Why artificial gravity is not possible?

[mheslep]

I really need to learn how to be more concise... care to teach me the Mheslep method?

Heh, well one can fine much better examples than my posts on PF.

 

[cjameshuff]

To be compact, light, and durable, can you imagine the strength of the materials needed for the flywheel, and given the amount of energy involved you'd need a casing like a vault. I believe you'd rapidly find that the safety requirements at this time would make it far too heavy, and to ignore safety would be... bad.

I can imagine a simple cast aluminum or steel case, yes. The scaling laws make lightweight, small radius, very high rotation rate flywheels preferable. They are essentially spools of carbon fiber, kevlar, etc with a binder. When they fail, a great deal of energy is released, but there are no chunks of high momentum material that would tear a hole in a reasonably strong casing as you describe. Your claims of dangerous shrapnel are incorrect.

mheslep refers to older style flywheels, but is correct, the main reason they are not often used is the size required...not danger of them exploding. A small vehicle powered purely by a steel or aluminum flywheel would largely be flywheel. More modern high speed composite flywheels have more promise for small vehicles...even if they're still too large for a primary energy storage medium, their ability to rapidly accept and supply energy gives them potential for use as temporary storage for regenerative braking and acceleration...for example: http://www.flybridsystems.com/

In comparison, capacitors have terrible energy density, and batteries have relatively severe limitations on charge/discharge rate and cycle count.

 

[mheslep]

... and batteries have relatively severe limitations on charge/discharge rate and cycle count.

Well charge rate is a problem, best addressed by battery swaps I believe. Discharge rate (i.e. specific power) is sufficient, and cycle life in a thermally controlled battery appears to be on the order of 2000-3000X, full depth of discharge.

 

[nismaratwork]

I can imagine a simple cast aluminum or steel case, yes. The scaling laws make lightweight, small radius, very high rotation rate flywheels preferable. They are essentially spools of carbon fiber, kevlar, etc with a binder. When they fail, a great deal of energy is released, but there are no chunks of high momentum material that would tear a hole in a reasonably strong casing as you describe. Your claims of dangerous shrapnel are incorrect.

mheslep refers to older style flywheels, but is correct, the main reason they are not often used is the size required...not danger of them exploding. A small vehicle powered purely by a steel or aluminum flywheel would largely be flywheel. More modern high speed composite flywheels have more promise for small vehicles...even if they're still too large for a primary energy storage medium, their ability to rapidly accept and supply energy gives them potential for use as temporary storage for regenerative braking and acceleration...for example: http://www.flybridsystems.com/

In comparison, capacitors have terrible energy density, and batteries have relatively severe limitations on charge/discharge rate and cycle count.

The flywheel detonates, the casing and other bits of car become shrapnel... and that's a much bigger issue with a smaller radius, rapid flywheel. That energy release is happening inside a CAR, concentrated and trapped by its casing... I'm sorry, but that's a bomb.

 

[mheslep]

The flywheel detonates, the casing and other bits of car become shrapnel... and that's a much bigger issue with a smaller radius, rapid flywheel. That energy release is happening inside a CAR, concentrated and trapped by its casing... I'm sorry, but that's a bomb.

So's a gas tank. As pointed out above the composite flywheels more or less vaporize on failure, as I understand without much risk to escaping the case. I believe the problem there is economic - the high RPM requires a near vacuum and exotic bearings. Fine for NASA, not so much in a Ford.

 

[nismaratwork]

So's a gas tank. As pointed out above the composite flywheels more or less vaporize on failure, as I understand without much risk to escaping the case. I believe the problem there is economic - the high RPM requires a near vacuum and exotic bearings. Fine for NASA, not so much in a Ford.

I don't know... it doesn't take much melted casing to kill you... I would add again that gasoline is flammable when mixed properly with oxygen. Without a bomb or large fire you won't have all of the energy in the gasoline released at once... not so with a flywheel.

Then... there's the NASA grade engineering, but that makes this less fun to talk about. :)

 

[DaveC426913]

This is sort of going off track. The simple point is that we utilize a technology with a ubiquity proportional to how manageable it becomes.

