Cylinders, drums and spheres

Large enclosed volumes (O'Neill cylinders, Kalpana-type drums, Bernal spheres) and the "non-rotating shell with internal centrifuges" family.

Key results

Drums and cylinders

  • Kalpana One (Globus et al., 2007): 250 m radius. Beamed microwave power is needed because rim cells are insufficient Shevek23, 2019-10-26. Its stability rule, length < 1.3 × radius, keeps the spin axis the maximum-inertia axis lamontagne, 2026-09-19.
  • Kalpana 2 / "Space Settlement: An Easier Way": 56 m radius, 4 rpm, ~8000 t, all launched from Earth, up to 500 people at 40 m² each, in equatorial LEO with minimal shielding lamontagne, 2019-09-13. It breaks the 1.3 rule (length about 100 m) lamontagne, 2026-09-19.
  • The High Frontier: An Easier Way (Marotta & Globus, 2018): a minimal O'Neill cylinder of 56 m radius × 112 m at 4 rpm in equatorial LEO Habitant, 2025-10-17.
  • Habitant scaled it down further: 37.8 m radius at 5 rpm and 19 m long, built from about 6 Starships of pre-bent titanium sheets. It offers 4512 m² of gravity floor and 85,287 m³, holding 73 t of air at 0.7 bar Habitant, 2025-10-17.
  • A short disc avoids long-axis tumbling Habitant, 2025-10-20.
  • Advantages claimed for cylinders:
  • 1 g at the rim, 0 g at the axis, and everything in between Habitant, 2025-10-17.
  • High ceilings (the other side's floor) feel less claustrophobic.
  • Most volume per unit of shielding Habitant, 2026-09-15.
  • Disadvantages:
  • Volume more than about 4 m above a walkable surface has little value, so the metric should be usable surface per unit of volume lamontagne, 2023-02-21.
  • The central "big sky" void wastes about 80% of the shielded volume Twark_Main, 2023-09-28.
  • A pressurized empty centre is expensive; fill it with a de-spun zero-g section Twark_Main, 2023-01-11.
  • Endcaps.
  • Concave (inverted) endcaps cut air mass, support mirrors and add stability lamontagne, 2023-02-16; Twark_Main, 2026-09-19.
  • A 2:1 ellipsoid uses about 1/3 less material than a hemisphere with more uniform load (UMd X-Hab study). Paul451 argues hemispherical caps can simply be thinned more Habitant, 2026-09-12; Paul451, 2026-09-12.
  • Habitant asked an AI to compare cap shapes for tumbling risk; it favoured ellipsoids Habitant, 2026-09-19.

Spheres and "Death Stars"

  • Twark_Main's family (2023–2026): a large non-rotating spherical pressure and radiation shell, a non-rotating zero-g core, and one or two counter-rotating toroidal centrifuges entirely inside Twark_Main, 2023-02-14; Paul451, 2023-02-16. Its claimed properties:
  • no rotating vacuum seals;
  • unlimited non-spinning docking ports;
  • the most efficient shielding geometry;
  • zero net L, so easy pointing and cheap spin-up;
  • air drag between sections generates wind for trees Twark_Main, 2023-01-16.
  • The final "Pokeball" version puts two counter-rotating pseudo-tori in the hemispheres of a spherical shield. With levels from 0.67 g to 1 g, the zero-g spine is only 16% of the volume Twark_Main, 2026-09-20.
  • Objections: FOD bouncing "like a pinball", churned air and friction losses Coastal Ron, 2023-03-02; mikelepage, 2023-03-02. Answers: FOD traps and boundary-layer physics Twark_Main, 2023-03-06.
  • Ashworth (JBIS, October 2020): a double torus inside an unpressurized spherical shield, the cheapest radiation-protected volume lamontagne, 2023-03-01; lamontagne, 2023-03-02.
  • Fill the sphere. Instead of an empty sphere, build a "solid skyscraper" with gym- and theatre-scale rooms. Subdivided space feels less claustrophobic ("Old World cities: 10 acres per acre") Twark_Main, 2023-03-09. lamontagne: spheres are claustrophobic lamontagne, 2023-03-07.
  • In-space fabrication. Large spheres cannot be made from Earth-built modules, so they need in-orbit fabrication (geodesic frames, inflatables plus coating, electron-beam welding) mikelepage, 2023-03-09; Habitant, 2025-10-17.

Early examples in the thread

  • A long spinning cylinder (120 m × 8–10 m) spun about its middle, with lab floors at every g Roy_H, 2014-02-17.
  • Bigelow Olympus with a contracting spine, inflating into a 20 m disc: 0.5 g at 6 rpm lkm, 2014-02-17.

