[Fun Maths]Centrifuge Fun

[Andy44]

The problem with an internal centrifuge like Discovery is that it is limited in diameter.

I always thought that pressure seals for external centrifuges were a problem, until I read about an idea that made me feel stupid for not thinking of:

Instead of one pressure seal between the spinning and despun pressurized sections, you have an intermediate section with a hatch on either end, which spins up to open the hatch into the centrifuge and spins down to open the despun hatch. It's basically a roatating airlock with a hatch on either end.

 

[Notebook]

The problem with an internal centrifuge like Discovery is that it is limited in diameter.

I always thought that pressure seals for external centrifuges were a problem, until I read about an idea that made me feel stupid for not thinking of:

Instead of one pressure seal between the spinning and despun pressurized sections, you have an intermediate section with a hatch on either end, which spins up to open the hatch into the centrifuge and spins down to open the despun hatch. It's basically a roatating airlock with a hatch on either end.

Does this mean the airlock is attched to the centrifuge while there is someone using it, and not attached when its empty?

N.

 

[JamesG]

Pretty much. The airlock is more like a elevator car that instead of going up and down, spins to match the rotating section, couples with it. Then detaches and then stops and then seals with the non-rotating section.

Seems very complicated and would be clumbersome to use. Would suck if you needed to go back and forth many times or if you went thru and then forgot something.

 

[Notebook]

Having a slight problem visualising this. If the centrifuge is disc-shaped, rotating about the centre of the disc, does the airlock connect at the circumference, or along the axis of rotation?

N.

 

[JamesG]

Centrifuge part is your normal wheel or whatever, connected by a bearing of some kind to the main structure of the ship (or at least what doesn't spin). Between a pressure door in the hub of the rotating part and another one on the facing bulkhead of pressure hull, there is a self contained airlock "cell" with its own doors that is just shorter than the space between the hub and main hull.
To get from the stationary hub to the centrifuge, the airlock is mated to main hull and you enter it. Close both doors and decouple from the main hull. Spin the airlock up to the rpm of the centrifuge and then couple with the rotating part (which from your perspectice isn't rotating anymore). "Dock" with it and open the doors.
Ta Da! you are now free to roam around in some form of simulated gravity.

Far simpler to just spin the whole ship and only counter rotate a antenna/sensor mast and a docking airlock at the center of rotation.

 

[T.Neo]

So split the difference. Build it with a flattened oval profile. This allows a curved pressure vessel for integrity, and a wider inside for a deck or two.

Well, that makes more sense. But you're going to end up with a even more complex shape to build; flat sheets are a lot easier to fabricate then oval sections.

The "wasted" space is actually useful for things like plumbing and ducting and power cables, as well as under-floor storage space.

Piping and ducts, sure, but I'm not so sure about loose storage. If you were, to say, place a crate (full of toothpaste, or some other mundane supply) it would still require a flat surface to rest on. Unless you had the shape of the container conform to the curved wall behind it.

Go talk to a structural or A/S engineer. They would probably be more interested than I in spelling out all of the technical reasons of "why things are the way they are".

Yes, "things are the way they are" because we build space habitats so often...

 

[JamesG]

Well, that makes more sense. But you're going to end up with a even more complex shape to build; flat sheets are a lot easier to fabricate then oval sections.

You are building a *space craft*. If you can't handle fabricating compound curves, you have no business in the space business.

Piping and ducts, sure, but I'm not so sure about loose storage. If you were, to say, place a crate (full of toothpaste, or some other mundane supply) it would still require a flat surface to rest on. Unless you had the shape of the container conform to the curved wall behind it.

Why? Very few things have to be stored absolutely "this side up", especally if you aren't under full Earth gravity. And even if you did want them nice and orderly, you can fit shelves and brackets.

Yes, "things are the way they are" because we build space habitats so often...

We build space habitats all the time (just not very many at a time). Space craft, especally pressurized manned craft are cylindrical for the same reason soda cans are cyliders with concave ends. It provides you with the maximum strength and volumn with the minimum weight and surface area.

 

[T.Neo]

You are building a *space craft*. If you can't handle fabricating compound curves, you have no business in the space business.

Uh, yeah. Does the idea of cost and complexity reduction ever enter your head? It's about avoiding complexity. Adding to that, the fact that factories on the Moon or cislunar space won't exactly be advanced.

Why? Very few things have to be stored absolutely "this side up", especally if you aren't under full Earth gravity.

Well, assuming that the storage areas are around the same distance from the axis of rotation as the living areas, you're at least going to require 0.38g.

We build space habitats all the time (just not very many at a time). Space craft, especally pressurized manned craft are cylindrical for the same reason soda cans are cyliders with concave ends. It provides you with the maximum strength and volumn with the minimum weight and surface area.

I meant large rotating habitats, sorry. Those of which we seem to build strikingly little of these days.

