Artificial gravity with a “can and pencil” spacecraft
Sunday, November 30th, 2025
Artificial gravity doesn’t need huge rigid wheel-like structures anymore, thanks to present-day computing power — the sort of thing that can keep a robot walking upright.
A “can-and-pencil” spacecraft would consist of two cylindrical rockets, each with a main thruster and several small manoeuvering thrusters pointing in all directions. One of the rockets (the “can”) is very big and heavy, and contains tanks of fuel, oxygen, water, waste, etc., and two drums of about 50m of wire strong enough to hold the weight of a bus (say). The other rocket (the “pencil”) is much lighter and narrower, and contains living quarters.
At the initial launch stage, the two are connected rigidly together. After refuelling in orbit, the can and pencil start to rotate and separate, connected only by a pair of wires, fore and aft. Using computer-coordinated firings of the small manoeuvering thrusters on both can and pencil, the can slowly pays out its wires, with the wires always kept taut. At the end of the process, the two cylinders are about 50 m apart. Because the can has a much larger mass than the pencil, the two rotate about an axis much closer to the can than the pencil, the latter in effect “suspended” from the can’s wires by centrifugal force. The pencil is like the gondola of an airship, suspended far below it, with opposed centrifugal forces keeping them apart rather than the opposed forces of buoyancy and gravity. At about 5 revolutions per minute, the pencil has artificial gravity all along its length similar to gravitational acceleration g on Earth.
Big accelerations (towards Mars, say) could be made together or separately, with the two main thrusters coordinated by computers. But the most intuitive procedure would involve the can hauling the pencil back in until the two could be rigidly connected again, as on launch. The speed of rotation would increase as the moment of inertia of the whole decreases, but the manoeuvering thrusters would slow it down to zero. The rockets would then be pointed in the right direction, and the required acceleration would be imparted by a burn. Then the two would separate and start rotating again for the long un-powered journey.