Sci-fi Mega-Engineering

So this is a thread about large scale civil engineering like space colonies and orbital engineering rather than those two other threads about military hardware and tactics.

So my first question: I'm trying to design a sterling engine with kilometer scale pistons to work on a tidally locked planet like Mercury. What could possibly go wrong?

1. Mercury isn't tidally locked in the traditional sense, it has a 3:2 resonance (rotating three times for every two orbits).
2. Why such a big engine?

The idea is to generate power for industrial use. Solar power is all well and good but solar panels wear out as part of their function. Thermocouples are better but between the high temperature demands and the low efficiency I discarded it.

Sterling engines have a remarkable efficiency and as a purely mechanical system your only limit is material science. The only hold back is the low specific power which I hope to overcome via scaling.

That and I like the idea of a flywheel that's as big as a city block.

Here's a thing, solar-thermal doesn't run out in the same way photo-voltaics do, and the modern parabolic dish collectors use Stirling Engines to drive their dynamos. This has the additional advantage of being able to maintain the things easier because if you need to repair it you can just rotate the dish away from the source and pull the power unit off.

edited 30th May '17 3:37:46 PM by MattII

So use a massive array of mirrors for the heating element. Gotcha. What about cooling?

Radiator fins on the other side of the engine.

Bel; Call me crazy but aren't Stirling engines a variety of external combustion engine?

The large reflector dish sounds a lot more practical and easier to handle. So what would you power with this and how would you transfer the power? Beamed energy supply? Space Elevator style cabling?

@Matt II That would work on earth but if we're working near the sun there's a very real chance of not having anywhere to put the heat.

I suppose we could daisy chain sterling engines together, hot end to cold end but that feels like delaying the inevitable.

@Tuefel https://en.wikipedia.org/wiki/Stirling_engine

It's actually a variation on the early steam engine but you just need heat to make it work. As for what to do with the power? Probably have the industrial sector built right next to it.

edited 30th May '17 7:08:28 PM by MattII

I can only imagine the catastrophic results of a fluid containment failure with a kilometer sized piston. More a weapon than an engine.

OK, I've been trying to design a viable STL interstellar ship that serves as the permanent habitat of it's crew (and their descendants), with travel time in centuries. These ships are not isolated, and have multiple opportunities to access resupply or repair/maintenance services. I am using near-future plausible tech. To make the time-line work, they need to achieve 5% of c.

The engine: Any objections to fusion (presuming someone can make it work)? The fuel is easily obtainable en-route, in the form of water ice. Or would some variation of solar sails (perhaps the laser-driven variety) be preferable (there is someone at the destination who can decelerate them)? Remember, they have the better part of a century to accelerate/decelerate, but for cultural reasons they want to be as independent of Earth/Sol as possible.

The habitat: Any problem with a rotating propeller design, surrounding a long, spinal structure that has the engine, supplies and primary sensors attached? (a "propeller" design utilizes independent modules arranged in a rotating circle around the central hub. Imagine multiple pairs of habitats connected by cables all rotating around a common axis). This saves weight, but limits interior space and requires cables capable of sustaining considerable weight and rotational tension. I have no idea if there is any appropriate material.

Food: They don't have to be self-sustaining eco-systems, since they aren't isolated from resupply, as I said, but the resupply has to come from deep space stations, so some form of artificial agriculture is going to be required. Most of these ships will try to cultivate a garden of some kind, and perhaps a small collection of domesticated animals (chickens?) to offset the need for resupply, so the design of the ships should include that capability.

The crew: About 100-200 people in most cases. Shuttles exist to exchange personnel, but they only arrive once every couple of years per ship, so they have to be largely self-sufficient in terms of labor, but not genetically.