Open questions

  • Is the psychological value of large open volumes worth their cost? "Cinematographers love it, CPAs hate it" Twark_Main, 2023-01-11.
  • At what scale does in-space fabrication (and a cylinder or sphere) beat Earth-built modules?

Sources

Forum posts

  • Roy_H, 2014-02-17 (reply #31): Spinning cylinder with floors at every g. Post.
  • lkm, 2014-02-17 (reply #40): Inflatable disc 0.5 g @6 rpm. Post.
  • lamontagne, 2019-09-13 (reply #1668, 1 like): Small Kalpana 56 m radius, 4 rpm, 8000 t, 500 people. Post. Attachments: Kalplana_Scene 1.jpg.
  • Shevek23, 2019-10-26 (reply #1694): Kalpana One vs Kalpana 2 power. Post.
  • Twark_Main, 2023-01-11 (reply #2857): Empty centre costly → de-spun zero-g core. Post.
  • Twark_Main, 2023-01-11 (reply #2859): Cinematographers love it, CPAs hate it. Post.
  • Twark_Main, 2023-01-16 (reply #2914): Counter-rotating shell; wind for trees. Post.
  • Twark_Main, 2023-02-14 (reply #3301): Stationary outer shell + internal AG rings. Post.
  • lamontagne, 2023-02-16 (reply #3316, 1 like): Concave Kalpana: saves N2, supports mirrors. Post. Attachments: Base5cut.png, Base5front.png, Base5 housing.png.
  • Paul451, 2023-02-16 (reply #3325, 2 likes): Paul451's clear explanation of internal centrifuge (2 likes). Post.
  • lamontagne, 2023-02-21 (reply #3389, 1 like): Volume >4 m above floor has little value. Post.
  • lamontagne, 2023-03-01 (reply #3446): Ashworth double torus in sphere (JBIS 2020). Post. Attachments: sphere.PNG.
  • Coastal Ron, 2023-03-02 (reply #3447): Loose objects pinball. Post.
  • lamontagne, 2023-03-02 (reply #3449): Not pressurized; cheapest shielded volume. Post. Attachments: JBIS-v73-no10-October-2020_r4pw21.pdf.
  • mikelepage, 2023-03-02 (reply #3454, 1 like): Air churned; friction (1 like). Post. Attachments: Torus with Dumbell air shield.gif.
  • Twark_Main, 2023-03-06 (reply #3493): FOD traps; boundary layer. Post.
  • lamontagne, 2023-03-07 (reply #3518): Spheres claustrophobic. Post. Attachments: Sphere1.jpg.
  • Twark_Main, 2023-03-09 (reply #3534): Fill sphere with skyscraper. Post.
  • mikelepage, 2023-03-09 (reply #3537): Spheres need in-space fabrication. Post.
  • Twark_Main, 2023-09-28 (reply #4035, 1 like): BIS Avalon wastes ~80% on big skies. Post.
  • Habitant, 2025-10-17 (reply #5010): High Frontier: An Easier Way (book). Post.
  • Habitant, 2025-10-17 (reply #5012): Disc cylinder 37.8 m r, 5 rpm, 19 m long. Post.
  • Habitant, 2025-10-20 (reply #5022, 1 like): Short cylinder avoids tumbling. Post.
  • Habitant, 2026-09-12 (reply #5095): UMd X-Hab report: ellipsoidal endcaps. Post. Attachments: Bildschirmfoto 2026-09-12 um 10.51.20.png, UMd X_Hab_Final_Report__Final_xx.pdf.
  • Paul451, 2026-09-12 (reply #5104, 1 like): Spherical caps can be thinned more. Post.
  • Habitant, 2026-09-15 (reply #5111): Cylinders give max volume per shielding. Post.
  • Habitant, 2026-09-19 (reply #5149): AI analysis: ellipsoid caps better vs tumbling. Post. Attachments: oneill_cylinder_stability_summary.pdf.
  • Twark_Main, 2026-09-19 (reply #5151, 2 likes): Inverted endcaps best. Post.
  • lamontagne, 2026-09-19 (reply #5159, 2 likes): Kalpana stability rule. Post. Attachments: 2007KalpanaOne (1).pdf.
  • Twark_Main, 2026-09-20 (reply #5160, 1 like): Pokeball station (1 like). Post.

External references

  • Marotta, T. & Globus, A. (2018). The High Frontier: An Easier Way.
  • Globus, A. et al. (2007). The Kalpana One Orbital Space Settlement Revised. AIAA (space.nss.org/kalpana-one-space-settlement/).
  • Globus, A. (2017). Space settlement: an easier way, The Space Review, article 3181.
  • Ashworth, S. (2020). Gas-giant cycler with double torus in sphere. JBIS 73(10).
  • University of Maryland ENAE484 X-Hab "Minimum Crew Cabin Studies" final report (2020), ssl.umd.edu/node/119.