 

[JamesG]

Uh, yeah. Does the idea of cost and complexity reduction ever enter your head? It's about avoiding complexity. Adding to that, the fact that factories on the Moon or cislunar space won't exactly be advanced.

Last time.

Its not any harder to create a toroidal structure than a slab sided one. Easier from an engineering stand point.

The factories that get sent/build in space will be the absolutely state of the art because the cost of getting them and keeping them there is an order of magnitude more expensive than the hardware itself. Same goes with the structures you build.

 

[perseus]

Peso total obtenido por la rotación, con diferentes grados de severidad de las cubiertas, A = V ^ 2 / R o A = W ^ 2.R (V = velocidad W = velocidad angular, R = radio)
Las secciones de la cubierta pueden ser rectangulares, mientras que el casco de presión es de esférica o geoide.

 

[T.Neo]

Last time.

Its not any harder to create a toroidal structure than a slab sided one. Easier from an engineering stand point.

No, it is harder to make curved pieces of metal, then it is to make flat ones. And if you're not willing to invest "effort" into design, well...

The factories that get sent/build in space will be the absolutely state of the art because the cost of getting them and keeping them there is an order of magnitude more expensive than the hardware itself. Same goes with the structures you build.

So "it's going to be expensive, let's make it more expensive"? -That's the mentality that has made spaceflight prohibitively expensive for the last 50 years.

Space-factories probably won't even be sent to space, btw. They'll be built from in-situ materials.

Seriously though, I'm not suggesting a flat-panel design is some sort of end-all, be all holy grail. But you are not entitled to dismiss it without some study and effort.

 

[Andy44]

Far simpler to just spin the whole ship and only counter rotate a antenna/sensor mast and a docking airlock at the center of rotation.

That restricts you to only one or maybe two docking ports, and the docking vessels must be balanced to rotate about their docking ports as well. The Perseus posted above is an example: It has docking ports off-center, which can only be used if vessels of identical masses are docked at the same time for balance, and they can only dock or undock when the vessel spins down.

This means that a spinning wheel station will have no space shuttle visits, for instance. And even if you manage to build a 2001-style wheel station, it can only handle one or two spaceliners.

On top of all that, you must balance the entire ship if you're spinning it, or else you will have wobble and nutation problems. Seems to me a despun section with not just antennae but also docking ports is a good idea.

By my way of thnking, only the parts that need to be spun should be spun. Easier to balance (by pumping less water around or whatever your balance mechanism is), and easier to construct the rest of the vessel, by not being required to strengthen it radially you can make it less massive.

Maybe what we need is to build a couple of demonstrators and see which one works best!:thumbup:

 

[T.Neo]

That restricts you to only one or maybe two docking ports, and the docking vessels must be balanced to rotate about their docking ports as well. The Perseus posted above is an example: It has docking ports off-center, which can only be used if vessels of identical masses are docked at the same time for balance, and they can only dock or undock when the vessel spins down.

What about an elaboration of your earlier idea, for partially rotating craft?

A small, self-propelled taxi to ferry supplies and people from a non-rotating shuttle to the station?

 

[Eagle]

A small, self-propelled taxi to ferry supplies and people from a non-rotating shuttle to the station?

That will work as long as you don't need to go back and forth between the two very often. The line to transfer between vessels might get backed up and there is a small expenditure of air and fuel each trip.

 

[Andy44]

Plus the two vessels now need to expend propellant to remain in formation.

In the Two Faces of Tomorrow by James P. Hogan, a large space station had a spinning and a despun section which were held together by some sort of magnetic bearing. I can't remember how the airlock integrity was maintained, though.

 

[T.Neo]

Plus the two vessels now need to expend propellant to remain in formation.

Indeed they would. But at least there would be no moving parts in terms of spun/despun sections.

 

[Andy44]

No, now you need an airlock that is essentially a complete spacecraft, with propulsion, control, life support, and docking systems.

A system of bearings may be simpler.

 

[T.Neo]

No, now you need an airlock that is essentially a complete spacecraft, with propulsion, control, life support, and docking systems.

Indeed. It would only have to travel a short distance though, so I'm not sure how much power/propulsion/life support it would require.

In the end you only end up docking two of these to the station at most, anyway. The only advantage to having no system at all would be the ability of docking with vessels without centered docking ports.

 

[Andy44]

The only advantage to having no system at all would be the ability of docking with vessels without centered docking ports.

No there is also no need to balance the entire despun section. Also, the ability of docking multiple vessels which do not need reinforced docking port structures to avoid G-loading under spin and which do not need to be balanced.

My own pre-fab space station add-on was designed with a despun sectior for this reason. It must dock many more than just two vessels, and those vessels cannot be easily designed to have strong docking ports aligned with the center of mass.

 

[T.Neo]

No there is also no need to balance the entire despun section. Also, the ability of docking multiple vessels which do not need reinforced docking port structures to avoid G-loading under spin and which do not need to be balanced.

I was not referring to a despun section, I was referring to a seperated taxi vessel.