A question about orbits

[kerbatron343]

Hello everyone,

First time posting here on the forums. I have been playing ksp on and off for the past year now. I just started playing again recently and started to build a space station. It's the largest I have built yet but I have noticed my periapsis seems to be slightly dropping since the first piece I sent up. When I started building the station the periapsis was around 99.8 km with my apoapsis being about 101 km. I have brought up quite a few pieces since then, I have a refueling adapter put up, 2 solar arrays, and around 6 or 8 living units (containers, pods, science labs/cuppola). Now my periapsis is currently at 98.973 km, which to me seems like a pretty significant drop. I will be in map mode and I can also see the periapsis slightly dropping. Is this a normal thing that I shouldn't worry about? Or is it something that will affect my station significantly like continually dropping till it de-orbits? I'm not anything close to a scientist, but it seems strange to me that it keeps dropping when I (believe that I am) am in a stable orbit quite a bit above the atmosphere so I shouldn't have to worry about falling back to Kerban. Any help would be greatly appreciated!

Thank you fellow kerbalnauts!

Kerbatron343

[Tortoise]

I am a complete noob, but I am assuming that the heavier it gets, the more that gravity has an effect on it? Therefore it gets pulled by earth's gravity slightly faster than when it was lighter?

[Starstrider42]

I am a complete noob, but I am assuming that the heavier it gets, the more that gravity has an effect on it? Therefore it gets pulled by earth's gravity slightly faster than when it was lighter?

kerbatron343, the best explanation I can think of is that you're just seeing random jitter in how KSP calculates orbits (in which case yes, you can safely ignore it). I thought maybe if you always docked station parts from the prograde direction, the accumulated momentum of each module would have pushed you backward a bit, but that would only be a few m/s change. I'm pretty sure a 1 km drop in periapsis requires at least a few dozen m/s.

EDIT: Just did some tests; a few m/s could totally have the effect you described.

Edited April 13, 2014 by Starstrider42

[syfyguy64]

Seems like you made a replica of MIR.

[kerbatron343]

OK thank you. I am hoping it is a jitter. It seems that I am losing about 1 meter to my periapsis around every 8 seconds. It is constant though, and the time between each meter stays about the same. The apoapsis jitters between 4 meters, but gradually increases a meter about every 20 seconds. I was going to circularize my orbit with the rcs, but with the size of my station I'm afraid I will rip it apart. I tried to move it a little bit and I was getting a ton of wobble.

Very cool video! I assumed weight didn't factor into the equation. Getting into ksp has gotten me really interested in anything space related, love watching those old apollo videos.

I will keep an eye on it as I add more to my station and start to create my mun station and base. Thanks for the feedback guys!

[cantab]

Yeah, there's no orbital decay in KSP, atmopsheres stop hard at their edge which is just below 70km up for Kerbin. Nudging the station when docking is the likely cause.

If you want you can always do a little stationkeeping to put it back in your desired orbit.

[Kasuha]

By docking another piece with the station, you give it small nudge in random direction and that can have the effect you see. Even by trying to orient your station by docking port towards incoming ship with RCS may have similar effect.

There's not much point in trying to have your station on perfect orbit, a few hundred meters difference does not mean anything. Time to time, you may pull it back to desired orbit with a few RCS burns. Or attach a small tug to it, perhaps using ion engines, which will get it to the position where you want it to be.

[?lackhole]

It will be okay if you are not controlling it real time.

[Othuyeg]

Could be a baby kraken. If you have parts clipping into eachother, they may be generating a phantom force and slowing you down.

However... if that were the case, you could not timewarp, because the station would be under acceleration. (correct me if I am wrong)

Edit: here is an illustration of the possible problem:

[astropapi1]

I am a complete noob, but I am assuming that the heavier it gets, the more that gravity has an effect on it? Therefore it gets pulled by earth's gravity slightly faster than when it was lighter?

Free-falling speed is not affected by the mass of an object, and orbiting is basically free-falling very fast.

[Anachronda]

Free-falling speed is not affected by the mass of an object, and orbiting is basically free-falling very fast.

Or, to quote Douglas Adams, the secret to flying is to throw yourself at the ground and miss.

[Dunbaratu]

I am a complete noob, but I am assuming that the heavier it gets, the more that gravity has an effect on it? Therefore it gets pulled by earth's gravity slightly faster than when it was lighter?

