Twark_Main: double-sphere barbell, 3-arm "tomahawk" stick

Twark_Main's argument that the barbell is "mathematically optimal" at near-term scale, and his minimal stable three-arm variants.

Design summary

  • Double-sphere barbell (2022). Two big spheres on a barbell (equal or unequal masses, e.g. Mars g and Earth g), built from small mass-produced parts Twark_Main, 2022-12-04; Twark_Main, 2022-12-05; Twark_Main, 2022-12-07; Twark_Main, 2022-12-07:
  • A 3D tension skeleton carries the pressure; light outer panels; a pressurized tunnel with a redundant elevator; cables in a tensile hyperboloid.
  • The "scaled barbell" is missing from the O'Neill 1975 comparison. It has low area/volume (cheap shielding) and high radius/volume (small minimum viable station, best Coriolis).
  • Example at 0.7 rpm and 1 g: 1800 m major radius, 180 m spheres, ~10 million m² of floor, ~6 Mt ≈ 60,000 Starship flights. Not near-term.
  • Recap of his optimal design (2023): a double barbell, ideally 100+ m radius, with a cellular internal pressure-isolation structure and 20–30 cm PE + boron shielding. For very large volumes: a non-rotating sphere with internal centrifuges Twark_Main, 2023-09-16.
  • For pure 1 g volume, several ~150 m radius barbells where shielding is only 5.6% of mass Twark_Main, 2023-09-17.
  • "Reasoning by drawing" makes barbell→torus seem natural; just scale up instead Twark_Main, 2023-09-18.
  • Three-arm stick / "tomahawk" (2024). Add a short third spoke (not at 120°; angles ~170/105/105 to put the CoM at the join). "Nothing left to take away" Twark_Main, 2024-08-27; Twark_Main, 2024-08-31.
  • Balance condition: short arm length = 2 sin((180° − angle between long arms)/2) Twark_Main, 2024-09-02. Iz/Iy against arm angle Twark_Main, 2024-09-06.
  • A ~3000 t tomahawk with a 3:1 mast at r = 300 m costs only ~14 t/yr for sun-pointing at Isp 2500 s Twark_Main, 2025-04-30.
  • Three arms suffice for intermediate-axis stability (−25% for an MVP) Twark_Main, 2023-06-09.

Critiques raised

  • Paul451: spheres waste volume at small scale; maximize floor area at the target g Paul451, 2022-12-06.
  • Coastal Ron doubts three-arm stability and prefers an "X" 4-arm testbed Coastal Ron, 2024-08-28.
  • JohnFornaro: 150 m radius 1 g = 2.5 rpm, so Coriolis concerns; the barbell as a stepping stone to a wheel JohnFornaro, 2023-09-17.

Open questions

  • What is the minimum radius at which a two-sphere barbell beats a torus on total cost?

Sources

Forum posts

  • Twark_Main, 2022-12-04 (reply #2539, 2 likes): Double sphere barbell. Post. Attachments: russianBola.JPG, russianBola2.JPG.
  • Twark_Main, 2022-12-05 (reply #2543): Detail. Post.
  • Paul451, 2022-12-06 (reply #2554, 1 like): Spheres waste volume. Post.
  • Twark_Main, 2022-12-07 (reply #2558): Scaled barbell. Post. Attachments: big_dumbell.png.
  • Twark_Main, 2022-12-07 (reply #2559, 1 like): Numbers. Post.
  • Twark_Main, 2023-06-09 (reply #3813): Three arms suffice. Post.
  • Twark_Main, 2023-09-16 (reply #3949): Optimal design recap. Post.
  • Twark_Main, 2023-09-17 (reply #3954): Several barbells. Post.
  • JohnFornaro, 2023-09-17 (reply #3959): Coriolis concerns. Post. Attachments: Ring-SK-207-Assembly-Sequence.jpg.
  • Twark_Main, 2023-09-18 (reply #3964): Reasoning by drawing. Post.
  • Twark_Main, 2024-08-27 (reply #4766): Third arm. Post.
  • Coastal Ron, 2024-08-28 (reply #4767): Doubts 3-arm. Post.
  • Twark_Main, 2024-08-31 (reply #4768): Angles. Post.
  • Twark_Main, 2024-09-02 (reply #4779): Balance formula. Post.
  • Twark_Main, 2024-09-06 (reply #4783, 3 likes): Iz/Iy plot (3 likes). Post. Attachments: moment_of_inertia_ratio_by_arm_angle.png.
  • Twark_Main, 2025-04-30 (reply #4919): Tomahawk sun-pointing cost. Post.