Why are ships rotating? (during warp)

[firda]

Why do they keep orientation to some global reference frame (e.g. keeping face-vector oriented towards Sun/Kerbol while orbiting Kerbin)?
Shouldn't the orientation be preserved according to gravity gradient, therefore keeping ship facing prograde facing it along the orbit?
...at least when perfectly circular, to simplify things. Theory of Relativity - the ship/station is following straight line (geodesic) in a curved space(-time). Why would it rotate?
If I have symmetrical object (sphere, box, ...) aligned properly (box having one face towards the body, one towards prograde and one in normal direction) moving on perfectly circular orbit (e=0 ... and hell that is causing KSP problems: apo/peri are jumping and inclination-changing maneuver nodes are jumping as well when I warp with e=0.0000) ... and the body I am orbiting is not spinning (rotating around me) in my local frame of reference (zero momentum), shouldn't it stay that way? (if we, of course, ignore gravity anomalies caused by irreguralities in the body and the ship has proper symmetry, is aligned the proper way and canceled the rotation).

TLDR: The orientation of the ship should be ruled by the nearest body (strongest gravity influence) and not by some very distant First Point of Aries/Libra or whatever Absolute Reference Frame.
Am I wrong?

Edited June 4, 2018 by firda
clarification

[linuxgurugamer]

There are several mods which do that, without seeing your log file, no way to tell

[firda]

Do what? Why log? No mods, place ship in orbit of Kerbin, e.g. 80 km apo=peri, facing The Sun... and it will keep facing it while orbiting. Why?
Is it limitation of KSP or am I wrong and it shouldn't keep prograde-orientation no matter what?

Edited June 4, 2018 by firda

[Freshmeat]

I am not entirely convinced that General Relativity prescribes that kind of motion. It is very much at odds with Newtonian motion at low speeds, where relativistic effects in general should not be noticeable.

[firda]

Ask yourself a question: Why should the orientation be dependent on the Sun and not Kerbin/Earth (which is closer and you are orbiting it, not the Sun)? Why any point anywhere else than the body that is binding you gravitationally?

It is not much about GR vs. Newton, but rather about ABSOLUTE REFERENCE FRAME. Why would you keep facing The Sun and not Libra? Why not center of the galaxy? Why not center of Universe? You see? That is absurd. THERE IS NO ABSOLUTE REFERENCE FRAME ... and no Aether

 

Is ISS rotating like that? Why / why not?

[Vexillar]

Hi firda.

If you mean why does the "bottom" of my spaceship not always point towards the surface of the nearest body, then that's how things behave IRL (at least in Newtonian mechanics, as Freshmeat just said).

True, the Moon shows the same face to the Earth (and the Mun is modelled to behave similarly), but that is because it is rotating, just (because of tidal forces) it does this at the precisely same angular velocity that it orbits.  More accurately, because the orbit is elliptical, it in fact appears to oscillate - "librate" - slightly as the orbital angular velocity varies with distance.  The Mun's orbit IIRC is perfectly circular, so there is zero libration.

A spaceship in orbit, once stabilised wrt to some other object, will point in the same direction, as you put it, in some "global" reference frame.  But as all things are relative (even in Newtonian Mechanics), there is no such frame - only that of the ship itself.

The International Space Station is caused to artificially rotate once per orbit, so that the "downward" and "upward" instruments continue to point at the surface or out into space - but this rotation has to be fine-tuned by thrusters to keep it in synch with its orbital period.

See explanation here: https://space.stackexchange.com/questions/12335/why-does-the-iss-rotate-exactly-once-per-orbit/12338?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

Where KSP departs from RL physics is that when you go into warp, any rotation of a ship is zeroed.  Hence even if it was spinning it will lose that angular velocity and behave as you describe.

 

Edited June 4, 2018 by Vexillar
Added reference to explain ISS rotation

[firda]

Thanks Vexillar, you have actually supported my claims, if I understood you correctly. Moon is rotating in Sun's FoR but not in Eearth's FoR. Same is true for ISS and the fine-tuning is needed because of disturbances (irregular gravity, drag), not because the rotation would be zeroed by time warp, so it is limitation of KSP, because it zeroes the rotation in ABSOLUTE frame of reference (Sun's / Kerbol's) instead of body's reference.

