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Various reaction drives in M-Space and their fuel


Thot

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15 minutes ago, Thot said:

There would be a colony (we would BE a colony) because the whole universe would be teeming with their colonies, nd there would simply be no matter left not used by their civilization(s). Billions of years of time!

Do a little compuation on how long we would need to settle the galaxy with 0.1c ships and a propulation growth rate of 2%. That's 400 billion stars. Please, do it. It's insightful.

That's assuming Malthusian population growth, a theory that doesn't quite work for civilizations.

I am a regular consumer of Isaac Arthur's SF channel on youtube, which means I am aware of those calculations. All of these come with the assumption that civilization doesn't change much.

Here's a fun setting theory for you - Dark Matter is actually all those solar systems or local clusters that have retreated into pocket universes, linked by wormholes, with their residual gravity holding together all the remaining galaxy. We're actually inhabiting the wastelands and spaces left out by that wormhole network, all the undesirable places. (Or our system is too young to have been included in that massive exodus?)

For a different explanation of the Fermi paradox, read the Schlock Mercenary webcomic. Great Filters and frantic exodus fleets away from the populated places in the galaxy to avoid that filter.

True, both of these are rather fantastic explanations. But your assumptions run on "barbarians in space", something that might be untenable.

15 minutes ago, Thot said:

Details. We know how gravity works and that matter causes gravity. Quantum gravity would just be a more detailed model that is reconciled with other models for other areas of physics.

Our definition of matter may be flawed.

 

15 minutes ago, Thot said:

Not just a few millennia. Millions of years.

My point, really. Our window of observation is too short.

 

15 minutes ago, Thot said:

At the very least, to exploit the planets' resources. Break them up and consume them. But we'd have noticed if that had happened.

While you're at it, take the matter from the suns as well, and increase Hawking radiation on the event horizons of black holes, evaporating them. Such a locust civilization would be leaving a void behind. (Or something like Dark Matter.)

And I mentioned before that exploiting an aggregation disk around a newly forming star offers much easier access to all the raw material - collect the stuff before it clumps into planets or blows off all that valuable gas and dust. If you have the choice between exploiting a mother lode and exploiting the slag and waste of a previous extraction operation, you are likely to go for the mother lode, right?

Stuff bound in planets are the waste heaps of the universe.

 

15 minutes ago, Thot said:

But that is what I am doing. I am extrapolating, and the result is: Sorry guys, no FTL, no interstellar flight, but a solar system teeming with life.

I'm with Isaac Arthur on this - if you manage to colonize the outer system, there is nothing to stop you from continuing into the Oort Cloud, and the interstellar space beyond until you reach the next star system. You can do so at ppm of C.

 

15 minutes ago, Thot said:

We do have evidence that no alien intelligent life has stripmined our planet.

Because that would be stupid. If you want heavy elements, go to mercury, or collect asteroids and meteorites. Way more bang for the energy put in.

We assume that Mercury originally came with a lithosphere. That isn't there any more. We have no idea whether all of that just evaporated or whether someone came and harvested.

If you talk about strip-mining our system over the course of 5 billion years, a superior civilization could have visited about 5 billion years ago to harvest the aggregation disk, and leave the rest as a waste pile.

Earth has nothing that you cannot get more easily elsewhere in our solar system, except for organisms which may hinder your extraction processes.

 

15 minutes ago, Thot said:

Sure, that is possible. I like to use that notion, too. Keep in ind, though, that we have found an awful lot of fossil fuels... which a prior civilization would likely have used.

Why bore deep wells and push the stuff up our considerable gravity well when you can just pump the liquid off Titan for a fraction of the effort? A visiting civilization would have come with space travel, and would have gone to Mercury for heavy metals, to various planetoids for regolith, to Jupiter or Saturn for water ice or methane, and to Titan for slightly more complex hydrocarbons. Take the oxygen from regolith (you'll want the metals the stuff was bound to, too) and react it with the methane, and even a first half 20th century chemist can produce all the basic organic chemicals you might ever wish to use from that (with a sprinkling of sulphur or nitrogen, e.g. taken from Venus atmosphere). Energy is there for the collecting - a fusion reaction just 8 light minutes from here.

There is plenty of coal about 3 km below the Cimbric peninsula - as much as there used to be in the Ruhr territory, for the same area. You don't go extracting that because there are much easier alternatives elsewhere. (And because we have a slight over-supply in re-mobilized carbon from sequestration traps that we need to deal with more urgently...)

