Author Topic: Large inflatables (and artifical gravity on Phobos)  (Read 18620 times)

Offline alexterrell

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Older science fiction is full of large domes on Mars and The Moon. These seem to have gone out of fashion, and there's a lot of work on semi-rigid inflatables.

Most of the Bigelow Habitat mass is accounted for by shielding - against micro-impacts and radiation.

But on the moon, or Phobos, that can be provided by regolith. A treble walled fabric can suffice.
- An inner layer to prevent accidental damage
- A pressure layer
- An outer layer, to contain the shielding

Just add regolith between the outer layer and middle layer to get wahtever thickness you desire.

The mass of the pressure vessel increases with the cube of the diameter. Using materials like spectra, 100m diameter is possible in 100 tons, 50m diamter 12.5 tons (though actually at that size, you'd stop shrinking wall thickness.

This makes a way to create comparitively massive habitats with very low mass, using regolith as shielding.

There's a focus on having Phobos (or Deimos) as an exploration destination. http://forum.nasaspaceflight.com/index.php?topic=18339.0, as it seems likely to have water, and plenty of regolith for shielding.

But no artifical gravity.

So, would it be possible to launch an inflatable dome or disc with a spinning wheel in the centre?

Assume a Jupiter class 100 ton payload. This is taken to Phobos collapsed. It's anchored to the surface, and inflated. Then a robot hopper gradually scoops up regolith (which is meant to be flaky), and covers the habitat, perhaps with 5-10m of regolith. (I'm not sure if the outer layer is needed to prevent regolith from falling off - as it the regolith weighs almost nothing, it might be better to just keep on pouring and bury the whole thing. The robot hopper could be mounted at the centreof the habitat.

The habitat is disk shaped, 50m in diameter, and perhaps 25m high. Inside, is an inflatable "tread-mill", axially mounted, supported on magnetic bearings. This rotates at 4rpm to provide Mars level gravity around the edge. (Humans are selected for withstanding this.) The floors are rigid inflatable.

Would it be feasible?



Offline rklaehn

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #1 on: 09/12/2009 10:01 AM »
Older science fiction is full of large domes on Mars and The Moon. These seem to have gone out of fashion, and there's a lot of work on semi-rigid inflatables.

Most of the Bigelow Habitat mass is accounted for by shielding - against micro-impacts and radiation.

But on the moon, or Phobos, that can be provided by regolith. A treble walled fabric can suffice.
- An inner layer to prevent accidental damage
- A pressure layer
- An outer layer, to contain the shielding

Just add regolith between the outer layer and middle layer to get wahtever thickness you desire.

The mass of the pressure vessel increases with the cube of the diameter. Using materials like spectra, 100m diameter is possible in 100 tons, 50m diamter 12.5 tons (though actually at that size, you'd stop shrinking wall thickness.

This makes a way to create comparitively massive habitats with very low mass, using regolith as shielding.

There's a focus on having Phobos (or Deimos) as an exploration destination. http://forum.nasaspaceflight.com/index.php?topic=18339.0, as it seems likely to have water, and plenty of regolith for shielding.

But no artifical gravity.

So, would it be possible to launch an inflatable dome or disc with a spinning wheel in the centre?

Assume a Jupiter class 100 ton payload. This is taken to Phobos collapsed. It's anchored to the surface, and inflated. Then a robot hopper gradually scoops up regolith (which is meant to be flaky), and covers the habitat, perhaps with 5-10m of regolith. (I'm not sure if the outer layer is needed to prevent regolith from falling off - as it the regolith weighs almost nothing, it might be better to just keep on pouring and bury the whole thing. The robot hopper could be mounted at the centreof the habitat.

The habitat is disk shaped, 50m in diameter, and perhaps 25m high. Inside, is an inflatable "tread-mill", axially mounted, supported on magnetic bearings. This rotates at 4rpm to provide Mars level gravity around the edge. (Humans are selected for withstanding this.) The floors are rigid inflatable.

Would it be feasible?

I see nothing impossible about this, but it would definitely be a longer term solution.

For an initial phobos base, you might consider putting a 3m radius centrifuge into a normal size bigelow habitat. There is evidence that such a centrifuge can be used to mitigate the effects of zero gravity.

The current publications on this topic all cost money. See for example this summary (link). But you can find a lot of summaries by doing a search for "short radius artificial gravity".

