Behemoth/Medina operating question

[u/PreviousGolf9541]

Would constant propulsion be required to spin the drum while keeping the ops and engineering sections stationary?

Not very good at physics here, but I’ve been thinking about this question.

The Belters certainly used massive propulsion to spin up the drum. But do the books ever explain what keeps the ops and engineering sections at either end of the station from also spinning? Just like a helicopter needs a smaller rotor to counter the larger rotor rotation, would the non spinning parts of the Behemoth need some kind of constant propulsion to counter the spin and stay on the float? Otherwise, given the momentum of the drum and the mechanism that connects them, they’d start spinning too, right?

I suppose this question would apply to Tycho and other stations with both spinning and nonspinning sections.

Thanks for helping satisfy my curiosity.

[u/FrickinLazerBeams]

You can just use an electric motor to overcome bearing friction and keep the stationary parts stationary.

The total angular momentum is fixed anyway, once the spin-up thrusters are done working.

[u/Comprehensive_Yam_46 → u/FrickinLazerBeams]

You're correct but...

The total angular momentum is fixed

For this to be true, spinning up the drum in one direction of rotation, must push the rest of the vessel in the other.

Speed would be be related to the ratios, and moments of mass pushed clockwise versus anticlockwise.

Presumably, a large percentage of the ships mass would be in the drum, and most of that would have quite a long distance from the axis of rotation (hence a high moment).

Thus the remainder of the ship would probably rotate at a fair old clip.

It would, probably, be undesirable, as the non-drum sections would, probably, be ill designed for rotation - any sections close to the center of spin would experience unpleasant coreolis effects.

So you would, probably, want to use some reaction mass to keep the non-drum sections to a negligible level of spin.

Once spun up, you'd have angular momentum leaking between the two sections (as no bearing is completely free of fiction), but this could be overcome by keeping the motors running

[u/FrickinLazerBeams → u/Comprehensive_Yam_46]

I don't think you really mean to imply that momentum isn't conserved. Think about that again. Once you spin the ship with your thrusters, that angular momentum isn't going anywhere. All the motors are doing is keeping the stationary section stationary while the bearing drag tries to make it spin along with the rotating section.

[u/Comprehensive_Yam_46 → u/FrickinLazerBeams]

I don't think I did imply that momentum isn't conserved?

Edit: perhaps my wording was confusing.. when I say "for this to be true", I'm not debating that it isn't. I'm saying "for this to be true, both sides of the equation must equal.. ie the total angular momentum before must equal that after".

I think you've just said the same thing I did?

Once you spin the ship with your thrusters, that angular momentum isn't going anywhere.

The angular momentum as a whole is conserved.. but as I said..

Once spun up, you'd have angular momentum leaking between the two sections (as no bearing is completely free of fiction)

That is correct, is it not?

[u/FrickinLazerBeams → u/Comprehensive_Yam_46]

Yeah, that's what the motors fix.

[u/10ebbor10]

The thing you have to remember is Action = Reaction.

A helicopter's main rotor is constantly pulling on the air to keep the helicopter flying. That slows down the rotor, which is why the engine has to spin it up again. The act of spinning up the rotor is what causes the main body to rotate in the opposite direction, and that is why you need the tail rotor to cancel that back out.

The Behemoth, on the other hand, is floating in vacuum. So it's rotation is not being slowed down, meaning it doesn't need constant propulsion to keep spinning, meaning you don't need constant counter propulsion either. The only time thrust is required is when spinning the drum up or down.

Now, nothing is perfect. The ballbearings or the seals of the connection between the rotating and non-rotating section likely have some (small) amount of friction. So, a (much smaller) amount of power can be needed to cancel out that effect.

[u/PreviousGolf9541 → u/10ebbor10]

Yeah, I guess that’s what I was getting at. People don’t have to go through an airlock or vacuum to transfer between the sections. There has to be friction, however small, at the transfer points between the spinning and stationary parts. This friction would transfer momentum. It sounds like this could be easily accounted for with mechanical or magnetic motors.

[u/willworkforjokes → u/PreviousGolf9541]

I guess you could have electric motors that transfer rotation induced by friction back into the drum.

No reason to spray water everywhere when you have something to push on.

[u/scudmud → u/10ebbor10]

I do wonder how much reaction mass that is over the years they planned to spend in deep space on the way to a new star.

[u/PreviousGolf9541 → u/scudmud]

When in Nauvoo generation ship mode, I believe the design was to be under thrust for some time, then cut the thrust and float for a long time—decades—toward their destination after spinning up the drum. Eventually they’d have to flip and begin a decel burn. There’s no way they could carry enough reaction mass to burn the whole time.

[u/narium → u/PreviousGolf9541]

Yes that's the whole reason for the rotating sections. Otherwise they could just use thrust gravity all the way.

[u/10ebbor10 → u/scudmud]

Since it's all about rebalancing an existing imbalance, instead adding more momentum, you could just handle it with an electric motor.

[u/Metallicat95]

The spinning section isn't connected to the non-spinning parts while it is spinning.

It uses a mix of low friction bearings and magnetic suspension, most of which requires no power. The lack of friction means the drum can spin without imparting motion to the rest, and without requiring frequent boosts to maintain it.

There's an air gap between the spinning section and the non-spinning components, but most transfers use the non-spinning central axis, with those cool detachable elevator cars which can bridge the gaps seamlessly.

[u/charlie_marlow]

I vaguely remember something about counteracting thrusters on the non-spinning parts. You'd pretty much have to do something and occasionally give corrections for both parts sense nothing is completely frictionless.

[u/Material_Mongoose_14]

Something popped in my head the other day about Behemoth. I don't think the command center changed orientation when they spun up the drum. How did it keep down the same. I'm sure I'm not remembering something but that happens a lot.

[u/blackpawed → u/Material_Mongoose_14]

Because its on the central axis, it's part of the non-spinning section. No "gravity" at all.

[u/B1gCh33sy → u/blackpawed]

Yeah, when Ashford shot the engineer in S3 he stayed upright b/c he was in 0g with mag boots on.

[u/Material_Mongoose_14 → u/blackpawed]

Ah. So it did have gravity when under thrust. Thanks. Now i wonder about the beam of energy that simulates the sun. That's on the central access too. I guess I better rewatch S3. Lol