[Shadowmage]

25 minutes ago, firda said:

ABSOLUTE frame of reference (Sun's / Kerbol's) instead of body's reference.

Which is the correct way to do it.

The only reason the ISS maintains an orientation relative to earth's surface, is that they expend energy and/or propellant to do so.

In the absence of gravity gradient influences or aerodynamic forces, an object will continue to be oriented relative to the -universe- (not the planet, star, galaxy, whatever, that it orbits).

TLDR: Your ship is not changing orientation.  The reference frame you are traveling along with -is-.

 

Edit:  Should add that AFAIK KSP only has limited 'frames of reference'.  Its 'universal' frame of reference would be the one used by the Sun/Kerbol (interplanetary space is the same as intergalactic in stock KSP).  Which is different from the 'planetary' frame-of-reference that you travel in while in the SOI of a planet.

Edited June 4, 2018 by Shadowmage

[Vexillar]

firda,

Sort of - the rotational cancellation in warp is definitely not RL physics.  And you are absolutely right in saying there is no absolute FOR and no aether - not even in KSP.

But sort of not - it is cancellation of a rotation that would not naturally be there.  (And it's a handy cheat to get a spinning or wobbling ship back under control - but that's another thing.)

The moon and ISS do rotate in the Earth's FOR.  Try this: launch a ship into a stable orbit and turn off SAS.  At this point it will be travelling prograde.  WITHOUT using warp, wait half an orbit and it will now be travelling retrograde.  That's correct by RL physics.  If you now apply warp, the ship will continue to behave in the same way, not showing the same side of the ship to the surface.  Hence the zeroing of rotation is not correct RL physics, but it is consistent with normal behaviour of orbiting ships in the absence of artificially induced rotation.  If you now turn on SAS to point prograde (or retrograde or out or in) then SAS will expend energy rotating the ship to align it with the planet's surface - like the ISS does.

Think of the Earth going round the Sun - it rotates 365.25-ish times faster than it orbits.  If you could stand on the surface of the Sun with heatproof binoculars, you'd see the Earth spinning frantically as it crossed the sky above you (again relatively speaking timewise).  It does not always show one side to the Sun, otherwise there'd be no night and day here.  In KSP, planets and moons do maintain their rotation in the FOR of their parent body even in warp, unlike ships.

If you need to model something like the behaviour of the ISS even in warp, there's a mod called "PersistentRotation".

Here:  https://forum.kerbalspaceprogram.com/index.php?/topic/102277-14-persistentrotation-185/

Happy gyrations!

 

Edited June 4, 2018 by Vexillar
Corrected name and added link to PersistentRotation

[OHara]

6 hours ago, firda said:

Why would you keep facing The Sun and not Libra? Why not center of the galaxy? Why not center of Universe? You see? That is absurd. There is no absolute reference frame... and no Aether

Even though physics is independent of motion through space (you cannot feel your velocity relative to any aether, only relative to other objects) experiments can measure absolute rotation.

If you throw a ball forward from your satellite, it will not follow the curve of your orbit, but rather go straight relative to what were once called 'the fixed stars'.  The path is at first surprising to anyone in a satellite that is keeping one face toward he parent body, and the apparent force causing the ball to deviate from orbit is called the 'Coriolis force'. (They call the Coriolis force a 'fictitious force' because it is a force that seems to be acting on moving object, to explain their motion when seen from a rotating point-of-view.) During your next orbital period, the ball loops around behind you in your orbit.

Satellites often do show one face always to the parent body ('tidally locked') but physical experiments on such a satellite can notice this absolute rotation -- usually experiments involving moving momentum like the ring of a spinning gyroscope.

People demonstrate the rotation of the Earth with the Foucault pendulum, which tries to swing in a plane stationary relative to the 'fixed stars', as closely as it can given the changing direction of gravity.  Freely gimbaled gyroscopes on a spacecraft do hold orientation with respect to their starting orientation in space-time. 