On Titan, all you need is some tubes with liquid methane or ammonia to pump some heat into the sedimented hydrocarbons and a pump to load them on whichever method you want to use to lift the stuff off a sixth of the gravity well you have to overcome when you have an oil extraction running on our planet. For the heat, use some solar mirrors built from stuff from the rockier smaller moons nearby. Now, which spacefaring civilization in their right mind would go extracting minimal amounts of dino juice from our dirtball with that place as a ready alternative? For less complex hydrocarbons, drop a floating methane extraction rig into the atmosphere of any of our gas giants and fling the stuff out from there. Meanwhile, enjoy the songs of the giant insects at the sunsets every eight hours on a beach resort on Carboniferous Earth on your shifts off from lifting crust and core matter off Mercury, or go cloud-diving on a much cooler Venus, or skiing on a rather wet Mars.

 

15 minutes ago, Thot said:

Our planet doesn't offer anything different from there, but more of it. Growth is exponential and needs EVERYTHING. If you can get it.

Our planet offers less of everything you find concentrated in these other places, and way harder to get at. It makes for a nice resort if you can avoid the native organisms, but so do the planets 2 and 4 during the Carboniferous, in all likelihood without those pests.

 

My point is that if some galactic civilization came here for a mining operation in a distant past (when the sun was less glaring than today), hardly anything would have been extracted from Earth. Strip mining on Mercury may have happened without us ever knowing about it. Gas extraction in the outer planets, and hydrocarbon harvesting on Titan? We won't know about that until we start such operations ourselves. Traces of resorts on our dirtball? Lost in the background noise.

The galactic civilization would know this as a former extraction area, and without much activity remaining, may have written the place off as too much trouble to extract more. Their remaining civilization (after sending out waves of colonists elsewhere) would have crumbled in the Ordovician gamma ray extinction event, possibly evacuating some survivors in their space habitats. The system could be registered in the data of that galactic commonwealth as mostly mined out, interrupted by a cataclysm, with possible survivors leaving for a different place. Maybe the system is regarded as haunted - who says that civilizations need to be all rational? While Germanicus went and cleared up the mass graves at Kalkriese, according to Tacitus, the Romans didn't return to settle Germania Magna after the Varus battle. Not worth the effort.

Telling how it is excessive verbis

 

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Space Force?  Check.  Moon base?  Check.  Curvy hard vacuum suits with bubble helmets?  Check.  The future is here, people, and it is ... the 1950s!

At the moment I'm not so concerned about FTL.  First we gotta dash across the solar syatem and colonize Yuggoth before the Mi-Go do it.

Edited by seneschal
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14 hours ago, Joerg said:

Me, I want me a form of space opera setting that makes a couple of unobtainium assumptions in a sufficiently logical form, much like I accept similar assumptions for magic, deities, or even an improbable universe like Glorantha.

Same here. As long as the assumptions and explanations are sufficiently logical and the universe abides by them (and not changing them on a whim with technobabble), and as long as the whole package is internally consistent, it's Good Enough (tm) for me. I basically want space opera with aspirations of hard SF without being utterly slaved to hard science.

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1 hour ago, Lurking Grue said:

Same here. As long as the assumptions and explanations are sufficiently logical and the universe abides by them (and not changing them on a whim with technobabble), and as long as the whole package is internally consistent, it's Good Enough (tm) for me. I basically want space opera with aspirations of hard SF without being utterly slaved to hard science.

And that's a reasonable thing to play and tell stories in. It is great fun, after all!

But sometimes I want to make a proper prognosis, and the result is : We'll use chemical rockets for large-scale space faring, and that is likely the best solution, all things considered.

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4 hours ago, Joerg said:

That's assuming Malthusian population growth, a theory that doesn't quite work for civilizations.

A society (or multi-society community) that can do FTL will use all available resources eventually. Given the age of the unvierse, "eventually" would have happened already.

4 hours ago, Joerg said:

 

My point, really. Our window of observation is too short.

Our window of observation is the age of the universe. That's hardly short.

We don't see giant interstellar civilizations consuming planets. We don't see Dyson spheres or swarms. We don't see space battles. We don't observe visitors, and our planet was full of natural resources when we started mining it.

4 hours ago, Joerg said:

While you're at it, take the matter from the suns as well, and increase Hawking radiation on the event horizons of black holes, evaporating them. Such a locust civilization would be leaving a void behind. (Or something like Dark Matter.)

