Physics Question

[Cryova]

This might sound a little bit strange, but bear with me!

 

So, you build a giant spaceship, that goes all the way around a gravitational body. To make it easy, we could say Gilly. So, we have now made an artificial ring around Gilly.

 

What would happen if we put two engines opposite of each other, and both have them fire in the same direction (if we get ride of the fact that the ship will probably crumple). How would the orbit change?

[FungusForge]

Well assuming one could build a ring ship around some celestial body, and in one way or another allow it to compensate the minute gravity fluctuations that could destabilize or destroy such a vessel, then what would probably happen is the "rear" section of the ring would smash into the planet.

[John FX]

one engine would slow down it`s side of the ring, the other would speed it up it`s side at least in the meaning of orbital mechanics. The slow side would fall toward the gravitational body while the faster side would move away. After a little while the ring would impact the body.

If the ring were infinitely strong it would then use the body as a pivot point for rotation, like a hula hoop.

Even without the engines though, due to gravity not being uniform on anything except a perfect sphere, after a while the ring would impact the body.

EDIT : I`m assuming the engines are pointing the same way, not both in the direction of the ring rotation. If they are speeding up the ring it would still impact the body, just with a higher rotational velocity.

Generally, orbits are more stable when both objects orbit a common barycentre. With a ring, it would not do that so it would be less stable. I`m sure someone here as the maths and most likely a more accurate explanation.

Pretty much every case though will lead to the ring hitting the body and then being a hula hoop. Of course in RL stuff is not that strong so it would just break up and either fly off or hit the body after a while.

Edited January 31, 2016 by John FX

[Gaarst]

Building a ring around any body is an extremely stupid idea anyway: gravity is an attractive force only (except if you involve negative masses but let's not involve negative masses), this means that at the slightiest movement in the orbit, one side of the ring will come closer, and the other side will get further; then the closer side will be more attracted, getting closer, and getting more attracted...

So engines or not, your ring will probably end up crashing anyway. In the case of a ring orbiting Earth, then you might want to step away from Himalaya.

Edited January 31, 2016 by Gaarst

[Cryova]

Perhaps that wasn't phrased well enough.

The engines would be pointed in such a way that if you drew a line through it, it would intersect at a 90 degree angle with the orbital line. I forget the technical name for this position, but it isn't prograde or retrograde.

[livefree75]

You are either talking about radial, antiradial, normal, or antinormal. If the line passes through the planet, it's radial (or antiradial)

Edited January 31, 2016 by livefree75

[*Aqua*]

You mean radial (to the center of the body you orbit) and anti-radial (the opposite way).

Nothing would happen. The pushes of both engines would cancel out each other.

Edited January 31, 2016 by *Aqua*

[suicidejunkie]

Normal?  As in towards the north pole for an equatorial ring?

Or Radial?  As in towards the sky/ground?

Which symbol on your maneuver node is each of the engines pushing towards?

 

If you're thrusting towards cyan (radial), you either squish or stretch your ring into a bit of an ellipse if the engines are opposing each other, or drive one side of it down into the planet if the two engines are cooperating.  If the ring is spinning, you'll need to thrust sharply or you'll cancel out your thrust as you progress in your orbit.

If you're thrusting towards purple (normal), you either change your inclination if the engines are opposing, or pop the ring due north or south if they are cooperating.  If you pop it due north, gravity will pull it back down towards the equator for it to oscillate back and forth.

 

Assuming it is rigid and uniform, and equatorial, the ring won't apply any net gravitational force on the planet, and vice versa.  This is a bad thing, since your ring is close to (and thus likely to randomly collide with) the planet, given tiny random perturbations.

In practice, lumpiness will induce tides and slowly wobble things around more chaotically.

 

Note: if the ring drifts to one side such that the left is closer to the planet than the right, that doesn't change things.  There is more force per unit of ring on the left side, but there is also more ring on the right side, and that balances out exactly.  The ring will keep drifting at the same speed until the fact that the planet isn't a point mass becomes important and you encounter tides, air and/or mountains.

[bewing]

You might want to read about Ringworld in Larry Niven's "A Mote In God's Eye". And the sequel, where he had to go back and add fusion ramscoops to Ringworld, because the physicists told him the concept was gravitationally unstable.

 

[Vanamonde]

Thread moved to Science discussions. 

[StrandedonEarth]

2 hours ago, bewing said:

You might want to read about Ringworld in Larry Niven's "A Mote In God's Eye". And the sequel, where he had to go back and add fusion ramscoops to Ringworld, because the physicists told him the concept was gravitationally unstable.

