Docking to rotating stations

How visiting vehicles, especially 100+ t Starships, attach to a spinning station. The thread identified five families of solution and never settled on one.

Key results

The problem

  • Loads. A 25 t BA330 at 1 g is 245 kN, against 100 kN for a standard docking adapter. Dock at the centre or stop rotation A_M_Swallow, 2015-11-10. A 200 t Starship berthed 10 m off-centre at station spin is ~39 kN (~4 t), needing a Mechazilla-class arm rather than Canadarm lamontagne, 2022-12-08.
  • Tolerances. Current standards tolerate 100 mm lateral and 4° angular misalignment and a 0.2°/s rotation mismatch. Paul451 proposed a separate grapple system to guide a wider berthing port Paul451, 2023-05-11.
  • CoM alignment.
  • The station's centre of rotation must coincide with the port. Adjust with a counterweight, internal movable mass or crew movement Paul451, 2015-03-02.
  • A sliding docking connector moves once to the current CoR, "it doesn't wobble" Paul451, 2023-05-04.
  • 500 kg off-centre on a stick station shifts the CoM by 7 cm lamontagne, 2023-05-04.
  • Stability of the combined body.
  • A docked capsule on the axis makes a T or + shape that prefers flat-plane spin, which is a problem for 2-lobed dumbbells; 3-armed and toroidal stations have enough inertia KelvinZero, 2015-11-12; Paul451, 2015-11-13.
  • Y or X shapes stabilize a docked ship Nilof, 2019-03-24. Ship mass in the ring plane means fore/aft docked Starships won't destabilize Paul451, 2024-02-25.
  • Starship specifics (Coastal Ron): >200 t with up to 1200 t of propellant, unknown CoM, no zero-g slosh baffles or sensors, and RCS lacking control authority Coastal Ron, 2023-01-26; Coastal Ron, 2023-01-27.
  • mikelepage: spin about the long (minimum-MoI) axis is short-term stable; the issue is "more like a boat on a swell" mikelepage, 2023-01-28.
  • Paul451: energy dissipation drives a slow drift from long-axis spin toward end-over-end; it is not intermediate-axis instability Paul451, 2023-01-29.

Solution families

  1. The vehicle spins to match and docks on the axis (Clarke's 2001).
  2. Paul451: capsules are already loaded with CoM in mind for attitude control; GNC in a rotating frame "is software" Paul451, 2015-09-14; Paul451, 2022-12-09.
  3. LMT cites autonomous tumble-docking research: Oestreich 2021 MIT thesis, docking in ~2 min with a ~40 s trajectory match LMT, 2023-01-18; LMT, 2023-01-26; LMT, 2024-02-24.
  4. A 3 kW motor spins up a 30–60 t vehicle in ~30 min A_M_Swallow, 2015-09-15.
  5. Salyut 7 was docked with as a "non-cooperative object" ZIP DUDE, 2015-03-02.
  6. A counter-rotating dock or gantry on the station. It captures the non-rotating vehicle and spins it up.
  7. Coastal Ron's roller-coaster circular track with an X-Y gantry Coastal Ron, 2022-11-29; Coastal Ron, 2019-10-30.
  8. MickQ's de-spun catch frame MickQ, 2023-05-27.
  9. Babylon-5-style hub capture Asteroza, 2016-01-12.
  10. An extensible spin-matching "Mechazilla cylinder" LarryCanuck, 2023-05-13.
  11. Coastal Ron: this works only for stout local shuttles, not Starship Coastal Ron, 2023-05-27.
  12. A short-term rotating berthing port or gangway.
  13. The port counter-rotates only while a ship is attached lamontagne, 2023-01-20; lamontagne, 2023-01-24.
  14. "Ron's rotating airlock": a non-rotating flexible gangway with redundant rotating seals lamontagne, 2023-01-30. See rotating airlocks.
  15. A zero-g transit station with dedicated shuttles (Coastal Ron's preference).
  16. Vehicle makers need not adapt; an "airport shuttle" or supply-chain analogy Coastal Ron, 2022-12-08; Coastal Ron, 2023-01-18; lamontagne, 2023-01-18.
  17. Paul451: a space-only taxi means 8 dock/undock events per visit, and a failure traps people in a small taxi Paul451, 2018-08-06.
  18. mikelepage: formation-flying stations must be tens to thousands of km apart, making transfers take hours to days. A near-term station should be one connected object that accepts Starship directly mikelepage, 2023-01-28.
  19. Catch and cradle. Catch Starship near the centre and move it into a side cradle, or berth it tail-first on a robust mount and bring an arm to its side hatch; the approach "sails on" if aborted lamontagne, 2022-12-08; lamontagne, 2018-08-06.

Other points

  • Roll into plane. Dock on the axis, then roll the dumbbell plus shuttle 90° into the rotation plane lamontagne, 2019-03-25. Paul451 opposes rotating a docked ship 90° into the plane: hanging loads, no emergency undock, ~5 t torque Paul451, 2024-02-25.
  • Undock by time-reversal. Twark_Main: dock by time-reversing an emergency "drop" undocking trajectory; abort Δv ≤ 2 × tangential speed Twark_Main, 2025-04-14.
  • Underway replenishment. Transfer only cargo pods between vehicles Coastal Ron, 2018-08-08.
  • Propellant stays elsewhere. Starships should arrive near-empty and refuel at a separate depot; "gas stations are not inside houses" lamontagne, 2023-01-24; Coastal Ron, 2023-01-25.