Exactly, and we are not observing this. So the only logical conclusion is: it is not possible. Because if it was possible, those civilizations who do it would outcompete those who don't.

4 hours ago, Joerg said:

Stuff bound in planets are the waste heaps of the universe.

When resources are used up everywhere, you start recycling waste. This isn't happening, despite the age of the universe.

4 hours ago, Joerg said:

I'm with Isaac Arthur on this - if you manage to colonize the outer system, there is nothing to stop you from continuing into the Oort Cloud, and the interstellar space beyond until you reach the next star system. You can do so at ppm of C.

You can never even get close to any speed worth  mentioning as a fraction of c.

4 hours ago, Joerg said:

Because that would be stupid. If you want heavy elements, go to mercury, or collect asteroids and meteorites. Way more bang for the energy put in.

Yet, mercury and the asteroids are still there.

4 hours ago, Joerg said:

We assume that Mercury originally came with a lithosphere. That isn't there any more. We have no idea whether all of that just evaporated or whether someone came and harvested.

We do know that. Such a group would have stripmined our asteroids and other planets, too.

4 hours ago, Joerg said:

If you talk about strip-mining our system over the course of 5 billion years, a superior civilization could have visited about 5 billion years ago to harvest the aggregation disk, and leave the rest as a waste pile.

That would be a waste (sorry for the pun).

4 hours ago, Joerg said:

Earth has nothing that you cannot get more easily elsewhere in our solar system, except for organisms which may hinder your extraction processes.

We already talked about this - not in quality, but in quantity. There is a lot of stuff on Earth, not to mention it is a free habitat.

4 hours ago, Joerg said:

Why bore deep wells and push the stuff up our considerable gravity well

But see, either our gravity well is "considerable", because technology does not allow to fly to orbit and beyond cheaply; in this case, interstellar flight is impossible. Or it is not considerable, in which case interstellar flight may be possible, but then we'd see civilizations doing it. Right in our backyard.

4 hours ago, Joerg said:

when you can just pump the liquid off Titan for a fraction of the effort?

Because someone already did that a million years ago. Or a billion.

4 hours ago, Joerg said:

My point is that if some galactic civilization came here for a mining operation in a distant past (when the sun was less glaring than today), hardly anything would have been extracted from Earth.

1. The other planets ae untouched and pristine, too.

2. When you make the journey, you take everything within reach, that's just efficiency. The whole inner system would be devoid of everything if interstellar flight was possible in this universe.

4 hours ago, Joerg said:

Strip mining on Mercury may have happened without us ever knowing about it.

(No, we'd have noticed that by now.) But why would they stop at Mercury?

4 hours ago, Joerg said:

While Germanicus went and cleared up the mass graves at Kalkriese, according to Tacitus, the Romans didn't return to settle Germania Magna after the Varus battle. Not worth the effort.

But given enough time, someone did. Wait another 20,000 years, and nothing will be left to use.

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9 hours ago, Thot said:

Details. We know how gravity works and that matter causes gravity. Quantum gravity would just be a more detailed model that is reconciled with other models for other areas of physics.

Once upon a time, planets as fixed objects in concentric spheres centered on the Earth was the valid, accurate model.

Then people noticed that it was a little bit off...  Not to worry, though, it's just a little bit.

Details.

They just needed to figure out the Motions of the Spheres.  It'll be figured out, eventually.  We just need a more detailed model.

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59 minutes ago, g33k said:

Once upon a time, planets as fixed objects in concentric spheres centered on the Earth was the valid, accurate model.

Nope. It was a fantasy, not a model by any standard.

59 minutes ago, g33k said:

Then people noticed that it was a little bit off...  Not to worry, though, it's just a little bit.

Actually, it was off by dimensions.

"We have made progress in the past" is a false analogy here, guys. Sure we can engineer a lot of stuff. We will eventually turn this solar system into a Dyson swarm, I have no doubt, though it may take longer than many here think. But FTL?

I am not saying FTL will not happen because I lack imagination. I am saying it because all we can OBSERVE in this universe points to the nonexistence of FTL or, in fact, interstellar travel.

 

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12 minutes ago, Thot said:

Nope.

<SNIP>

 I think we're just going to have to agree to disagree.

I find too many of your fundamental assumptions to be either invalid or unsubstantiated, and this venue probably isn't the space to work out all those details.

