Using a skimmer for propulsion (with airships or rockets)

[DrPhysics he/him]

I spend too much time thinking about physics in the Cosmere and invented a form of propulsion. 

Background:

We know from the books and Brandon himself that momentum is conserved when a feruchemist changes their mass (weight) in the books.

Quote

Brandon Sanderson:

1). Wax changing his weight doesn't change the pull of gravity on him, or the rate at which he falls. 2) He DOES follow the laws of conservation of momentum. Source:Reddit

There's been a few discussion here on the shard about how this breaks down with reference frames, but every situation in the books works if you assume the mass that is stored/tapped is at rest with respect to the skimmer's reference frame. That's why Wax getting heavy on a train doesn't slow him down, because he sees himself as sitting still on the train.

In the books, if you decrease mass while flying through the air, you will speed up. If you increase mass, you will slow down. That works if you assume the mass you are storing/tapping moves at the speed of the room you are in. Really, what you are doing is stopping the mass when you store it, giving the momentum that it had to you, so you speed up. When you tap the mass, you have to speed it up to your speed, making you slow down. 

However, if you tap mass that is already moving at your speed, you don't change speed (this is why when Wax gets really heavy while sitting still, he doesn't slow down relative to the rotation of Scadrial).

Propulsion:

Now imagine that you could change reference frames either using perception (opening/closing a window with some sort of metal mantra or concentration aid) or using Connection. You could do propulsion this way:

1. Put a skimmer on an airship (or rocket) and convince them that they are moving in the airship/are connected to the ground.
2. They store mass that isn't moving in the reference frame of the planet, speeding them (and the ship) up.
3. Close the windows/change Connection so that the skimmer is convinced that they are at rest with respect to the ship,
4. Tap mass that is moving at the speed of the ship and therefore won't slow it down.
5. Repeat from step 1

This works similar to a rocket - you jettison mass in a way that increases your momentum - but has the advantage that you don't have to carry all the fuel with you. If you could repeat this process fast enough, after a small starting push any skimmer would be able to reach orbit (or beyond) for very little fuel cost, instead just alternating storing/tapping weight and their perception of their surroundings.

 

We can even come up with the relevant thrust equation:

Force = change in mass/change in time * speed of the ship (F=-dm/dt*v) (negative denotes that force is opposite velocity)

 

Launching the rocket - aka the Bendalloy Railgun

If we can find ways to manipulate Connection, you could launch the ship by having it enter a bendalloy time bubble (while attached to a sturdy rail so that it doesn't deflect) while the bubble is connected to the planet, making the rocket speed up. Then you switch the connection so that the thing making the bubble is connected to the rocket and the bubble travels with it, so that the rocket doesn't leave the bubble until it reaches its destination and you attach the bubbles to the new planet and let the rocket slow down as it exits each one.

I'm imagining a charged ettmetal grenade in a magnetic casing that you could just drop onto the ship as it goes by, or use a system that lets you turn an iron pull on/off to bring them in.

Stacking bubbles (so that you enter several) could increase the speed further.

Edited June 17, 2025 by DrPhysics
Adding in thrust equation and Bendalloy railgun

[KaladinsSenseOfHumourSpren He/Him]

13 hours ago, DrPhysics said:

I spend too much time thinking about physics in the Cosmere and invented a form of propulsion. 

Background:

We know from the books and Brandon himself that momentum is conserved when a feruchemist changes their mass (weight) in the books.

There's been a few discussion here on the shard about how this breaks down with reference frames, but every situation in the books works if you assume the mass that is stored/tapped is at rest with respect to the skimmer's reference frame. That's why Wax getting heavy on a train doesn't slow him down, because he sees himself as sitting still on the train.

In the books, if you decrease mass while flying through the air, you will speed up. If you increase mass, you will slow down. That works if you assume the mass you are storing/tapping moves at the speed of the room you are in. Really, what you are doing is stopping the mass when you store it, giving the momentum that it had to you, so you speed up. When you tap the mass, you have to speed it up to your speed, making you slow down. 

