On Sickness, Games, and Dyson Spheres

[elfric]

So, after 3 months of being off work and completely unable to concentrate, my liver tests are back to only mildly alarming and I'm going back to work on Monday. I've been bored out of my skull and have played more RPGs/strategy games in the last three months than I think I have my whole life before that

Comments

[hallerlake]

...if you're moving any reasonable fraction of lightspeed, how do you stay in orbit instead of just shooting off into deep space?

[elfric]

Hmmm, well, I think you found the (in hindsight) obvious flaw in Version 1's configuration. I suspected that the high orbital speed would be a problem.

Escape velocity from the Sun at 1 AU is calculated by the formula

v_e = sqrt(2GM/r)

Where
G is the Universal Gravity Constant (6.67428 x 10^-11 (m^3/(kg * s^2))
M is the mass of the Sun (2 x 10^30 kg)
r is the distance from the Sun (1 AU or 1.49598 x 10^11 m)

Or, 29789.06669 m/s. Interestingly, this is only very slightly more than the orbital velocity of the Earth itself.

Ah well, that explains why I haven't seen that configuration anywhere else. Thanks for the insight, as always :)

[elfric]

Actually, I just remembered one of the rules for stable orbits is that the orbital velocity must be right around the actual escape velocity. Less and the orbit will decay and eventually fall into the sun, more and the gravity of the sun won't be enough to hold the object in orbit.

Obvious in hindsight, I suppose. Ah well, it's always the stupid mistakes that get ya.

[hallerlake]

In double retrospect, you could THEORETICALLY solve this by tethering the orbiting objects. Clearly you can't tether them to the Sun, but if you have a ring of them you could perhaps attach them to the ones across the way (at some angle of offset so the connecting tether doesn't go through the Sun). Picture a multi-headed bola.

But even if that's a reasonable solution, I don't think any plausible materials technology is up to it. You're back to tractor beam handwaving. Also, the failure of any of those attaching tethers would be pretty catastrophic.

[elfric]

yeah, I thought about the tether system after your first comment, but I rejected it for the same reasons you did. Additionally, not only would a tether collision be catastrophic (in fact, it already would be even without the tethers due to how closely the habitats are packed together), but I'm not sure that a viable tether configuration is even possible with as many habitats as would be in use. Not only would there be millions of "ropes" connecting various habitats across ~2 AUs, but I suspect the sheer amount of ropes would get in the way of the sunlight, blocking a probably not insignificant portion of the energy output from the star in getting to the habitats.

If I really wanted to prove how impossible the tether system would be, I could calculate the tensile strength needed for each tether. I suspect that with that orbital velocity, the tensile strength would be literally impossible.

At that point, coming up with artificial gravity seems easier :)

[hallerlake]

For solution 2, I'm not clear what keeps the cylinders in position. Are you presuming they are in orbit around the star, or are they drifting with some level of minimal thrust (eg the lightsail) to keep them from eventually falling down the gravity well?

[elfric]

No, they're stationary in relation to the star - it's a Dyson Bubble with statites. The light-sails on the wikipedia page are used to keep the habitats stable in relation to the star. My configuration would require some form of propulsion to do the same thing - I'm just "assuming" that I'll have a method that doesn't require a footprint on the stellar sphere as big as a light-sail (or, in fact, as big as the habitat itself).

In fact, after my initial post (and your initial comments), I already added a note about this topic to the main post. For reference, here's the note I added:

After some more thought, I suspect this need for constant station-keeping acceleration may end up being the primary flaw for this configuration. Even if we could find a way of propulsion that didn't use up internal mass (or could possibly recapture and re-use it), it seems likely that it would still require energy. The amount of energy needed to keep the cylinder in place around the star may be more than is gathered by the habitat. Since we don't