In the language of 4D space-time, you cannot feel your orientation; neither absolute orientation in the space coordinates, nor your absolute velocity which is orientation in a space-time plane.  You can feel your change in orientation; either rotation in space using gyroscopes, or acceleration which is rotation of your reference frame in a space-time plane.

KSP has one big flaw in that it suddenly stops rotation during time-warp; at first we guessed you wanted the mod 'Persistent Rotation' that repairs that.  It has a tiny flaw in that its gyroscopes hold orientation in a frame rotating with the planet rather than the fixed stars, if you are close to the planet.

[Reactordrone]

Without any rotation your ship stays in the same orientation in much the same way as the Earth stays in the same orientation. The axis of rotation always points in the same direction (discounting precession) as the earth goes around the Sun which gives us the seasons.

[firda]

19 hours ago, Vexillar said:

... launch a ship into a stable orbit and turn off SAS.  At this point it will be travelling prograde.  WITHOUT using warp, wait half an orbit and it will now be travelling retrograde.  That's correct by RL physics.  If you now apply warp, the ship will continue to behave in the same way, not showing the same side of the ship to the surface.  Hence the zeroing of rotation is not correct RL physics, but it is consistent with normal behaviour of orbiting ships in the absence of artificially induced rotation.  If you now turn on SAS to point prograde (or retrograde or out or in) then SAS will expend energy rotating the ship to align it with the planet's surface - like the ISS does.

And if you give it the proper amount of rotation (hit "W" a bit), then it will keep rotating and pointing in prograde forewer (with e=0) ... or not?

[Shadowmage]

14 minutes ago, firda said:

And if you give it the proper amount of rotation (hit "W" a bit), then it will keep rotating and pointing in prograde forewer (with e=0) ... or not?

As long as you don't hit timewarp (or otherwise unfocus the craft; e.g. by going to the space center) -- yes.  You'll need a rotation rate that matches your orbital period, but it will work.

[firda]

22 hours ago, Shadowmage said:

As long as you don't hit timewarp (or otherwise unfocus the craft; e.g. by going to the space center) -- yes.  You'll need a rotation rate that matches your orbital period, but it will work.

On 6/5/2018 at 6:08 AM, OHara said:

Satellites often do show one face always to the parent body ('tidally locked') but physical experiments on such a satellite can notice this absolute rotation -- usually experiments involving moving momentum like the ring of a spinning gyroscope.

On 6/4/2018 at 11:51 PM, Shadowmage said:

The only reason the ISS maintains an orientation relative to earth's surface, is that they expend energy and/or propellant to do so. 

On 6/5/2018 at 12:14 AM, Vexillar said:

then SAS will expend energy rotating the ship to align it with the planet's surface - like the ISS does

And that about ISS confused me, because it sounds like ISS needs A LOT of energy to keep it's momentum, like if somebody was hitting the time-warp, that violates the Conservation of angular momentum. It sounded to me like contra-argument to me stating, that ISS just needs small correction because of disturbances (drag, gravity gradient changes, non-circular orbit, solar wind etc.), but not because of something really inherent:

On 6/4/2018 at 11:26 PM, firda said:

... ISS and the fine-tuning is needed because of disturbances (irregular gravity, drag), not because the rotation would be zeroed by time warp

Thank you all, I think I have learned something today. It wasn't easy to see what is wrong with all the possible reference frames (and I have started with me on the ground looking up at the ship, thus rotating together with Kerbin/Earth calling this FoR the Kerbin's/Earth's, but you use that name for different FoR, where Kerbin is rotating), but I think I got it. It is really best to use the biggest reasonable FoR (the one where Sun rotates and orbits around CoM of the system, which is somewhere inside the Sun, but not exactly in its center) to describe the things, which makes sense why it is used in KSP (well, probably CoM assumed in the center of the Sun), not that it would be the ABSOLUTE, or the only possible or correct, but it does not have as many fictitious forces in it, like the Coriolis force.

Footnote: If SAS was not canceling rotation (angular momentum) but instead keeping the proper rotation (non-zero angular momentum in Sun's FoR, but ZERO in FoR of anybody on the ground) and KSP was either keeping it or reseting to this stable state, then it would be no problem to build ISS that would keep one window towards the earth, without needing to spend any energy on it (because there would be no real-world disturbances in KSP that real ISS has to correct for).

