Polished version of some illustrations I made for Alan Rominger's Gravitational Space Balloons project. Gravity balloons are a type of space habitat created by hollowing out or inflating the interior of an asteroid with breathable air, such that the outward pressure of the gas is "balanced" by the inward pressure of the asteroid rock's own self-gravity. This provides a stable habitat solution capable of supporting populations of 100+ million in space, providing breathable air at 1 atmosphere or more of pressure, and ample protection from radiation without a need for large support structures.

Axles are a means of "spinning up" space habitats that spin to provide artificial gravity (centrifugal force). Since the inside of a gravity balloon will have virtually zero gravity as far as humans are concerned, cylindrical habitats must be built to house the balloon's citizens. Since these habitats are built in open air and are therefore subject to friction and turbulent flow, it would be extremely inefficient to use jets or fans to keep the cylinder spinning, because these use large amounts of energy to push against gas for momentum. Instead, mounting cylinders on an axle that's attached to the colossally heavy asteroid rock, or attached to another cylinder spinning in the opposite direction (thus canceling out each others' angular momentum), allows cylinders to use a much more efficient "train" system, since it pushes against a solid surface.

If you're interested in the worldbuilding behind this, check out Rominger's blog and the post on my new content creation Tumblr that this comes from.