Any powerplant capable of propelling a starship and all its occupants on an interstellar journey will have to harness far more than enough energy to vapourize everything in a medium sized volume if it fails.

This hypothetical black-hole-engine is no different. Worse case, it's mounted at the end of a ten mile long extension rather than a one-mile long extension.

 

[nismaratwork]

This is sort of going off track. The simple point is that we utilize a technology with a ubiquity proportional to how manageable it becomes.

Any powerplant capable of propelling a starship and all its occupants on an interstellar journey will have to harness far more than enough energy to vapourize everything in a medium sized volume if it fails.

This hypothetical black-hole-engine is no different. Worse case, it's mounted at the end of a ten mile long extension rather than a one-mile long extension.

I'm not sure that's necessarily true... consider VASIMIR for instance. I think my point about a black hole is that it can scale beyond our capacity to manage it by its very nature. Even antimatter would be devastating, but it would annihilate, killl everyone... the end! A black hole wouldn't explode... in fact it would persist and potentially GROW. I agree with you in general, but a black hole is a unique... object... and anything that has the capacity to scale between microscopic and galactic is fundamentally dangerous in ways no other technology I can think of is.

 

[thedeester1]

you could place the space station in one of the gravity neutrals. therefore you wouldn't need fuel to spin the station it would all be done by repulsing gravity and ceterfugal force! Could scarifice gravity for supplies by using thruster engines to move you to supplies that were launched into space...then you would have to find the neutral zone again...Maybe 90% of t he supplies would be fuel?

 

[thedeester1]

pardon my spelling but its very possible! Gravity as a force has to exist between all large objects and the math of the middle is key. Put an object in the neutral zone and it will spin at the rate to create gravity

 

[dkotschessaa]

Of course whatever means we come up (rather than the rotational solution) is going to have an effect on nearby bodies, i.e. earth, if we are talking about putting this thing in orbit. What crazy tidal effects might we expect? Surfs up?

-DaveKA

 

[cjameshuff]

you could place the space station in one of the gravity neutrals. therefore you wouldn't need fuel to spin the station it would all be done by repulsing gravity and ceterfugal force! Could scarifice gravity for supplies by using thruster engines to move you to supplies that were launched into space...then you would have to find the neutral zone again...Maybe 90% of t he supplies would be fuel?

pardon my spelling but its very possible! Gravity as a force has to exist between all large objects and the math of the middle is key. Put an object in the neutral zone and it will spin at the rate to create gravity

...where did you get this idea?
The only "gravity neutral" I can think of in Earth's neighborhood is the point where Earth's and the moon's gravity is equal, just a bit on the far side of the Earth-Moon L1 point. Gravity doesn't repel and an object placed at this location (or the nearby L1 point, where you can actually have a closed orbit, albeit an unstable one) won't start spinning on its own...not due to being at any sort of neutral point, anyway.

And aside from that "neutral zone" stuff not working at all as described, it sounds like you're suggesting saving the small amount of fuel needed to spin up the station by using massive amounts of fuel to chase supplies around this "neutral zone"...

 

[DaveC426913]

you could place the space station in one of the gravity neutrals. therefore you wouldn't need fuel to spin the station it would all be done by repulsing gravity and ceterfugal force! Could scarifice gravity for supplies by using thruster engines to move you to supplies that were launched into space...then you would have to find the neutral zone again...Maybe 90% of t he supplies would be fuel?

pardon my spelling but its very possible! Gravity as a force has to exist between all large objects and the math of the middle is key. Put an object in the neutral zone and it will spin at the rate to create gravity

Yeah, I don't know what you're getting at. This describes nothing in physics that I know of.

There are Lagrange points, but they don't cause any spinning.

 

[thedeester1]

...where did you get this idea?
The only "gravity neutral" I can think of in Earth's neighborhood is the point where Earth's and the moon's gravity is equal, just a bit on the far side of the Earth-Moon L1 point. Gravity doesn't repel and an object placed at this location (or the nearby L1 point, where you can actually have a closed orbit, albeit an unstable one) won't start spinning on its own...not due to being at any sort of neutral point, anyway.

And aside from that "neutral zone" stuff not working at all as described, it sounds like you're suggesting saving the small amount of fuel needed to spin up the station by using massive amounts of fuel to chase supplies around this "neutral zone"...

