Saturday, February 20, 2016

What About Transport of Utilities Between Space Colonies?

Much of what I write here is an extension of the idea that our conventional vision of orbital space colonies involves an impractically small population size. This is natural to think about in terms of movement of people, but it’s even more painful if you compare it to how we provide mundane utilities to cities on Earth today.

What Utilities Are Involved?

Space colonies may have somewhat different needs than Earth cities, but we're all human, after all. Provision of utilities is all very scale-dependent. In my reference size of an artificial gravity tube, there are about 20,000 people. Compare to a city on Earth, what utilities are distributed on a scale larger than this? The answer is “just about everything”.

Things that must be distributed between tubes:
  • Electricity
  • Communication
  • Goods
  • Industrial Fluids
  • People
  • Water / Sewage
I’m using a catch-all of “industrial fluids” to denote anything that is remotely similar to the role that oil plays in the world today. We transport oil through pipelines because there is SO MUCH of it to move that we need good efficiency. A space colony wouldn’t use hydrocarbons in the same way, but they may use fuels like Hydrogen. All these things would necessarily need to be managed among several tubes at once, and possibly throughout the entire gravity balloon. But I do want to make mention of at least one thing I can imagine for which the multi-tube assumption is not (can not be) true for.

Things produced and consumed inside a single tube:
  • Light
The psychological impact of light can’t be discounted, but when we’re talking about a colony that has a wall over 10 km thick, then it’s pretty impractical to pipe natural sunlight through this barrier, and then pipe it through complicated snaking channels into a cluttered and rotating tube.

But Physically, Where Will These Go?

Artificial gravity tubes have an access limitation around the outside due to the flow dividers to reduce drag. That means that all utilities, like people, need to go in through the tube openings on either side. For most of these utilities, they will need to access some connection points near the axial line, and then be distributed to the inner surface through “vertical” pipes or elevators (elevators are in the case of batch processes).

The challenge is that you want to avoid moving things in batch processes as much as you possibly can. Batch processes are tremendously economic. A bath process going through an airlock would be much worse - thus, the entire motivation for a gravity balloon.

How Can They be Moved?

Electricity probably has the simplest answer, and is a total cop-out. We use slip rings all the time for electric machinery, some of which are the largest units on the grid, supplying more than the population of one of these tubes. The power constraint itself should not be a problem, but there will likely be a voltage constraint. Several kV shouldn’t be a problem, and this might constrain the transformer architecture a little bit. Let me elaborate a little more specifically. Imagine that there are 2 levels of transformers to get electricity from the zero-gravity high-voltage electric transmission system to someone’s home on the inside of the tube. You will need one transformer to step it down from, say, 750 kV outside the tube to, say 10 kV, going into the tube. After that, you’ll need another transformer to take it down to 100-200 V for residential use, as needed. This is because the slip-rings connect the tube electric distribution system to the outside by brushes that slide along a conductor while the tube spins. These brushes wear out, and they will wear out faster at higher voltages. There is a practical limit, and also other hazards due to the larger footprint of the slip rings.

Communication also has an easy cop-out, which is wireless technology. Again, this might be a slight headache for the experts (in this case, networking experts) who build the system. Alternatively, you could (again) use slip rings in this case as well. Even better, it might be possible to create a coupling for a fiber-optic cable that allows both ends to rotate relative to each other. This wouldn’t even necessarily have to connect near the tube’s axial line (I guess the same could be said for electricity, but it’s a harder sell).

People - I’ve mentioned moving people in and out in other posts. In short, you will need elevator transport to and from the axial line, although I am partial to the idea of entering the tube through a literal slide.

Goods - again, there’s no other choice but to move shipping containers through the axial line and lower/raise through elevators. Combined with people, the staging areas for these are sure to occupy the majority of the bottleneck of the tube-end openings.

Air - I have not mentioned among the others, because it must be handed in the same distribution system that temperature control operates in. It’s not a “visible” distribution system, although barriers to control the movement of air will be a major source of clutter in the tubes.

Industrial fluids - this is where the problem gets hard. If you did need to transport oil or natural gas, you would likely have to do it in a batch process along with the rest of the goods transported. There is still the possibility for a local distribution system on the inner surface of the tube with storage just for that 20,000 person (or however many) population. A rotating joint for this type of stuff is not easy.

Water / Sewage - You would think that the same principle might apply here - that there’s no other option but to ship in water trucks through the axial line and have them disembark, unload, and exit alongside sewage trucks taking the used water back out to shared process facilities among many tubes. But I think this is where it gets interesting.

Water is different than something like natural gas because contact with ambient air isn’t necessarily bad, and because it has a definite phase difference. This allows for a different kind of coupling… a weird one that I can’t imagine would ever come up except for in situations like the tubes in a gravity balloon. In short, I think you would use something like a toroidal water-garden at a partial gravity level. There, you can allow a glorified straw to add water to the partially-filled torus, or to suck up sewage.

At this point, it’s getting weird, and I like that. But the conversation about water distribution is going to be more involved. This is something I hope to cover in my next post, and I think it touches on some very cool and very novel concepts.

