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


Spacescifi

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In lieu of actually making a huge torus rotating station which is harder still... do we have the ability to make tethers that could do 1 RPM at 1g at about a km length?

Max weight load on both ends without breaking?

 

 

Edited by Spacescifi
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1 hour ago, SpaceFace545 said:

Rope?

 

I mean really any tensile material works, just depends on the parameters. Would you mind going into more depth about load and such.

 

7 hours ago, Spacescifi said:

In lieu of actually making a huge torus rotating station which is harder still... do we have the ability to make tethers that could do 1 RPM at 1g at about a km length?

Max weight load on both ends without breaking?

 

 

 

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37 minutes ago, benzman said:

I don't know what material would be best for a space tether, but i do know that if it broke it would be seriously bad. I think a torus or similar rotating station would be vastly safer.

 

Not as bad as you may think... if I am right in assuming that you thought I intend the the tether for a space station. I only mentioned stations since all are more familiar with that than spinning coasting spaceships on tethers on interplanetary trips.

 

Two ways: Separate a ship in halves with a tether. More risk involved. Or use two ships, one with the tether equipment and backup mission supplies, and the other with crew and main mission supplies. One ship attaches to other and spins.

 

If a separation does occur no problem... that's what rocket engines are for that all spaceships worthy of the name have.

 

 

Edited by Spacescifi
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1 hour ago, Spacescifi said:

 

Not as bad as you may think... if I am right in assuming that you thought I intend the the tether for a space station. I only mentioned stations since all are more familiar with that than spinning coasting spaceships on tethers on interplanetary trips.

 

Two ways: Separate a ship in halves with a tether. More risk involved. Or use two ships, one with the tether equipment and backup mission supplies, and the other with crew and main mission supplies. One ship attaches to other and spins.

 

If a separation does occur no problem... that's what rocket engines are for that all spaceships worthy of the name have.

 

 

Wait.... no....     First off, this would imply that you would have 2 identical vessels on either ends of the tether, which requires a tether of roughly twice the length to get the same g force than a single ship that has the majority of it's mass on one end of the tether, with the section that requires gravity on the other.   The twin ship scenario is not what you are describing, but rather the single ship.  If the tether does break, this lighter section is 'doomed', as the angular momentum budget is now converted to linear motion.    This means one section will fly off in one direction, and the other section, in  a perfect world, has to go get it, burning a lot of fuel.    Then it has to either speed up or slow down to get back on course. 

These manuevers burn a lot of fuel.   Most missions that would utilize a tethered centrifuge rather  than a fixed arm are budget missions.   They've had to make sacrifices to figure out the best way to get a small vessel to their target, and part of that is a limited fuel budget.   Having the tether fail is not a "teehee!  We'll just go get them!" moment.   That's a mission critical failure and cannot just be written off as minor.  

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6 minutes ago, Gargamel said:

Wait.... no....     First off, this would imply that you would have 2 identical vessels on either ends of the tether, which requires a tether of roughly twice the length to get the same g force than a single ship that has the majority of it's mass on one end of the tether, with the section that requires gravity on the other.   The twin ship scenario is not what you are describing, but rather the single ship.  If the tether does break, this lighter section is 'doomed', as the angular momentum budget is now converted to linear motion.    This means one section will fly off in one direction, and the other section, in  a perfect world, has to go get it, burning a lot of fuel.    Then it has to either speed up or slow down to get back on course. 

These manuevers burn a lot of fuel.   Most missions that would utilize a tethered centrifuge rather  than a fixed arm are budget missions.   They've had to make sacrifices to figure out the best way to get a small vessel to their target, and part of that is a limited fuel budget.   Having the tether fail is not a "teehee!  We'll just go get them!" moment.   That's a mission critical failure and cannot just be written off as minor.  

 

Ideally you don't want a tether to break... but two vessels of equal weight is arguably easier than having to worry about docking... if the ship needed to because it was halved by tether and needed extra supplies from the other half.

 

Granted I know weights will change with the mission, if the tether is good enough, which is a major challenge in of itself... it may be possible.

 

In theory it works on paper.  But in reality wear and tear and radiation and heat will add it's own input to the challenge to make it... just that.

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9 minutes ago, Gargamel said:

But.... if you want a tether... you have to dock....

Not ship to ship directly you don't.

 

They don't have to kiss docking port to port. Just need the tether to attach or secure itself to the other and then back off by thrusting as it unspools the rest.

Several ways of doing it, one is using a mechanical arm once ships are close to attach the tether to latching area... or you could design a port area to stick the tether to as well.

 

Docking is fine, but getting as close as possible to the point of kissing port to port is not absolutely necessary.... for tethering anyway.

