Non-rotating hubs, bearings and de-spun sections

Should part of the station stay still, and if so how does it connect to the rotating part? This is one of the thread's longest-running debates, from the opening post in 2014 to the Peter_GST and "zero-g core" discussions of 2025.

Key results

  • Why a non-rotating hub.
  • Microgravity work, a docking target that visiting vehicles need not spin-match, and stationary solar arrays and antennas Roy_H, 2014-02-16; JasonAW3, 2014-03-19; mikelepage, 2014-03-22.
  • Twark_Main's taxonomy: a station with ≥3 ports ("Class 3") effectively needs de-spun docking. The trivial version is a standard 5-port node on a despin bearing Twark_Main, 2023-02-07; Twark_Main, 2023-02-11.
  • The case against.
  • A rotating hub avoids a failure-prone bearing; only mass entering or leaving needs to rotate Coastal Ron, 2015-02-24; blasphemer, 2014-02-16.
  • With a non-rotating hub, the centre of mass must stay precisely on the bearing axis or there is "horrific vibration", so mass balancing becomes critical lamontagne, 2018-08-04.
  • A non-rotating axial section on a bearing transmits wheel tilt. Put the bearings at the CoM Coastal Ron, 2025-04-14.
  • Peter_GST's conclusion: a hub on a bearing vibrates unless perfectly balanced, so true µg is best on small free-flyers Peter_GST, 2025-04-11.
  • Bearing technology.
  • Magnetic bearings and slip rings are "1950s tech" RanulfC, 2014-03-19. Magnetic levitation is unstable at low rpm lamontagne, 2022-10-07.
  • Very large roller bearings are mechanically bad: cost and resistance grow with diameter Robotbeat, 2023-02-19. Barley: move the bearing to the rim (wheels on a track) so the larger lever reduces the forces Barley, 2022-11-29.
  • Bearing drag must be cancelled by thrusters or flywheel desaturation Paul451, 2015-11-15.
  • lamontagne's failure mitigation: 4 bearings (2 needed) and two linear induction motors to cancel bearing friction lamontagne, 2018-08-06.
  • A 100 m electromagnetic bearing with steel-wheel backups raises manufacturing questions JohnFornaro, 2023-02-17.
  • Power and data across the joint. Slip rings, roll rings, inductive and microwave options are covered in seals and joints A_M_Swallow, 2015-11-13; lamontagne, 2022-10-07.
  • Designs with the rotating/non-rotating interface inside the pressure vessel. These need no vacuum-tight bearing:
  • A counter-rotating room floating on air bearings inside the hub Aussie_Space_Nut, 2018-10-01.
  • Internal centrifuges in a stationary shell Paul451, 2023-02-16.
  • Two counter-rotating AG sections giving a stationary micro-g core, L = 0, and a stationary outer shield Twark_Main, 2023-01-18.
  • The "zero-g core": a de-spun section inside the pressure hull, or a lighter inverted option of a de-spun rack on a Stewart platform Twark_Main, 2025-09-14.
  • redneck: a 100 m hub volume enclosing a de-spun 95 m µg facility; air drag at 15 m/s needs power, but "a minute from 1 g to µg" redneck, 2025-09-20.
  • A de-spun zero-g section inside Kalpana's empty central volume lamontagne, 2023-01-09.
  • Counter-rotating stationary shields. An outer counter-rotating or stationary shield ring on magnetic bearings combines shielding and angular-momentum cancellation. Island One also had a fixed shield 1, 2023-01-10; Twark_Main, 2022-11-30.
  • A counter-rotating pair still needs strong bearings to transmit opposing torques mikelepage, 2023-09-12. Non-coplanar masses precess and put torque on the bearings Coastal Ron, 2023-01-09.
  • Hubless roller-bearing designs. lamontagne's hubless station has two counter-rotating rings with an intercalary element holding rollers and transfer elevators lamontagne, 2022-11-29. Rim magnetic bearings appear in "Babylon 4" rakaydos, 2019-03-30.