The idea is to basically let the crew spend one or two hours per day doing exercise on a short-radius centrifuge at ~2g to mitigate the effects of zero gravity. It has been shown to work to reduce bone loss.
« Last Edit: 09/12/2009 10:21 AM by rklaehn »
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Offline tamarack

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #2 on: 09/12/2009 10:33 AM »
No.
-If only the circumference is habitable, why waste weight building the interior? Just rotate a hoop within an inflated tube covered in shielding.
-"The floors are rigid inflatable." First; I have to question just how rigid they'd be and if they could support the systems. Second; to keep the structure balanced as crew/supplies move about, sufficient static rotational weight or fluid counterweight is needed. Which comes back to an inflatable floor's insufficient load bearing.

For any large structure on a body covered in regolith, I'd recommend a concrete plant http://forum.nasaspaceflight.com/index.php?topic=18641.0 to produce the foundations, shielding tiles, main structural components, launch/landing pad, etc.

-Any extraterrestrial base is a major undertaking unless it's launched pre-fab like the ISS or Altair. If artificial gravity is also an objective, IMO it would be better to send and spin completed habitats in an optimal orbit around the chosen body. Shield tiles, water, fuel, crops, science, etc could still be retrieved from the surface base, but habitat and base construction is vastly simpler.

$0.02

Offline rklaehn

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #3 on: 09/12/2009 10:48 AM »
No.
-If only the circumference is habitable, why waste weight building the interior? Just rotate a hoop within an inflated tube covered in shielding.

You need some way to transfer from the zero-gravity portion of the base to the circumference. So it would make sense to let the center be pressurized. But you are right that you would not want the center section to be too thick since it is basically wasted volume. A torus-shaped inflatable connected to a disc-shaped central section might be the best approach.

Quote
Second; to keep the structure balanced as crew/supplies move about, sufficient static rotational weight or fluid counterweight is needed. Which comes back to an inflatable floor's insufficient load bearing.

If you bury the whole thing in phobos soil as proposed by alexterrell, you would not need any moving counterweights for crew movements. The forces acting on the hub are trivial compared to what industrial bearings can survive, and any vibrations would be effectively dampened by the entire mass of phobos.

Of course when moving major pieces of equipment around you would want to keep them roughly balanced.

Quote
-Any extraterrestrial base is a major undertaking unless it's launched pre-fab like the ISS or Altair. If artificial gravity is also an objective, IMO it would be better to send and spin completed habitats in an optimal orbit around the chosen body. Shield tiles, water, fuel, crops, science, etc could still be retrieved from the surface base, but habitat and base construction is vastly simpler.

I think putting several meters of shielding in the rotating section of a habitat is not practical. But burying an inflatable with an internal rotating section under 10m of phobos soil would be possible.

Such a habitat would offer complete protection against both galactic cosmic radiation and solar flares. And you would not have to worry about zero gravity effects. So it would really be a place where you could stay pretty much indefinitely.

I think there is also a psychological aspect: a habitat that is buried below a large layer of regolith would not only be a very safe place, but also feel that way.

Edit: I found this thing (link) to be very useful for calculating artificial gravity habitats.
« Last Edit: 09/12/2009 11:09 AM by rklaehn »
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Offline randomly

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #4 on: 09/12/2009 11:38 AM »
No.
-If only the circumference is habitable, why waste weight building the interior? Just rotate a hoop within an inflated tube covered in shielding.
-"The floors are rigid inflatable." First; I have to question just how rigid they'd be and if they could support the systems. Second; to keep the structure balanced as crew/supplies move about, sufficient static rotational weight or fluid counterweight is needed. Which comes back to an inflatable floor's insufficient load bearing.

I believe Alexterrell implied that the inflated treadmill was housed within an outer structure that was not inflated but just provided a protected volume for the rotating system and thus could be light in weight since it's not load bearing. Having a continuous interior also simplifies the bearings and drive mechanism because you can have a single axle and bearings at the center, which also gives you a much smoother 'ride'. The habitable volume may be hoop shaped with a connecting passage to the core all inside a secondary outer skin that is inflated to low pressure (< 1psi) just to provide an unobstructed volume to rotate within the regolith 'sand pile'. At 4.7 psi the inflatable structure should be quite rigid. if you need more local rigidity for equipment you can spread the load with panels or design the equipment to spread the load to the skin. If you restrict the amount of habitable volume you could also probably just make it rigid.
Quote