Also... I'm taking just a bit of relax time now, but too busy to engage in an extended and multifaceted debate.

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I think we've veered quite far from the original thread, which concerned drives in the M-Space games. As interesting as these discussions are around the physics of space flight, likelihood of First Contact and (now) the heliocentric model, perhaps continue such things in Alastor's Skull Inn?

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I'd like to see a playable M-space version of a solar sail drive.  It was a big part of Larry Niven's "The Mote In God's Eye" but I've never played an RPG that had ship design rules that would allow PCs to build and operate a spacecraft with one.

(How's that for getting back on track?)

And it is non-FTL.

Edited by seneschal
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I have thought a bit about the original question. So, if the "modules" in M-Space refer to mass (not volume), which would make sense, you could easily define fuel tank systems that have a certain delta V, depending on the share of fuel tank modules compared to the whole ship.

For a chemical rocket like SpaceX's new methane-oxygen drives, that would probably look a bit like this, assuming a 10-module ship (and easily scaleable from there)

1       381
2       807
3       1,289
4       1,847
5       2,506
6       3,312
7       4,352
8       5,818
9       8,324
9.9     16,648
9.99    24,972
9.999   33,295

 

The first number is the amount of modules reserved for fuel tanks in a 10-module vessel, the second, larger number is the delta v (the total amount of speed change the ship can carry out before refuelling) in m/s.

For reference: In order to be in an Earth orbit, you need at least 7,800 m/s of actual speed.                                                                                  

Edited by Thot
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Same numbers for an ion drive with an exhaust velocity of 30 km/s :

1       3,161
2       6,694
3       10,700
4       15,325
5       20,794
6       27,489
7       36,119
8       48,283
9       69,078
9.9     138,155
9.99    207,233
9.999   276,310

(Keep in mind, acceleration for this type of drive is way, way, way below 0.1 G, so no liftoff with this.)

Edited by Thot
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And for a fusion rocket, with an exhaust velocity of 100,000 m/s:

1       10,536
2       22,314
3       35,667
4       51,083
5       69,315
6       91,629
7       120,397
8       160,944
9       230,259
9.9     460,517
9.99    690,776
9.999   921,034

Note, however, that this would require a reactor in the terawatt range, for any meaningful acceleration numbers.

Edited by Thot
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M-Space Solar Sail Drive

Before we worry about game mechanics, let's talk about what a solar sail is, how it works, and how that might affect the lives of our player-characters.  A solar sail is a slower-than-light spacecraft drive powered by sunlight.  Light consists of tiny packages of energy that push very gently against whatever it touches.  You don't feel the push when you are enjoying a sunny day because the surface area of your body is relatively tiny.  A solar sail, on the other hand, is a massive thing designed to capture as much light as possible.  It is a bag-, parachute- or umbrella-shaped  construct of tough material, stretched out to be as thin as possible.  As such, it would have to be built and launched in the vacuum of space, couldn't enter a planet's gravity well, and would have a small payload compared to the acres of sail required to lug the crew and cargo along.  But the fuel is free and doesn't take up space inside the vehicle.  As with Earthbound watercraft, the Cutty Sark of space would accelerate slowly -- no fast getaways -- but since there is no friction in space it could rival the Millennium Falcon's sub-light velocities once it got going.

What about the crew?  The cabin would need a separate, independent power source to provide life support and operate equipment (diffuse sunlight just isn't going to do it), so there might be fuel tankage requirements there.  The crew would also need some sort of landing craft to reach a planet's surface.  They would steer the ship via sturdy guide lines that would change the sail's shape just like a sailboat.  Going outbound from the sun would be straightforward.  Returning to the inner planets of a solar system might be slower and more difficult.  I will have to research that.

Finally, Larry Niven suggested that such a vehicle's initial launch might be facilitated by firing massive Death Star type lasers into the sail to give it a good shove.  So, anybody want to go sailing with me ... in spaaaaaaaace?

https://en.m.wikipedia.org/wiki/Solar_sail

In Classic Traveller terms, our solar sailor is a big craft (100 metric tons plus) with no star drive and a maneuver drive that doesn't require fuel.  It does need a power plant and fuel sufficient to support the crew during journeys of 3 years or longer.  So it is even more of an Age of Sail in Space craft than those of that game, where a ship's boat can zip across a solar system at 6 gs.

