I May Have Solved the Centrifuge Problem!!!

[Ael Syd]

Okay, big dramatic title to draw you in. But seriously, I need someone much smarter than me to tell me why I'm talking crazy here. (and I guess I should admit right off the bat that I'm not fully "in the know" about these things, so this could be an idea that's already been thought about, and proven nutty)

From what I understand, it's very difficult to build a spacecraft that generates gravity-simulating acceleration by rotating a long arm with a habitat module attached, or a wheel design for that matter. There are several issues, but the biggest seems to be energy usage. If you have a spinning section, it's going to lose energy due to friction and so you'll need to keep spinning it back up to speed with thrusters or an engine of some sort, and that's a huge fuel penalty.

I know that the Mars Direct people came up with a novel concept of rotating the entire spaceship around a common axis to generate artificial gravity. Pretty clever idea; since the spaceship is not losing any energy to friction then, like an asteroid spinning in empty space, it can rotate for a very long time without applying much if any additional energy. But for a larger spacecraft, it doesn't seem practical to have to constantly spin up the entire ship between maneuvers.

So, at some point thinking about this, I had my eureka moment: You could have a traditional non-rotating craft. You get it set on its trajectory, and then you detach a companion habitat module and just spin that up. Now, since the spaceship is already on its trajectory and traveling at a constant velocity, the detached section should (in theory) stay with the main craft, but it's not in contact, so it should be able to rotate freely without losing energy to friction. When the ship needs to maneuver, the hab module can be retracted, spun down, and reattached to the main body of the spaceship.

Is this a nutty idea? It sounds brilliant to me, but I'm not an engineer.

---------- Post added at 02:09 AM ---------- Previous post was at 02:08 AM ----------

Also, apologies for the dull, text-heavy OP. Next time I throw out a "concept" post like this, I'll not be lazy and actually include some diagrams or something!

 

[malisle]

I'm not sure I follow. You would detach a habitat module from the main craft and rotate it independently of the main craft (no tether lines etc)? Why not simply rotate the entire craft around it's axis without any disassembly?

 

[Linguofreak]

From what I understand, it's very difficult to build a spacecraft that generates gravity-simulating acceleration by rotating a long arm with a habitat module attached, or a wheel design for that matter.

It's really too early to say. The issue at this point is that current space programs don't have the funding to operate far enough away from Earth to make it worthwhile or necessary to even try building such a thing, so we've not put much effort into figuring out how to do it.

I know that the Mars Direct people came up with a novel concept of rotating the entire spaceship around a common axis to generate artificial gravity. Pretty clever idea; since the spaceship is not losing any energy to friction then, like an asteroid spinning in empty space, it can rotate for a very long time without applying much if any additional energy. But for a larger spacecraft, it doesn't seem practical to have to constantly spin up the entire ship between maneuvers.

You have a lot of dead time between maneuvers for any spacecraft it would matter for, and it can be done with reaction wheels.

 

[Urwumpe]

Also, you can actually maneuver even while spinning. It is not easy if the connection is not rigid, but possible.

Still, physics won't change: If you separate a module, your base module will also move, the CoG of the whole connected vehicle will stay on the trajectory, but both modules will rotate around the CoG.

 

[jedidia]

I know that the Mars Direct people came up with a novel concept of rotating the entire spaceship around a common axis to generate artificial gravity.

Novel? That Idea is probably older than the idea of spinning a centrifuge by itself without the rest of the ship :lol:

 

[statickid]

I propose a simple reason: the fuel needed to maintain position as well as to keep docking and redocking might be more than what is needed to fight friction. This might be especially true when it could be spun up with some tiny dedicated ion engines as opposed to using valuable RCS systems for docking.

I also simply do not like the overly complicated nature of the setup. Is the unspun portion manned or unmanned during these flight sections? If manned, it will require extra dockings for shift changes. I also don't like the fact that if something unexpected happens then maybe you've already lost half the ship. I think it's just too complicated.

 

[Ael Syd]

In theory, it shouldn't require too much fuel. You could pretty much just have the hab section held up by a set of tripod legs that retract and leave it hovering some safe distance above the main body. Since constant velocity is indistinguishable from being at rest, it should stay put. Re-docking would just be a matter of reattaching the legs. If you wanted to get really funky, just for safety's sake, you could even engineer the thing so that a section of the hab descends into a caged off area below where it's shaped so that if it does drift off course, it catches and comes back to where it should be.

It's a wacky untested concept, but then so are all the others at this point. I think it sounds pretty valid. You don't want to have a spin section attached to the rest of your rigid ship. I guess for any ship we're likely to build in the near future, it will be small enough that we could spin up the whole craft with reaction wheels like Linguofreak was saying, and not use up an immense amount of time or fuel in doing so. The main reason this came up was because I was designing a larger science-fiction style craft for something I was working on and the ship seemed too large and unwieldy to spin the whole thing (not to mention it's a cargo vessel which could be transporting volatile materials).