Tuesday, February 16, 2016

But Would the Nested Flow Dividers Really Work?

People coming from different backgrounds will have trouble with different parts of the gravity balloon concept for space colonies. However, the one criticism I am most excited to receive is that the flow dividers might not work. This came up on a reddit thread sharing this blog. I am overjoyed to hear this criticism - because it means that the critic has understood all the big details. They are up to speed, and that means they are ready for the meat of the conversation.
Now, having something spin in an atmosphere presents issues of its own, and the author proposes a complicated scheme involving nested shells to avoid turbulence. This feels like the sketchiest part of it to me - there's a lot of handwaving involved.
For too long, I have neglected to argue the core mechanical details on this blog. I can't give it the full treatment it deserves in limited time, but I'll break out the big guns (even if that only means labeling them).

Why the Balloon is Not Disputed

Other scientists and engineers have already covered the basic physical mechanism of a gravity balloon with no rotating structures inside. In every case, educated people who looked at the problem said "yeah, of course that would work". The core prediction comes from Newtonian gravity.

Take a moment to appreciate this fact: A gravity balloon construction has never existed. Even if someone tried to replicated it within a present-day space station, the other (mostly molecular) forces would dwarf self-gravitation. It is a purely hypothetical construction. Yet we are all agreed (all of the informed, for whatever it matters) with 100% certainty that it would work.

Conservative Approach to Flow Dividers

What is the "sketchy" part of the flow dividers? Like any engineering, the concept originates directly from the equations, given specific assumptions.
  • Equations - Parallel plate turbulent flow (or laminar, if needed)
  • Assumptions - The geometry and movement of the flow dividers

You probably need a fluids expert to comment on this. One problem might be that those equations are not exact... but this is unconvincing. Turbulent flow models don't run the risk of dramatically underestimating the drag. The transition point from laminar to turbulent is also highly uncertain. That would chip away at the laminar flow designs I have entertained before.

Also, there's more to flow than the global sheer forces. We have eddy currents. Those can form resonant patterns of certain kinds, you could posit that those might be destructive. But that claim is just plain wrong - because the exact problem has been studied before. It's called Taylor-Couette flow. For the most part, this leaves the flow circling in cylinders between the sheets. No, I don't have the exact flow description for the (very turbulent, very big) geometry described here, but there's nothing spectacular about the flow regime.

Geometry is the most challenging part of this all. The flow solution is all well-and-good, but it assumes that the sheets are in certain places. This requires them to be held there. That could be difficult, maybe even impossible. That might demand large steel scaffolding holding the flow dividers in place, along with mechanical joints and wheels to maintain separation between the nested sheets. This could become quite expensive. I'm not even willing to concede that this scenario makes it totally nonviable in all foreseeable circumstances.

Just take a moment to accept, however, that demanding assumptions of large structural supports (to resist air currents) is the most conservative academically honest position you could take. The flow regimes have already been in literature. All I'm asking is to apply them to a fictional geometry.

Very Liberal Approaches

The sell gets difficult when we start attempting to strip down those supports for maintaining the geometry. As I've argued, you can try using flimsy sheets. Perhaps you apply some positive pressure to them so that they hold a pseudo-rigid cylinder shape. But maybe not. We can just handwave these complications away.

In fact, there are two components to maintaining the geometry.
  1. Keeping the flow dividers from colliding or jostling
  2. Keeping the flow dividers shape in tact
Violating #2 might also imply a violation of #1, but I'm not worried as much about #2. Balancing the pressure in each layer will likely maintain shape as a side effect.

I've received one interesting response that seems to argue that the sheets may not even be necessary because the transition to laminar flow isn't clearly defined and may not necessarily exist if certain precautions are made (what exactly, I don't know). That sort of position is too liberal for me.

You might even build flow dividers with massive holes in them. Flow dividers which are more of a suggestion for the flow than a solid rule might be entirely sufficient. As for myself, I pull back a little bit from that vision. There is a lot of energy in the system, and the movement makes it difficult to identify a clear lines to the isobars in a mostly open system. If this were simply cylindrical geometry, I would be more inclined to the idea, but the end tapers wreck havoc on the flow complexity. I'm mainly speaking from intuition here, and I think that partially-open flow dividers are in the engineering battleground.

Mechanical Stability (the meat of the discussion)

There is something called the "wedge effect", but it might not be called exactly this depending on the source. Lots of large machinery levitate a rotor on a fluid. Some of that machinery rotates at tremendously high speeds. Essentially, the combination of the rotation in conjunction with the

You can find plenty of literature on the subject. Go look at chapters 2 and 3 of this book for some basic theory. This is what makes me largely an optimist, because it argues for a mostly passive answer to component #1. Additionally, it's an answer that is quite hopeful to aid in the concerns of component #2. Oscillations aren't really going to lead up to a Bernoulli effect like a naive reading might seem to suggest. The forces in involved have most to do with friction and a roughly static pressure profile.... if its fully laminar. As we get into turbulent territory, there is some wiggle room for pessimists, but it's more unknown than anything else.

Well, I just wanted to get that out there. Consider the surface to be scratched.