Mass balance, "wobble" and vibration

What happens when crew, cargo and fluids move, and how to keep the rotation axis where docking ports and bearings need it.

Key results

What actually happens

• A free-flying station rotates about its centre of mass. Shifting mass within the rotation plane just moves the axis to a new stable position. That gives a fixed floor tilt or a CoR offset, not a rolling "wobble" or vibration Paul451, 2015-11-09; Paul451, 2023-01-17; Paul451, 2023-11-06.
• A barbell with ten times more mass on one end is still stable; the imbalance can even be deliberate, to get two g-levels Paul451, 2023-11-06.
• Axial (out-of-plane) imbalance gives a fixed lean or precession of the axis. That is the case that matters Paul451, 2015-03-03; Paul451, 2023-11-08. The correct terms are nutation (from internal mass shifts) and torque-free precession (principal axis ≠ L) Barley, 2023-11-08.
• When CoR offset matters:
• For docking: the port must be at the CoR, and current docking standards tolerate 100 mm lateral and 4° angular misalignment and 0.2°/s rotation mismatch Paul451, 2023-05-11.
• For a non-rotating hub on a bearing: any offset causes "horrific vibration" or a whip in the non-rotating section Paul451, 2018-08-05; Paul451, 2025-04-22.
• A free-flyer is far less sensitive.
• Magnitudes (lamontagne's spreadsheets):
• A 500 kg off-centre mass in a stick station shifts the CoM by 7 cm; a 120 kg person, ≤8 mm lamontagne, 2023-05-04; lamontagne, 2023-05-09.
• 100 people grouped in a 3000 t station shift it by ~30 cm lamontagne, 2024-01-30.
• A 1000 t Starship docked at the hub of a 6.4 Mt station is ~0.0015% of its mass Coastal Ron, 2023-01-16.
• Floor slope is about 1% per 1% radius offset Paul451, 2022-10-26.
• Targets: Roy_H 1–2 cm eccentricity Roy_H, 2022-10-04; earlier ~1 m tolerance Roy_H, 2014-03-16.

Passive balancing

• Free-flowing water in a torus settles at the "lightest" (farthest, downhill) point and so partly corrects imbalance. Baffles prevent sloshing resonance Paul451, 2015-03-12; Paul451, 2019-01-09; Paul451, 2023-05-11.
• Toroidal stations damp themselves if anything can transfer momentum parallel to the axis: flexing, or water shielding moving. No pumps or power are needed Paul451, 2023-11-08.
• Eu:CROPIS flew a passive water-tank nutation damper LMT, 2023-11-09.
• Fluid tuned-mass dampers act at slosh frequency, faster than a quarter rotation Twark_Main, 2023-10-31. Coastal Ron objected that dampers eventually saturate and need thruster dumping Coastal Ron, 2023-10-31. Twark_Main: the same is true of CMGs, which the ISS still uses because they cut propellant use Twark_Main, 2023-11-03.
• Hinged solar panels can counteract wobble and trim orientation Coastal Ron, 2023-01-22. Pivoting solar-array arms north/south adjust axial inertia Paul451, 2023-11-06.

Active trim systems

• Water tanks.
• Three tanks at 120° correct any 2D imbalance, as in industrial laundry machines Twark_Main, 2023-01-16. A fourth, central tank also varies angular momentum Twark_Main, 2023-01-17.
• N tanks give N−1 degrees of freedom, so about 8 tanks is the practical minimum for a torus Twark_Main, 2023-10-31.
• A ringed-cross station with 12 tanks (North/South low on each arm, one Top tank per arm) gives all six axes without pumping around the ring Twark_Main, 2023-10-31.
• Use ≥3 tanks per module in a triangle, with tanks part-way down the spokes Twark_Main, 2023-10-31.
• lamontagne: 12 carbon-fibre bladder tanks, ~20 t of water for a ~250-person station; pumps faster than elevator transit; strain and acceleration gauges lamontagne, 2018-08-05.
• Water vs oil vs mercury.
• Oil: lower freezing point, lubricant Roy_H, 2018-08-05.
• Water: also serves life support, shielding and electrolysis Coastal Ron, 2018-08-05; lamontagne, 2018-08-05.
• Mercury: rejected. It amalgamates aluminium and a leak creates dangerous MMOD; it costs ~$2.4M/t, so 100 t would be 5% of world production Paul451, 2023-11-08; lamontagne, 2023-11-07.
• Dual use. Ballast water can double as fire-sprinkler reserve (NFPA allows combined systems) lamontagne, 2023-11-06. Coastal Ron wants safety systems kept independent Coastal Ron, 2023-11-06. Ballast water can be pumped into shelter walls during solar storms Paul451, 2023-11-06.
• Other movers:
• Moving weights (r, θ, z sets) with AI control Greg Hullender, 2023-11-05. Twark_Main prefers tanks: multi-axis, small pipes rather than shafts, no falling-weight hazard, and standard model-based control rather than AI Twark_Main, 2023-11-06; Twark_Main, 2023-10-31.
• Battery packs on rails along the arms Aussie_Space_Nut, 2019-03-31.
• Counterweight elevators JohnFornaro, 2023-01-15.
• A movable hub on reelable cables mikelepage, 2016-01-12.
• Density-sorted storage: denser items on lower floors of the short arm Twark_Main, 2024-09-02.
• Stewart platforms at zero-g core access holes for small, fast transients Twark_Main, 2025-09-14.
• Human behaviour. For small and medium stations, crews stay put during docking (ISS/Mir practice), so people become part of the control system Paul451, 2024-01-30; Paul451, 2022-09-30. Rules or "Terms of Service" can ban mass gatherings and synchronized marching JohnFornaro, 2023-05-09. Paul451's layered approach: main system, redundant pumps, mechanical centring, PA and training, quick-disconnect lamontagne, 2024-01-30.