Open questions

  • Will autonomous rotation-matching GNC be certified for crewed Starships, or will stations have to provide the rotation interface?
  • Is the station's port a city-centre dock or an airport outside town? lamontagne, 2023-01-24

Sources

Forum posts

  • ZIP DUDE, 2015-03-02 (reply #465): Salyut 7 precedent. Post.
  • Paul451, 2015-03-02 (reply #467, 1 like): Align CoR with adaptor. Post.
  • Paul451, 2015-09-14 (reply #636): Visiting craft match rotation. Post.
  • A_M_Swallow, 2015-09-15 (reply #643): 3 kW motor, 30 min. Post.
  • A_M_Swallow, 2015-11-10 (reply #744): 245 kN vs 100 kN adaptor. Post.
  • KelvinZero, 2015-11-12 (reply #755): Docked capsule T-shape. Post.
  • Paul451, 2015-11-13 (reply #760): Power arrays at 90° stabilize. Post. Attachments: Station solar arrays.png.
  • Asteroza, 2016-01-12 (reply #801): Babylon 5 hub frame. Post.
  • Paul451, 2018-08-06 (reply #1168): Taxi = 8 dock events. Post.
  • lamontagne, 2018-08-06 (reply #1181): Tail-first berthing + pivoting arm. Post.
  • Coastal Ron, 2018-08-08 (reply #1194, 1 like): Underway replenishment analogy. Post.
  • Nilof, 2019-03-24 (reply #1384): Y or X shape stabilizes. Post.
  • lamontagne, 2019-03-25 (reply #1392, 2 likes): Controlled roll. Post.
  • Coastal Ron, 2019-10-30 (reply #1703): Counter-rotating grabber dock. Post.
  • Coastal Ron, 2022-11-29 (reply #2516): Circular track gantry. Post.
  • Coastal Ron, 2022-12-08 (reply #2604, 1 like): Zero-g transit + local shuttles. Post.
  • lamontagne, 2022-12-08 (reply #2605, 1 like): Catch into side cradle. Post. Attachments: dock.png.
  • lamontagne, 2022-12-08 (reply #2616): 200 t Starship 10 m off-centre ~39 kN. Post.
  • Paul451, 2022-12-09 (reply #2622): Not more burdensome. Post.
  • lamontagne, 2023-01-18 (reply #2963): Airport shuttle analogy. Post.
  • Coastal Ron, 2023-01-18 (reply #2964): 1st-gen: transit station; 2nd-gen: hangar. Post.
  • LMT, 2023-01-18 (reply #2965, 1 like): Oestreich 2021. Post. Attachments: Oestreich 2021 Fig3-2.png.
  • lamontagne, 2023-01-20 (reply #3007): Short-term rotating berthing port. Post. Attachments: canport.png, can baton.png.
  • lamontagne, 2023-01-24 (reply #3048): 3 m UBM rotating port, 1–5 rpm. Post. Attachments: IAC-21,D3,IP,4,x64597 Universal Berthing Mechanism Standard.pdf, airlock.jpg.
  • lamontagne, 2023-01-24 (reply #3054, 1 like): Starship arrives near-empty. Post.
  • Coastal Ron, 2023-01-25 (reply #3101): Separate zero-g depot. Post.
  • Coastal Ron, 2023-01-26 (reply #3121): Starship docking problems. Post.
  • LMT, 2023-01-26 (reply #3123): Tumble docking ~2 min. Post.
  • Coastal Ron, 2023-01-27 (reply #3129): Slosh, CoM, RCS authority. Post.
  • mikelepage, 2023-01-28 (reply #3158): Long-axis spin OK; single connected object. Post.
  • Paul451, 2023-01-29 (reply #3167): Correct term: energy dissipation. Post.
  • lamontagne, 2023-01-30 (reply #3189, 2 likes): Ron's rotating airlock (2 likes). Post. Attachments: can flex.png, can flex 2.png.
  • lamontagne, 2023-05-04 (reply #3665): 500 kg → 7 cm CoM shift. Post. Attachments: Airlock design (1).xlsx.
  • Paul451, 2023-05-04 (reply #3666, 1 like): Docking connector on sliding rail (1 like). Post.
  • Paul451, 2023-05-11 (reply #3676, 1 like): Docking tolerances; grapple system (1 like). Post.
  • LarryCanuck, 2023-05-13 (reply #3699): Spin-matching Mechazilla cylinder. Post.
  • MickQ, 2023-05-27 (reply #3759): De-spun catch frame. Post.
  • Coastal Ron, 2023-05-27 (reply #3762): Only stout local shuttles. Post.
  • LMT, 2024-02-24 (reply #4484): Rotating ports outdated; autonomous tumble docking. Post.
  • Paul451, 2024-02-25 (reply #4495): Ring mass in plane OK. Post.
  • Twark_Main, 2025-04-14 (reply #4864): Time-reversed drop undock. Post.

External references

  • Oestreich, C. (2021). Robust control for docking with tumbling objects of unknown inertia. MIT thesis.
  • Silvestrini, S. & Lavagna, M. (2022). Drones 6(10), neural GNC.
  • Ikeya et al. (2023). J. Spacecraft & Rockets, learning reference governor for rendezvous.
  • NTRS 20110008387 (NDS power/data connectors).