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Kilometer Long Tether Rotational Gravity.... Do We Have Material That Won't Break?


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8 hours ago, Spacescifi said:

Had not considered static versus variable dynamic loading.... thanks.

I just assumed... build a super awesome tether guys!

Problem is... you can't just... do that.

Because like another poster said, one thing effects another, meaning you either get superheavy but redundant tether and lower thrust ship, or less heavy tether but doomed if something goes wrong but no problem!  Ship has better thrust since tether is lighter!

This is not an either/or scenario.  Read @RCgothic's post again, since that's the most thoroughly worked out example on the thread.

Looking at the tethered Starship example, there are several options for a lighter tether:

  • Accept a less redundant system with a lower safety factor.
  • Use lighter tether materials than steel ropes.
  • Go for a lower artificial gravity than 1g.

Any tether system is going to add mass to the spacecraft and there's always going to be a risk of something going wrong. This is not a binary choice between 'superheavy but redundant tether' and  'less heavy tether but doomed if something goes wrong'. This is a whole set of choices and trade-offs, depending on how much risk is acceptable.

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7 minutes ago, KSK said:

This is not an either/or scenario.  Read @RCgothic's post again, since that's the most thoroughly worked out example on the thread.

Looking at the tethered Starship example, there are several options for a lighter tether:

  • Accept a less redundant system with a lower safety factor.
  • Use lighter tether materials than steel ropes.
  • Go for a lower artificial gravity than 1g.

Any tether system is going to add mass to the spacecraft and there's always going to be a risk of something going wrong. This is not a binary choice between 'superheavy but redundant tether' and  'less heavy tether but doomed if something goes wrong'. This is a whole set of choices and trade-offs, depending on how much risk is acceptable.

 

I know... less heavy I remarked is not doomed because if it breaks it is easier to thrust back and reconnect it BECAUSE it weighs less, making the ship weigh less which means less fuel burned during recovery.

 

That's the irony of spaceship design. The heavier and more redundant a ship becomes,  the more it becomes too big to fail like the Titanic.

It's arguably easier to fix what is less redundant and lower mass than what is heavy and redundant to the nth degree.

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Btw the tether needs a drum, the drum needs a motor, and this motor should be able to pull (read: "lift") the half-mass of the ship in the 1 g gravity.

So, the splittable tethership should carry a motor-drum-tether from the coastal crane able to lift hundreds-to-thousands tonnes from water.

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1 hour ago, kerbiloid said:

The half of the ship which has flown away is not required?

The point is that the velocity difference between the two crafts are not significant as in around 50 m/s.
Now an larger problem could be the whiplash effect, an long heavy duty wire who snaps is very dangerous, the tip can easy reach supersonic velocity, it will easy kill you, the load snap you hear from an long whip is an supersonic crack. An close to an kilometre long snapped wire in vacuum while you was spinning around an center of mass is something you don't want to be close to. 
Well I would drop my connector asap and take evasive action. 

Now this is unlikely to happen as we have lots of experience making wires, but recover the wire after an break would be challenging. 

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55 minutes ago, Spacescifi said:

I know... less heavy I remarked is not doomed because if it breaks it is easier to thrust back and reconnect it BECAUSE it weighs less, making the ship weigh less which means less fuel burned during recovery.

That's the irony of spaceship design. The heavier and more redundant a ship becomes,  the more it becomes too big to fail like the Titanic.

It's arguably easier to fix what is less redundant and lower mass than what is heavy and redundant to the nth degree.

You’re contradicting yourself. Originally the a ship with the lighter tether was doomed if something went wrong, then it wasn’t doomed because it would require less propellant to recover from a tether break.

Ditch the unhelpful ‘doomed’ language and I would say that both are correct. A less redundant tether system is lighter but riskier (tether might break) but the mass saved by using a lighter tether means that it’s possible to have a greater propellant reserve to recover from tether breaks.

Theres’s also no reason why the two concepts (redundant tether and enough propellant reserve ) can’t be combined - but that extra mass requirement then has to come at the expense of a less capable spacecraft (in terms of achievable acceleration or delta-V) or a mass saving that has to be found somewhere else in the spacecraft design.

Compromises. Compromises all the way down.