---------- Post added at 09:59 AM ---------- Previous post was at 09:51 AM ----------

...Still, physics won't change: If you separate a module, your base module will also move, the CoG of the whole connected vehicle will stay on the trajectory, but both modules will rotate around the CoG.

Ah, always a fly in the ointment! So the two bodies would probably not maintain the exact relative position for a very long period, because even though they're on the same trajectory, they'd have different rotational properties. I'm trying to visualize a workaround for that, but it's pretty clearly becoming a much more complicated animal than I had initially thought.

 

[Face]

In theory, it shouldn't require too much fuel. You could pretty much just have the hab section held up by a set of tripod legs that retract and leave it hovering some safe distance above the main body. Since constant velocity is indistinguishable from being at rest, it should stay put. Re-docking would just be a matter of reattaching the legs.

The problem with this approach is that a habitat in space will actually change the position of its walls w.r.t. a rigid stationary object due to the moving of interior parts. Of course this will not result in propulsion (and therefore relative velocity), but in shifting around the center of mass of the whole hab including people and material within.

If you wanted to get really funky, just for safety's sake, you could even engineer the thing so that a section of the hab descends into a caged off area below where it's shaped so that if it does drift off course, it catches and comes back to where it should be.

I think this might actually work. Imagine the hab as kind of bouncing ball inside of a net attached to the stationary parts... with a clever reel-in procedure, you could eliminate almost all fuel spending (provided that spin-up/down is achieved by reaction wheels).

 

[Urwumpe]

Ah, always a fly in the ointment! So the two bodies would probably not maintain the exact relative position for a very long period, because even though they're on the same trajectory, they'd have different rotational properties. I'm trying to visualize a workaround for that, but it's pretty clearly becoming a much more complicated animal than I had initially thought.

Well, if you operate without any tether holding things together, your problem will also be simple relative orbital mechanics, "Hill's equations" to be precise.

This paper here tries to do exactly what you are talking about:

http://hohmann.mit.edu/papers/ecc_orb_jgcd.pdf

And also explains what will happen.

 

[Ael Syd]

Thanks you for that, Urwumpe! I'll check it out.

I guess usually when you think you have a new idea, that just means you haven't done enough research!

 

[Donamy]

Been there, done that.

https://www.youtube.com/watch?v=k7PjSDhjRxA

 

[Ael Syd]

Well, that one had a tether and a rotating section on the ship. That's exactly what I was trying not to do!

---------- Post added at 10:34 AM ---------- Previous post was at 10:33 AM ----------

I was trying to come up with an idea that avoids the fuel cost of rotating an entire section of the ship or hab modules. Having the hab freely spinning in space alongside the main body is almost costless, but right now it looks like there are some pretty good engineering reasons why it's not a good idea.

 

[Donamy]

Electro magnets eliminate the friction.

 

[Ael Syd]

How does that work? Where are you putting the magnets?

 

[Urwumpe]

How does that work? Where are you putting the magnets?

Look for magnetic levitation or [ame="http://en.wikipedia.org/wiki/Magnetic_bearing"]magnetic bearing[/ame].

 

[Ael Syd]

Okay, so I was trying to come up with an inventive way to solve a friction problem... that doesn't really exist. Greeeeat! Well, this is all good stuff I'm learning, even if my brilliant idea went down in flames in the process!

 

[N_Molson]

Personally I would try to power the rotating device with electric motors... In a Earth->Mars scenario, solar panels should give enough power to counter the friction... Or you add a fission reactor to the stack (there have been several realistic proposals) which solves your energy problems. Maybe I would add a set of RCS hydrazine thrusters to give the possibility to stop or control the rotation during a contingency.

 

[Capt_hensley]

My solution to GWS gravity wheel was as simple as it was elegant. 4 electric motors place 90 degrees apart, spinning a bearing seal rack and pinion style inside the graviton module.

the issue comes from connecting electrical (somewhat simple) and liquid transfer couplings, (much more difficult) without causing more friction than a 20 watt motor can absorb and still operate freely.

as for CG issues the GWS has many GNC(CMGs) and Prop modules to keep station so while GWS architecture is constantly out of perfect CG, the station will be in proper orbit configuration. The cost is balanced by spreading it out over the electrical and fuel capacity of the entire station.

next the wheel spins parallel to the main station lending a tendency to roll the entire configuration, but with all the GNC and Prop modules the torque of the roll is kept in check with very little cost in energy.

I'm sure all would agree, it's not perfect, but it's better than no gravity hab at all.