Dumbbells, barbells, batons and sticks

Two (or more) habitable masses on a rigid connector spinning end-over-end. The thread's two poles were Twark_Main's mass-efficient spheres on a bar and Vast's "spinning stick".

Key results

Why barbells

  • Easiest geometry to start with. A dog-bone (two habitation sections on a truss) is easier than a torus NovaSilisko, 2014-02-23.
  • The cheapest station is a single module attached to its upper stage, spinning end-over-end: "people overthink spin stations" Paul451, 2015-03-31.
  • Paul451's tumbling pigeon: BA-330 + node + upper stage, radius 14–15 m, Mars g on the bottom deck at 5 rpm and 1 g below 8 rpm, about 25 t Paul451, 2016-01-13; Paul451, 2016-01-13.
  • Every g-level at once. A baton offers every g from hub to tip, so many customers and long-term experiments can share it Twark_Main, 2022-08-27; Twark_Main, 2024-09-12.
  • Scales down gracefully. The minimum barbell pressurized volume is about 67 m³, against ~3100 m³ for a minimum 1 g torus (2 m minor radius, 4 rpm): "a torus can't be shrunk" Twark_Main, 2022-10-30. "A barbell is a shrunken torus: barbell → cross → add modules → sausage-link torus" Paul451, 2022-10-30.
  • Mass efficiency (the geometry argument).
  • Spheres on a bar minimize surface area per unit volume, so shielding, pressure hull and MMOD mass are all lower Twark_Main, 2022-12-04; Twark_Main, 2023-02-21.
  • For the same volume (2.1×10⁷ m³) and 7.5 t/m² shielding, a 2-sphere barbell needs ~3.5 Mt of shielding, half that of a banded torus mikelepage, 2023-03-06.
  • lamontagne's steel check (500 m radius torus with 10 m rim radius, versus 160 × 20 m spheres at 100 kPa and SF 3): the barbell is ~33% lighter, or ~25% lighter if the torus uses its spare longitudinal capacity lamontagne, 2023-09-21. With filament-wound materials a torus is roughly equal to the beads Twark_Main, 2023-09-27.
  • At a modest 150 m radius, shielding is only 5.6% of barbell mass Twark_Main, 2023-09-17.
  • High-thrust capable. Sticks and barbells tolerate axial burns for interplanetary transfer, and each is useful before a larger station is complete mikelepage, 2023-09-18; mikelepage, 2024-09-17.

Problems and fixes

  • Stability. A symmetric barbell has two equal maximum axes. Mass on the axis (a docked capsule) makes it unstable Paul451, 2019-03-23. Fixes: arrays at 90° in the spin plane; a third arm (≥60% of the length) or an X shape; an elongated habitat. See stability. A thin Vast-type stick is not passively stable (ratio ≈1.01) and needs CMGs Twark_Main, 2024-09-20.
  • Docking on the axis of a two-lobe dumbbell is unstable. Options:
  • Stop the rotation to board Aussie_Space_Nut, 2019-03-24.
  • Roll the dumbbell and shuttle 90° into the rotation plane under control lamontagne, 2019-03-25.
  • Dock off-axis in the rotation plane near the hub rakaydos, 2019-03-25.
  • Avoid docking at the centre altogether Twark_Main, 2024-01-30.
  • Circulation. A bola is like "two office buildings separated by a long tunnel", with an elevator whose gravity reverses mid-trip Coastal Ron, 2024-07-30; Twark_Main, 2024-07-31.
  • Module clustering. Instead of spreading 60 identical modules in a ring, cluster them as two 2×3×4 blocks on two arms. Only the outer walls need shielding, corridors shrink, and there are more escape routes Paul451, 2023-06-05. Tin-can modules in a honeycomb at each end lose only ~9% of volume Twark_Main, 2024-07-23.
  • Two g-levels by imbalance. A 76 m, two-Starship-launch station balanced 1/3 to 2/3 by mass gives lunar g at one end and Mars g at the other at 2.5 rpm Paul451, 2022-10-11.