Edited by Spacescifi
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But.... that's docking.....     99% of docking is intercept and rendezvous.    Once you get velocities zeroed, the mating of the ships is 'trivial'. 

5 minutes ago, Spacescifi said:

Just need the tether to attach or secure itself to the other and then back off by thrusting as it unspools the rest.

No, you'd need to rigidly brace each ship off each other, start the rotation of the whole system, and then unspool the tether.   If you don't brace, then tether system will either get shock loaded or end up wrapping around something it shouldn't.    If you don't start spinning when they are close, it'll require a lot more fuel and coordination to do it when they are at full extension.   And the chances for shock loading are higher again.   It's dynamic loads that break cables, not usually static.   It's much easier to design a system that handles a static load, rather than a variable dynamic one.   

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But to be fair @mikegarrison, our hypothetical spaceship probably has the same mass as one little section of that bridge, and doesn't need to be weather proofed.   Although that pic does put it into perspective some. 

We're not saying a tethered ship won't work, it's just that it will only work for a couple very specific scenarios.    Most likely that scenario will be a single ship with a tether system built in, and only a lightweight crew section.   The ultralight backpacking version of interplanetary missions.    Anything heavier than that will require tether materials of insane strength.   And if we had such materials, then the fabled space elevator is not out of the question.   And if we have a space elevator, we have 'easy' orbital construction.   And with that, we can build heavier ships with rotating tori.

Another scenario that a tether system would be useful would be like the one seen in SevenEves.   Crew habitation pods with life support and tiny manuevering thrusters, and not much else.  Tether two of them together to provide long term living quarters.    But these are houses in space, not spaceships.  

 

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The snarky answer is No.   We don't  have the ability to make tethers that could do 1 RPM at 1g at about a km length, unless you're assuming fairly generous definitions of 'about' a km, or that we've managed to rewrite the laws of physics. Tether length, rotation speed and centripetal force are all linked and one can't just arbitrarily define all three. 

The serious answer is that there are online calculators for these kind of artificial gravity questions.  A 1km cable (assuming rotation about the midpoint for simplicity and because OP proposed this instead of a rotating torus) spinning at 1 rpm gives an artificial gravity of 0.56g. Tangential velocity of whatever is tied to the cable ends is 52 m/s.  For a 1g artificial gravity and an angular velocity of 1 RPM,  an 1.8km cable is required. Alternatively, if you want to stick with a 1km cable,  an angular velocity of approximately 1.34 RPM is required.

But @mikegarrisonmade the most relevant point. The situation we're describing is equivalent (I think) to hanging a 2km cable from a building on Earth and then attaching weights to it. 

I'm not inclined to look into this in any more detail but a useful place to start might be to look up the specific strength of various  materials, otherwise known as breaking length, or "the maximum length of a vertical column of the material (assuming a fixed cross-section) that could suspend its own weight when supported only at the top."  There appear to be plenty of materials with a breaking length of over 2km, so there's plenty of choice for tether materials, depending on how thick you want to make it and how much mass you're attaching to the end.

Amusingly, in the context of this conversation, American eastern white pine wood apparently has a breaking length of 22.7 km if you want to go for a more natural aesthetic for your spacecraft.

 

Edited by KSK
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@KSK Wooden spaceships? :0.0: 

You don’t actually need 1g of gravity for this sort of thing though, and reducing the gravity means less strain on the whole system and most likely less material to be transported up into space in the first place- like that massive cable, for example, and you’re going to want to have a backup because single points of failure for critical systems are a big NOin aircraft, never mind spacecraft. One micrometeor strike that damaged the cable and the whole thing could tear itself apart in seconds, but with lots of smaller cables the load is spread and there’s a lot of redundancy, so losing one cable isn’t such a big deal; though this also makes it heavier.

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58 minutes ago, KSK said:

The situation we're describing is equivalent (I think) to hanging a 2km cable from a building on Earth and then attaching weights to it. 

That's if each end of the cable has equal masses.   We just want 1km from the barycenter of the system.    So if we leave all the fuel that we won't be using, engines, and all the other heavy stuff on one end, and the light weight crew hab section on the other, we could get away with a cable that is just over 1km.   

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15 minutes ago, Gargamel said:

That's if each end of the cable has equal masses.   We just want 1km from the barycenter of the system.    So if we leave all the fuel that we won't be using, engines, and all the other heavy stuff on one end, and the light weight crew hab section on the other, we could get away with a cable that is just over 1km.   

Agreed.  I was just assuming equal mass on each end of the cable because the tether system  was initially proposed as an alternative to a rotating torus.

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3 hours ago, jimmymcgoochie said:

Wooden spaceships?

