Tethered bolas and counterweight stations

A habitat and a counterweight (or two habitats) joined by cable and spun end-over-end. This was the thread's opening concept, and it lost ground to rigid designs over time.

Key results

• Basic geometry. At 1 g, 2 rpm needs about 223–225 m of cable per side and 1 rpm about 890 m Roy_H, 2014-02-16.
• The "simplest realistic answer" is two capsules on a rope JohnFornaro, 2014-02-17.
• The cheapest demo is two small modules on a tether of at least 225 m at 2 rpm Coastal Ron, 2015-02-21; ppnl, 2018-07-08.
• daveklingler: 900 m cables are off the shelf daveklingler, 2015-10-08.
• Counterweights.
• A spent upper stage: DragonLab tethered to its own F9 second stage, with a cable reel in the nose mikelepage, 2014-03-14.
• The AGNEP concept (selenianboondocks) nacnud, 2014-02-17.
• Propellant depot tanks A_M_Swallow, 2015-08-30.
• A nuclear reactor on a NASA TransHab-tensegrity study Lampyridae, 2016-01-13.
• Cable mass and strength.
• Steel cable, 2 inch, ~10 kg/m: 2.2 t at 220 m, 8.9 t at 890 m gbaikie, 2014-03-16.
• Dyneema or Spectra have 8–15× steel's strength-to-weight gbaikie, 2014-03-17.
• A 156 mm Dyneema rope masses 14.4 t/km with 13,300 kN break load. As a 225 m radius hoop it supports ~960 kg per metre of rim, a third of a BA330's linear mass, so many cables are needed Coastal Ron, 2015-09-12.
• Cable is lighter than a truss for the same load Twark_Main, 2022-10-14.
• Carbon-fibre Hoytethers (tethers.com) give redundancy Roy_H, 2018-06-23.

Problems identified

1. No rigidity. A cable system is a flexible gyroscope; out-of-plane forces go unresisted, so it cones and wobbles DMeader, 2014-02-20; JohnFornaro, 2014-03-16.
2. Resonance. A pure tether is a spring. A compression element such as a truss or pressurized tube is needed for damping mikelepage, 2015-02-17; RanulfC, 2014-03-21.
3. Moving mass. Elevators change the moment of inertia and curve the tethers dror, 2014-03-17. Cable-suspended modules twist when crew move, like a swinging bridge platform Paul451, 2019-01-07.
4. Failure modes.
5. If spin slows or a module fails, tensioned cables go slack or pull the structure apart mikelepage, 2018-07-03.
6. A cable-supported ring can "unzip" once a rim link and its cables are cut, so designs need sacrificial "breaker" modules mikelepage, 2018-08-06.
7. After a cable break the parts stay in similar orbits, because rotation velocity ≪ orbital velocity rklaehn, 2014-02-17.
8. Tensioned cables are "a big yikes": their failure modes are unkind to pressurized cans Twark_Main, 2024-09-02.
9. Operations.
10. Long cables beyond ~200 m are beyond the human-spaceflight state of the art Paul451, 2015-10-13.
11. Gemini 11 showed that gravity gradient alone did not keep a tether taut, and manual stabilization was hard muomega0, 2014-03-16; mikelepage, 2019-03-29.
12. An elevator must be treated as a crewed spacecraft, with each move a docking Jim, 2014-02-20.

Hybrids that survived

• Inflatable tunnel compressed by internal tensile cables. It is effectively rigid, pressurized, shielded and self-deploying Paul451, 2016-01-17; Paul451, 2022-10-11.
• Telescoping or truss compression members with tethers mikelepage, 2015-02-17.
• Superconducting magnetically inflated cables (Powell and Maise, NIAC 2006) for rigid lightweight structures Cedalion, 2014-03-12; Cedalion, 2015-10-21.
• Joe Carroll's tether research facility (2010), with nodes at the CoM and at 0.06, 0.16 and 0.38 g (ratios about e) kdhilliard, 2015-02-24.
• Twark_Main's "Simplest Dumbest AG" tether service module between two nose-docked Starships. See concept page.

Open questions

• Can automated cable control (the lesson from Gemini) make pure-tether stations safe enough for crews?
• How do tethers survive micrometeoroids over years? Hoytether-style redundancy is the usual answer.

