Angular momentum, flywheels and counter-rotation

How to spin a station up and down, and how to change its orientation, without spending large amounts of propellant.

Key results

Conservation

• Angular momentum is conserved. Counter-rotation does not save propellant against external torques such as drag, gravity gradient and docking impulses. Its benefit is internal exchange Twark_Main, 2019-10-28; Twark_Main, 2019-10-31.
• Counter-rotating segments.
• Spin-up and spin-down become purely electrical, using motors between the segments 1, 2018-07-04.
• Energy is recoverable when stopping, so the spin-up cost is "much much lower" than with reaction engines lamontagne, 2026-09-18.
• Such a station can despin itself for maintenance or to accept a craft that cannot dock while rotating 1, 2023-02-17.
• Net-zero L makes reorientation free. A counter-rotating pair can point anywhere and precess once a year with no torque. That is why O'Neill used paired cylinders, and the Stanford Torus has its big mirror lamontagne, 2022-11-28; lamontagne, 2026-09-17. The individual rings still exert opposite torques on the frame, so the bearings must be strong Paul451, 2023-02-17; mikelepage, 2023-09-12.

Flywheels and CMGs

• To despin a station without propellant, a flywheel must store the entire station angular momentum. For large stations this becomes a counter-rotating ring Barley, 2022-11-28.
• The flywheel mass needs to be comparable to the spinning mass unless it spins much faster RanulfC, 2014-03-21. Material strength limits rim speed, so a fast small flywheel ends up with about the same mass lamontagne, 2023-01-14.
• Small, super-strong flywheels can still buffer docking transients KelvinZero, 2019-01-07; mikelepage, 2019-01-08. Shevek23 proposed Kevlar flywheels at 1/10 radius, 1/10 mass and 400 rpm to despin a docking platform and absorb the angular momentum of added cargo Shevek23, 2019-10-28.
• Control moment gyros saturate under constant-direction torque. To walk the station's angular-momentum vector around a circle (for sun-pointing) a CMG would have to store as much L as the station Twark_Main, 2025-05-01.
• Oversized CMGs in each module could buffer a good fraction of station L during spin and despin, giving "nearly twice the total gyro AM capacity for free" Twark_Main, 2024-08-31.
• Useful hardware as flywheel. Solar panels as counter-rotating flywheels A_M_Swallow, 2015-09-07; A_M_Swallow, 2019-01-08; modules, axle modules or engines sanman, 2019-01-07.
• Magnetorquers are too weak and slow for docking loads and serve only for CMG desaturation Jim, 2018-07-05. LMT's counterpoint: electrodynamic cables on the spokes can spin up a 3000 t, 100 m station to Mars g in about 2 months (see spin-up) LMT, 2023-04-02.

Reorientation tricks

• Cat-righting reflex: reorientation through internal gimballed reaction masses mikelepage, 2018-08-18. This works only for L ≈ 0. A spinning monolithic station precesses if torqued 1, 2018-08-28.
• Cancel the slow yearly rotation with a small fast disc: proposed by lamontagne lamontagne, 2023-01-12. Refuted: after 6 months the ring appears to spin the opposite way, so the equivalent of two full spin-down/spin-up cycles per year is needed Twark_Main, 2023-01-13; 1, 2023-01-13. lamontagne: "cancelling the main angular momentum with very rapidly spinning smaller masses isn't working out" lamontagne, 2023-01-14.

Counter-rotating ring hardware

• Nested counter-rotating rings each get g from their own rpm relative to the inertial frame Paul451, 2016-01-14.
• Bearing options:
• Magnetic suspension with wheel backups (BIS Avalon) lamontagne, 2023-09-29.
• 50,000 train bogies at 240 km/h lamontagne, 2023-02-18. Robotbeat called this "mechanically bad", since a large-diameter roller bearing's cost and resistance grow with diameter Robotbeat, 2023-02-19. Twark_Main: the rotating structure should be self-supporting, with EM actuators only for alignment Twark_Main, 2023-02-18.
• Tracks and bogies distribute torquing loads better than a central bearing Twark_Main, 2023-09-12.
• Magnetic bearings are unstable at low rpm lamontagne, 2022-10-07.
• Seizure risk. Coastal Ron: "Two masses moving past each other with lots of inertia would be something to see" Coastal Ron, 2023-12-29. Nested same-axis rings on a shared shaft precess Coastal Ron, 2019-03-30.
• Loose items inside counter-rotating shells receive torque only through contact and friction. Rotation of the whole structure is so slow (1 rev/yr) that its torques are negligible Roy_H, 2023-02-17; Paul451, 2023-02-17.
• All electric motors on the station should have counter-rotating masses so that they cause no net torque Roy_H, 2014-03-16.

Momentum bookkeeping

• A payload lifted up a spoke must be spun up (the "ballerina effect"). Symmetric elevators change the moment of inertia and hence the spin rate dror, 2014-03-17. When a hub arm plucks an elevator off a cable, its angular momentum passes to the hub, which needs a flywheel Roy_H, 2014-02-20.
• Cargo exchange between station clusters can transfer momentum, acting as discrete "ballistic coupling" Lampyridae, 2024-03-04.