For any large structure on a body covered in regolith, I'd recommend a concrete plant http://forum.nasaspaceflight.com/index.php?topic=18641.0 to produce the foundations, shielding tiles, main structural components, launch/landing pad, etc.
A concrete plant seems unnecessarily complex and unneeded for this situation. You just want to bury the hab. You don't need a landing pad, a 100 mt (almost a quarter of a million pounds) spacecraft only weighs about 40 kg on Phobos and could be picked up by a single astronaut.
Quote
-Any extraterrestrial base is a major undertaking unless it's launched pre-fab like the ISS or Altair. If artificial gravity is also an objective, IMO it would be better to send and spin completed habitats in an optimal orbit around the chosen body. Shield tiles, water, fuel, crops, science, etc could still be retrieved from the surface base, but habitat and base construction is vastly simpler.
Absolutely you want to launch it prefabbed and just bury it. The whole raison d'etre of a Phobos base is dealing with the intractable problem of GCR. You need meters of shielding, so you go where the shielding is.

As to burying it, digging and moving the regolith seems problematic in such low gravity. It might require a system of drilled in anchors and cables to hold the excavator down. Or possibly the excavator would drill an anchor in and then excavate from opposite sides of itself simultaneously to minimize lateral thrust requirements. I would think some kind of 'snowblower' arrangement to just jettison the regolith in an aimed shower over the hab would be workable, and it would also provide some small additional downward thrust on the excavator.

nice link rklaehn  :)
« Last Edit: 09/12/2009 12:35 PM by randomly »

Offline rklaehn

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #5 on: 09/12/2009 03:18 PM »
I think covering the habitat in regolith would be simplified by just finding an appropriately sized crater and putting it into the crater. You would also have significant radiation protection before you finished covering the habitat with regolith.

Here is a high resolution image of phobos taken from mars express:
phobos (link).

That bar on top with the 5km marking is about 400 pixel wide. So the resolution is about 12.5m per Pixel. A 50m diameter toroidial habitat like the one proposed by alexterrell would be just four pixels across. There are literally hundreds of craters that would have exactly the right size.

This also provides some sense of scale for those that think of phobos as a tiny object. Compared to everything we can hope to achieve in the next few decades it is so large that it might as well be a planet.
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Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #6 on: 09/12/2009 03:46 PM »
Thanks for the responses. I attach a picture to show my thinking:

- Rather than a sphere, I've gone for a filled in torus, with a tensile unit down the middle (At 0.4 bar, there is some 3000 tons of tension on this bar), and an ellisoidal minor cross section (if that's the right term).
- The payload is placed in a crater, and inflated. (Before inflation, the payload fits in a 10m wide, 20m long fairing (100 tons)
- A second unit is placed on top to provide access
- The "treadmill" (colured red) is about 6m deep. It consists of multiple 6m rigid slats, attached to the centre by high strength filaments. The centre rotates using electric motors and magnetic bearings. I think this is better than an inflatable unit, and in storage the slats line up parallel to the payload axis.
- The treadmill rotates at 4 rpm, to give about .38g on the outer floor, and less on the inner floors. The outer floor has 900m2 of space. In total there's 3,000m2 of floor space. There will be some holes in the floors for ladders or stairs to be inserted.
- Other space is used for zero-g stuff. Life support equipment is placed in here later.
- A little away, a scoop is tied down. This digs up dirt, and throws it on top, until the crater is filled in.

The point is, with no need for armour, the entire structure could fit in one heavy lift payload.

Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #7 on: 09/12/2009 08:53 PM »
Or, on the moon, you don't need a centrifuge, so you can go smaller. A 25m diameter dome-

One of the requirements is that in the event of pressure loss, the inside is not destroyed. The dome may sag, but the structures need to support it.
« Last Edit: 09/12/2009 08:58 PM by alexterrell »

Offline randomly

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #8 on: 09/12/2009 09:01 PM »
I think covering the habitat in regolith would be simplified by just finding an appropriately sized crater and putting it into the crater. You would also have significant radiation protection before you finished covering the habitat with regolith.

