### Gravitational assumptions

I was working on some fluid flow at work the other day, and while trying to determine whether I could use gravity flow between a series of tanks or whether I had to put a pump in there somewhere, I wondered how a chemical plant built in microgravity would work.

Fluids or solids move because they have potential energy. (Once they're moving they also have kinetic energy.) So what kind of potential energy could, say, a pipe full of water have, if it didn't have gravitational potential and thus couldn't do any kind of gravity flow?

The first thing I thought of was pressure; this is how pumps overcome gravity and lift liquids uphill. Absent gravity, building up pressure at one end of a pipe would push the fluid toward the other, lower pressure end.

Quite an extensive system could be built with just one pump, with every pipe and tank sealed. Each tank would be at a slightly lower pressure than the one before. The pump wouldn't even have to be right at the beginning; pumps have a certain amount of suction head available to them. In a gravitational field this is usually used to draw liquid up out of a container and into the pump, and so is limited by the vapour pressure of the liquid in question. In microgravity the suction head would also be limited by the vapour pressure of the liquid, but because it isn't battling gravity it could draw quite a lot of liquid through quite a lot of equipment. Without gravity, head loss / pressure loss is determined by friction. However, without gravity, you also can't get a free ride just by building on the side of a hill.

But what about the initial and final stages? All material processed by the plant has to come from somewhere and go to somewhere. I've worked at plants that used ponds at either end, but those depend on gravity to keep the liquid in place. Similarly, a treated water discharge to a river depends on gravity to keep the water moving away from the end of the pipe.

But it's not just water. Liquid and gas reagents can be delivered in batches and pumped out of their tanks; the tanks would have to either be flexible or have an air inlet to keep them at constant pressure for consistent pumping, but that actually seems fairly straightforward.(Actually, the air inlet assumes gravity too - what keeps the liquid by the pump suction and the air elsewhere? Gravity!) Solid reagents or solid products are often delivered to or removed from the process by conveyor belt, screw conveyor, rotary valve, or pneumatics. Of those, only pneumatic conveying doesn't depend on gravity - it depends on pressure, and it works best on stuff light enough to be moved around by hitting it with air. In microgravity, those particles could be bigger than with gravity, because there's nothing pulling them to settle at the bottom of the pipe, but there is also a practical size limit dictated by how heavy the particles shooting toward a corner in the piping system are! A conveyor belt depends on gravity to keep the material sitting on the belt. A screw conveyor and rotary valve (and conveyor belts too) usually have a hopper in which the solid material is stored, and as the unit removes solids more falls down into the moving parts. Yeah—falls down. There's that gravity again.

Then I started wondering about dump trucks. There has been science fiction talking about asteroid mining for decades; I'm sure I'm far from the first person to wonder what the asteroid belt equivalent of a haul truck would look like. How would you load and unload it with no gravity to keep stuff in the bed of the truck? It would have to be closed to keep the rocks inside during transport, and you can't just tilt the thing to slide the load out, because that uses gravity. You'd have to push or pull the rocks out somehow. I don't think air would do the trick; haul trucks can move anything from a single rock of multiple tonnes (how many tonnes depends on the truck's capacity), to a big pile of gravel.

What else assumes gravity? I'm sitting on a chair. My drink stays in its cup. My toilet flushes. My furniture stays where I put it. Hot air rises away from the candle flame, and water from the shower head falls into the drain. NASA has been thinking about this from the start, it seems—and it looks like everything we do assumes gravity.