Incidentally, there’s no irony in your last comments. The choice between using redundant systems (and therefore requiring a heavier spacecraft) or making the spacecraft repairable (and potentially lighter) has been understood since the earliest days of crewed spacecraft design.

So far, redundant systems have  mostly* been the preferred option, at least in US spacecraft. Making systems repairable in flight adds a whole other bunch of compromises to the design, not to mention the need to take along the necessary tools and spare parts - which add mass and also take up valuable storage space.

*The ISS is a different matter of course but that’s a very long duration flight with a whole logistics chain to resupply it with consumables and spare parts. 

Edited by KSK
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1 hour ago, kerbiloid said:

 this motor should be able to pull (read: "lift") the half-mass of the ship in the 1 g gravity.

No it absolutely shouldn't. Conservation of angular momentum would spin a 2 starship system up to over 60RPM as the tether retracted. 2 Dragons would be much worse due to the closer finishing position - over 10,000RPM.

The system needs to be stopped for deployment and retraction, in which case the winch motor only needs to be strong enough to make the tether coil up neatly (which is not zero force - wire rope prefers to be straight).

 

Also I'd also say the tether is not such a significant mass that having a lighter one would be significantly less propellant expended for recovery. But it is relatively trivial to have double or triple redundancy. Quickly get to a situation where failure is not credible.

We regularly trust lives to wire rope systems. Elevators, Cranes. It's not a big deal. The only novel factors for space travel are mass budget (which is becoming less important), and vacuum operation, which is just double checking material properties of the tether are appropriate.

Edited by RCgothic
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34 minutes ago, RCgothic said:

No it absolutely shouldn't.  <...>
The system needs to be stopped for deployment and retraction, in which case the winch motor only needs to be strong enough to make the tether coil up neatly (which is not zero force - wire rope prefers to be straight).

The stopping is not as interesting as starting.

Without preliminary tether tightening the ships connected with the weakened tether would get a strong side shock of 1 g average acceleration and delta-V up to the mentioned 50 m/s before their rotation stabilize. While a human safely withstands 5..10 m/s.

So, the winch should be able to smoothly put out several g of acceleration without harming the crew, and that's why it should actually be able to have enough strong motor to pull the ship at the final 1 g before switching it off.
Unlike a jumping ropes, they can't hope on the elasticity.

And btw the elasticity. The tether has one, too. And it will extend, then retract. And the winch should put out this pull, too. (Let alone the waves running in the tether and whipping the ship.)

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Please explain?

The tether can be paid out slowly and then gently braked to a stop. No excessive acceleration required.

Start up of the rotation  is as gentle as the thrusters that initiate the spin up manoeuvre. This is generally very much less than 1g. A Dragon capsule masses ~15t. A Draco thruster provides 0.04tf thrust. Even as a combination together, this is very much less than 1g.

There's no need to do anything quickly. In fact falling objects is a very good reason not to.

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Why would you accelerate the ships up to full tangential velocity before taking the slack out of the tether? Get them going at some small velocity tangentially with the tether near-straight, then once the tether pulls taught and stabilizes at some small fraction of a G centrifugial force, accelerate the system up to speed.

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44 minutes ago, RCgothic said:

The tether can be paid out slowly and then gently braked to a stop. No excessive acceleration required.

Start up of the rotation  is as gentle as the thrusters that initiate the spin up manoeuvre.

If they keep the tether tightened while getting away, this means that they need to accelerate perpendicularly to the thether, keeping it tightened, and having the tether gently expanding/retracting for that.
If it's weakened, they will get a radial shock when it gets toghtened.
This means they need a motor comparable to the sudden shock acceleration, i.e. comparable to the average 1 g, and still keep perfectly synchronize the ships at 1 km distance. And the halfships are hubdreds tonnes heavy at least.
So, this is a thought-out scheme for ideal conditions never tested even on lesser crafts. (Although Voskhod-6 was planned to do it, it was never sent.)

And anyway the shock will cause a wave of stress in the tether, as the tether is elastic.
So, even once they have started rotating, they will need some time to let the oscillations in the tether shade out and dissipate the mechanical energy in the tether winches.
This means that they will withstand a sequence of shocks in the winches before the tether stops elastically expanding/retracting and gets the fixed length.
This series of shocks should be put out by a spring or so.  As the ships are too big for springs, this means that they should use magnetic springs, i.e. the winch should be equipped with electric motor enough capable to hold a series of ~ 1 g  shocks.