That's what I originally proposed in this thread http://forum.nasaspaceflight.com/index.php?topic=18339.msg463094#msg463094
But I think we are talking about 2 different Phobos scenarios. One is where you arrive at Phobos in your transfer vehicle/Hab and park it in a crater on the Mars side of Phobos to mitigate GCR radiation and live there for the duration of the mission, then 'take off' from Phobos in the same vehicle to take you home. The other scenario is to make a permanent base structure and send it to Phobos and bury it. You use a separate vehicle for Mars - Earth transits. The buried base is reused for multiple missions perhaps even continually manned with rotating crews from earth.  Mars is explored by landing a telepresence robotic lander and a small sample return vehicle at every point of interest. Missions eventually including manned sorties to Mars surface.
Quote
Here is a high resolution image of phobos taken from mars express:
phobos (link).

That bar on top with the 5km marking is about 400 pixel wide. So the resolution is about 12.5m per Pixel. A 50m diameter toroidial habitat like the one proposed by alexterrell would be just four pixels across. There are literally hundreds of craters that would have exactly the right size.

This also provides some sense of scale for those that think of phobos as a tiny object. Compared to everything we can hope to achieve in the next few decades it is so large that it might as well be a planet.

Great picture, thanks  :)
Btw any idea what point on that Phobos picture would be the closest towards Mars?

These types of systems and large EDL shields are why I lean towards a launch vehicle with capacity for a very large fairing size like Jupiter even though it may not be the most economical $/kg to LEO.
« Last Edit: 09/12/2009 09:11 PM by randomly »

Offline docmordrid

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #9 on: 09/12/2009 11:46 PM »
Bigelow has some ideas on how to do this patented or in the process. 

When reviewing the below patent data keep in mind the modification to their space station propulsion bus - landing legs added - in one of their concept images.  Not exactly as in the patents images, but then it's an early version.

For low gravity locales I guess pitons could be fired and the base reeled down and secured.




http://www.freepatentsonline.com/7469864.html
http://www.freepatentsonline.com/7469864.pdf

(Images at bottom of this post)

Quote
Method for assembling and landing a habitable structure on an extraterrestrial body

Document Type and Number: United States Patent 7469864

Kind Code: A1

Abstract:

A method for assembling and landing a habitable module on an extraterrestrial mass is claimed. At least one inflatable module and a second module are placed into orbit about an extraterrestrial mass. Connecting nodes, propulsion busses, and landing pads are also placed into orbit. A habitable structure is constructed from the modules, busses, pads, and nodes. The structure can be robotically constructed. The habitable structure is then landed onto the surface of the extraterrestrial mass. The landing can be remotely controlled and the modules can be non-occupied.

Inventors: Bigelow, Robert T. (US)
Application Number: 11/363346
Publication Date: 08/30/2007
>

http://www.freepatentsonline.com/y2008/0296435.html
http://www.freepatentsonline.com/20080296435.pdf

(note: STF's are being developed by DARPA as a liquid armor that can be included in fabric combat jackets.  On contact they harden enough to stop bullets.)

Quote
Shear-thickening fluid reinforced fabrics for use with an expandable spacecraft

Document Type and Number: United States Patent Application 20080296435
Kind Code: A1

Abstract:

A shear-thickening fluid is used in conjunction with fabrics utilized in an expandable spacecraft. The combination of the fluid and the fabric allows the fabric to resist penetration by hypervelocity particles in space.

Inventors: Cohen, Dan (Las Vegas, NV, US)
Application Number: 11/807830
Publication Date: 12/04/2008
Filing Date: 05/30/2007

Assignee: Bigelow Aerospace
>

http://www.freepatentsonline.com/y2008/0000168.html
http://www.freepatentsonline.com/20080000168.pdf

Quote
Regolith container for use with a structure on an extraterrestrial mass

Document Type and Number: United States Patent Application 20080000168
Kind Code: A1

Abstract:

A regolith container for use with regolith on an extraterrestrial mass is disclosed. The filled container covers a portion of a spacecraft to provide the spacecraft with a measure of protection against space debris and radiation. The container can be compartmentalized to reduce the loss of regolith should from the container if a single compartment is penetrated.

Inventors: Bigelow, Robert T. (Las Vegas, NV, US)
Application Number: 11/479276
Publication Date: 01/03/2008
Filing Date: 07/03/2006
>








« Last Edit: 09/13/2009 12:29 AM by docmordrid »
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Offline KelvinZero

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #10 on: 09/13/2009 03:55 AM »
Cant the ares V launch something with a radius of 10 or so?

What about a bycycle-powered treadmill that fits in this. That way you have it for the journey as well. Then you bury it when you get there.