P.S.: I hope that i have used angular momentum correctly, I mean the rotation and hope you get what I mean, if it is not angular momentum.

EDIT: Actually, I was working (in my mind) with FoR, where neither Kerbin nor the ship are moving, that is why I imagined the ship rotating when warping, but it can also be made non-rotating in that frame, until you hit warp. That frame is not inertial, btw, and Kerbin IS rotating in it.

Edited June 6, 2018 by firda
Added quote of Vexillar about ISS as well; 2nd EDIT: See last line and the strike.

[Aphobius]

37 minutes ago, firda said:

I think I got it. It is really best to use the biggest reasonable FoR (the one where Sun rotates and orbits around CoM of the system, which is somewhere inside the Sun, but not exactly in its center) to describe the things, which makes sense why it is used in KSP (well, probably CoM assumed in the center of the Sun), not that it would be the ABSOLUTE, or the only possible or correct, but it does not have as many fictitious forces in it, like the Coriolis force.

Unfortunately you din't got it. You don't have to use any "biggest reasonable". Rotation is absolute.

A non-rotating (as observed by a gyroscope) sphere orbiting Earth would maintain its orientation to distant stars for an extremely long time (the "distant stars" will eventually move xD), it would not rotate towards the Sun or whatever you imagine.

Also, your use of "proper rotation" is extremely irritating, I have absolutely no idea why you think that specific angular velocity is "proper".  

[MechBFP]

Can someone explain in laymen’s term what exactly the issue in this thread is?

[Tyko]

30 minutes ago, MechBFP said:

Can someone explain in laymen’s term what exactly the issue in this thread is?

I'll take a shot at that <puts on flame retardant suit>

There was a question raised about whether a craft that's, 1) not rotating and 2) in orbit pointing prograde, would continue to point prograde as it moved along it's orbit. The answer is "no" the craft will continue pointing in the same direction, so if it goes halfway around the orbit the craft will be pointing retrograde.

Edited June 5, 2018 by Tyko

[MechBFP]

2 hours ago, Tyko said:

I'll take a shot at that <puts on flame retardant suit>

There was a question raised about whether a craft that's, 1) not rotating and 2) in orbit pointing prograde, would continue to point prograde as it moved along it's orbit. The answer is "no" the craft will continue pointing in the same direction, so if it goes halfway around the orbit the craft will be pointing retrograde.

But the op is saying the ship is rotating during warp, as per the title. But it doesn’t, as everyone who has played the game is well aware. So I don’t understand why he thinks it does?

[Atkara]

1 hour ago, MechBFP said:

But the op is saying the ship is rotating during warp, as per the title. But it doesn’t, as everyone who has played the game is well aware. So I don’t understand why he thinks it does?

If I believed vessels should be pointing at prograde all the way along their orbit, I too would propably think that it's my spacecraft which "rotates" by itself -with or without warp 

[bonyetty]

A summary of my current understanding of some of the difference between an orbiting space craft in real life and in KSP.

In KSP the mass of a craft is a single point therefore no tidal influence is coded into a crafts motion. Also, in the coding of KSP, the active craft is the center of the Kerbal universe. I believe this is to reduce rounding errors in the game engine and/or current computer limitations when using very large and small numbers in the solar system sized game environment. In KSP the gravitational interaction is only calculated between 2 objects, the active craft, and the planet’s/moon’s sphere of influence it is in. In real life everything has it’s own frame of reference and everything has many many more things effecting their motion in the universe like tidal forces, everything affecting everything gravitationaly, relativism....

When I first started playing KSP I had similar questions of why and how it’s different than real life. Still do. These differences have pushed me to such a greater understanding of real world physics and coding. Thanks again, to the marvelous creator Felipe Falange, aka HarvesteR.

 

[MechBFP]

2 hours ago, Atkara said:

If I believed vessels should be pointing at prograde all the way along their orbit, I too would propably think that it's my spacecraft which "rotates" by itself -with or without warp 

Ahhhh I get what he is thinking now. Thanks!