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10 hours ago, RCgothic said:

No it absolutely shouldn't. Conservation of angular momentum would spin a 2 starship system up to over 60RPM as the tether retracted. 2 Dragons would be much worse due to the closer finishing position - over 10,000RPM.

The system needs to be stopped for deployment and retraction, in which case the winch motor only needs to be strong enough to make the tether coil up neatly (which is not zero force - wire rope prefers to be straight).

 

Also I'd also say the tether is not such a significant mass that having a lighter one would be significantly less propellant expended for recovery. But it is relatively trivial to have double or triple redundancy. Quickly get to a situation where failure is not credible.

We regularly trust lives to wire rope systems. Elevators, Cranes. It's not a big deal. The only novel factors for space travel are mass budget (which is becoming less important), and vacuum operation, which is just double checking material properties of the tether are appropriate.

Good point about slowing down the spin before reeling in. 

However we don't trust wire much outside of wire lift systems like ski and gondola lifts. Elevators has had an emergency brake if the wire breaks for 150 years. 
It was the thing who enabled tall buildings.  The area below lifting cranes are not safe areas on an construction site, now this is more about stuff falling off than the cable snapping and is often ignored or some lifts are safer than other, like then nothing can fall off, do not know the rules here for tower cranes just the small ones. 
Yes in emergency or war the safety procedures get ignored, spaceflight is a lot like that in this century 

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42 minutes ago, kerbiloid said:

If they keep the tether tightened while getting away, this means that they need to accelerate perpendicularly to the thether, keeping it tightened, and having the tether gently expanding/retracting for that.
If it's weakened, they will get a radial shock when it gets toghtened.
This means they need a motor comparable to the sudden shock acceleration, i.e. comparable to the average 1 g, and still keep perfectly synchronize the ships at 1 km distance. And the halfships are hubdreds tonnes heavy at least.
So, this is a thought-out scheme for ideal conditions never tested even on lesser crafts. (Although Voskhod-6 was planned to do it, it was never sent.)

And anyway the shock will cause a wave of stress in the tether, as the tether is elastic.
So, even once they have started rotating, they will need some time to let the oscillations in the tether shade out and dissipate the mechanical energy in the tether winches.
This means that they will withstand a sequence of shocks in the winches before the tether stops elastically expanding/retracting and gets the fixed length.
This series of shocks should be put out by a spring or so.  As the ships are too big for springs, this means that they should use magnetic springs, i.e. the winch should be equipped with electric motor enough capable to hold a series of ~ 1 g  shocks.

A gradual start would not produce shocks of the order 1g.

Particularly not with a tether which, as you've pointed out, is reasonably elastic. Roughly 3m extension over 1km for a 20x 16mm diameter strand ribbon for 3x Starships as I suggested calculated above.

Yes, there may be some small oscillations. Tangential velocity at 1g at 1km is ~70m/s.  Assume  the spacecraft has a 1 deg deviation from tangent and hits the end of the tether. Radial velocity is ~1.2m/s. Kinetic energy is ~187kJ. Extension is ~ 0.7m. Peak acceleration is ~0.23g. That's a very big oscillation because we're starting from full speed. Very likely oscillations would be damped out by the time it got up to speed. Most would occur as the slack in the tether is taken up at under 1 tenth that velocity.

A small damping shock absorber and a brake are sufficient. The motor doesn't need to see any dynamic load at all.

Edited by RCgothic
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1 hour ago, RCgothic said:

Tangential velocity at 1g at 1km is ~70m/s.  Assume  the spacecraft has a 1 deg deviation from tangent and hits the end of the tether. Radial velocity is ~1.2m/s. Kinetic energy is ~187kJ. Extension is ~ 0.7m. Peak acceleration is ~0.23g.

When you can ideally synchronize the ships movement, at the same time turning their velocity vectors , to keep the tether tightened.

Even the modern ship have to wait several minutes after dockng to let the oscillations stop. And a kilometer-long tethered system of thousand tonnes would be a yo-yo.