Also you can have passenger seats (plus desks and terminals) so that you get a couple of hours exercise each day, and some more hours doing desk work (eg teleoperating robots) also under gravity.

And for that matter, if the whole thing breaks down, you could probably get some exercise and impacts just running around the circuit. I think a high impact competitive sport could also help. The strength of an impact doesnt depend on the gravity, but how hard you are trying. Perhaps something like basketball, but using a medicine ball and a drum instead of a court.

Offline KelvinZero

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #11 on: 09/13/2009 04:20 AM »
One issue: how does phobos rotate on its axis? You would want the centrifuge to have the same axis of rotation or there would be a lot of twisting on the bearing.

Does everything in space have an axis of rotation? Or can they tumble in a more complicated way?
« Last Edit: 09/13/2009 04:29 AM by KelvinZero »

Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #12 on: 09/13/2009 07:04 AM »
One issue: how does phobos rotate on its axis? You would want the centrifuge to have the same axis of rotation or there would be a lot of twisting on the bearing.

Does everything in space have an axis of rotation? Or can they tumble in a more complicated way?
I'm pretty certain Phobos will be tidally locked. So wil rotate every 10 hoursor so.

If you had a large rotating structure, yes, you'd probably want it at a pole. The structure envisioned above though is very light weight - probably 20 tons or so.

Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #13 on: 09/13/2009 07:12 AM »
Cant the ares V launch something with a radius of 10 or so?

What about a bycycle-powered treadmill that fits in this. That way you have it for the journey as well. Then you bury it when you get there.

Also you can have passenger seats (plus desks and terminals) so that you get a couple of hours exercise each day, and some more hours doing desk work (eg teleoperating robots) also under gravity.

And for that matter, if the whole thing breaks down, you could probably get some exercise and impacts just running around the circuit. I think a high impact competitive sport could also help. The strength of an impact doesnt depend on the gravity, but how hard you are trying. Perhaps something like basketball, but using a medicine ball and a drum instead of a court.
I think Ares V would be about 16m diameter max fairing. With a "soft inflatable" (as opposed to a Bigelow "hard inflatable") you really can get it all in a 10m fairing.

The inflatable structures here can't be inflated in space because they're very vulnerable to micro-impacts - probably 2mm thick Spectra 2000 sheet, with a 1mm kevlar outer layer.

High impact exercise helps a lot. But a Mars mission is going to be over 2 years.


Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #14 on: 09/13/2009 07:17 AM »
Bigelow has some ideas on how to do this patented or in the process. 

Not sure I can see much inventive here.

The US Patent Office will look at previous patents and decide it's inventive. But when it comes to a challenge, just look through the Sci Fi literature, or better still, sci.space.policy, or nasaspaceflight, and you'll see almost everything has been invented, at least at the macro level.

There will be inventions in sub-systems, but I doubt the use of patents in the major systems.

Offline KelvinZero

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #15 on: 09/13/2009 07:50 AM »
synchronous and 7h40m according to wiki
http://en.wikipedia.org/wiki/Phobos_(moon)

It also mentions:
Quote
As seen from Phobos, Mars would appear 6400 times larger and 2500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere.
What a view :)

however it also seems to include this information:
Quote
names Jake!!! this guy in my class called curtis has a massive egg head and likes men!!!! hahahahahahahahahahahahahahahhahahahahha
I hope they update that shortly :)

..anyway..

Even for only twenty tons I reckon you would not want it twisting on its axis every 7 hours. Maybe a polar location could give you easier line-of-sight to earth anyway.

Also if they are not landing on mars, two years probably isnt too bad. But we would have to solve this for a permanent base anyway, and that doesnt seem a bad goal.

Offline randomly

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #16 on: 09/13/2009 05:09 PM »
You can put the Hab anywhere you want by cancelingl out the rotational inertia with a counter rotating mass at higher rpm to minimize it's weight. This also allows you to cancel out the torque from masses (people) moving from the core out to the periphery and back. This may not be a necessary feature for a buried hab, but would probably be required for a ship on the surface with AG.

The patents seem pretty worthless as patents often are, easily invalidated due to obviousness to a practitioner of the arts or prior art. They'll probably be expired before they ever get used anyway.
« Last Edit: 09/13/2009 05:13 PM by randomly »

Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #17 on: 09/13/2009 06:47 PM »

Even for only twenty tons I reckon you would not want it twisting on its axis every 7 hours. Maybe a polar location could give you easier line-of-sight to earth anyway.