These ships are spaced about a month of travel time apart, each traveling at the same speed along the same route to the same destination, and again on the return trip (thus they are constantly passing ships going the other way), so imagine a light-years long 2-way conveyor belt like arrangement, and you aren't far off. There are hundreds of ship on these deep space "lines", and tens of thousands of people on these ships. They are their own nation in space, and the assumption is that most of them will remain on board for the return voyage (though not necessarily on the same ship). Any "colonists" they drop off at the destination system are extra.

Economics: The purpose is to exchange technological items with an alien race. The aliens want to build up their manufacturing capacity at the destination star system, and they are paying humanity to help with this. The paying cargo consists of small, high value items that can be reproduced at the destination, not bulk materials.

Thoughts, ideas, questions? I have a couple of short stories built on this idea so far.

edited 31st May '17 8:00:42 PM by DeMarquis

I'm not sure what you mean by 'the fuel is obtainable en-route in the form of water ice'. Collecting anything from your environment at 5% of lightspeed is not so easy. Are we talking about some kind of ramscoop here? It's not entirely clear why 'collecting fuel en-route' is even important; if the engine is efficient enough, you might as well just take all your fuel right from the start.

Solar sails can't really reach relativistic speeds, the light from a star just isn't intense enough. Laser sails are feasible as long as you have lasers at the destination to slow down with, but for such a large vehicle the laser battery itself would need to be pretty big. Theoretically you could also use a hybrid system, accelerating with a laser sail and then using some kind of nuclear drive to decelerate (or vice versa, but the advantage of the hybrid system is that you don't need a laser battery at the destination).

The rotating modules on cables sound fine, as long as they don't have to be super long and the ship's acceleration/deceleration is reasonably gentle. For a small ship, even steel cables would work. Otherwise, go with carbon nanotubes.

Regarding resupply and crew exchange: Frankly, it doesn't seem worth it. The shuttles bringing the new supplies/crew to the ship would need to accelerate all the way to the ship's own speed, and then decelerate afterwards. This is pretty expensive in terms of energy requirements. The more stuff you just carry the whole way (or at least until the point where you start decelerating), the more stuff you're only accelerating/decelerating once, which is generally cheaper than doing it multiple times, especially since your main energy sources are likely to be in star systems and not deep space.

Regarding chickens: Although they are more energy-efficient than cows/pigs, any animal food is always going to be less efficient than plants. If the crew absolutely insists on having meat available, growing it in a vat would use less energy.

The other thing you really have to think about when it comes to relativistic vehicles is protection from impacts. It's not common to run into anything out in space, but at such a high speed even one little pebble can really mess things up. The most weight-efficient approach is probably to have sensors and lasers on the front of the ship, so that you can see debris a few thousand kilometers ahead and shoot it out of the way. At 5% of lightspeed this is probably doable, but it does become more difficult at higher speeds.

Habitat: Needs a layer of water a couple of meters thick around the habitation areas, to protect against radiation. And once you've got that, you might as well have an enclosed O'Neill-style cylinder to make the most of the space between the outer hull and the inner foundation. (Spare gas storage comes to mind.)

Food: Crickets, modified for the rigors of space and industrial production ('natural' crickets won't cut it; even on Earth they're not a good replacement so much as a supplement). Less work for the CO 2 scrubbers and a means of getting some further use out of plant material.

The crew: I'm assuming that fuel supply depot for the ship involves orbiting a single point rather than docking to it directly, while shuttles ferry out goods. This being the case, I would have the ship depart Earth with crew for the ship and relief crew for the supply depots, as otherwise you'd have depot crew stuck waiting for a dedicated relief ship. Why not kill two birds with one stone?

Economics: I'd plan on a 10-year mission, including round-trip travel time. Anything that can be created can be delivered via drone (why send a watchmaker, food, water, and air - when you can just send the watches?), and the difference in cost kinda kills the point of a manned starship. Instead, I would trade workers - one alien for a human, for a set number of well-compensated years - as a labor exchange program.