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On 10/11/2021 at 12:50 AM, RCgothic said:

Alternatively a quad-tether of such wire ropes could sustain 2x10t modules with multiple redundancy for 22t.

interestingly, this is incorrect!  :)  while a single, double, and triple rope setup can hold 1x, 2x, and 3x the mass, a quad rope setup can only hold 2x the mass.  this is because in real life one rope will always be slightly shorter than the others, which will cause the load to be supported entirely by the short rope and it's opposite, while teetering between the 3rd and 4th rope with no guaranteed lift from either.

this is also the reason why 4 legged café tables are always unstable! 

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46 minutes ago, Blasty McBlastblast said:

interestingly, this is incorrect!  :)  while a single, double, and triple rope setup can hold 1x, 2x, and 3x the mass, a quad rope setup can only hold 2x the mass.  this is because in real life one rope will always be slightly shorter than the others, which will cause the load to be supported entirely by the short rope and it's opposite, while teetering between the 3rd and 4th rope with no guaranteed lift from either.

this is also the reason why 4 legged café tables are always unstable! 

1) Real materials strain in response to stress, so you will end up with the load distributed among all four ropes. Maybe not distributed evenly, but it definitely will not be two ropes taking all the load and two ropes slack, unless you deliberately make it that way.

2) The really key point that was being mentioned there anyway was the concept of redundancy.

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3 hours ago, RCgothic said:

Particularly not with a tether which, as you've pointed out, is reasonably elastic. Roughly 3m extension over 1km for a 20x 16mm diameter strand ribbon for 3x Starships as I suggested calculated above.

I wonder what the equilibrium temperature for steel in sunlight in vacuum is? IIRC I've heard ~ +200deg C quoted for as being representative of the sunlit "space environment".  Space background temp in the shade is 4K = ~ -270C. I expect the high surface area to volume ratio of a cable would cause it to radiate pretty quickly.

For 316 Stainless (a good cryo-steel)Using a linear thermal expansion coefficient of 16x10^-6 m/(m*degC) * 1000m * ~470C = ~7.5m

So, if I've pulled the numbers out of my butt appropriately, orbiting around a planet from day to night might do some fun things to a steel wire rope.

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Really cool you guys are using math to figure this all out.

 

So we know what to expect with a few surprises still when 'we' (mankind' do it for real.

And we can also chew apart any scifi movie for accuracy that  uses rotating tethers.

 

Edited by Spacescifi
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A kilometer-long tether would strain about 3m per strand under load, so that's a pretty effective load-equaliser even without resorting to a tension equaliser, which I have personal experience of designing for an industrial 4-strand lifting application. More strands is just more equaliser complexity, but the equaliser's probably unnecessary. 

The day/night cycle is an interesting point, but what matters is the rate of change. 7.5m is only a big deal if it takes just seconds to equilibriate, which sounds unlikely.

Edited by RCgothic
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7 hours ago, Spacescifi said:

Really cool you guys are using math to figure this all out.

So we know what to expect with a few surprises still when 'we' (mankind' do it for real.

And we can also chew apart any scifi movie for accuracy that  uses rotating tethers.=

I really don't understand  this need to prove sci-fi wrong which seems to come up a lot on your threads. I mean, I could understand looking at a bit of sci-fi and trying to figure out if it could be replicated in real life (and being disappointed if it couldn't) but chewing apart that same sci-fi just to declare that 'haha - they got it wrong!' seems particularly mean-spirited.

As far as rotating tethers are concerned, if I was planning to write a story which involved one, I'd be overjoyed at reading this thread, and particularly @RCgothic's post which I referred to previously.  Five hundred tons of spacecraft spinning on a 1km tether to create 1g of artificial gravity - and all using plain old steel rope? Fantastic! That gives me plenty of leeway to design and describe a fictional spacecraft knowing that the numbers are about right. If I need a more massive craft, I can go away and look for stronger tether materials to see if those craft would be feasible.

The fine details of winch mechanisms, cable elasticity and hysteresis, damping vibrational modes on the tether, thermal properties of tether materials, effects of radiation on cable materials, etc. etc. ?  Those I don't give half a damn about unless they're at all relevant to the story. For example,  I might want to have the tether break or otherwise malfunction in a plausible way without making my behind-the-scenes spacecraft designers look like complete idiots for not thinking of the obvious failure modes ahead of time.

