth’s climate and environment problems should not expect a magical rewriting or human nature to succeed. We should plan for increased energy use. But make it clean, and move the supply chain off-Earth
18/ To make that approach really sustainable, we will need even more recycling, which requires even more energy! The beauty of SBSP is that, in the long run, about 95% of the energy sector *and the supply chain that supports it* can be moved off planet. And with it… Image
19/ …most of the computing sector and its supply chain can be moved off-planet. By 2100 that could be ~half of our environmental burden moved off planet. One thing will become inexorably more costly: real estate on Earth. Everything else drops in cost. SBSP becomes viable.
20/ But that is really long-term, decades in the future requiring ongoing progress in robotics and AI for off-Earth industry. I am convinced we will get there in the long term. But what about near term? Should we be convinced of SBSP like Mankins and now apparently Pete Worden?
21/ I think we need continued tech progress to really prove it — that’s the way it always is with technology —, but ESA just commissioned two studies which both came back with positive responses that it is achievable. It may help the climate crisis so it is a tiny cost to try it.
22/ I am convinced it is a no-brainer to place this bet. Even if it takes a bit longer to become economic than what we think, programs like SBSP communicate confidence in the future. This motivates education among young people and a more optimistic world.
23/ And because it is a space project focused on saving Earth instead of a minority of people settling on Mars, it is likely to gain broader political support, which helps space overall (and therefore also helps the economics of settling Mars). End 🧵

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More from @DrPhiltill

Mar 29
Here’s something I think is cool in the new papers that I linked yesterday.

My research group over the years has run many, many small scale experiments where a jet digs a crater against a window so we can see into it.
2/ Something weird we see in these experiments is that the depth of the crater is perfectly described by the logarithm function. Like I mean, perfectly. There are two parameters: a and b, the length scale and (inverse) time scale.
Image
Image
3/ You can use different gas speed, molecular weight, diameter jets, grain sizes, mineral density, gravity, etc. The crater depth is always a perfect logarithm of time. In fact, if you plot it versus the logarithm of time, the depth turns out to be a perfectly straight line. WHY? Image
Read 15 tweets
Mar 18
I’m not so sure. The link to the prior estimate is a paper that measures the “blast zone”, which is the region around a lander where the reflectivity of the surface has changed. We have never known exactly what causes this change. Is it from gas blowing the dust texture flat? /1
2/ Or is it from blowing dust plowing across the surface? Or from engine shutoff when the last sputter of the engine cause a low velocity blanket of dust to fly out to a much smaller distance than normal? The problem has always been that this blast zone is *too small* to be…
3/ …the area where the dust finally lands, because the dust in rocket exhaust is going far to fast in low gravity to travel only that far. (We think we might know the cause now, but I don’t want to tell here since we will probably write a paper on it.)

So the prior estimate…
Read 10 tweets
Mar 17
Lots of discussion today on space radiation including errors like this one. This has confused water with regolith. Using too thin a layer of *regolith* creates secondaries, increasing the dose. But using water, or PTFE (lots of hydrogen), even very thin, always reduces the dose/1
2/ The thing about using regolith for shielding is that you use it when you are on the surface of a planet, and there’s so much available and you do t carry it on a spaceship so you have no reason to use a thin amount. It entirely solves the problem!

And…
3/ …if you want shielding on your spaceship you aren’t going to use regolith. You’ll use water, food, rocket propellant — mass you need to carry anyhow — and low-density material designed for shielding with lots of hydrogen. And you can travel *faster* to reduce the dose, too.
Read 12 tweets
Feb 25
I could write a 50 page paper answering this :)

A few points in outline form only:

1) The rocket exhaust is expanding into vacuum, so viscosity breaks down, so the gas does not obey the Navier-Stokes equation, which is the basis of CFD (computational fluid dynamics) models. /1
2/ When I was at NASA, one of the things I was doing was writing solicitations to industry to write physics-based code to do CFD without Navier-Stokes. There are many ways to treat the fundamental physics (the Boltzmann Transport Equation) and they all work for different…
3/…approximations, but it is really hard to write a code that will handle the full range of conditions from dense gas inside the rocket nozzle all the way to rarefied gas on the Moon far from the rocket.

2) We don’t understand turbulence when the gas becomes rarefied.
Read 22 tweets
Feb 24
About how the lunar environment makes everything tippier…

1) I’m sure the CLPS contractors know this and designed for it. My point is that the Moon does this to your hardware, so when things go wrong (as they do) then tipping happens more often than on Earth. /1
2/

2) There are different ways you can tip. For static stability, gravity makes no difference. You fall when you are so tilted that the center of gravity (cg) is outside of your footpad. I don’t know where the Nova-C has its cg, but crudely it could handle ~54 degrees tilt. Image
3/

3) But for dynamic stability, gravity does make a difference. Imagine your vehicle is accidentally moving sideways at touchdown with velocity v. The energy of that motion is (1/2)m v^2 where m is the vehicle’s mass. The vehicle will fall over if that energy exceeds…
Read 17 tweets
Feb 3
I finally submitted this paper to Icarus (planetary science journal). I split it into two papers: “Erosion rate of lunar soil under a landing rocket, part 1: identifying the rate-limiting physics” and “…part 2: benchmarking and predictions.” The breakthrough was in part 1.
1/N
2/ It took 8.5 months from the breakthrough while sitting at McDonalds until I got the paper done. 😭 I had to re-do it several times. 💀

I’m not keeping the info secret before publication, so I’ll go ahead and tell a little here.
3/ We tested jets of gas blowing soil in reduced gravity about 13 years ago. I did about 450 parabolas of lunar, Martian, and zero g, plus 2-g pullouts between parabolas where we did additional experiments. So we got 4 gravity levels.
Image
Image
Read 35 tweets

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