Notable instances

  • Paul451 BA-330 barbell and roadmap (page).
  • Twark_Main double-sphere barbell (page).
  • Vast spinning stick (page).
  • lamontagne dumbbell fleet (page).
  • Janhunen's shielded dumbbell L5 settlement: 97% of its mass is shielding at a population of 200. Twark_Main noted this is near the paper's worst case; at a population of 105, total mass per person falls from 400 t to 50 t lamontagne, 2023-04-26; Twark_Main, 2023-04-29.
  • mikelepage's asteroid-pit bola (page).

Open questions

  • How to dock large ships to small barbells without despinning.
  • Can the elevator transit between the two ends be made routine and safe (gravity reversal, emergency egress)?

Sources

Forum posts

  • NovaSilisko, 2014-02-23 (reply #120, 1 like): Dog-bone easier than torus. Post.
  • Paul451, 2015-03-31 (reply #554): Cheapest: single module + upper stage end-over-end. Post. Attachments: Station.png.
  • Paul451, 2016-01-13 (reply #806): Tumbling pigeon: 14–15 m radius, Mars g @5 rpm. Post.
  • Paul451, 2016-01-13 (reply #819): Tumbling pigeon ~25 t. Post.
  • Paul451, 2019-03-23 (reply #1358): Capsule on dumbbell axis unstable. Post.
  • Aussie_Space_Nut, 2019-03-24 (reply #1374): Dumbbell summary: stop to board; lifeboat per hab. Post.
  • rakaydos, 2019-03-25 (reply #1390): Dock in rotation plane near hub. Post.
  • lamontagne, 2019-03-25 (reply #1392, 2 likes): Controlled roll after docking. Post.
  • Twark_Main, 2022-08-27 (reply #1733): Baton offers every gravity level. Post.
  • Paul451, 2022-10-11 (reply #1977, 2 likes): Unbalanced dumbbell: lunar + Mars g at 2.5 rpm. Post.
  • Twark_Main, 2022-10-30 (reply #2258): Minimum barbell 67 m³ vs minimum torus 3100 m³. Post.
  • Paul451, 2022-10-30 (reply #2260): Barbell is shrunken torus. Post.
  • Twark_Main, 2022-12-04 (reply #2539, 2 likes): Double-sphere barbell concept. Post. Attachments: russianBola.JPG, russianBola2.JPG.
  • Twark_Main, 2023-02-21 (reply #3381): Minimize area/volume with thick shields. Post.
  • mikelepage, 2023-03-06 (reply #3494): 2-sphere barbell half the shielding of banded torus. Post.
  • lamontagne, 2023-04-26 (reply #3637): Janhunen dumbbell paper link. Post.
  • Twark_Main, 2023-04-29 (reply #3651): Correction: 97% near worst case; 105 people → 50 t/person. Post.
  • Paul451, 2023-06-05 (reply #3784, 3 likes): Cluster 60 modules as two blocks (3 likes). Post. Attachments: Modules as dumbbell.jpg.
  • Twark_Main, 2023-09-17 (reply #3954): 150 m barbells: shielding 5.6% of mass. Post.
  • mikelepage, 2023-09-18 (reply #3968): Barbells tolerate high-thrust burns; useful before torus complete. Post.
  • lamontagne, 2023-09-21 (reply #4007): Barbell ~33% (or 25%) lighter than torus in steel. Post.
  • Twark_Main, 2023-09-27 (reply #4032): Filament materials: torus ≈ beads. Post.
  • Twark_Main, 2024-01-30 (reply #4446): Barbells limit CoM displacement; don't dock at centre. Post.
  • Twark_Main, 2024-07-23 (reply #4658): Honeycomb of cans at each end, 9% loss. Post.
  • Coastal Ron, 2024-07-30 (reply #4669): Bola = two buildings joined by tunnel. Post.
  • Twark_Main, 2024-07-31 (reply #4675): Tunnel = elevator shaft; torus is long thin corridor. Post.
  • Twark_Main, 2024-09-12 (reply #4793): Stick stations for variable-g experiments. Post.
  • mikelepage, 2024-09-17 (reply #4796): Sticks carry axial thrust. Post. Attachments: IMG_2976.jpg.
  • Twark_Main, 2024-09-20 (reply #4804): Vast stick ratio ≈1.01. Post.

External references

  • Janhunen, P. (2020). Shielded dumbbell L5 settlement. arXiv:2004.02241.