Otherwise, I'm not going to care about the numbers at all. Mostly because the chances of my characters being in a situation where they need to explain those numbers in any detail are pretty slim.  Partly because the more precisely I pin down a particular aspect of my spacecraft, the more constrained I am when talking about the rest of it.  And partly because any numbers I put in the story are hostages to fortune for precisely this kind of nitpicking. 

(Plus it's tremendously easy to disappear down the rabbit hole of research, for the sake of a throwaway story detail, in lieu of actually getting any writing done. Ask me how I know.)

At the end of the day, I'm writing a science-fiction story, not a monograph on spacecraft design. Unless the story requires a deep dive into the details, my aim would be to describe a plausible sounding tethered spacecraft with  plausible capabilities, and to try not to include anything too boneheaded that would break my readers' suspension of disbelief.

Edited by KSK
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9 hours ago, Blasty McBlastblast said:

interestingly, this is incorrect!  :)  while a single, double, and triple rope setup can hold 1x, 2x, and 3x the mass, a quad rope setup can only hold 2x the mass.  this is because in real life one rope will always be slightly shorter than the others, which will cause the load to be supported entirely by the short rope and it's opposite, while teetering between the 3rd and 4th rope with no guaranteed lift from either.

this is also the reason why 4 legged café tables are always unstable! 

The way you equalise 4 strands is you equalise two adjacent strands by allowing them to float until the slack is taken up. 

O   O

X    X

By moments about an axis X-O, if the floating legs O and O are made the same then the fixed legs X and X are also the same, because X must equal O.

I was doing a spell as an integration engineer on a job and the designers just before a deadline "helpfully" gave us a heavy component we had to cryo-fit flush underneath with no lifting features and refused to let us modify it. Had to lift it into a nitrogen bath with magnets, but the magnets individually weren't very capable (they could have been made stronger, but we'd start damaging the component). I had to do a deep dive into all the "de-rating" factors to see which ones could be made not applicable to the specific process.

Other than non-equal legs, the other major de-rating factor for multiple legs is included angle between them. If they're pulling horizontally then capacity is being used on that and not vertical load. It's common for general purpose multi-legged lifting equipment to assume an included angle of 90-120 degrees, which de-rates each leg by the cosine of the half-angle.

Magnets in particular do *not* like being pulled sideways though. They'll slide until the pull is mostly vertical, except now that's a smaller lifting footprint so the load probably becomes unstable, the lifting surface angles to the horizontal, and then the magnets slide again until the load is dropped.

So if you're only using a very small included angle on the legs anyway to prevent sliding, that de-rating factor also drops out almost completely.

Also had to account for cryo-conditions as well. All-stainless steel equipment, and samarium-cobalt magnets that would only lose ~10% of their magnetism unlike neodymium one which become basically useless at LN2 temperatures. Got some beautifully shiny stainless steel D-shackles that became prized office paperweights after fulfilling their function.

And then the magnet supplier's courier let us down, so I had to get the magnets taxi'd across the country overnight on my own buck to make the start of testing the next day.

Fun little job.

Edited by RCgothic
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12 hours ago, FleshJeb said:

So, if I've pulled the numbers out of my butt appropriately, orbiting around a planet from day to night might do some fun things to a steel wire rope.

Good point! But if that turns out to be an actual problem then just keep them heated. ;)

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13 hours ago, mikegarrison said:

1) Real materials strain in response to stress, so you will end up with the load distributed among all four ropes. Maybe not distributed evenly, but it definitely will not be two ropes taking all the load and two ropes slack, unless you deliberately make it that way.

2) The really key point that was being mentioned there anyway was the concept of redundancy.

If we use a single strand woven through a block and tackle system, the single strand would be able to handle as much as we want, in theory.   In this example, we'll just use 4x system for simplicity. 

Something like this:

untitled1.png

In our setup, instead of anchoring to the bottom pulley, we could have each end of the strand on a motorized spool.    This would serve two major functions.   One, redundancy, with two motorized spools, failure of one would not mean system lock up, as the remaining spool would be able to reel in the entire length.   And, secondly, each spool, when both are functioning, would only have 1/4 of the total mass/force applied to them.  

