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Post by walterscientist on Jan 3, 2019 19:48:43 GMT
A realistic spacecraft for long missions should probably include a module with centrifugal artificial gravity. After reading Atomic Rockets it seems systems with diameter ~20 meters and ~0.5g of acceleration are fairly feasible. One issue I ran into when figuring out the specifics is the design of the connection enabling the module to rotate while the rest of the craft is static. Atomic Rockets say that issue of twisting-off cables and piping can be overcome using Canfield joints. The website doesn't show a specific way how to incorporate the joint, but I assume it has to connect the module in a way that the linkage goes through the axis of rotation. The problem I see comes from the fact that people also need to go through the axis in order to avoid danger of being crushed. It seems the simplest solution is to connect the axis from one side of the module and access passage from the other side. That however creates the need for the structure of the craft reaching around the centrifuge module at some point. It would also require a passageway to go around the centrifuge if you need regular access to halves of the craft on both sides of the centrifuge. Other solution might be to mount the linkage into the center of an oversized passageway, similar to how to a wind tunnel turbine is mounted. That would enable people to cross off-axis without crushing danger, but it seems it might be excessively large and possibly not very structurally sound. Structural strength could be an issue in itself as the joints on the centrifuge module might be the only thing holding together the two halves of the craft. I tried to sketch several solutions, but I feel that maybe I am missing something simple and obvious, since I can't find this addressed anywhere on the internet. Any thoughts?
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Post by bigbombr on Jan 3, 2019 22:10:27 GMT
A realistic spacecraft for long missions should probably include a module with centrifugal artificial gravity. After reading Atomic Rockets it seems systems with diameter ~20 meters and ~0.5g of acceleration are fairly feasible. One issue I ran into when figuring out the specifics is the design of the connection enabling the module to rotate while the rest of the craft is static. Atomic Rockets say that issue of twisting-off cables and piping can be overcome using Canfield joints. The website doesn't show a specific way how to incorporate the joint, but I assume it has to connect the module in a way that the linkage goes through the axis of rotation. The problem I see comes from the fact that people also need to go through the axis in order to avoid danger of being crushed. It seems the simplest solution is to connect the axis from one side of the module and access passage from the other side. That however creates the need for the structure of the craft reaching around the centrifuge module at some point. It would also require a passageway to go around the centrifuge if you need regular access to halves of the craft on both sides of the centrifuge. Other solution might be to mount the linkage into the center of an oversized passageway, similar to how to a wind tunnel turbine is mounted. That would enable people to cross off-axis without crushing danger, but it seems it might be excessively large and possibly not very structurally sound. Structural strength could be an issue in itself as the joints on the centrifuge module might be the only thing holding together the two halves of the craft. I tried to sketch several solutions, but I feel that maybe I am missing something simple and obvious, since I can't find this addressed anywhere on the internet. Any thoughts? I very much recommend using proper formatting so your questions are easier to read and asking your question on the ThoughSF discord: discord.gg/gBJKvr |
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Post by airc777 on Jan 3, 2019 23:25:00 GMT
Inflatable doughnut shaped crew habitation module, roll the entire craft with maneuvering thrusters. Discard the inflatable hab or store it in in an otherwise unused volume inside the craft before combat, have the separate command on control module buried deep in the craft under the armor. Have the crew in individually pressurized suits during combat so you aren't worried about the command and control module being punctured. Much simpler solution to the problem, fewer complex joints. Rotating solid habs on civilian craft because luxury.
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Post by AtomHeartDragon on Jan 3, 2019 23:51:35 GMT
I would either make an internal centrifuge completely enclosed within pressurized volume (there will be energy losses from aerodynamics!), or spin the entire craft, with possible exception of unmanned de-spun platform(s) for stuff you wouldn't want to be spinning.
Fun variants of spinning the entire craft include tumbling pigeons (just needleships tumbling end over end with only minimal engineering considerations needed for artificial gravity) and ships/modules tethered together to make very large centirfuges with very modest means.
Dividing your craft, including pressurized volume, into rotating and non-rotating parts seems like just giving yourself a bunch of engineering headaches for no adequate reason. It's like trying to make a mecha when a tank would do.
