Would it be possible to build an "Inverted Dyson Shell"

[PB666]

11 hours ago, magnemoe said:

he talk about lots of things, some pretty realistic at least in an long term, other not so. H-H fusion will be very hard for one, 

Still an I do not get the idea of using an thermal fusion reactor to run an ion engine it makes no sense. using the fusion plasma directly will give an way higher exhaust velocity.
Unless you replace the ion engine with an particle accelerator and says goodbye to trust, not that in this setting you can use anything as reaction mass, ionization energy can be ignored. 
Yes its more wasteful with fuel as its also reaction mass but it will have far higher trust and less heating issues. 

Now if you have a faction of an dyson sphere you could use laser pumped solar sails or ions powered by lasers on solar panels, you can make an solar panel designed for just one frequency very efficient 
 

Lets not talk about vaporware. I have asked here in threads for everyone to provide suitable example of fusion direct propulsion (and not the boiler plate big-bang machines). I just watched a lecture today about Liouvilles application to energy. The problem with energetic systems that have chaotic components is that the phase-space becomes convoluted and difficult to manage. This is so-true with fusion plasma generated propulsion, because you have to put energy into the system (its actually alot of energy for plasma) to keep the plasma ordered. The particulates like to dissociated and go their separate ways.

If your machine is a heat machine, then as a heat machine you idealize the transformation of radiant heat into electric power. but once that is done you know what the efficiency is. We can model that with a fission reactor, with a lense pointing focused sunlight on a black body, whatever. If you are machine is a plasma machine then you idealize the plasma containment. I don't idealize either, a machine that does not exist is simply vaporware. But I can argue if you had fusion in space (and you will need it gathering in the outer solar system) then you also use it for ION drives, but fission does exist, and it simply would be feckless. So the question about fusion is will it work in space, maybe . . . .200-300 years from now. Without that dyson swarms are nothing more the heliocentric orbiting habitats.

Follow the energy, always, if your are going to get in a discussion with me, tell me were the energy comes from . . .if its mass, tell me what energy that was used to derive the mass, if its propulsion, the same thing. If the source of energy goes undisclosed then there is no conversation to be had. If the mass is in the Mars-Pluto band, then mentioning solar is only going to prompt me to say where did you get the super lightweight super efficient panels from and how are you going to use these to carry the cubic kilometers of mass around?

[Snark]

On 2/22/2018 at 5:37 PM, YNM said:

L-points are also "statically stable", ie. it's only stable for that one point, if nudged it'll tend to deviate.

• L-4 and L-5 are stable, like a marble sitting at the bottom of a bowl.  Deviate slightly, and the object gets pulled back.  That's why there are "Trojan asteroids".
• L-1, L-2, and L-3 are un-stable, like a marble perched on top of a hill.  Deviate slightly, and it gets pushed away.

The case of a spherical shell around a central math is like neither of the above cases:

On 2/22/2018 at 5:37 PM, YNM said:

So is a skin/shell of equal gravity (equipotential surface). Deviate the slightest and it's no longer that way.

No, that's not how it works.  It doesn't matter whether it's centered or not.  It's neutral, like a marble sitting on a perfectly flat tabletop.  There's nothing holding it in place... but there's nothing pushing it in any direction, either.  Roll the marble to a slightly different spot, and it's in the same situation-- the marble doesn't care where it is.

Deviate in the slightest... and it makes no difference.  It is neutral, not getting pushed or pulled in any direction.  And that's true whether it's perfectly centered, or slightly off-center, or a lot off-center.  As long as the central mass is inside the sphere, there is zero gravitational coupling between the two, because of the shell theorem.

In short:  your statement is correct for L1, L-2, and L-3.  It is incorrect for L-4 and L-5, and also for the case of the Dyson sphere.

[YNM]

33 minutes ago, Snark said:

There's nothing holding it in place... but there's nothing pushing it in any direction, either.

This.

There's a good difference between statically stable and dynamically stable. A statically stable position is as you mention, whereas dynamically stable means any slight nudge will result in a returning force that will be dampened overtime.

A good returning force is a dampened returning force - getting smaller and smaller overtime. If you have otherwise it'll be a harmonic motion, not very good unless you don't mind your sphere to wobble continuosly.

Also, in real world, there's something called perturbation. True as it maybe that trojan asteroids exist - but that's just because of the relatively long periods. Over very much longer periods it will still goes away from stability.

So yeah, L-points are extremely stable for 2-body or limited 3-body case, but real world is always n-body, hence there's no true stability at those points.

Same goes to perfectly spherical equipotential surface - they only exist in 1-body or limited 2-body problems.

[magnemoe]

On 23.2.2018 at 5:12 AM, PB666 said:

Lets not talk about vaporware. I have asked here in threads for everyone to provide suitable example of fusion direct propulsion (and not the boiler plate big-bang machines). I just watched a lecture today about Liouvilles application to energy. The problem with energetic systems that have chaotic components is that the phase-space becomes convoluted and difficult to manage. This is so-true with fusion plasma generated propulsion, because you have to put energy into the system (its actually alot of energy for plasma) to keep the plasma ordered. The particulates like to dissociated and go their separate ways.

If your machine is a heat machine, then as a heat machine you idealize the transformation of radiant heat into electric power. but once that is done you know what the efficiency is. We can model that with a fission reactor, with a lense pointing focused sunlight on a black body, whatever. If you are machine is a plasma machine then you idealize the plasma containment. I don't idealize either, a machine that does not exist is simply vaporware. But I can argue if you had fusion in space (and you will need it gathering in the outer solar system) then you also use it for ION drives, but fission does exist, and it simply would be feckless. So the question about fusion is will it work in space, maybe . . . .200-300 years from now. Without that dyson swarms are nothing more the heliocentric orbiting habitats.

Follow the energy, always, if your are going to get in a discussion with me, tell me were the energy comes from . . .if its mass, tell me what energy that was used to derive the mass, if its propulsion, the same thing. If the source of energy goes undisclosed then there is no conversation to be had. If the mass is in the Mars-Pluto band, then mentioning solar is only going to prompt me to say where did you get the super lightweight super efficient panels from and how are you going to use these to carry the cubic kilometers of mass around?

http://www.projectrho.com/public_html/rocket/enginelist.php#mcfusion
here is an list of some fusion concepts, note that its easier to make an simple fusion engine than an reactor, we could build an fusion engine today as in z-pinch, we can not make an fusion reactor. 

The solar panels I mentioned would not use sunlight but laser, an single frequency solar cell can be made much more efficient as in 95% letting you send lots of power to them, only limited by them getting to hot.