Related pages

• Cables vs trusses · Dumbbell/barbell · Roy_H tethered hub · Gemini 11

Sources

Forum posts

• Roy_H, 2014-02-16 (reply #0, 7 likes): Opening proposal: BA330s on 223 m (2 rpm) or 890 m (1 rpm) cables. Post. Attachments: Bigelow.png.
• nacnud, 2014-02-17 (reply #13): AGNEP counterweight link. Post.
• rklaehn, 2014-02-17 (reply #22, 4 likes): Cable break leaves parts in similar orbit (4 likes). Post.
• JohnFornaro, 2014-02-17 (reply #35, 3 likes): Two capsules on a rope. Post.
• DMeader, 2014-02-20 (reply #81): Cable system = flexible gyroscope. Post.
• Jim, 2014-02-20 (reply #86): Elevators = crewed spacecraft. Post.
• Cedalion, 2014-03-12 (reply #149): Magnetically inflated cables. Post.
• mikelepage, 2014-03-14 (reply #151, 1 like): DragonLab + own upper stage counterweight (mikelepage first post). Post.
• muomega0, 2014-03-16 (reply #171): Gemini 11 lessons. Post.
• gbaikie, 2014-03-16 (reply #172): Steel cable mass. Post.
• JohnFornaro, 2014-03-16 (reply #182): Cable-only resists only in-plane tension; coning. Post. Attachments: Tensegrity-Sphere.png.
• gbaikie, 2014-03-17 (reply #191): Dyneema/Spectra 8–15× steel. Post.
• dror, 2014-03-17 (reply #196): Elevators curve tethers, change MoI. Post.
• RanulfC, 2014-03-21 (reply #242, 1 like): Tension + compression damps (Tacoma). Post.
• mikelepage, 2015-02-17 (reply #285): Tether = spring → resonances; add compression element. Post.
• Coastal Ron, 2015-02-21 (reply #358): Cheapest demo: 2 modules, ≥225 m tether, 2 rpm. Post.
• kdhilliard, 2015-02-24 (reply #392): Carroll 2010 tether AG facility (key source). Post.
• A_M_Swallow, 2015-08-30 (reply #573): Tank station: hab + 3 cryo tanks as counterweights. Post. Attachments: Artifical_g_spacestation.jpg, Artifical_g_spacestation_tank.jpg.
• Coastal Ron, 2015-09-12 (reply #629): Dyneema hoop capacity calc. Post.
• daveklingler, 2015-10-08 (reply #688): 900 m cables off-the-shelf; simple station designs. Post.
• Paul451, 2015-10-13 (reply #702): Long cables beyond state of art. Post.
• Cedalion, 2015-10-21 (reply #707): MIC NIAC 2006 link. Post.
• Lampyridae, 2016-01-13 (reply #808): NASA TransHab + tensegrity + reactor counterweight ~30 m. Post.
• Paul451, 2016-01-17 (reply #827, 2 likes): Inflatable tunnel-truss with internal cables. Post.
• Roy_H, 2018-06-23 (reply #842): Roy_H wheel with Hoytether cables. Post. Attachments: MainCutaway.png, AgriculturalModule3.png, StateroomLayout.png.
• mikelepage, 2018-07-03 (reply #850): Cable-stayed ring collapses if spin slows. Post.
• ppnl, 2018-07-08 (reply #899): Hab + counterbalance on 225 m cable. Post.
• mikelepage, 2018-08-06 (reply #1187): Ring unzipping; breaker modules. Post. Attachments: Severed capsule+cables.png.
• Paul451, 2019-01-07 (reply #1271): Cable-suspended modules twist when crew move. Post. Attachments: People on platform.png.
• mikelepage, 2019-03-29 (reply #1421, 1 like): Gemini tether hard to stabilize; automate. Post.
• Paul451, 2022-10-11 (reply #1977, 2 likes): Inflatable tunnel + cables; 2 Starships → 76 m station. Post.
• Twark_Main, 2022-10-14 (reply #2081): Cable lighter than truss for same MN. Post.
• Twark_Main, 2024-09-02 (reply #4780): Tensioned cables bad failure modes. Post.

External references

• Carroll, J. (2010). Design Concepts for a Manned Artificial Gravity Research Facility. SSI Space Manufacturing 14 proceedings.
• Powell, J. et al. (2006). Magnetically inflated cable (MIC) structures. NIAC.
• AGNEP, selenianboondocks.com/2010/06/agnep1/.
• Tethers Unlimited, Hoytether.