Keep in mind, we are a species that eat capsaicin for funsies and then use it for pain relief, all the while shrugging off three or four rhinovirus infections a year - and some of us use the first as a remedy for the second. Our hands are dexterous and sensate in ways other species on our own planet can't match, and we mass-produce culture by doing... well, not much more than facilitating comfortable waking and sleeping.

On the flip side, our fine sensitivity comes at the cost of having relatively low tolerance among Earth species for physical trauma (pick a fight with an animal of equal weight, if you don't believe me), and our predilection for culture is coupled to a tendency to break off into violently opposing tribes far too easily. Clearly, there's space for a being who can do what we have some trouble doing.

We might do something like trade our best watchmakers for their best deep-sea miners, or our best football coaches for their best diplomats (the ones willing to travel, anyway, but who are nonetheless superior to the locals in their specialties).

Thank you for the input!

I see that I didn't explain enough. The water ice would be collected at the beginning and end of the journey, while the ship is still within a system's oort cloud, there is comparatively little in the remote inter-stellar spaces. So they would not have reached maximum speed yet. Still, that counts as a disadvantage of fusion relative to a sail design. I'll keep that in mind.

There could very easily be a laser battery at both ends, as it was the alien's who invented the thing (if that's what I end up using). The one at the destination star is probably larger and better than the one we have here. Maybe fusion within the Sol system and laser deceleration at the other end?

I visualize the hub to habitat radius being about 100 meters or so, with three rotations a minute. I am not nearly enough of a mathematician to calculate the rotational momentum, but that seems modest enough for modern materials. Scott Manley mentioned a type of "granite fiber" that might work as a cabling material. I need to find out what the engineering parameters for this sort of thing are.

Resupply and crew exchange- the shuttles are coming from other ships in the same line, so they only have to accelerate/decelerate enough to catch up. There are deep space stations that are stationary with respect to the ships, and those will be expensive to exchange materials and personnel with, but I think you underestimate the natural human desire to interact with new people and environments. People will spend a lot to access new opportunities. In my story, a young girl saves up enough credit to buy passage on a space-station bound shuttle, but the relative velocities are such that she knows she can never return (her ship of origin isn't stopping there, it's doing a flyby). It's quite poignant.

So it's doable, but not easy or cheap.

Modified insects- I love it! Again, you may be underestimating the human need for something that resembles a dynamic ecosystem. People love animals, and dietary variety, and I think they are likely to take along some small critters if the can.

Collision avoidance- of course, I forgot about that. I'm wondering if a whipple shield makes sense.

They do need some layer of protection against cosmic radiation, but I dont understand the reference to "make the most of the space between the outer hull and the inner foundation." Could you explain that? Individual modules are going to require a heck of a lot less mass to protect themselves than a full wheel design. One of the reasons, in fact, that I chose it.

People live permanently at the supply depots- they are like little cities, with populations in the low thousands. People are born and live their entire lives there. Otherwise they are just big habs, spinning in space.

As for the economics- these missions last multiple generations. At .05c, it takes about 200 years to get to Epsilon Eridani. Maybe I didn't explain the scale of this story universe very well. The people on these ships aren't really crew so much as residents. Most of them are born and die on a ship (though not always the same one). They aren't employees of a space agency, they are an independent community in deep space.

Psychologically, this means that they are not so much striving to reach the destination, as attempting to keep their ship inhabitable while it circles endlessly between two star systems. Dropping off cargo pays for the supplies they need, but it really isn't the purpose of their existence (from their point of view. How the folks back on Earth see it is a different matter).

Now, your point about trading workers in a human-alien labor exchange program is an excellent one, I shall immediately adopt that. So what they want is not so much devices as people who can independently invent and design devices. Makes sense. So the primary "cargo" becomes human skill sets. And they would be sending the equivalent back. Hmm, got to think about the implications of that.

edited 2nd Jun '17 7:35:23 AM by DeMarquis

Speaking of mega structures and massive engineering what about arcology or arcology like structures?

For quick and dirty refrence.