Open questions

• Is a counter-rotating shield ring (Kalpana, Avalon) worth its mass, bearing risk and friction losses compared with paying propellant for pointing? (See orientation.)
• How do counter-rotating segments behave when flexible, with delayed coupling across hundreds of metres? 1, 2023-01-10

Related pages

• Orientation and sun-pointing · Non-rotating hub · Spin-up and reboost

Sources

Forum posts

• Roy_H, 2014-02-20 (reply #91): Elevator momentum transferred to hub; hub flywheel. Post.
• Roy_H, 2014-03-16 (reply #187): Counter-rotating masses on all motors. Post.
• dror, 2014-03-17 (reply #196): Elevators change MoI (ballerina effect). Post.
• RanulfC, 2014-03-21 (reply #242, 1 like): Counter-rotating flywheel mass ~ spinning mass. Post.
• A_M_Swallow, 2015-09-07 (reply #626): Solar panels as counter-rotating items. Post.
• Paul451, 2016-01-14 (reply #823): Each ring's g depends on own rpm. Post.
• 1, 2018-07-04 (reply #868, 1 like): Counter-rotating segments allow electric spin-up; L=0. Post.
• Jim, 2018-07-05 (reply #879): Magnetorquers too weak (Jim). Post.
• mikelepage, 2018-08-18 (reply #1195): Cat-righting reflex reorientation. Post.
• 1, 2018-08-28 (reply #1197, 1 like): Works only for L≈0. Post.
• KelvinZero, 2019-01-07 (reply #1266): Small fast flywheel for spin up/down. Post.
• sanman, 2019-01-07 (reply #1272): Use useful hardware as flywheel. Post.
• mikelepage, 2019-01-08 (reply #1283): Flywheels need not be heavy. Post.
• A_M_Swallow, 2019-01-08 (reply #1286): Solar panels as flywheels. Post.
• Coastal Ron, 2019-03-30 (reply #1430): Counter-rotating on shared shaft precesses. Post.
• Twark_Main, 2019-10-28 (reply #1698): No propellant advantage to counter-rotation vs external torques. Post.
• Shevek23, 2019-10-28 (reply #1699): Kevlar flywheel buffers for docking platform. Post.
• Twark_Main, 2019-10-31 (reply #1704): Build-phase mass import handled by rockets. Post.
• lamontagne, 2022-10-07 (reply #1893): Magnetic bearings unstable at low rpm. Post.
• lamontagne, 2022-11-28 (reply #2479, 2 likes): Counter-rotating pairs L=0 can sun-point (Stanford Torus mirror). Post.
• Barley, 2022-11-28 (reply #2481, 1 like): CMGs would need station's whole L → counter-rotating rings. Post.
• 1, 2023-01-10 (reply #2847, 1 like): Large structures not rigid; coupling delay (1 like). Post.
• lamontagne, 2023-01-12 (reply #2873): Cancel yearly rotation with small disc. Post. Attachments: Station (4).png.
• Twark_Main, 2023-01-13 (reply #2878): Doesn't work: axis rotated 180° after 6 months. Post.
• 1, 2023-01-13 (reply #2885, 2 likes): Equivalent to spin down and reverse twice/yr (2 likes). Post.
• lamontagne, 2023-01-14 (reply #2892): Material strength limits flywheel rpm → same mass. Post.
• Paul451, 2023-02-17 (reply #3327): Opposite precession of counter-rotating wheels cancels; structural torques remain. Post. Attachments: counter-rotation and precession.png.
• Roy_H, 2023-02-17 (reply #3332): How do forces reach loose items?. Post.
• Paul451, 2023-02-17 (reply #3334): Via contact; negligible. Post.
• 1, 2023-02-17 (reply #3337, 1 like): Counter-rotation lets station spin up/down for energy only. Post.
• lamontagne, 2023-02-18 (reply #3340, 1 like): Counter-ring on 50,000 bogies. Post. Attachments: counter ring1.png, counter ring2.png, counter ring3.png, counter ring4.png, counter ring5.png.
• Twark_Main, 2023-02-18 (reply #3342): Self-supporting ring + EM actuators; bogies unnecessary. Post.
• Robotbeat, 2023-02-19 (reply #3353): Large roller bearing mechanically bad. Post. Attachments: D4F99FDF-FFC8-4337-BC1E-305B51AB87BC.png.
• LMT, 2023-04-02 (reply #3571, 1 like): EM stationkeeping example (1 like). Post. Attachments: 400 km geomagnetic field at 0 N 0 E.png.
• mikelepage, 2023-09-12 (reply #3923): Strong bearings needed. Post.
• Twark_Main, 2023-09-12 (reply #3925): Distribute torques via tracks/bogies. Post.
• lamontagne, 2023-09-29 (reply #4060): Avalon: magnetic suspension + backup rails. Post.
• Coastal Ron, 2023-12-29 (reply #4382, 1 like): Failure modes of counter-rotating masses. Post.
• Lampyridae, 2024-03-04 (reply #4536): Cargo exchange as momentum transfer between stations. Post.
• Twark_Main, 2024-08-31 (reply #4768): Oversized CMGs buffer station L during spin ops. Post.
• Twark_Main, 2025-05-01 (reply #4927): CMG saturates; must store station L. Post.
• lamontagne, 2026-09-17 (reply #5124): O'Neill double cylinders for sun-facing. Post.
• lamontagne, 2026-09-18 (reply #5130, 2 likes): Counter-rotating recovers energy. Post. Attachments: Arcadia.xlsx.

External references

• Walter Lewin, MIT 8.01 Lecture 24 "Rolling Motion, Gyroscopes" (1999), used to settle the precession debate.
• Smarter Every Day, cat-righting reflex video.