I got this off of Wikki...not the most reliable source I know...But its kinda what i was getting at...Im not a student of astro physics its kinda a hobby for a geek.

http://en.wikipedia.org/wiki/Lagrangian_point

other neutral gravity points must exist between all mass in the solar system and the universe...otherwise fast colapse would be occurring NOW.

 

[nismaratwork]

I got this off of Wikki...not the most reliable source I know...But its kinda what i was getting at...Im not a student of astro physics its kinda a hobby for a geek.

http://en.wikipedia.org/wiki/Lagrangian_point

You've utterly misunderstood everything I can imagine in regards to what Langrangian points are, based on what seems to be a complete lack of understanding of gravity. I would recommend some reading on those subjects.

 

[russ_watters]

I got this off of Wikki...

Could you quote the passage in the wiki that says putting an object at a Lagrange point will make it spin?

 

[nismaratwork]

I got this off of Wikki...not the most reliable source I know...But its kinda what i was getting at...Im not a student of astro physics its kinda a hobby for a geek.

http://en.wikipedia.org/wiki/Lagrangian_point

other neutral gravity points must exist between all mass in the solar system and the universe...otherwise fast colapse would be occurring NOW.

re the bolded portion: OK, you added this... why? Lagrangian points are not 'gravity neutral' in the way you're portraying them. Your conclusion that somehow these points are producing a repulsive force that keeps universal expansion from being overwhelmed by mass... then WHAT THE **** are you talking about?!

 

[thedeester1]

Could you quote the passage in the wiki that says putting an object at a Lagrange point will make it spin?

no i cant...sorry...But i think that it will spin. this is my thinking as a novice.. You could try it out place 2 metal ojects on a table then rotate the outer object. given our gravity your going to have to spin the object very quickly.. In space however i don't think that's the case. The passing centrifugal force will spin the said space station...i think

 

[cjameshuff]

no i cant...sorry...But i think that it will spin. this is my thinking as a novice.. You could try it out place 2 metal ojects on a table then rotate the outer object. given our gravity your going to have to spin the object very quickly.. In space however i don't think that's the case. The passing centrifugal force will spin the said space station...i think

I'm sorry, but...I don't think any of what you've posted can clearly be said to be correct. A full explanation of what you got wrong would be a physics primer that starts from a clean slate. I'm not even sure what you're trying to illustrate with your metal objects on a table. Aside from air currents, vibrations, etc, spinning one won't do anything to the other...regardless of spin or amount of gravity. (lets not bring frame dragging into this, let the guy get a grip on Newtonian mechanics first...)

The Wikipedia article you linked is fine, but your interpretation of it is totally off the mark. Some better starting points:
http://en.wikipedia.org/wiki/Newton's_laws_of_motion
http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation

But given how badly confused the L-point article got you, I'm not sure how much help this will be. You might be better off checking out an introductory physics testbook from a library.

 

[nismaratwork]

no i cant...sorry...But i think that it will spin. this is my thinking as a novice.. You could try it out place 2 metal ojects on a table then rotate the outer object. given our gravity your going to have to spin the object very quickly.. In space however i don't think that's the case. The passing centrifugal force will spin the said space station...i think

This isn't your thinking as a novice, this is just some random thinking based on concepts you seem to only just have learned. You clearly don't understand the most basic models of gravity, never mind Relativity's view. cjameshuff is right, you just need to start from square one... carefully.

 

[tggokulesh]

you are wrong . it is easy to create an artificial gravity.this is what einstein's(actually its mach's) equivalence principle states.if you accelerate upwards it would produce a natural gravity which is induced.to know more read general relativity basics

 

[nismaratwork]

you are wrong . it is easy to create an artificial gravity.this is what einstein's(actually its mach's) equivalence principle states.if you accelerate upwards it would produce a natural gravity which is induced.to know more read general relativity basics

Really? Can you cite a single reference to support that?

 

[DaveC426913]

Really? Can you cite a single reference to support that?

Well, he's simply saying a ship under constant acceleration (say, 1g) will give its occupants the same effect as gravity. And he's quite right.

