Home / Engineering / Spin gravity and counter-rotation
Spin gravity and counter-rotation
The Long Passage turns her habitat drums once every 20 seconds, and the floor presses on your feet with 0.56 times your Earth weight.
See it in the tour: chapter long passage, shot 0.2.
Weight from motion
A spinning drum holds you against its wall. You try to travel in a straight line, the wall curves away, and the wall pushes you back. That push is your weight physics.
The acceleration is a = ω²r: the spin rate squared times the radius. The Long Passage's drums have a floor radius of 56 m and turn at 3 rpm design. That gives 5.53 m/s², or 0.56 g physics. One turn takes 20 s, and the floor moves at 17.6 m/s.
Weight falls in a straight line toward the axis. At the axis you float.
Why the drums come in pairs
A spinning drum holds angular momentum, and angular momentum is conserved physics. Spin one drum up against a ship and the ship turns the other way, like a person spinning a bicycle wheel on a stool.
So the habitat is a counter-rotating pair. One drum turns clockwise and the other turns counter-clockwise at the same rate. Their angular momenta cancel and the ship stays pointed where the thrusters put it physics. The drums sit about 400 m from the reactor, behind its shadow shield design.
What the floor does to you
Spin has a side effect: the Coriolis force physics. In the drum, at 3 rpm:
- Standing still weighs 0.564 g.
- Walking spinward at 1.3 m/s weighs 0.650 g.
- Walking against the spin weighs 0.483 g.
- A straight-up jump of 0.76 m lands 0.35 m spinward.
The floor load changes by about 15% with your walking direction, plenty to feel. A jumper keeps the angular momentum of the floor. Closer to the axis that means moving faster than the floor, so the jump lands spinward of where it began physics.
The Gateway ring
The Gateway is a non-rotating truss with a rotating habitation ring. The truss was assembled in 2071 and the ring opened in 2076 fiction. The same relation sets its weight: a bigger ring needs a slower turn for the same g. The truss does not spin, so cargo work happens on a still structure.
Titan is not a drum
Titan's surface gravity is 1.352 m/s², 0.138 g physics. Nothing spins. The settlement lives in a steady pull of about one seventh of Earth's.
The walking is different. The same legs that jump 0.45 m on Earth jump 3.26 m on Titan physics. A walker is limited to about 0.78 m/s before the gait becomes a lope physics.
Nobody knows what decades at 0.14 g do to a body challenge. Residents exercise daily, and many rotate back to spin-gravity habitats every few years fiction. Enceladus is lighter still, 0.0115 g physics.
Key numbers
| Quantity | Value | Basis |
|---|---|---|
| Drum radius / spin rate | 56 m / 3 rpm | design |
| Floor gravity | 5.53 m/s² (0.56 g) | physics |
| Walking spinward / against | 0.650 g / 0.483 g | physics |
| Jump deflection (0.76 m jump) | 0.35 m | physics |
| Reactor distance from drums | about 400 m | design |
| Gateway truss / ring | 2071 / 2076 | fiction |
| Titan gravity | 1.352 m/s² (0.138 g) | physics |
| Enceladus gravity | 0.113 m/s² (0.0115 g) | physics |