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Post by airc777 on Jan 4, 2019 5:56:00 GMT
Had a thought.
If you wanted specifically to not use the axis as the access hallway for whatever reason (perhaps a hundred(s) of meters long spinal railgun is in the way.) You could achieve matching velocity with the rotating section of the ship with a sort of 'radial elevator' or train. Just a room with safety doors on a track that accelerates to match rotational velocity with the relevant part of the craft. This could be done either externally as an airlock or internally inside the pressurized volume as AtomHeartDragon suggests.
If this was a military ship they probably wouldn't even bother with the safety concerns, in modern main battle tanks you very much can be caught between the turret basket and the turret ring and have a limb eaten by 'the turret monster'. I'm guessing there have been studies done that show that it's safer for the crew to make the vehicle as compact of a target as possible then it is to invest the mass of increased internal volume that it would take to engineer a safer system, or at least more combat effective if not safer.
If the rotary part of the centrifuge was internal, and running on tracks on the outside edge instead of being suspended from a bearing at the center then the entire 'overhead' volume in the center could be non rotational access way from 'in front' of to 'behind' the centrifuge. Please excuse my arbitrarily assigned directions.
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Post by apophys on Jan 4, 2019 9:42:14 GMT
Since the highest RPM that can be comfortably adjusted to is 6 RPM (giving 1 G at 25 m radius), with 2-4 RPM normal, the rim speed near the axis is necessarily low, and you can simply step off (no need for a train). This means you don't need your connecting hallway exactly at the axle, but can work with a toroidal crossing passage around it. www.artificial-gravity.com/sw/SpinCalc/Sketch by me in MS Paint:  Green: spinal weapon, as well as propellant lines. Purple: non-rotating ceiling Red: rotating floor (held with bearings), & spokes to edge Blue: non-rotating "floor" (Mind the gap!  ) Yellow: ladder/elevator access from edge It's possible to add an intermediate-speed ring on each side for an even gentler transition. IMO, it would be simplest to do this hallway joint in vacuum, to avoid the generated wind that AtomHeartDragon mentions (and also to allow the rotating section to stick out far from the ship in some designs, to get 1 G with very low RPM). Note that no fluids can be piped between the ship & the hab with this construction, only brought across in tanks. So they will need separate plumbing systems (and separate air if the joint is in vacuum). |
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Post by airc777 on Jan 4, 2019 10:29:49 GMT
...Note that no fluids can be piped between the ship & the hab with this construction, only brought across in tanks. So they will need separate plumbing systems (and separate air if the joint is in vacuum). You're making me google if anyone has ever tried to pump hydraulic fluid through a ring gasket sealed turret ring on a tank for the purposes of elevating the gun. Probably not, it's much easier to use brushes to transfer electrical power from the hull to the turret to run a separate hydraulic pump, but I'll get back to you. I think it's a stupid idea, but I don't think it's impossible.
