Wait a mo... since things fall in a straight line...

[Starwhip]

...why do stations rotate in orbit?

On every single station I have researched, they have an "earth-facing" port. That would imply it always faces earth. But in KSP, the station rotates around its axis, so the only stationary ports are the "top" and "bottom" ones... if you are facing that way. The way things fall IRL is in a line; say you tied a string to a metal rod, in space. Fling the metal rod about your head now. It would constantly face the same way in relation to the center, with one end pointing "prograde" and one "retrograde". This is how things should work in KSP.

Could SQUAD fix this?

[r4pt0r]

i too would like that so my nsa cupola module can stare down into the astronaught complex and determine who is being a good citizen and who is being a free thinker.

but i think it has to do with things ceasing all rotation in timewarp. what happens when you are perfectly still, disable all rcs/sas and watch an orbit realtime?

[Starwhip]

That's a good question. I have no idea. Hmm... I think it still happens, but I can't remember off the top of my head.

[Red Iron Crown]

This doesn't need fixing, it is accurate. Stations with Earth-facing ports have to rotate the station to keep it that way.

The metal rod on a string analogy is not very good for visualizing orbits.

[UbioZur]

It is not turning, it is an appearance of turning because it is rotating around Kerbin.

Take a point on your table, and get a pen, point the pen toward the point, and make the pen do circle around the point (like an orbit) without ever rotating the pen, and you will understand.

I do believe in real life it is the same thing if the vessel is inerte (without inertial rotational velocity). I think ISS have been given an inertial rotation to keep the cupola pointing to earth. Most satellite have RCS engines to keep them pointing where needed.

[Endersmens]

This doesn't need fixing, it is accurate. Stations with Earth-facing ports have to rotate the station to keep it that way.

Yeah i was going to say that they would have gyros on them to slowly rotate the station to match their orbital period. The rotation makes sense if you think about it. Using Starwhips model of a string attached to a rod, it seems that they should always face earth. However, in space, the string isnt attached to the end of your station or vessel, its attached to the center of mass. Attach a string to the center of the rod on both sides and spin it. (since the string cant pass through the rod, unlike in space where the string is smiply a force and can pass through the station) the rod will stay facing the same way all the way around (assuming it doesnt have a lot of friction on the string) this is is closer to what orbit is like.

Edited April 28, 2014 by Endersmens

[J.Random]

The way things fall IRL is in a line; say you tied a string to a metal rod, in space. Fling the metal rod about your head now. It would constantly face the same way in relation to the center, with one end pointing "prograde" and one "retrograde".

Shouldn't it be outward/inward radial instead of prograde/retrograde, because of tidal forces?

[Dunbaratu]

You have it 100% backward.

When a real space station has a side that always faces earth, it's not because it doesn't rotate. It's because it does rotate.

It rotates very slowly at the rate of one revolution per orbit. If it takes 2 hours to finish an orbit, it's rotating at 1/7200th of an RPM, or 1 rotation per 2 hours.

Just like the Moon, which orbits once every 27 days, and is rotating at a rate of 1/27th of a rotation per day. Thus the same side always faces earth.

And the reason this 'coincidence' happens a lot in astronomy is no coincidence at all. The same effect that causes the Moon to distort Earth's shape, resulting in ocean tides, also causes the Earth to distort the Moon's shape, and when an orbiting object is no longer entirely 100% spherical, but has been squished into an egg shape, there's forces causing it to tend to rotate toward an orientation where the longer dimension is pointed toward the primary body pulling on it. This, over a LONG time (and the moon has had a few billion years of time to work with), will tend to make the orbiting body conform to a rotational rate that exactly matches its orbital period, so it keeps its longer dimension radially aligned toward the primary body all the time.

More here: http://en.wikipedia.org/wiki/Tidal_locking

And the reason KSP stations don't do that isn't because they do rotate. It's because they don't rotate (the game doesn't remember rotational inertia when a ship goes on rails). That, and tide locking isn't implemented in KSP, but even if it wasn't you could still try to manually make the ship rotate at the right rate to keep it facing the right way, if not for the fact that that rotation gets killed by going onto rails.

[pizzaoverhead]

The reason for this is that stations can set their rotation to match their orbit around the Earth and then leave it that way. In KSP, timewarping removes all rotation from the craft. The Timewarp Rotation Fix fixes this. Once you have that installed, the only obstacles in your are having fine enough controls to adjust rotation (RLA Stockalike has some tiny RCS ports that can help in this regard) and the rotation information to tell if your rotation rate is accurate.