However, if you tap mass that is already moving at your speed, you don't change speed (this is why when Wax gets really heavy while sitting still, he doesn't slow down relative to the rotation of Scadrial).

Propulsion:

Now imagine that you could change reference frames either using perception (opening/closing a window with some sort of metal mantra or concentration aid) or using Connection. You could do propulsion this way:

1. Put a skimmer on an airship (or rocket) and convince them that they are moving in the airship/are connected to the ground.
2. They store mass that isn't moving in the reference frame of the planet, speeding them (and the ship) up.
3. Close the windows/change Connection so that the skimmer is convinced that they are at rest with respect to the ship,
4. Tap mass that is moving at the speed of the ship and therefore won't slow it down.
5. Repeat from step 1

This works similar to a rocket - you jettison mass in a way that increases your momentum - but has the advantage that you don't have to carry all the fuel with you. If you could repeat this process fast enough, after a small starting push any skimmer would be able to reach orbit (or beyond) for very little fuel cost, instead just alternating storing/tapping weight and their perception of their surroundings.

 

We can even come up with the relevant thrust equation:

Force = change in mass/change in time * speed of the ship (F=-dm/dt*v) (negative denotes that force is opposite velocity)

Wow...

Now only if this could be automated by fabrial or something. Or if you had someone raised from birth to do this, then it could work really well. The only thing is that the ship is going to be much heavier than the skimmer, so even storing to the point of air-like mass, then you wouldn't speed it up that much. This would be more effective if, say, a skimmer were going for a space walk with just a space-suit on.

Or, even better, a full feruchemist that is also tapping or maybe compounding cadmium and gold, so that you only need a form of propulsion and the metalminds themselves.

[DrPhysics he/him]

5 hours ago, KaladinsSenseOfHumorSpren said:

The only thing is that the ship is going to be much heavier than the skimmer, so even storing to the point of air-like mass, then you wouldn't speed it up that much.

The force depends on (1) how quickly the skimmer can store mass and (2) how fast the ship is going but does not depend on the skimmers mass, unless that somehow impacts #1.

So, if the ship has a decent jump start and/or the skimmer can cycle quickly, they would be able to create a significant push.

 

Our real rockets work by ejecting a tiny mass very very quickly, so could this one.

[Through The Living Ash he/him]

I'm pretty solid at physics, but there's something I'm not getting here. I can see how you could propel the skimmer in this way, but how would that affect the propulsion of the airship? 

[DrPhysics he/him]

27 minutes ago, Ashkaloda said:

I'm pretty solid at physics, but there's something I'm not getting here. I can see how you could propel the skimmer in this way, but how would that affect the propulsion of the airship? 

You'd have to strap in the skimmer. If you can create a force on the skimmer (or propel them in any way) they can in turn exert a force on the ship. It's like strapping someone with a jetpack onto the top of the ship. The jetpack pushes the person who in turn pushes the ship.

[Through The Living Ash he/him]

Ah, for some reason I was imagining a skimmer just floating in the center of the ship. 

[DrPhysics he/him]

If they were, they wouldn't stay there long...

[Nitpicking]

Create a medallion or fabrial that Connects the allomancer to the ship, the way

WIND AND TRUTH SPOILER

Spoiler

Ishar connected the Radiants to the planet.

That would let the allomancer affect the mass of the entire vessel.

[Through The Living Ash he/him]

41 minutes ago, Nitpicking said:

Create a medallion or fabrial that Connects the allomancer to the ship, the way

WIND AND TRUTH SPOILER

  Reveal hidden contents

Ishar connected the Radiants to the planet.

That would let the allomancer affect the mass of the entire vessel.

Connection isn't going to work that way. The Feruchemist will have Connections to countless things, but the Connection alone will not allow them to affect other things, especially using Feruchemy. You would also have to alter the Intent of the Investiture used, which would be significantly harder to do, or much easier would be to somehow include the ship in the Feruchemist's Identity.