But the biggest advantage of this system in the built in redundancy for the material strength of the strand.  Given our system would be perfectly balanced, as it should with proper design, each length of strand would only carry 1/4 of the total load.   Each of the fittings would be designed to handle the appropriate load. 

If the strand would happen to fail, it would be contained by some automatic "prussik" devices, as a prussik minding pulley locks down on the rope as it travels if needed. 

a2.jpg

These would have to be some automated device that locks down when a break is detected, and one on each end of each pulley.   In case of a break, each spool would then have to wind up at individual rates to keep the load balanced. 

One major downside to this system is that you need to carry 4x the length of strand than a single strand.   But since it's fairly obvious that a single strand is not a smart design, we're going to have redundant strands anyways, so we might as well reduce the number of spools needed (but not size), and make it automatically load balancing.    A minor downside is that, using two spools, the unwinding and winding would take twice as long as single strand spools. 

5 hours ago, KSK said:

I really don't understand  this need to prove sci-fi wrong which seems to come up a lot on your threads. I mean, I could understand looking at a bit of sci-fi and trying to figure out if it could be replicated in real life (and being disappointed if it couldn't) but chewing apart that same sci-fi just to declare that 'haha - they got it wrong!' seems particularly mean-spirited.

My comments here and in other threads might have come across as such, but that's not my intent.  More of Devil's advocate.   Poke holes in the idea so that Scifi can find the appropriate fix. 

But I do think a tethered system is only feasible in a few specific mission scenarios.   A larger craft would be better served by a fixed torus. 

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6 hours ago, KSK said:

I really don't understand  this need to prove sci-fi wrong which seems to come up a lot on your threads. I mean, I could understand looking at a bit of sci-fi and trying to figure out if it could be replicated in real life (and being disappointed if it couldn't) but chewing apart that same sci-fi just to declare that 'haha - they got it wrong!' seems particularly mean-spirited.

As far as rotating tethers are concerned, if I was planning to write a story which involved one, I'd be overjoyed at reading this thread, and particularly @RCgothic's post which I referred to previously.  Five hundred tons of spacecraft spinning on a 1km tether to create 1g of artificial gravity - and all using plain old steel rope? Fantastic! That gives me plenty of leeway to design and describe a fictional spacecraft knowing that the numbers are about right. If I need a more massive craft, I can go away and look for stronger tether materials to see if those craft would be feasible.

The fine details of winch mechanisms, cable elasticity and hysteresis, damping vibrational modes on the tether, thermal properties of tether materials, effects of radiation on cable materials, etc. etc. ?  Those I don't give half a damn about unless they're at all relevant to the story. For example,  I might want to have the tether break or otherwise malfunction in a plausible way without making my behind-the-scenes spacecraft designers look like complete idiots for not thinking of the obvious failure modes ahead of time.

Otherwise, I'm not going to care about the numbers at all. Mostly because the chances of my characters being in a situation where they need to explain those numbers in any detail are pretty slim.  Partly because the more precisely I pin down a particular aspect of my spacecraft, the more constrained I am when talking about the rest of it.  And partly because any numbers I put in the story are hostages to fortune for precisely this kind of nitpicking. 

(Plus it's tremendously easy to disappear down the rabbit hole of research, for the sake of a throwaway story detail, in lieu of actually getting any writing done. Ask me how I know.)

At the end of the day, I'm writing a science-fiction story, not a monograph on spacecraft design. Unless the story requires a deep dive into the details, my aim would be to describe a plausible sounding tethered spacecraft with  plausible capabilities, and to try not to include anything too boneheaded that would break my readers' suspension of disbelief.

 

Pretty much what Gargamel said is how I feel.... since in spite of my name.... not EVERY thread am I considering for story use.

 

Sometimes I just want to know what is possible and what is impossible.

 

The link between them is what currently is impossible but won't always be, and the problem with scifi solutions is it is a true rabbit hole, since a simple solution to any problem can make ten more greater possible dangers and break all plausibility of civilization existing as is.

 

That said... the future reality will be made of the same stufg as the present, so it is fun to see how good our tech can do when put to the test.

Edited by Spacescifi
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