Some currently known sci-fi examples are the "Hives" in Warhammer 40k and similar structures in Shadow Run, also by Larry Niven and Jerry Pournelle, "Oath of Fealty" has an arcology structure.

The real world example is the experimental and still being built Arcosanti and a partial example is the Begich Towers in Alaska.

The big catch most have pointed out is that regardless of general efficiency any arcology building will still need outside goods to fully support their internal markets which has led many to also point out that they could never be fully self sufficient and instead would likely not be a singular building but a collection of buildings that support the population of the arcology. Basically people mostly live in the arcology but use some sort of self maintained commuter system to work or use technology to do remote operations as much as possible. The idea being you rarely need to leave the arcology.

Some of the other problems are already above in social tendencies and the generally tribal nature of humanity.

I am of the school you really need external structures that feed material to the arcology but can utilize some of the same concepts to make them as efficient and eco and economical as possible.

What about the rest of you? What do you think?

The main issue is power - ensuring you have enough of it to actually do the things you need is job one with an arcology. (Which is really stating the obvious.) Any means of generating it will probably need to be external to the arcology, for safety and size reasons as much as plain logistics. The thought experiment isn't worth considering without a source of power, after all.

Overall, Domed Hometown is how I think about it, as that gives you a certain sense of escapism to work with. (It's hard to pull off convincingly in live-action, which is why, I think, you largely see it in literature and animated stuff. Raises too many questions, for one.) And there's a couple of ways to go about it, I think.

Option A is to give each dome a certain mix of sustaining units (residential, agriculture, recycling, etc.) to improve redundancy in a dome cluster. This is a recipe for tribalism, naturally, and also reduces efficiencies of scale; at the same time, this forces the group to either work together or die together. Better for smaller populations, I think.

Option B is to dedicate each dome to a single purpose, within reason; agriculture gets its own domes, there's a recycling dome, but each habitation dome in the cluster is a mix of residential, commercial, administrative, etc. with a special focus on one type. Kills redundancy, but improves efficiency through scale and forces inhabitants to move between areas to keep things running properly.

And I have to say, I personally like the Paradigm City approach, which shows the artificial sunlight units on the exteriors of the domes that would need them, like the agricultural ones.

I had forgot about the Big O Arcology structures. Which is a shame as that show was a favorite years back.

Power is always going to be a big issue for any endeavor and better yet where exactly to locate it. Too far and you lose power over enough transmission distance, too close any power plant(s) capable of serious output having an accident can possibly damage your hab areas. Perhaps smaller power generation where you can get away with it and save the big power projects for sectors that have to have it like industrial and agriculture.

For real life examples, dont forget Biosphere II.

What about the City of Blame? The Dyson Sphere that engulfs the Solar System?

Should be physically impossible for the material costs alone.

Then again, we've got Gravity Guns so toss out the physics textbook.

Well there is the fact they deconstructed several planets for raw materials.

At the point where you are encapsulating an entire solar system you are beyond Mega Engineering. That is the kind of civ that could possibly just build their own damn planets for fun.

Well they could, if the City wasn't too occupied continually destroying and rebuilding itself, and killing off any "squatters" seen.

...Why would they care about squatters? With the sheer amount of space, resource access, and technology that should be a non-issue.

edited 10th Jun '17 5:32:54 AM by TuefelHundenIV

The original designers were picky about who got to live there but genetic drift has kicked in so everyone falls outside the parameters of the automatic security system and there's no one left with admin privileges to update the settings.

Fun fact: If you took the mass of the Earth and pounded it flat like aluminum foil, you could completely envelop the Sun in all directions.

That's the interesting thing about Tabby's Star. There's a theory that an alien civilization is building (or has built given the distance) at least a Dyson Swarm around it. It's the one theory that refuses to die despite numerous attempts at natural phenomena trying to explain it. (None yet succeeding.)

edited 10th Jun '17 5:16:05 PM by MajorTom