So in short, yes, SQUAD could fix this.

[Red Iron Crown]

That, and tide locking isn't implemented in KSP, but even if it wasn't you could still try to manually make the ship rotate at the right rate to keep it facing the right way, if not for the fact that that rotation gets killed by going onto rails.

I wonder about this, because I thought the "wobble" in orbits when not on rails was caused by the slightly different orbits of a craft's constituent parts. I think I'll do some testing tonight when I get home, any thoughts on how a craft designed to maximize tidal effects should be built?

[r4pt0r]

very long with masses on both ends and one end starting pointing at kerbin?

[BudgetHedgehog]

So in short, yes, SQUAD could fix this.

By 'this', I hope you mean retaining inertial rotation while on rails, because there's nothing else to fix here that would keep stations stationary.

[J.Random]

That, and tide locking isn't implemented in KSP

Actually, it is. It just doesn't work with point mass. If you try to reenact tether experiment with two satellites connected by KAS tether (undocked mode), it will work (until you timewarp, of course).

[pizzaoverhead]

By 'this', I hope you mean retaining inertial rotation while on rails, because there's nothing else to fix here that would keep stations stationary.

Retaining inertial rotation on rails, and some tools to assist in accurately setting a specific rotational speed. Sun-tracking SAS modes or a rotational speedometer for example.

[Dunbaratu]

Actually, it is. It just doesn't work with point mass. If you try to reenact tether experiment with two satellites connected by KAS tether (undocked mode), it will work (until you timewarp, of course).

But does it work with KSP unmodded? I've never gotten a long spaceship to tide lock, no matter how long it is.

[J.Random]

But does it work with KSP unmodded? I've never gotten a long spaceship to tide lock, no matter how long it is.

Long unmodded ship won't lock because, again, it's a single point mass. KAS doesn't change anything here, except that it gives you means to have two vessels (two point masses) connected to each other by long tether (without docking and merging two point masses into one). It doesn't change orbital physics.

[r4pt0r]

Long unmodded ship won't lock because, again, it's a single point mass. KAS doesn't change anything here, except that it gives you means to have two vessels (two point masses) connected to each other by long tether (without docking and merging two point masses into one). It doesn't change orbital physics.

but 2 ships separated by a dozen or so long trusses with claws on either end? leave the claws unlocked? doesnt the claw read the targets cg seperately from its host ship?

[Red Iron Crown]

but 2 ships separated by a dozen or so long trusses with claws on either end? leave the claws unlocked? doesnt the claw read the targets cg seperately from its host ship?

I thought the claw worked like a docking port, merging two ships into one. If KSP simplifies to point mass for orbital calculations, it should behave like one point mass for clawed objects.

[J.Random]

but 2 ships separated by a dozen or so long trusses with claws on either end? leave the claws unlocked? doesnt the claw read the targets cg seperately from its host ship?

Hmmm. Didn't think about the claw. It may probably work, especially with pivots unlocked. I doubt it, but it's worth a try.

Edited April 28, 2014 by J.Random

[Chrisd857]

Also, depending on the craft, gravity-gradient stabilization can be used to orient toward Earth, but that is probably outside the scope of physics simulation for KSP.

[Red Iron Crown]

Also, depending on the craft, gravity-gradient stabilization can be used to orient toward Earth, but that is probably outside the scope of physics simulation for KSP.

That's tidal locking, if I am understanding it correctly.

[Dunbaratu]

Long unmodded ship won't lock because, again, it's a single point mass. KAS doesn't change anything here, except that it gives you means to have two vessels (two point masses) connected to each other by long tether (without docking and merging two point masses into one). It doesn't change orbital physics.

I would argue that you are violating the meaning of "Kerbal space program does it" when the limitation preventing it is the standard way the game works. It does change the way the game models orbital physics to make it model a craft as more than one point mass.

[R0cketC0der]

This is how it works in reality. As already said, it doesn't work in ksp because all ships are treated as point masses and ships don't rotate during time warp or while further away than 2.5km.

[xrayfishx]

Starwhip, you have helped NathanKell and probably other modders with their projects, you always have good arguments in threads, you've always seemed like a smart person to me... and now you say something like this? What happened?

[rkman]

things fall in a straight line...

..in curved space-time.

I could be mistaken but i vaguely recall a discussion about an older version of KSP (before my time, pre 0.13.x) where ships were not simplified to a point mass, and the gravity gradient thing did sort of work.

Edited April 28, 2014 by rkman