Previously, I did an agenda post. Many things I went on to complete, but others I abandoned because I took a different direction.
I'm going to do the same thing here. Why?
The pivot to cislunar space by NASA and SpaceX has happened. I believe they will accomplish their goal, and the revolution in space development will happen. Artificial gravity won't come into play anytime soon, but the SpaceX plan was a 1 million city on Mars. I would ask the question of what the equivalent accomplishment between Earth and the Moon would look like, and that involves artificial gravity. Because of that, I could see a scenario (even if not imminent) where these writings become relevant.
This scenario requires an approach where flow dividers in an integrated atmosphere shows an obvious advantage both at small scales and at large scales, in addition to demonstration of basic physical viability.
I started writing about the Bare Metal Sphere Habitat (BMSH) to talk about things that could be built in cislunar space. The overlap of thermal & shielding functions are really the basic things to work out there, and I'm not entirely finished either. So with this, I will break things into sections with brief abstracts of unwritten work. Finished posts will have pictures, but this will not.
Cislunar Habitat Design Reference
The BMSH is a thing to build out technical specifics of what it would look like to use the friction-buffer idea, but without the self-gravitating walls. This imagines rotating tubes inside of a larger pressurized vessel that is mostly stationary (probably sun-tracking), and built out of materials from Lunar sources. This might seem self-obvious, but thermal management is still a thing, as well as other considerations.
A Complete Description of Internal Shielding
The last post on this gave a throw-away geometry for the porous internal shielding. However, I want to get something that:
- a) Isn't just intended to be awful but might be a competitive design and
- b) Have specific numbers and materials for the shielding, pinning down the specific penalty for making it porous
For (b) I already have some numbers. It's 2.1x the mass usage. Realistically, I still believe it will come out to less when using full, proper, radiation shielding calculations. I did my calculations with really basic attenuation mechanics, which are still non-trivial.
With porous internal shielding, air can pass through it. I envision this working with the "apple core" flow pattern. A large flow goes through the center of the sphere, and turns around to return flow in the other direction along the edges (behind the shielding).
Umbrella Protection and Thermal Control of Space Habitats
A bare sphere still seems very unprotected. So I need to take time to write a post that covers due diligence for operating in cislunar space. This needs to answer the question of radiation embrittlement (of Aluminum probably) and impact risk. The embrittlement question is just a matter of running the numbers. But the impact risk issue gets weird in the way I like.
For such a large station, it would be hard to do maneuvering to avoid debris in space. Even if you did so, your threshold for moving the station would have to be awful high. So you would have to accept impact, and in our case that would hit the structure directly (same could be said for other designs). Because of that, I had a wacky image in my head of a catchers glove held by an arm that could move in front of debris. However, as I thought more about this wacky idea, it quickly becomes apparent that accurately predicting the impact time is much easier (basically already solved) than the impact location. So we could have a time-based control to go into temporary "turtle mode" where the station pulls its head into its shell. What would this look like?
If you look at what debris shielding looks like, its goal is spallation and tends to be multiple layers separated by distance. This works well with our approach, because we can use plenty of space for "turtle mode" because it's not frequently deployed. So this system would extend to potentially a large radius, multiple sheets, and physical separation between those sheets. It is also very interesting to note the similarity between missile shielding and thermal radiation blockage, which is layers of multiple sheets (in vacuum) with separation between them. We do have a need, already, for thermal control. So it makes sense to have a single structure that solves both of these problems. This would likely take the appearance of multiple umbrellas around the sphere, partially deployed for thermal control, and sometimes fully expanding for missile protection (turtle mode). We already have some spacecraft references that so this type of folding and unfolding, although not on the scale or speed involved. However, the idea is simple and effective, and mechanical & engineering needs modest.
To get full coverage, we might divide up the sphere into a platonic solid, where each face would correspond to an umbrella. The umbrellas might overlap with each other at slightly different radii, or their shapes might interlock fairly exactly at the same radius. Even when collapsed, we would have the poles of the folded umbrellas extending outward like a spiky space station.
This is a tremendously fun design, which I think would be generally useful. There is still a specific articulable motivation for the BMSH, since it will use mongrel Alumnium from the moon, and we are worried about tiny impacts tearing up its surface, causing large cracks, potentially compromising. The good thermal conductivity casts some doubt on how good spot-welding would be for fixing impacts. The umbrella won't fully stop impacts, but will dramatically decrease the depth disturbed which could easily get us into a safe-forever territory.