Open questions

  • Can a stationary hub be balanced well enough for useful microgravity work, or does µg research belong on free-flyers? Peter_GST, 2025-04-11
  • At what station size does an internal de-spun core beat a separate co-orbiting zero-g station?

Sources

Forum posts

  • Roy_H, 2014-02-16 (reply #0, 7 likes): Opening proposal: non-rotating hub. Post. Attachments: Bigelow.png.
  • blasphemer, 2014-02-16 (reply #7, 2 likes): Rotate hub too; ships match. Post.
  • RanulfC, 2014-03-19 (reply #208): Magnetic bearings & slip rings old tech. Post.
  • JasonAW3, 2014-03-19 (reply #214): Rotary coupling simplest. Post.
  • mikelepage, 2014-03-22 (reply #244, 1 like): Stationary antennas/star trackers issue. Post.
  • Coastal Ron, 2015-02-24 (reply #391): Rotating hub preferable. Post.
  • A_M_Swallow, 2015-11-13 (reply #769): Slip rings easy; fluids hard. Post.
  • Paul451, 2015-11-15 (reply #772): Bearing drag → thrusters/flywheel. Post.
  • lamontagne, 2018-08-04 (reply #1144): CoM must stay at hub. Post. Attachments: Station3c.jpg, Station3d.jpg.
  • lamontagne, 2018-08-06 (reply #1186): 4 bearings, 2 LIMs. Post.
  • Aussie_Space_Nut, 2018-10-01 (reply #1223): Inner lab on air bearings. Post.
  • rakaydos, 2019-03-30 (reply #1444, 1 like): Rim magnetic bearings. Post.
  • lamontagne, 2022-10-07 (reply #1890): ISS roll rings. Post. Attachments: 19940029729.pdf, 19920014958.pdf, 19940028788.pdf.
  • lamontagne, 2022-10-07 (reply #1893): Magnetic bearings unstable at low rpm. Post.
  • Barley, 2022-11-29 (reply #2514): Bearing at rim reduces forces. Post.
  • lamontagne, 2022-11-29 (reply #2517, 1 like): Hubless roller-bearing station. Post. Attachments: Extrusion construction (6).png, Extrusion construction (5).png.
  • Twark_Main, 2022-11-30 (reply #2522): Stationary shield around rings. Post.
  • lamontagne, 2023-01-09 (reply #2830, 3 likes): De-spun section inside Kalpana. Post. Attachments: Image16.png, Image15.png, Image17.png, Image12.png, cut2.png.
  • Coastal Ron, 2023-01-09 (reply #2843): Non-coplanar counter-rotation torques. Post.
  • 1, 2023-01-10 (reply #2847, 1 like): Counter-rotating shield ring (1 like). Post.
  • Twark_Main, 2023-01-18 (reply #2967): Split AG into two counter-rotating sections. Post.
  • Twark_Main, 2023-02-07 (reply #3256): Station class taxonomy. Post.
  • Twark_Main, 2023-02-11 (reply #3282): Trivial Class 3: node on despin bearing. Post.
  • Paul451, 2023-02-16 (reply #3325, 2 likes): Centrifuge inside open air; bearings needn't be vacuum-tight (2 likes). Post.
  • JohnFornaro, 2023-02-17 (reply #3338): 100 m bearing manufacture?. Post.
  • Robotbeat, 2023-02-19 (reply #3353): Large roller bearings bad. Post. Attachments: D4F99FDF-FFC8-4337-BC1E-305B51AB87BC.png.
  • mikelepage, 2023-09-12 (reply #3923): Counter-rotating pair needs strong bearings. Post.
  • Peter_GST, 2025-04-11 (reply #4850): Hub vibrates; µg best on free-flyers. Post.
  • Coastal Ron, 2025-04-14 (reply #4866): Bearing transmits wheel tilt; align bearings with CoM. Post.
  • Twark_Main, 2025-09-14 (reply #4955): Zero-g core. Post.
  • redneck, 2025-09-20 (reply #4996): De-spun µg facility inside hub. Post.