My starship design – going all-out geek in the name of science (fiction)

I wanted my sci-fi novella ‘Missionary‘ to be not just a story about people finding strength in themselves – I also wanted it to be a homage to the golden age of sci-fi.


The "Star Witch" sans fuel tanks.
The “Star Witch” sans fuel tanks.

The stuff Heinlein and Clarke wrote, particularly. Ever read Clarke’s Rendezvous with Rama? It’s a story about people – set against a backdrop that is ultra-hard sci-fi – all the physics work out (I know, I checked the math of some of it. No really. I am that much of a geek…).

Endless Worlds cover 1So when it came to designing my starship, I cut loose with total geekism. As I mentioned in a recent post, realistic deep-space ships will likely create ‘artificial gravity’ by spinning their hab-module like a ferris wheel. Most Hollywood versions so far – including the Discovery from 2001 – haven’t done that properly, because the radius of the centrifuge is too small – the coriolis effect means the crew will get dizzy and fall over, just standing up.

This system doesn’t induce true gravity, but does produce a force that feels like it. It’s a function of conservation of momentum. Somebody standing on the inside rim of the centrifuge is moving sideways (tangential velocity), but prevented from moving along that vector because the floor prevents them. Instead, the floor moves their velocity vector around in a curve. The acceleration felt as a result of that vector-shift – centripetal, not centrifugal – creates the illusion of gravity.

You can check it out yourself on a merry-go-round – this is exactly the same principle. And it can be calculated. To do that you need to know the radius and spin rate – the equation is a = v^2/r. And here’s what I came up with: a two-deck centrifuge with ‘gravity’ shown both at floor level and when standing up, when your head is at a different radius from your feet (the difference in tangential velocity makes you dizzy.When applied to a sphere on planetary scale, it’s what causes wind systems. Just saying).

Diagram I drew of the Mission Ship Star Witch's hab module... complete with the maths needed to figure out the induced gravity on spin.
Diagram I drew of the Mission Ship Star Witch’s hab module… noting the geek-out frenzy I went into to calculate every detail from radius to rate of rotation and the induced gravity on spin at various distances from the hub.

Structurally, such a centrifuge could be considered as a suspension bridge with no end points – hence, it could be strung like one to keep the mass down.

It’s also important to have a counter-rotating balancer. This could be a second centrifuge, rotating the other way – or, more efficiently in terms of mass, a dense balance-weight that spins faster in the opposite direction. A ship without a balancer will always try to rotate around the axis on which the centrifuge is spinning, owing to the angular momentum transmitted to the non-rotating structure as a result of friction losses in the bearings. A ship without a balancer also won’t turn easily in certain directions, owing to gyroscopic and precession (‘torque’) effects. That might not be a problem for a space-ship, unless swift orientation-changes are needed for some reason (I can’t immediately think of one).

The "Star Witch" departing, with her fuel tanks.
The “Star Witch” departing, with her fuel tanks.

As an aside, the physics of that specific issue was a problem to pilots of rotary-engined aircraft such as the Sopwith Camel in WWI – which, mechanically, were exactly what I’m describing, because the main mass of the motor spun. They were flying in air with large surfaces (wings and side-area of fuselage) to damp the rotation and precession effects against air resistance, but they still turned faster one way than the other, something German pilots soon learned. And, of course, German rotary-engined aircraft such as the Fokker Dr.1 had the same issue. Yah – this isn’t just a sci-fi geek problem. Some people, a century ago, stayed alive by understanding it.

Spacecraft don’t usually operate in an atmosphere, so there’s nothing to damp the conservation of angular momentum if you have a centrifuge. That’s why a counter-rotating mass is a Good Thing If You Don’t Have Unlimited Roll Thruster Fuel. And yes, 2010: Odyssey Two got it wrong to the tune of 90 degrees, confusing frictional with torque effects (Discovery could have ended up spinning end-for-end when the centrifuge transmitted its angular momentum to the main structure, because of precessional torque; but the majority effect would still have been a short-axis roll).

Man, I am such a geek. As for more? One obvious plot-point – what happens if the bearings jam? The angular momentum of either the centrifuge or its counter-balance gets transferred into the structure of the whole ship, making it rotate. Or, more likely – given these things will be built down to the lowest mass and engineers like to think their bearings won’t fail, the torsion over-stresses the structure and the ship tears itself apart.

OK, you say, where is the humanity in this, the story? Other than the hilarity of describing wannabe gym-jocks doing the technicolour yawn after a session of jumping jacks? Let me put it this way. Robert A Heinlein wrote a brilliant psychological story about a guy doing EVA from a spinning ship who ended up hanging by his fingertips, ‘above’ an endless void where to let go was certain death… And, as I portray in ‘Missionary’, there’s always the possibility of the centrifuge being haunted. (Or something – heh heh….)

Does this happen? Well, you’ll have to read the story… It’s out now, on Amazon. Bwahahahaha!

Copyright © Matthew Wright 2015

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