 

Um, you must mean Larry Niven's Ringworld books. A Mote In God's Eye is in a different universe of his.

[K^2]

2 hours ago, StrandedonEarth said:

Um, you must mean Larry Niven's Ringworld books. A Mote In God's Eye is in a different universe of his.

I think it was The Ringworld Engineers that introduces fusion rockets around the perimeter.

Here is the thing, though, a rigid ring around a planet/star is gravitationally unstable. If the ring is flexible, interesting things can happen, depending on how flexible it is. Larry Niven ran into the problem because he needed a world built on the inner side of the ring. That required something that's pretty rigid if he didn't want terrain being constantly devastated by flex of the ring. In fact, it required material which was impossibly strong and impossibly rigid, which he invented. But that's why he needed the fusion rockets.

Something built from more realistic materials, however, would behave more like a swarm with tethers. Such a structure can be made to have a dynamically stable orbit. Of course, if you put jets on that thing and try to change its angular velocity, it will either be torn apart by centrifugal forces or collapse inward. Either way, it's not a good idea.

[cantab]

12 hours ago, Cryova said:

How would the orbit change?

There isn't an orbit to begin with. There is no overall gravitational force between a central mass (Gilly) and a sphere or ring surrounding it (the spacecraft) - this is known as the "Shell theorem". So they'll just drift, each behaving as though the other isn't even there - right up until they bump into each other. (In the case of a ring if the central mass moves out of the ring plane there will be a restoring force in that direction, but there is nothing to oppose the central mass moving in the plane of the ring.)

[Temstar]

1 hour ago, K^2 said:

it required material which was impossibly strong and impossibly rigid, which he invented

Said material is held together by the strong force (the same force that holds protons and neutrons together in the nucleus and is the source of fusion and fission, so you know it's really strong!), not electromagnetic force of normal matter. It doesn't even start to soften until it's heated to a point that it's black body radiation is mainly in the UV side of the scale.

[bewing]

Heh. Did I mess up my book reference?  Oh well, it's been 30 years since I read those silly books.

 

[Yourself]

50 minutes ago, cantab said:

There is no overall gravitational force between a central mass (Gilly) and a sphere or ring surrounding it (the spacecraft) - this is known as the "Shell theorem".

The shell theorem only applies to spherical shells of uniform mass, not rings.  The ring will feel a gravitational force due to the central body if it's not perfectly centered.  The shell theorem does allow us to treat the central body as a point mass (under the usual assumptions).  See here for why this is unstable: http://larryniven.net/physics/img13.shtml

[K^2]

17 hours ago, Yourself said:

The shell theorem only applies to spherical shells of uniform mass, not rings.  The ring will feel a gravitational force due to the central body if it's not perfectly centered.  The shell theorem does allow us to treat the central body as a point mass (under the usual assumptions).  See here for why this is unstable: http://larryniven.net/physics/img13.shtml

Same exact problem persists if you try to put a shell "in orbit" around a planet/star. It's not about shell vs. ring, but about what happens the moment it becomes slightly off center. And while shell theorem does tell you that there is equilibrium, both for ring and the sphere, that equilibrium is unstable. Which means that tiniest deviation from center would result in that deviation increasing, until one end of the ring/shell comes crashing down into the planet/star. This will happen regardless of whether the structure rotates or not.

There are some really cool orbits where statically unstable equilibria can still be dynamically stable^*. Unfortunately, the amount of symmetry exhibited by both rings and shells is the very thing that prevents you from doing anything creative. For a shell, absolutely any rotation makes no difference whatsoever. For a ring, rotating on axis perpendicular to axis of symmetry could be interesting, but still won't prevent the ring from crashing into the planet.

^* Lissajous Orbit is a cool example of dynamic stability at statically unstable equilibrium.

[fredinno]

 

20 hours ago, cantab said:

There isn't an orbit to begin with. There is no overall gravitational force between a central mass (Gilly) and a sphere or ring surrounding it (the spacecraft) - this is known as the "Shell theorem". So they'll just drift, each behaving as though the other isn't even there - right up until they bump into each other. (In the case of a ring if the central mass moves out of the ring plane there will be a restoring force in that direction, but there is nothing to oppose the central mass moving in the plane of the ring.)

Wouldn't the ring just orbit the parent body like a normal ring, but solid?

[K^2]

16 minutes ago, fredinno said:

Wouldn't the ring just orbit the parent body like a normal ring, but solid?

No, and pretty much the entire thread has been about why it would not.

[AlextheBodacious]

What if it were vertical? If it went up, instead of in? It would, for a second, be a sort of... hula hoop?