Aliens 3 initially was planned in a wooden asteroid.

3 hours ago, KSK said:

Tangential velocity of whatever is tied to the cable ends is 52 m/s.

And now they should try to accelerate/decelerate and lengthen/shorten the rope absolutely synchronously to avoid a loop or a whip.

***

Why lift buildings? Lift a seaship.

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A standard steel kilometre long 1x19 wire rope 10mm in diameter would mass half a ton.

It could sustain a minimum breaking load of 10t at each end, of which self-weight would contribute 0.125t. With a safety factor of just under 2, there could be a 5t module at each end, a 10.5t spacecraft total.

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

Tethering two 250t loaded Starships together would take a ribbon of 20 16mm 1x19 wire ropes for a safety factor of 2, and would be very tolerant of individual rope failures. They'd weigh 25t (total spacecraft weight 525t).  Stronger ropes are available that would reduce the number of strands required at the cost of less redundancy, but flexibility and compactness of the reeling mechanism becomes an issue with larger diameter ropes. 

Although very large amounts of redundancy are probably unnecessary - multi-strand wire ropes are already self-redundant, and if any failures are detected you'd just reduce the speed of rotation of the combined craft.

It's also quite unlikely that a full 1g is required for crew comfort. 

So tethers are actually quite easy with standard engineering materials if you have the mass budget for the mechanism.

Edited by RCgothic
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with a ring station i think the counterintuitive part is that you need more material close to the hub than you do near the deck. most materials are better in tension than in shear, and you got lots of tension near the deck and lots of shear near the hub. im not sure that this is a huge problem with cables though, as they tend to be flexible. also a tether system is going to mean a significantly more light weight solution. construction cranes might be the model to go after as they lift heavy loads long distances on relatively thin cables. its unlikely you would have a kilometer of cable on a construction crane, but some of the cranes used for undersea operations got some reach. 

 

needless to say its an engineering problem, and not really a very hard one when compared to something like a rocket engine or nuclear reactor. 

wrt needing 1g. it makes sense for large stations built for long term habitation but not really for crew gravity on space craft especially in transit to a planet/moon with something else. one of the advantages of a tether system is you can tweak the parameters of the gravity to acclimatize astronauts to their destination. a large centrifuge can too, its just you can only tweak rpm and cant control radius and you likely have to build it beefier if you want 1g or higher (not really a problem with a light weight tether system though). 

Edited by Nuke
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And just to repeat something from up thread, of the cable breaks the separation speed is only in double digit m/s, well within the rounding errors of your maneuvering engines.

If the cable breaks you just re-dock, bolt the ends of the cable back together and wind out again

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15 hours ago, Gargamel said:

But.... that's docking.....     99% of docking is intercept and rendezvous.    Once you get velocities zeroed, the mating of the ships is 'trivial'. 

No, you'd need to rigidly brace each ship off each other, start the rotation of the whole system, and then unspool the tether.   If you don't brace, then tether system will either get shock loaded or end up wrapping around something it shouldn't.    If you don't start spinning when they are close, it'll require a lot more fuel and coordination to do it when they are at full extension.   And the chances for shock loading are higher again.   It's dynamic loads that break cables, not usually static.   It's much easier to design a system that handles a static load, rather than a variable dynamic one.   

 

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!

Edited by Spacescifi
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i think that all centrifuge types have a place on spacecraft depending on purpose and size of ship.  i came up with 10 different gravity classes in order of least to most massive:

1. no gravity (everything we have done so far)
2. tether gravity (ship from stowaways)
3. single module on boom (with counter weight, like the edward israel from expanse s4)
4. multi module on boom (ship from europa report) 
5. modular ring (endurance from interstellar)
6. small ring (discovery from 2001)
7. spun up ship (avalon from passengers)
8. medium ring module (ship from the martian, propulsion, fuel, etc at hub)
9. ring ship (with distributed propulsion, fuel, etc in the ring. may be hubless using structural cables or may locate a reactor or nuclear propulsion in hub to reduce radiation shielding requirements).
10. massive cylinder (think orion powered interstellar generation ship)

early interplanetary missions would likely be  2-5, and maybe 1 (though i figure thats only useful for a mars/venus flyby or missions to asteroids and small moons with low gravity). military will probibly use 1, 6, and 7. cargo ships will likely be 3 or 4, though 7 can be used if cargo requires gravity. bulk passenger transport would be 6-9 depending on capacity. colony ships would be 8-10 with 10 for long range colonization missions. mining ships could be anything depending on what you are mining and where the refinery is. i dont see us getting past 5 or 6 in our life time. 

Edited by Nuke
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