Vibration and resonance

• No wind in space, so Tacoma-Narrows-type vortex shedding is unlikely. Station-keeping thrust must avoid structural resonances Coastal Ron, 2022-11-29; lamontagne, 2022-11-29; ppnl, 2022-11-29.
• Stiffness ∝ depth³, so a tube tens of metres deep is orders of magnitude stiffer than Tacoma's 2.4 m girders Barley, 2022-11-29.
• Sources in flexible structures: motors, fans, people, liquids, transfer rings, solar pressure, gravity gradient, drag. Electric turnbuckles or shock absorbers on cables lamontagne, 2018-08-04. Power-line-style dampers on Island Zero's tension rods lamontagne, 2026-07-08.
• A single rigid rotating structure has no vibration source. Bearings between masses introduce vibration Coastal Ron, 2023-01-24.
• Coastal Ron's summary: narrow wheels (diameter/width ≥10) need little attention. Wobble is mainly a concern for "fat" wheels Coastal Ron, 2023-11-01.
• The construction phase, when large masses are added, is the hardest balance problem JohnFornaro, 2023-10-31.

Open questions

• How large an axis offset do inhabitants and docking tolerate over months? Coastal Ron, 2023-11-08
• Should the design be passive-only (Coastal Ron's preference Coastal Ron, 2024-08-03)? Twark_Main counts simple trim tanks as acceptable Twark_Main, 2024-08-03.
• Is active control needed at all for large wheels with water shielding?