Edit: en.wikipedia.org/wiki/Deep_wading seems to suggest that most modern tank turret rings are or can be made water proof to several meters depth for the purpose of fording rivers. Haven't found an example of someone trying to pump fluid though a turret ring. |
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Post by AtomHeartDragon on Jan 4, 2019 11:01:45 GMT
Had a thought. If you wanted specifically to not use the axis as the access hallway for whatever reason (perhaps a hundred(s) of meters long spinal railgun is in the way.) You could achieve matching velocity with the rotating section of the ship with a sort of 'radial elevator' or train. Just a room with safety doors on a track that accelerates to match rotational velocity with the relevant part of the craft. This could be done either externally as an airlock or internally inside the pressurized volume as AtomHeartDragon suggests. If this was a military ship they probably wouldn't even bother with the safety concerns, in modern main battle tanks you very much can be caught between the turret basket and the turret ring and have a limb eaten by 'the turret monster'. I'm guessing there have been studies done that show that it's safer for the crew to make the vehicle as compact of a target as possible then it is to invest the mass of increased internal volume that it would take to engineer a safer system, or at least more combat effective if not safer. If the rotary part of the centrifuge was internal, and running on tracks on the outside edge instead of being suspended from a bearing at the center then the entire 'overhead' volume in the center could be non rotational access way from 'in front' of to 'behind' the centrifuge. Please excuse my arbitrarily assigned directions. I would imagine safety concerns to be very important if you are running with minimal crew (because monkey tax) and the nearest replacements are a year away. Meanwhile two of your ships had their combat efficiency drop by half ever since their main battery is manned by one-armed gunners, one started to handle like a freighter (due to one handed tactical pilot) and one has been close to mutiny ever since the (allegedly essential) cook stuck his head into the joint. Accidents do happen and once you cannot reduce the accidents below the threshold you need via training, you need a redesign. IMO, it would be simplest to do this hallway joint in vacuum, to avoid the generated wind that AtomHeartDragon mentions (and also to allow the rotating section to stick out far from the ship in some designs, to get 1 G with very low RPM). Note that no fluids can be piped between the ship & the hab with this construction, only brought across in tanks. So they will need separate plumbing systems (and separate air if the joint is in vacuum). Keeping vacuum between parts people need to to regularly move between is an awful idea. Not only it will require suiting up every time leading to inefficiency and cutting corners, but it will also cause air losses every time system is cycled. It's much better to just deal with some extra power requirement and waste heat, possibly using it to drive ventilation as well, it's even better to just sidestep all problems by spinning up the whole ship. If you do want internal centrifuge (enclosed in pressurized volume), small one could be conventional one, with access through or near hub. Large one could be a train or moving floor riding in a toroidal tunnel. Accessing large one could be done by a succession of moving walkways at safe relative velocities. Yes, it would waste some space and add some complexity, but you ARE already making a separate centrifuge so that is a foregone conclusion. |
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Post by airc777 on Jan 4, 2019 11:22:14 GMT
I would imagine safety concerns to be very important if you are running with minimal crew (because monkey tax) and the nearest replacements are a year away. Meanwhile two of your ships had their combat efficiency drop by half ever since their main battery is manned by one-armed gunners, one started to handle like a freighter (due to one handed tactical pilot) and one has been close to mutiny ever since the (allegedly essential) cook stuck his head into the joint. Accidents do happen and once you cannot reduce the accidents below the threshold you need via training, you need a redesign. I agree, but mind the gap.
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Post by apophys on Jan 4, 2019 13:34:05 GMT
Keeping vacuum between parts people need to to regularly move between is an awful idea. Not only it will require suiting up every time leading to inefficiency and cutting corners, but it will also cause air losses every time system is cycled. It's much better to just deal with some extra power requirement and waste heat, possibly using it to drive ventilation as well, it's even better to just sidestep all problems by spinning up the whole ship.
To me, it seems most sane for everyone who might leave the rotating hab on a daily basis (of which there shouldn't be many) to be in skin-tight mechanical counterpressure suits at all times, meaning all you need to do is put on your helmet and wait for the airlock to cycle. Clothing can be worn on top if desired.
I don't see the need to pressurize the main ship to begin with, other than a compact, heavily protected central module (used for combat or bad solar storms). Too much pressurized surface area invites tiny leaks that go unnoticed for months; air is a fire hazard, it lets shockwaves travel, etc. The rotating hab is made for humans to live and work in; it might as well get used for that purpose to the exclusion of the rest, as much as feasible.
My other reason for considering a connection in vacuum is to be able to have practically no friction, using magnetic bearings. For combat situations, after evacuating everyone from the hab to the ship, this allows using the hab as high-RPM flywheel energy storage for weaponry (you've got all that mass there; might as well get your money's worth out of it).
For purely civilian ships, the energy savings should be worth it if the airlock is not used much (really, how often does one need to leave the hab while in transit?... ).
Spinning the whole ship works, and is the simple answer. But it causes issues with precisely pointing spinal weaponry. Spinning down the ship for combat wastes dV (more mass than airlock losses, I bet), unless you happen to have a really heavy flywheel somewhere...