Bare Sphere Habitat with External Shielding
The umbrella seems to be a shoe-in for the reference design. However, the combined structural & heat rejection function are highly suspect. Using the wall as your radiator requires exposing the Aluminum directly to space, and this might be unwise. It also strains our ability to remove heat at large sizes at which point we'll need a new method anyway.
So I should describe the obvious solution of putting heat pipes into the wall. However, the specifics of what the heat pipe connects (thermally) is an important question. We want a leak to not be a big deal, but also want to use the heat pipe physics to spread out heat as much as possible on the radiator surface. Because of this, I envision the heat pipes expanding into an external radiator structure, but possibly not going into the habitat itself. If it just comes close enough to the internal wall of the pressure vessel, simple radiator fins can do the rest of the job, and no internal leak becomes plausible.
Philosophically, we need to salvage a principle from the overall BMSH approach, which is to reduce the distance the heat travels. I have been running some numbers, and long-distance heat transfer quickly gets insane.
Vacuum Long-Distance Heat Transfer
After thinking and writing about this topic for years, I've become convinced that long-distance heat transfer would have to be done in vacuum. That introduces all kinds of new problems for the heat transfer from vacuum to atmosphere, and becomes a bit of a nightmare with extremely non-obvious solutions. No matter though, for basically all sizes of BMSH we are interested in, heat pipes work well. That is, we just have to keep the pipes limited to traversing from one side of the wall to the other, and no more. This requires a very large number of heat pipes, and they would have to be mass-manufactured.
To give spoilers, I envision a space "train" with blocks that look like the monolith from 2001 Space Odyssey. As these go through the radiating region, they would be spread out. However, as they approach the habitat to dump their coldness, they would need to fold up multiples into a single unit and then physically touch the habitat to conduct heat. The need for folding is a consequence of scale. It also appears to be operationally awful and something to be avoided if at all possible. I don't think you can avoid it. At some scale, you would have to use this.
Reference Design
The discussion up to this point is about the design space, not just one specific design. Provided I can ever finish talking out the details of how such a habitat would be built in the abstract, we need to move onto pinning down certain scales and describing what operation would be like in practice. So, moving on from physics to more practical concerns... ultimately getting a named reference that people care about and will reference. One day.
Habitat Molting
I have some images sitting around I want to share of how I envision this approach to going from one size to a bigger size. In this conception, you start with a sphere, and build a bigger sphere around it. For multiple reasons, I think this is a bad idea, but I still need to put some pictures on my blog to have something to point to.
Joining of habitats
A better idea that habitat molting is to join spheres together. This means that there will be a circular opening on the surface of the sphere which is docked to another sphere. With 2 equal sizes, you have an internal volume in a barbell like space.
My original idea is that you would mass produce identical spheres with docking ports, eventually creating a potentially regular 3D pattern. However, this is god-awful for heat removal. Doing this would require transferring heat through vacuum. However, getting the heat pipeline to move heat out of the habitat in any sane way was worse than I thought. The 3D lattice of identical spheres is an idea I thought on for a long time, and eventually rejected.
So what's the alternative? Connecting spheres of different sizes. This means that volume additions are slightly less incremental, and more exponential. Every time you make a new sphere, maybe that new sphere has double the volume of the last sphere. Continue this until you block heat transfer from the prior spheres significantly... I'm thinking 4 spheres. So you have spheres at radii of 1km, 1.3km, 1.5km, and 2km. At this point, the 1km sphere would only be 7% of your total volume. So you would tear it down. Throw it away and recycle the materials of the smaller sphere. This would open up space to add a new sphere of 2.5km. And so on, this pattern can be continued forever to continue growing space. The rest of the details, like the heat transfer, the umbrellas, and everything will remain. There will be some de-construction happening as needed. However, new additions are always significantly larger than the parts being tore down, so this adapts well to exponential growth. It answers the big questions about the lifecycle of the habitat, while not pinning people down to an original decision about the number of inhabitants. This is fundamentally a very growth-oriented approach.
More broadly, as long as you can do permanent docking of spheres, the ability to do incremental growth is a massive argument in favor of the integrated rotation, enabled by flow dividers. This is even more-so a powerful argument in cislunar space.
Back to design of flow dividers
I'm happy to write all of that, and there's much more I want to share, but right now I need to keep my focus on the truly novel part of this blog which is the flow dividers. All above topics will be shelved for a while.
Consideration of Flow Divider Spacers
Referring back to the simulation results (and its associated theory) we don't have a good claim to think that the flow dividers will be naturally held in place, and we do have some reason to believe that fluid forces will want to either cause a wobble or break it apart. Even if we have an acceptable wobble (likely IMO) there can be resonance modes that break it apart.