 

 

Edited by seneschal
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My previous comments were based on my reading "The Mote in God's Eye" by Larry Niven.  However, after actually skimming the online article I had referenced, I realize some of my descriptions were off.  A rigid sail design is best, with steering and altitude control done electronically rather than physically.  The good news is that there are few moving parts to break or wear out, but you'd better have a dependable electrical system aboard and really good computers to constantly adjust and control the complex changes needed to keep the ship on course.  So far, square or kite or disc-shaped designs with the payload located in the center on one side have been successfully tested since 2010 by NASA, Japan's space agency, and others.

So our solar sailor might look something like a giant RuneQuest shield, kilometers across, with the crew cabin clinging to the backside like a mushroom cap's stem.  It couldn't, after all, tack back and forth like a sailboat to return to the sun because solar wind acts differently than air wind.  There are ways to compensate for that electronically but the star's massive gravity is the main force that would draw the ship back to the inner planets.  We might need some sort of secondary drive to get home after all.

Such a ship would be slower than I first thought but would be perfectly capable of reaching nearby stars ... eventually.  More later.

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As for the argument that they aren't out there because we haven't seen them, that's such utter lazy logic.  WE are "out there" and they haven't seen US.  Consider what the odds of intelligent life existing in any given solar system.  Even if we discovered FTL, we might not find other intelligent life out there because the odds are against it.  What if it's a billion to 1?  How long to explore a billion worlds?

Of course, the odds go up greatly for an encounter if both species have FTL.

There IS other intelligent life out there.  It's a mathematical certainty.

Edited by Pentallion
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For solar sails, the table here is also highly relevant:

https://en.wikipedia.org/wiki/Radiation_pressure#Pressures_of_absorption_and_reflection

At Earth's distance from the sun, the radiation pressure is about 9 Newton per square kilometer of solar sail surface. In other words, if you had a 100 ton ship with a solar sail 100 by 100 km, you'd be able to accelerate at roughly 1 m/s², assuming 100% efficiency of reflection.

So, you need an incredibly light material to make a solar sail worth doing, obviously.

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On 10/19/2019 at 7:11 PM, seneschal said:

M-Space Solar Sail Drive

[...]

What about the crew?  The cabin would need a separate, independent power source to provide life support and operate equipment (diffuse sunlight just isn't going to do it), so there might be fuel tankage requirements there.  The crew would also need some sort of landing craft to reach a planet's surface.  They would steer the ship via sturdy guide lines that would change the sail's shape just like a sailboat.  Going outbound from the sun would be straightforward.  Returning to the inner planets of a solar system might be slower and more difficult.  I will have to research that.

The light reflected from the sail might be harvested by photovoltaic cells, or (parts of) the sails could be such. If the sail can be kept in a convex shape, concentrating a lot of diffuse sunlight reflected on the collectors makes this viable way farther out in a solar system even without active irradiation.

Returning inward probably requires slingshot maneuvers around massive objects for change of direction and weaker lateral acceleration for additional tack.

A developed system will have mirrors or sun-powered lasers to provide rivers of light that can be sailed inward. These mirrors might be rocket ships or railgun missiles which carry only an array of light sails into position.

On 10/19/2019 at 7:11 PM, seneschal said:

Finally, Larry Niven suggested that such a vehicle's initial launch might be facilitated by firing massive Death Star type lasers into the sail to give it a good shove.  So, anybody want to go sailing with me ... in spaaaaaaaace?

Actually, those are in all likelihood something like Stellasers - basically pairs of mirrors in moderately low solar orbit using the photosphere of the sun as laser medium. A third mirror directs the output passing through the center of one of those mirrors. Yes, these are weapon grade when held on a planetary target or similar.

 

 

Telling how it is excessive verbis

 

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4 hours ago, Joerg said:

Returning inward probably requires slingshot maneuvers around massive objects for change of direction and weaker lateral acceleration for additional tack.

You just turn the sail 45° from the star so that it slows your orbit around it, and then the star's gravity does the rest. Likewise, you can turn the sail so that your orbital speed increases.

 

 

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4 hours ago, seneschal said:

Good input, Joerg!  Thanks.  So how do we make these concepts a playable part of M-Space?

There are many setting-specific variables. You have to decide how much mass a solar sail will have per square km. Something like 3 grams per square meter seems realistic; that would mean 3 tons per square km. So e.g. a 30 ton ship would have 1 module for 1 km² of solar sail, providing the ship with an acceleration of  0.0003 m/s² at 1 AU distance from the sun.