[firda]

10 hours ago, MechBFP said:

Ahhhh I get what he is thinking now. Thanks!

Really?

22 hours ago, firda said:

And if you give it the proper amount of rotation (hit "W" a bit), then it will keep rotating and pointing in prograde forewer (with e=0) ... or not?

22 hours ago, Shadowmage said:

As long as you don't hit timewarp (or otherwise unfocus the craft; e.g. by going to the space center) -- yes.  You'll need a rotation rate that matches your orbital period, but it will work.

The above explains everything.

P.S.: Is the ship rotating or the world around it? ...when you hit warp. I never said that FoR must be inertial (well, no FoR is really inertial, because universe is expanding).

[MechBFP]

2 hours ago, firda said:

Really?

The above explains everything.

P.S.: Is the ship rotating or the world around it? ...when you hit warp. I never said that FoR must be inertial (well, no FoR is really inertial, because universe is expanding).

If the ship isn’t rotating (the actual ship itself in reference to itself), then using the word rotation doesn’t make any sense. Things are just orbiting. 

Edited June 6, 2018 by MechBFP

[Tyko]

14 hours ago, bonyetty said:

A summary of my current understanding of some of the difference between an orbiting space craft in real life and in KSP.

In KSP the mass of a craft is a single point therefore no tidal influence is coded into a crafts motion. Also, in the coding of KSP, the active craft is the center of the Kerbal universe. I believe this is to reduce rounding errors in the game engine and/or current computer limitations when using very large and small numbers in the solar system sized game environment. In KSP the gravitational interaction is only calculated between 2 objects, the active craft, and the planet’s/moon’s sphere of influence it is in. In real life everything has it’s own frame of reference and everything has many many more things effecting their motion in the universe like tidal forces, everything affecting everything gravitationaly, relativism....

When I first started playing KSP I had similar questions of why and how it’s different than real life. Still do. These differences have pushed me to such a greater understanding of real world physics and coding. Thanks again, to the marvelous creator Felipe Falange, aka HarvesteR.

 

I don't think that tidal influence really matters for craft, even in real life. It's a very subtle effect that wouldn't get the craft tidally locked in any reasonable period of time. There'd be an ever so slight nudge that would probably very slowly start the craft rotating on a planetary timescale, but not a Kerbal time scale.

I guess you could build ships with a chunk of mass on the end of a long boom to get enough leverage to make it work, but it's easier just to set the craft rotating slowly so that it's rotation keeps the same side pointing "down".

[firda]

1 hour ago, MechBFP said:

If the ship isn’t rotating (the actual ship itself in reference to itself), then using the word rotation doesn’t make any sense. Things are just orbiting. 

And what are you doing, when you hit 'W'? If the ship is not rotating, then the universe is. Orbit is something else, you orbit around some body, but rotate around your own CoM. That rotation can be zero in some reference frame and non-zero in another, but at least one of these frames won't be inertial and there would be some fictional forces acting on objects (making Kerbing appear like orbiting around the ship and moon doing some crazy flower-shaped orbits around you ... people actually described geo-centric systems like that, before moving to helio-centric).

I have started this, becase I was observing my ship in warp and seen, that Kerbin is rotating around the ship (you can enter the cockpit to see same thing),
but I had SAS locked to prograde and the only reasonable thing for me was, that the prograde-locked SAS should actually make the ship rotate (in global reference frame, where the ship is orbiting Kerbin and not the other way around) to match the period of the orbit, becase that is the most stable result, requiring least amount of energy to maintain (zero in ideal world),
but KSP is doing something else - it cancels the rotation in global reference frame ... and maybe even the coding of prograde-SAS is somehow wrong, to treat zero-rotation in global frame as stable, which it is not (becase the SAS needs to repeatedly cycle between readjusting to prograde and pseudo-stabilizing in global reference frame, that is just wrong).

So, my "if you give it the proper amount of rotation (hit "W" a bit)" and Shadowmage's "You'll need a rotation rate that matches your orbital period." is exactly what I expected prograde-locked SAS to do