How Thurst Creates Artificial Gravity.


I’m currently reading through The Expanse series (on Caliban’s war at the moment) and the always talk about the feeling of gravity as the ship moves at 1G. I understand that higher gravity forces make the feeling of an elephant on your chest but the say they can walk around the ship as if everything is normal while under 1G of thrust. Wouldn’t any forward momentum stick you the the opposite wall or is their engine at a weird angle?

In: 9

I’m trying to remember which ship you’re discussing specifically, but The Expanse does quite well with physics in general.

In the Expanse they generally have two systems for artificial gravity, a spinning thing (like Ceres) and a thrusting object (like the Roci).

Spinning – Imagine spinning a bucket of water on rope around your head – the water doesn’t leave right? Because the water always wants to fly *away* in a straight line with the rope, but the bucket resists that flying away. (the same way just holding a bucket of water keeps the water from just hitting the ground). So on Ceres they have a system of tunnels carved in circles around the center of the planetoid and the “ceilings” of the tunnels (the sides closest to space vs. the center of Ceres) act like the bucket in the string example. So people are actually walking around on the *ceilings*, being pressed against the ceiling with the ceiling pushing back against them keeping them from flying away into space. That’s the push that mimics gravity. The strength of the push depends on the speed the planetoid spins so they discussed in detail earlier in the novels that step 1 was a “spinning up” Ceres in the first place to a speed that produces a force that mimics gravity. Math is hard here, but this also implies that the distance from the center of Ceres also impacts the strength, so they also mention how people who live in the tunnels closest to the center of Ceres experience *less* gravity than those closest to the surface. In theory in the center of Ceres you’d just be floating like you were in deep space.

Thrust – In the case of the Roci, they are not spinning. The decks of the Roci are actually at a right angle to the centerline of the ship. So not like an airplaine, the opposite as an airplane. If you were sitting in a chair the top of your head would point to the nose cone and your butt would point towards the engines. This system *only works* when the Roci is under a specific amount of thrust AND in a straight line nose-forward. If the Roci could “reverse” they’d all fly up and the hit the ceiling. When the Roci twists and turns in fights the people will rag-doll around. This is exactly why they make such a big deal of always getting into their couches prior to maneuvering and I’m pretty sure several characters do die from being flung around due to maneuvering. In an ideal flight, and you see this often, the Roci (or any of the maneuvering space ships) will fly at a specific thrust nose-forward (so people are under 1G) and then *flip around* (people in couches for a moment) and then re-engage the engines (really slowing them down) again, mimicking gravity.

For a final point, it’s not *thrust* that’s key here, is *acceleration*. When we feel as gravity is our constant acceleration down the center of the Earth, only we usually have the Earth resisting our fall. In the case of the spinning ship, you are accelerating in a constant circular motion because nature hates circles, it wants straight line movement, to make something spin in a circle you need to be constantly accelerating in a loop away from a straightline, so that’s the how “gravity” works on Ceres. In the case of the Roci, people want to “fall back” behind the engines and the couches/ship are pushing them up exactly like the Earth keeps us from falling into the Core.

If you are on a ship with thrust, “down” will be toward the back of the ship. To put it another way, the ship is always moving “up”.

> Wouldn’t any forward momentum stick you the the opposite wall

Any forward thrust would indeed stick you to a wall, and you would name that wall “Floor” and you would walk around on that wall and put your furniture on that wall.

The wall opposite “Floor” would be called “Ceiling” and the thrust would make it very hard for you to reach out and touch “Ceiling” unless you are very tall.

Ships in *The Expanse* are built like skyscrapers with the engine on the bottom. This means that under thrust, you feel like gravity is pulling you towards the floor. For stations using spin gravity, the levels are arranged with “down” towards the outside of the station.