Related pages

• Stability · Docking · Life support and water

Sources

Forum posts

• Roy_H, 2014-03-16 (reply #173): Oil ballast; ~1 m tolerance. Post.
• Paul451, 2015-03-03 (reply #484): Real problem: mass moving parallel to spin axis. Post.
• Paul451, 2015-03-12 (reply #513, 1 like): Water in baffled channels self-corrects. Post. Attachments: Spin - mass flow.png.
• Paul451, 2015-11-09 (reply #743): Off-centre vibration myth; free-flyer rotates about CoM. Post.
• mikelepage, 2016-01-12 (reply #803): Movable hub on cables. Post.
• lamontagne, 2018-08-04 (reply #1144): Vibration sources in flexible structures. Post. Attachments: Station3c.jpg, Station3d.jpg.
• Paul451, 2018-08-05 (reply #1153, 1 like): Non-rotating hub requires CoM exactly at hub. Post.
• Roy_H, 2018-08-05 (reply #1155): Oil for balance tanks. Post. Attachments: SpaceStation.pdf.
• Coastal Ron, 2018-08-05 (reply #1157): Water better. Post.
• lamontagne, 2018-08-05 (reply #1158): 12-tank water system ~20 t. Post. Attachments: water balance.jpg.
• lamontagne, 2018-08-05 (reply #1162, 1 like): Water better (lamontagne). Post.
• Paul451, 2019-01-09 (reply #1298): Free water settles at lightest point. Post.
• Aussie_Space_Nut, 2019-03-31 (reply #1451, 1 like): Battery packs on rails to move CoR. Post. Attachments: 0002-Realistic-Near-Term-Rotating-Spacestation-HubAndArmOnly.jpg, 0002-Realistic-Near-Term-Rotating-Spacestation.jpg.
• Paul451, 2022-09-30 (reply #1828): Crew at evac points during docking. Post.
• Roy_H, 2022-10-04 (reply #1854): Target eccentricity 1–2 cm. Post.
• Paul451, 2022-10-26 (reply #2225): Floors tilt 1% per 1% radius offset. Post.
• Coastal Ron, 2022-11-29 (reply #2494, 1 like): No periodic energy source → resonance unlikely. Post.
• lamontagne, 2022-11-29 (reply #2507): Avoid thrust resonances. Post.
• Barley, 2022-11-29 (reply #2513): Stiffness ∝ depth³. Post.
• ppnl, 2022-11-29 (reply #2520, 1 like): Tacoma was vortex shedding; start small. Post.
• JohnFornaro, 2023-01-15 (reply #2909): Counterweight elevators in spokes. Post.
• Twark_Main, 2023-01-16 (reply #2913): Three tanks at 120° (laundry machines). Post.
• Coastal Ron, 2023-01-16 (reply #2935): Starship 0.0015% of station mass. Post.
• Paul451, 2023-01-17 (reply #2940): No wobble in free flight; CoR moves to new stable spot. Post.
• Twark_Main, 2023-01-17 (reply #2946): Fourth central tank varies L. Post.
• Coastal Ron, 2023-01-22 (reply #3026): Hinged panels as wobble dampers. Post.
• Coastal Ron, 2023-01-24 (reply #3064): No vibration in single rigid body; bearings introduce it. Post.
• lamontagne, 2023-05-04 (reply #3665): 500 kg → 7 cm CoM shift. Post. Attachments: Airlock design (1).xlsx.
• lamontagne, 2023-05-09 (reply #3669): 120 kg → 8 mm. Post.
• JohnFornaro, 2023-05-09 (reply #3670): ToS ban synchronized marching. Post.
• Paul451, 2023-05-11 (reply #3676, 1 like): Docking tolerance 100 mm, 4°, 0.2°/s; passive water trick (1 like). Post.
• Twark_Main, 2023-10-31 (reply #4244): Fluid tuned mass damper. Post.
• Twark_Main, 2023-10-31 (reply #4246, 2 likes): Model-based control; 4 weights minimum (2 likes). Post.
• Coastal Ron, 2023-10-31 (reply #4247): TMD saturates; thrusters needed. Post.
• JohnFornaro, 2023-10-31 (reply #4249): Construction phase biggest balance problem. Post.
• Twark_Main, 2023-10-31 (reply #4256): N tanks → N−1 DoF; ~8 tanks for torus. Post.
• Twark_Main, 2023-10-31 (reply #4257): 12-tank ringed cross controls 6 axes. Post.
• LMT, 2023-11-01 (reply #4262): Hendricks 1971 control systems. Post. Attachments: Hendricks 1971.png.
• Coastal Ron, 2023-11-01 (reply #4263): D/W ≥10 wheels needn't worry. Post.
• Twark_Main, 2023-11-03 (reply #4270): Trim systems reduce, not eliminate, thruster use. Post.
• Greg Hullender, 2023-11-05 (reply #4275): Moving weights with AI. Post.
• Twark_Main, 2023-11-06 (reply #4278): Tanks beat weights. Post.
• Paul451, 2023-11-06 (reply #4281, 1 like): In-plane mass distribution doesn't cause instability (1 like). Post.
• Paul451, 2023-11-06 (reply #4282): Pump ballast into storm shelter walls. Post.
• Paul451, 2023-11-06 (reply #4283): Pivoting array arms for axial balance. Post.
• lamontagne, 2023-11-06 (reply #4286): Ballast + fire water. Post.
• Coastal Ron, 2023-11-06 (reply #4287): Keep safety systems independent. Post.
• lamontagne, 2023-11-07 (reply #4301, 1 like): Mercury cost. Post.
• Coastal Ron, 2023-11-08 (reply #4320): Limits for inhabitants unresolved. Post.
• Paul451, 2023-11-08 (reply #4326): Mercury rejected. Post.
• Paul451, 2023-11-08 (reply #4335, 1 like): Axial imbalance → fixed floor tilt. Post.
• Barley, 2023-11-08 (reply #4336, 1 like): Nutation vs precession terms (1 like). Post.
• LMT, 2023-11-09 (reply #4341, 1 like): Eu:CROPIS passive water damper (1 like). Post. Attachments: EuCROPIS water tank.png.
• Paul451, 2024-01-30 (reply #4437, 1 like): People as part of motion control (1 like). Post.
• lamontagne, 2024-01-30 (reply #4438): 100 people in 3000 t → 30 cm. Post.
• lamontagne, 2024-01-30 (reply #4442): Layered balancing approach. Post.
• Coastal Ron, 2024-08-03 (reply #4699): Passive stability only. Post.
• Twark_Main, 2024-08-03 (reply #4701): Trim tanks OK. Post.
• Twark_Main, 2024-09-02 (reply #4780): Density-sorted storage instead of ballast. Post.
• Paul451, 2025-04-22 (reply #4893): CoR ≠ geometric centre → whip. Post.
• Twark_Main, 2025-09-14 (reply #4955): Stewart platform trims small wobbles. Post.
• lamontagne, 2026-07-08 (reply #5072, 1 like): Damping in tension rods?. Post. Attachments: Module Assembly_e.jpg, Image15_000.jpg.

External references

• Queijo et al. (1988) and Garrett et al. (1991), NASA studies with water trim and counter-rotating inertial balance tanks (NTRS 19880010196, 19910010864).
• Hendricks (1971), Martin Marietta, AG station control systems LMT, 2023-11-01.