by Marie Vibbert
From the reactive properties of cement, to the use of local materials, learn all about the scientific potential for building a bicycle track on Mars in this fascinating essay by Marie Vibbert, whose story “The First Velodrome on Mars” appears in our [July/August issue, on sale now!]
My story in the July Analog, “The First Velodrome on Mars,” began when I was riding my bike, and my thoughts wandered to what it would be like to bike on another planet. Of course, it would be hard to operate a bicycle in an environment suit; they aren’t known for their flexibility nor their full field of vision, and if I have to watch the ground carefully for potholes on city streets, just think how much more I’d have to watch on Mars! A Velodrome, I thought: a bike arena. Yes. You’d get the pleasure of motion in an area that can be custom-smoothed and pressurized.
Ah, but how to build it?
Using local materials as much as possible makes sense. We won’t want to ship bulky, heavy things like steel or even plasterboard to another planet. Martian pioneers would rely on the materials of their landers, and whatever they could find in situ. Concrete is a likely choice, since it is made largely of rock and sand, which are plentiful, available materials.
The science of concrete is more complicated than people realize. My father was a construction worker who often poured concrete foundations and walls, and I learned a lot from talking with him as a kid. At its simplest, you have some form of glue and some stuff to glue together. Most concrete on Earth has a glue of lime (calcium oxide) with water, gluing together aggregate (small rocks and/or sand). The lime reacts with the water to bind the aggregate. It requires a period of curing with steady temperature, pressure, and hydration. It may require heating. Or cooling! The reaction with water that turns lime into slaked lime, (calcium hydroxide), is is exothermic. You have to design for that heat to escape if you don’t want exploding concrete.
You can see the problems with Mars: lower pressure, variable temperatures, and very little water.
Cement may need to be pressed to adhere to its pebbles in low pressure. Well, okay, we put clamps on the molds.
The curing stage should have a controlled, even temperature. The temperatures on Mars, with no blanket of atmosphere to even things out, can range from a balmy 70 F to a bitter -153 F. Frozen water won’t dissolve your chemical binder. Rapid temperature changes will cause the material to expand and contract, cracking it and further spoiling the bind. Add a heating blanket to the story, draped over the mold while curing. Also, with a little insulation, the exothermic concrete will warm itself a bit.
Lack of water is the biggest problem, and I gloss over it in my story. Ideally, my cyclists have put their settlement somewhere with sub-surface ice (which we’ve found in lots of places on Mars) and are mining it via a “Rodriguez Well.” (https://ntrs.nasa.gov/api/citations/20210015807/downloads/Rodriguez%20Wells%20for%20Space%20Resources%20Roundtable%20Jun%202021%20v1%20revised.pptx.pdf)
Given how much water we humans need to keep humaning, and the water needed to grow food, my cyclists might not want to spare too much of their supply for extra-curricular building. If they want to save that water for other applications, my Martians could substitute blood, sweat, and tears. No, really! Also urine. Scientists have investigated using such “biomaterials” for concrete on Mars. Food wastes from animal products could be used, too. (https://www.sciencedirect.com/science/article/pii/S0094576524006301)
But let’s just assume they have the water and there isn’t a long, icky sequence of saving up biomaterials.
Mars doesn’t have a lot of limestone lying about like we have on Earth, that we can tell, so far. Some carboniferous rock has been found, but not much. Scientists are actually confused why there isn’t more, given what we know of Mars’ geologic past. However, Mars has other minerals that can do the binding job, such as sulfur or magnesium. Lots of different compounds can be mixed together on the surface of Mars, though you might have to trek a while to get all the minerals you need. It would help to have a soil survey of Mars, tracking where the mineral-rich deposits are.
(Oo . . . or, I’m imagining a mineral-mining rover that snuffles along like a truffle pig. Excuse me, writing a new short story now.)
Oh! And concrete needs to have a uniform pebble size, too. Think about it—if you have larger and smaller pebbles in there, you’re creating locations for preferential weathering, expansion, and wear. On Earth, concrete companies have these large towers full of shaking screens shorting their sand into finer and finer particle sizes, with conveyor belts carrying gravel to the top and removing sorted material at different levels. Again, that’s lots of building parts required that we wouldn’t want to ship to Mars.
Well, to get past it all in my story, I magicked up a “Quickcrete Machine” that would eat up regolith, sift it into similar particle sizes, and mix the binder, extruding the result from a hose the operator can direct. Pay no attention to the handwavium inside the box. It could happen, given enough energy and engineering. The machine could even have a spectrometer and sort minerals as it scoops up dirt.
Of course, the track still needs some reinforcement. You’d think at first you could just build things out of concrete like mud castles. Structures of pure concrete can be done—look at the Pantheon—but concrete, while strong in compression, is weak in tension; it can’t take stretching as well as it can take smooshing. Enclosing a breathable atmosphere on Mars would mean creating a higher pressure area inside than outside, and lo, that high pressure would want to expand the concrete like a balloon. Something needs to hold it together against that stretch.
We use iron rebar on Earth, mostly. Iron is plentiful on Mars, but making it into rebar would require, well, a lot more than a small, portable machine. (Though now I’m imagining a “Mr. Bessemer” you could drag around to pick up iron particles, maybe with a magnet? And a wee crucible?)
Digression! My heroes had to find something in their environment, and went with shredded packing material.
My builders would also need removable molds to pour the concrete into. On Earth, these are usually steel or plywood concrete forms, but on Mars, I have them use a more-easily-obtained section of plastic barrel.
If the concrete is excreted slowly, layer by layer, like the material in a 3d printing machine, allowing each later to harden before the next is laid down, they could do away with a form. They could use patterns from nature to create cellular structures which would increase the strength of the wall and provide pockets for insulation from cold and radiation. (I had read some lovely articles on biology-inspired concrete structures, as well as cellular concrete, here’s one: https://worldarchitecture.org/architecture-projects/hfchp/cellular-ecosystem-2f-concrete-calligraphy-project-pages.html—I really love the shapes, and like to imagine this is what the inside of the velodrome track looks like.)
Calculating the angle of the velodrome track was another bit of fun I had. Velodromes have a banked, or tilted surface, so that they even out the forces acting on the cyclist, making it easy to stay on the track at speed. The forces at work are the radius of the turn, velocity of the bike, and the force of gravity. The surface gravity on Mars is about 38% of that on Earth, so my protagonist can’t just copy the standard dimensions of an Earth velodrome.
The formula is θ = tan−1 (v²/rg), where θ is the angle the cyclist will lean at, v is the velocity of the bike, r is the radius of the turn, and g is gravity’s acceleration. You tilt the surface to be perpendicular to the cyclist’s lean as he travels through the curve. Because of the lighter gravity, the Martian velodrome would need a higher bank to compensate, so my heroes may find it hard to get started on the track with low velocity. (Hence I have them fall over on their first attempts in the story.) They will also have to decide whether it’s worth it to have a larger velodrome for larger turns with lower banks. There would be some point at which the cost of materials outweighs the benefit to the cyclist, and I imagine the final dimensions of the velodrome were carefully worked out before the Quickcrete machine was taken from its storage bay.
If you find these sorts of questions as fun as I do, I think you’ll enjoy “The First Velodrome on Mars.” You can find a list of all my past stories (many in Analog!) at my website, marievibbert.com. My latest novel, Andrei and the Hellcats, comes out July 14!
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