Instead of accepting wobble or designing around resonance modes, we would be much more likely to add another physical structure to prevent them (very speculative all-around here). So I need to describe some options for that. This need to be a tour of just all possible geometries.
You could have simple scaffolding put between the sheets. If they touch the sheets, they bounce off (with friction, sure). The more contact, the more drag penalty you incur, so you minimize the area of contact. This might work well making limited parts of the flow divider sheets rigid. The minimum contact points would be simply 2. Maybe 1 but that seems dicey. 2 contact points would also help the axial thrust. You would place these at the end of the cylindrical region, right where the tapering starts. This might be all that you need - 2 hoops on each flow divider and scaffolding in the shape of hoops in between layers.... I will really need to draw this out sometime.
Next, we will get fancier. Instead of scaffolding that occasionally touches neighbor sheets at velocity (like 5-10 m/s) you could put the scaffolding in the middle of the channel (0 relative velocity to the fluid) and attach rotating structures to the scaffolding. This could be wheels which have an axis in the same direction as the whole tube, or they could be cylinders. At the extreme, you could have cylinders running the entire cylindrical geometry section. This would provide extremely good stability but it is unclear how much flow would be negatively impacted.
The next geometry would be to throw some spheres into it, and let them move around. The obvious geometry selection is that there sphere's diameter is the channel width (approximately), although this incurs some risk of bounce, and corresponding stress on the dividers. This seems wacky, but too early to rule things out.
Serious Discussion of Negative Pressure Designs
For probably the last year or two, I have been extremely bound to the idea that each flow divider much hold some pressure. Otherwise, you can't have a coherent boundary condition where positive pressure is maintained up until the connection point at the access tunnel.
However, at the point that we are adding spacer structures, this notion deserves to come under attack. If we have a network of cylinders or spheres floating around in-between the flow dividers, why not just get rid of the flow dividers? What are the consequences of that?
I can very easily tell you the elephant-in-the-room consequence. You can't taper in the same way. This dramatically changes the image of what we're building, but maybe not for the worse. You can have the poofy ballon type shape as it goes up the edges. There wouldn't be a nice 45 degree angle from the surface where people live up to the access point, but there might be a -45 degree angle in its place. Let me explain.
There will be a free-air condition from a microgravity point up to the outside of the tube's hull. Combined with the fact that the air is rotating, this means we will be at less pressure there than in the microgravity. This means that the "ends" of the flow divider region will be sucking. This can be accomplished by using rigid sheets for the ends, or by using a flexible sheet attached to a ring. This flexible sheet will bow inward.
Related, I've also accepted that what I call the "negative pressure" designs might actually be easier to test that the flow dividers themselves, and so they might be the first things to actually post results.
Experimental Designs
For real experiments, I don't have anything to show yet. I am gaining better clarity on the "bucket" scale of experiments, which I always envisioned as the widest variety of experiments. I have multilple sizes, and many flow divider configurations in mind. Towards the end of this, some end taper designs should be getting tested, which is far from where I am now.
As for the purpose of the experiment, I'm becoming increasingly focused on the near-term goal of showing some effect at all. That is, lower power input for the same speed, enabled by passive flow manipulation. Any result of this kind proves stability uncompromised by fluid forces exists. What it doesn't show is that the particular flow divider design will scale.
Like I was arguing in the negative pressure designs, the number of flow divider solutions might be more numerous than I had thought. I have been studying the Taylor-Couette flow charts more, and realize that the flow patterns are donut-like for the regime I care about but also extremely chaotic. I still believe that the viability of a divider structure is almost a direct function of Reynolds number. It's possible that one will work at the bucket scale but not at larger scale. Developing multiple viable solutions now will help speed up future work.
Thoughts on a Space Station Test
In my experimental design plan, I penciled in 2 versions of air experiments. I am leaning towards saying this is overkill. There is the possibility of a scenario where we go straight from some University-level work at the room scale (ceiling fan stuff) straight to experiments on a space station. I think this, in large part, because activity on space stations (and particularly commercial ones) will probably increase in frequency in the next few years.
This is extremely appealing because the materials you need to make it happen are dramatically less than what you need on Earth. To minimize work up there, you would have to combine the motor and speed/torque measurements, and the motor would be the heaviest part by far. Aside from the motor, everything else would be inflatable, taking up relatively little mass. Then the size you can deploy for the experiment would be a large fraction of the habitat module where they conduct the experiment. The more I think about it, this isn't actually that crazy.