Of course, if you went for a 50m diameter sphere, you could put the rotating ring on any axis you want.

Of course, if you have it on a horizontal (phobos) axis, you'd get a slight variation in g. I suspect this might feel like a gentle , long frequency swell in an ocean liner. Probably not an issue compared to the coriolis effect.

If you cancel out the inertia with a counter rotating mass, you still have the torque in the bearings, so if you're on Phobos, it's not really a help. If in orbit, then you'd put the spin in the same axis as the orbit.

Offline randomly

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #18 on: 09/13/2009 07:20 PM »
Of course, if you went for a 50m diameter sphere, you could put the rotating ring on any axis you want.
You think bigger than I'm allowed to.
Quote
Of course, if you have it on a horizontal (phobos) axis, you'd get a slight variation in g. I suspect this might feel like a gentle , long frequency swell in an ocean liner. Probably not an issue compared to the coriolis effect.

If you cancel out the inertia with a counter rotating mass, you still have the torque in the bearings, so if you're on Phobos, it's not really a help. If in orbit, then you'd put the spin in the same axis as the orbit.
With a 7h 40m orbit I doubt you'd notice anything.

Yes you'd still have the torque in the bearings but that should not be a problem for the bearings to handle. I'm more concerned with the problem of a ship with AG sitting on the surface being tipped over by gyroscopic forces in the very low gravity. Even under several meters of loose regolith the gyroscopic forces could possible move the habitat around since the downforce of 2 meter thick layer of regolith over a 15 m diameter hab is only a few hundred pounds.

Offline alexterrell

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Re: Large inflatables (and artifical gravity on Phobos)
« Reply #19 on: 09/14/2009 09:14 AM »
Of course, if you went for a 50m diameter sphere, you could put the rotating ring on any axis you want.
You think bigger than I'm allowed to.

Why? Do you work for NASA?

Unfortunately, a 25m radius is about the minimum for any useful artificial gravity. That said, a 50m sphere, with a light weight internal structure, designed for 0.4 bar (0.2 O2 + 0.2 N2), with a safety factor of 10, will fit into a SD HLV payload.

Total force = Pi x 25^2 x 40000 = 80MN.
2mm thick wall area = 2E-3 x 50 x Pi = 0.155m2
F/A = 250MPa. Spectra http://en.wikipedia.org/wiki/Spectra_%28fiber%29 has a yield strength of 2.4GPa.
Assuming total thickness 4mm (1mm inner layer, 1mm outer layer), Volume=Pi x 50^2 x 4E-3 = 31m3
Mass = 31 tons.

The Phobos design above has 3,000m2 of floor space. This can be made of Spectra slats, average thickness 3mm (10mm/1mm thick I girders), Mass = 10 tons.

Add in internal supports and electrical connectors etc, and it easily comes to less than 100 tons.

It can be inflated and tested to 1 bar in any hanger on Earth.

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Of course, if you have it on a horizontal (phobos) axis, you'd get a slight variation in g. I suspect this might feel like a gentle , long frequency swell in an ocean liner. Probably not an issue compared to the coriolis effect.

If you cancel out the inertia with a counter rotating mass, you still have the torque in the bearings, so if you're on Phobos, it's not really a help. If in orbit, then you'd put the spin in the same axis as the orbit.
With a 7h 40m orbit I doubt you'd notice anything.

Yes you'd still have the torque in the bearings but that should not be a problem for the bearings to handle. I'm more concerned with the problem of a ship with AG sitting on the surface being tipped over by gyroscopic forces in the very low gravity. Even under several meters of loose regolith the gyroscopic forces could possible move the habitat around since the downforce of 2 meter thick layer of regolith over a 15 m diameter hab is only a few hundred pounds.
So you need an outer layer that sticks to the regolith. Nothing the Norwegian cross country ski team can't sort out for you :)

That said, for Phobos, I'd go for the Open Torus design I showed above. Being flatter, you can get the regolith on top of it. If it does deflate, the internal structures will need to support a tons or so of weight, whilst the crew fix it in their spacesuits.

This scale also avoids the need for transfer of services to the rotating part. Only electricity needs a commutator. Clean Water and Waste Water would be provided by a daily transferring of tanks. Data will be WiFi. Air will be natural ventilation. Stale air will be pumped out, cleaned, cooled, and pumped back in. The rotating section (outer wall speed 22mph) ensures plenty of air circulation all over.

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