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Post by AtomHeartDragon on Jan 4, 2019 16:24:26 GMT
To me, it seems most sane for everyone who might leave the rotating hab on a daily basis (of which there shouldn't be many) to be in skin-tight mechanical counterpressure suits at all times, meaning all you need to do is put on your helmet and wait for the airlock to cycle. Clothing can be worn on top if desired. You'd probably want to avoid that whenever possible. Mechanical counterpressure is difficult do do properly, may lead to all sorts of health problems if not, and is by no means convenient either way. I would have everyone in those in combat or whenever major penetration would be otherwise likely (actually, I would also have air out while in combat), but not all the time. While there is no point to pressurize whole ship, having separate centrifuge implies you want some non-spinning parts for whatever reason. For combat situations having your centrifuge rotate at all is a grade A insanity. Not only is it going to kill your manoeuvrability with gyroscopic effects, but it is also going to mess you up really badly if damaged in combat. If you have a pile, energy is effectively free and waste heat is something you need to deal with anyway, so energy losses are not much of an issue - doubly so if you can channel the aerodynamic effects to do something useful, like drive parts of your ventilation system. You really don't need to spin up and despin rapidly, so you don't need huge flywheels, or might use electrical propulsion for that (or tethers, or electromagnetic tethers, etc), and you definitely don't want to be tumbling all the time in combat.
I would imagine safety concerns to be very important if you are running with minimal crew (because monkey tax) and the nearest replacements are a year away. Meanwhile two of your ships had their combat efficiency drop by half ever since their main battery is manned by one-armed gunners, one started to handle like a freighter (due to one handed tactical pilot) and one has been close to mutiny ever since the (allegedly essential) cook stuck his head into the joint. Accidents do happen and once you cannot reduce the accidents below the threshold you need via training, you need a redesign. I agree, but mind the gap. Nearest reserve crews are not a year away.
You are also likely to have more tanks than space warships in any case.
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Post by lucubratory on Jan 9, 2019 0:03:14 GMT
Could you have an open axial corridor (have to walk up a ramp or ladder to access) with some sort of circular joint so the device can spin in vacuum on magnetic bearings but still allow suitless transfer? Could the circular joint be something like a long and sinuous 2D Tesla valve, or would that leak too much atmosphere? What about a plasma window, could you make those in a circle between the rotating and non-rotating parts of the ship? If you could get tolerances tight enough maybe the joint could only have a millimetre clearance around a circle a metre wide, giving an area for the plasma window of 31.4cm^2, I think. That would require an energy for the plasma window of 50.6kW according to Wikipedia and some Google calculating. I think that's peanuts compared to most ship systems in COADE? If manufacturing tolerances can be made that tight. Current magnetic levitation trains can apparently levitate with as little as 1cm clearance from the guideway. I don't know how much they vary from that height (obviously well under a cm for safety factor) and how engineerable that is. Maybe you could combine a plasma window with a 2D Tesla valve to lower the atmospheric pressure to be contained so the plasma window draws less power (if that does draw less power). Would be interested to hear people's thoughts.
Edit: I tried figuring out what energy a 1cm clearance would require for maintain its plasma window, it is 159.192kW. So the ships regularly built here could easily power a system where a rotating hab spins on magnetic levitation rails and has a plasma window seal in the centre of a decent size with achievable tolerance. So the only stumbling block is whether or not it's possible at all to run a plasma window between moving surfaces.
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Post by airc777 on Jan 9, 2019 15:38:47 GMT
I'm not familiar enough with the topic to debate weather or not you possibly could, but on weather or not you probably should:
What you are suggesting is building an actively supported structure, instead of a passively supported structure, for the purposes of containing atmosphere, in the context of a game where we build warships. You have a lot of faith in your power stability.
I'm not normally opposed to the idea of actively supported structures, I think low altitude orbital rings are a really cool mega structure, but I don't think an actively supported anything would make ideal wartime infrastructure. Civilian spaceships would be fine though, just octuple redundant all of your power supplies.
If I had to do it I think I would just use several ring gaskets, and then maybe I would have something like a mercury vacuum pump or some other low pressure pump process recollecting gas as it leaked passed the first gasket if it needed it. Instead of spending a large amount of power actively holding in the atmosphere I would just speed a little power combating the friction on the ring gaskets.