Problem is:
1] It's a huge fuel cost, accelerating all the way.
2] The occupants would go floating away every time the ship wanted to stop.

 

[dkotschessaa]

Does the inverse square law apply to the the forces created in this way (acceleration or rotation?)

-DaveKA

 

[DaveC426913]

Does the inverse square law apply to the the forces created in this way (acceleration or rotation?)

-DaveKA

No.

In the case of rotational AG, you won't experience it unless you are in contact with the deck.

In the case of accelerative AG, you will experience the acceleration of the deck toward you, but it will be independent of your distance from the deck.

 

[nismaratwork]

Well, he's simply saying a ship under constant acceleration (say, 1g) will give its occupants the same effect as gravity. And he's quite right.

Problem is:
1] It's a huge fuel cost, accelerating all the way.
2] The occupants would go floating away every time the ship wanted to stop.

Yes, but that's not gravity, but a totally different pseudo-force. Obviously we're long past spinning up stations, magnets, and the like. It isn't a "natural gravity" as he said, nor (and this is crucial as you've already pointed out the issues) is it "easy". I'm not arguing against centrifugal force or 1g constant acceleration... which STILL doesn't solve the problem. Even if you have massive amounts of fuel and don't want to stop, you eventually start getting close to c, and are no longer able to sustain 1g.

[STRIKE]dkotschessaa: Well, it applies to Gravity, which by the equivalence principle means that any related pseudo-force should too. I'm not sure that it's that simple, but that's my guess.[/STRIKE]
edit: The above is clearly wrong (having read Dave's post). My bad!

 

[DaveC426913]

Yes, but that's not gravity, but a totally different pseudo-force.

tggokulesh actually says "artifical gravity":

"...it is easy to create an artificial gravity..."

Granted he then says that "...it would produce a natural gravity..." but I think he's means natural-feeling gravity - as in "indistinguishable from real gravity", which is true (contrarily, rotational AG is experientially quite unlike real gravity.)

Even if you have massive amounts of fuel and don't want to stop, you eventually start getting close to c, and are no longer able to sustain 1g.

What makes you think this??

You can continue to accelerate, experiencing 1g in your spacecraft , for your entire natural life, your children's lives and the rest of eternity.


Care to retract that comment?

 

[DrGreg]

Yes, but that's not gravity, but a totally different pseudo-force.

Not totally different; according to the equivalence principle, in a small enough region there's virtually no difference.

Even if you have massive amounts of fuel and don't want to stop, you eventually start getting close to c, and are no longer able to sustain 1g.

As I recently said in another thread...

In relativity, acceleration is relative too. Although all inertial observers agree whether an object is accelerating or not, they disagree over the value of a non-zero acceleration. An inertial observer who is momentarily at rest relative to an accelerating object ("comoving inertial observer") will measure a larger acceleration than observers who have non-zero relative velocity, and the acceleration tends to zero as the relative velocity approaches the speed of light.

The acceleration measured by a comoving inertial observer is called "proper acceleration". It the "g-force" that the object experiences and what is measured by an accelerometer attached to the object.

If your rocket was accelerating with a constant proper acceleration of 1 g, the acceleration measured by an inertial observer would gradually decrease [STRIKE]to[/STRIKE] towards* zero. So you can continue at 1 g as long as you like, fuel permitting.

EDIT *corrected in view of DaveC's comment in #119

 

[DaveC426913]

If your rocket was accelerating with a constant proper acceleration of 1 g, the acceleration measured by an inertial observer would gradually decrease to zero. So you can continue at 1 g as long as you like, fuel permitting.

Just in case Dr Greg's explanation is ambiguous: an observer external to the spaceship would see its acceleration approach zero (though, DrGreg, it would never actually reach zero as you imply) - but the spaceship occupant would happily experience 1g for a long as he wishes.

 

[DrGreg]

In the case of accelerative AG, you will experience the acceleration of the deck toward you, but it will be independent of your distance from the deck.

Actually if you were supported at a constant height of h above the deck, you'd experience a proper acceleration of

a = \frac{g}{1 + \frac{gh}{c^2}}​

so it's not an inverse square law, just an inverse law.

(This is a consequence of Rindler coordinates.)