Image from Steve Bowers
First conceived during the Information Age, the skywheel works on similar principles to those of the rotovator. It consists of a ring-shaped satellite orbiting a celestial body with a diameter ranging from tens to thousands of kilometres. The ring rotates such that its bottom portion (at any given moment in time) moves slower than orbital velocity while its upper portion moves faster than orbital velocity. This rotation subjects the ring to hoop stress, so it must be made of a high-strength material (generally carbon nanotubes).

For ascending to orbit, a spacecraft attaches to the bottom portion of a skywheel's rim, remains there until it reaches the highest point, and then detaches to be thrown into orbit. This can also be done in reverse (attaching to the top and detaching from the bottom) for a spacecraft to shed its velocity for docking with a habitat or hab cluster or landing on the surface of a world. Lifting a spacecraft into orbit reduces a skywheel's orbital velocity slightly, while landing a spacecraft does the opposite. Finally, a skywheel can also be used by a moving spacecraft to change its direction without altering its speed

In practice, skywheels also include multiple spokes leading to a central hub, which in turn has photovoltaic arrays, communication equipment and other attachments. The spokes are made of conductive materials, allowing the skywheel to control its orbital velocity using electrodynamic propulsion, provided it orbits a world with a magnetic field. If there is no magnetic field, then ion thrusters are used instead.

Skywheel Diagram
Image from Avengium and Steve Bowers
While a rotovator only has one of its tips available for docking for a brief period of time, a skywheel always has a portion of its circumference ready to dock with incoming spacecraft. This makes it easier for spacecraft to attach to a skywheel over a rotovator. Additionally, it is possible for many spacecraft to attach to a skywheel at once, whereas a rotovator's tips have much less room. A skywheel can thus handle more traffic than a rotovator of comparable size and rotation speed.

Despite their many advantages, skywheels do suffer from one major disadvantage: They require more material to build than a comparable rotovator. Specifically, it requires 2π (or about 6.28) times more material.

Because of this, skywheels are not normally used around newly settled worlds or in lightly settled systems, which prefer smaller infrastructure such as rotovators or the still-smaller bolos. Only after a world or system is developed and its orbital traffic becomes sufficiently busy are skywheels implemented. Materials used to construct skywheels can be launched into orbit via the same rotovators or bolos that they are to replace. Skywheels may in turn be replaced by beanstalks or orbital rings, which allow efficient transport directly to and from a planetary surface, or a civilization may opt to skip skywheels and progress directly to beanstalks/ orbital rings.

A few MPA systems continue using skywheels on a long-term basis, amused by the whimsy of gigantic wheels transporting objects through space.
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Development Notes
Text by Tardigrada
Initially published on 21 December 2023.

Concept of the sky wheel based on the report Capture-ejector Satellites - Macconochie, Eldred and Martin (1983)