Might be a kind of cool tourist attraction if you could up scale the tech to the point where you could do safe 'suit-less' spacewalks through plasma window tunnels. Depending on how bright and/or transparent the tunnel is and how much heat they would radiate to the passengers.
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Post by AtomHeartDragon on Jan 9, 2019 18:58:53 GMT
Could you have an open axial corridor (have to walk up a ramp or ladder to access) with some sort of circular joint so the device can spin in vacuum on magnetic bearings but still allow suitless transfer? Could the circular joint be something like a long and sinuous 2D Tesla valve, or would that leak too much atmosphere? What about a plasma window, could you make those in a circle between the rotating and non-rotating parts of the ship? If you could get tolerances tight enough maybe the joint could only have a millimetre clearance around a circle a metre wide, giving an area for the plasma window of 31.4cm^2, I think. That would require an energy for the plasma window of 50.6kW according to Wikipedia and some Google calculating. I think that's peanuts compared to most ship systems in COADE? If manufacturing tolerances can be made that tight. Current magnetic levitation trains can apparently levitate with as little as 1cm clearance from the guideway. I don't know how much they vary from that height (obviously well under a cm for safety factor) and how engineerable that is. Maybe you could combine a plasma window with a 2D Tesla valve to lower the atmospheric pressure to be contained so the plasma window draws less power (if that does draw less power). Would be interested to hear people's thoughts. Edit: I tried figuring out what energy a 1cm clearance would require for maintain its plasma window, it is 159.192kW. So the ships regularly built here could easily power a system where a rotating hab spins on magnetic levitation rails and has a plasma window seal in the centre of a decent size with achievable tolerance. So the only stumbling block is whether or not it's possible at all to run a plasma window between moving surfaces. Uh, but what problem are you actually trying to solve?
If you tumble a needleship, or hang an inflatable hab (directly or on a tether) from needleship's nose or tie two such habs or two needleships together, you get artificial gravity without any sort of centrifuges, rotating joints (hermetically sealed or not), engineering complexity or large diameter structures that are unfriendly to armour and shadowshields alike. You also get something that is going to be the easiest to remake back into something usable from badly damaged parts (you can, for example, create ugly and lopsided inflatable hab from the pieces of the old one after its storage compartment got remodelled by high calibre k-slug - try field fixing normal hermetic centrifuge or even a toroidal hab meant to be used as one and needing to be very carefully balanced, meanwhile a single long mass or two tethered masses spinning about their centre of gravity are self-balancing).
If you want a more conventional ring shaped rotating hab, you can still just rotate the whole ship/station, without needing rotating joints. Depending on risk of dings docking can be accomplished by just flying into the hub and manoeuvring there (basically landing on the inner surface of the dock), or parking near the hub and getting picked by a robotic manipulator and berthed. Beyond that there is no clear reason for despun platforms, let alone large, pressurized ones.
If you *really* want a separate centrifuge on your ship and are ready to take the mass and complexity penalties, then rotating inside cylindrical or toroidal pressurized volume is not going to cause severe enough energy losses to make you lose any sleep over the energy (you do have a pile, after all) or additional waste heat and you might repurpose resulting airflow for ventilation, reclaiming a lot of wasted energy for practical use. For combat you are going to de-spin any centrifuge you have anyway, and it's easier to not have to design for both rotation and hermeticity at the same time.
Tesla valve is very clever but far too leaky for this purpose, plasma windows are antithesis of failsafe. Why not just not make things harder for yourself.
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Post by lucubratory on Jan 9, 2019 22:39:07 GMT
[/div][/quote] I didn't realise it was exclusively a warship. I probably wouldn't suggest that for a small warship, I was thinking of a primarily civilian vessel. If it was a warship, would it be okay on a very large capital ship to have those sort of connections for their residential quarters? Like one that was ~16km long and ~8km wide? My rough goal is to minimise maintenance by trying to make as many things as possible solid-state, and to maximise self-sufficiency by minimising the amount of atmosphere and other volatiles you leak. Edit: Also as for failsafe, what if it also had some emergency gaskets that could extend to cut off atmosphere loss (at the cost of drag and maintenance) in the case of a power outage or plasma window failure? |
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