 

There are, by the way, also concepts of solar sails that are not actual sails, but magnetic fields filled with thin hydrogen, for the same effect, but with potential of a much larger sail - if you have the energy.

Edited by Thot
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22 minutes ago, Thot said:

There are many setting-specific variables. You have to decide how much mass a solar sail will have per square km. Something like 3 grams per square meter seems realistic; that would mean 3 tons per square km. So e.g. a 30 ton ship would have 1 module for 1 km² of solar sail, providing the ship with an acceleration of  0.0003 m/s² at 1 AU distance from the sun.

Graphene might be the ideal monomolecular foil, although for the desired reflectivity a single layer of the stuff might be insufficient. As a carrier material for a thin film of metal (e.g. Lithium or Beryllium) it might work, too.

Manufacturing  Graphene sheets of that size is an engineering unobtainium at the time being, on par with spacelift or sky hook technology.

22 minutes ago, Thot said:

There are, by the way, also concepts of solar sails that are not actual sails, but magnetic fields filled with thin hydrogen, for the same effect, but with potential of a much larger sail - if you have the energy.

I am considering plasma shields on the same principle as a first line of defence against impacts and  irradiation, but having to bring the energy is the limiting factor for this, and the field strength is a matter of distance squared between magnetic coils. Give me a flexible high temperature supra-conductor (like the stuff used by the Puppeteers in the Ringworld novels) and I might believe in something like that for a solar sail, but that just provides another unobtainium in addition to the flawless graphene manufacture at near-continental sizes.

For the magnetic fields to be planar with a payable amount of energy, you'd need to have the magnets on a lightweight mesh of fabric (possibly monomolecular) or more rigid profiles of some light matter strong enough to keep those magnets apart.

But then, the individual magnets might be maser-powered, from a light source similar to that of the wavelength chosen to propel the sail, external to the ship and powered by the system primary.

 

Setting parameters would include how long distance sailing vessels interact with objects in defined orbits - do they adjust all of their payload to that orbital speed, or will there be some sort of lighter system or just cargo pods dropped in the keel"water" to be salvaged either by the sailing vessel or by the stationary object.

I think there would be mirror farms and probably stellasers feeding distant relay lasers with solar energy. Keeping those mirrors in decent reflectivity is probably a major maintenance task.

You wouldn't quite get a Kardashev 2 situation out of such a setting, but at least a step way beyond Kardashev 1 by dimming and concentrating the light emitted from the primaries of this civilization. Spectra of such stars might be a lot more metallic than expected due to interaction with the mirror material when viewed from a sufficient distance. Externally powered interstellar travel at sublight speeds might become possible, although the travelers would be dependent on long term stability of their propulsion beams in their system of origin.

Telling how it is excessive verbis

 

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https://www.nasa.gov/sites/default/files/files/YOSS_Act1.pdf
 

Our NASA link says, among other things, rhat a spacecraft traveling as fast as the International Space Station would take about 18 years to go from Earth to Neptune.  That's a long trip but perhaps doable in a human lifetime.  It would be a 40-ish year round trip.  Problem is, our articles on solar sailors seem to indicate that they would be slower than the International Space Station..  What does that mean for our doughty spacehands?  A voyage to the outer planets would be at least a career-length commitment if not a lifetime commitment.

Jim Hawkins signs on as cabin boy of the Space Hispaniola at age 8 or 10 (the Treasure Island character was about 12).  He is 50-ish when he returns to Earth.  What happened to all the adult crewmen he shipped with?  They are in their 90s or older.  Do we assume advanced medical care will keep them spry?  Or perhaps we staff our spaceships with virile 20-year-olds.  They are in their early 60s upon return while Jim is in his early 50s.  Sounds more practical and explains all those grizzled Classic Traveller veterans.  Still you've got to account for training and education time so they can operate the ship.  So mid-60s upon return.  Would you sign up for a space voyage straight out of high school or college, knowing that you might never see you parents or favorite aunt alive again?  That your high school or college sweetheart wouldn't wait 40 years for you (and why should she)?  That your friends and siblings will marry, raise families, and be spoiling their grandkids while you are toddling around in the void?  That's a heck of a commitment to make, especially at an age when most people are still trying to figure out who they are and what they want to do with their lives.  Once they seal the hatch behind you, you are stuck.  No turning back.

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