No. Galileo.

Longer Answer: The mass cancels out. The more mass, the more force is pulling on it, but the more mass, the more force it needs pulling on it to achieve the same effect to its motion. Push on something twice as massive with twice as much force, and its movement is the same as pushing on something half as massive with half as much force.

[Yoha]

No. Galileo. Longer Answer: The mass cancels out. The more mass, the more force is pulling on it, but the more mass, the more force it needs pulling on it to achieve the same effect to its motion. Push on something twice as massive with twice as much force, and its movement is the same as pushing on something half as massive with half as much force.

I think a simpler way to see it is to consider sub-parts of the satellite. If your satellite is orbiting around Kerbin, its two halves are as well, and at the same speed, even though they each weight less than the total.

[Dunbaratu]

I think a simpler way to see it is to consider sub-parts of the satellite. If your satellite is orbiting around Kerbin, its two halves are as well, and at the same speed, even though they each weight less than the total.

That is classically how the principle was derived, pre-Newton, but it doesn't convince me at all why it supposedly makes sense. Consider a different case - the case of blowing a wind on something. You glue two objects together, both are 1 kg, but one of them is a large piece of cardboard, and the other is a small lump of lead. You put them in a wind tunnel. Do they separate apart? No, They're held together and thus constrained to go the same speed. The differential effect on the two parts can cause the combined object to rotate but that's it. But if you separated them they would not be affected the same. The cardboard piece would be moved a lot more.

That's an example where, unlike with gravity, the two objects *are* moved differently when separated and yet they can still be attached together into one object.

The notion that "If two objects affected by some phenomenon went two different speeds when unattached to each other, then they'd have to be going two different speeds if attached to each other as one object, which is impossible, and that's why they have to go the same speed when not attached to each other" is a fallacious argument. Once you attach them together all that would mean is that there's tension pulling at the attachment point and if the attachment was strong enough to handle it, they would be forced to go the same speed even if they wouldn't when separated.

[blizzy78]

The problem is the shifting center of mass. Scott Manley did a video where he had two orange tanks in space, one empty and one full. He then spun the whole thing up, then pumped the fuel from one tank into the other. Half a rotation around, he pumped it back. Another half, he pumped it again. He was able to alter the orbit doing that.

[SRV Ron]

The problem is the shifting center of mass. Scott Manley did a video where he had two orange tanks in space, one empty and one full. He then spun the whole thing up, then pumped the fuel from one tank into the other. Half a rotation around, he pumped it back. Another half, he pumped it again. He was able to alter the orbit doing that.

That is most likely the reason for the orbit change which the wobbling can also influence as a result of docking or an attempt to use RCS to adjust the orbit.

[Guest]

Hmm, this looks like it could be a bug reported some time ago. Vessels were experiencing a phantom acceleration when focused, with no good explanation. Turn off all the RCS, SAS, let the station tumble and see if the orbit is still changing. If it is, you've hit a bug.

[Javster]

You must have knocked it while docking.

[Claw]

I tend to agree. The 900 m drop is probably mostly to do with docking.

As far the gradual decay, I can't say that I've sat and watched an AP/PE over very extended periods of time but you're probably just seeing jitter (as pointed out earlier) or possibly the rounding errors that happen while the game calculates all the physics.

When you aren't actively watching your station (i.e. you're working with another craft more than 2.5 km away from it), it should not decay. The game puts ships "on-rails" which means they simply move about their orbit without much in the way of physics calculations. You could even put a craft "on-rails" in Kerbin's atmosphere (at high altitudes) and it won't decay as long as you're not looking. So I guess my point is that there's no need to worry if you leave it for a while.

[Tinyboss]

That is classically how the principle was derived, pre-Newton, but it doesn't convince me at all why it supposedly makes sense. Consider a different case - the case of blowing a wind on something. You glue two objects together, both are 1 kg, but one of them is a large piece of cardboard, and the other is a small lump of lead. You put them in a wind tunnel. Do they separate apart? No, They're held together and thus constrained to go the same speed. The differential effect on the two parts can cause the combined object to rotate but that's it. But if you separated them they would not be affected the same. The cardboard piece would be moved a lot more.

That's an example where, unlike with gravity, the two objects *are* moved differently when separated and yet they can still be attached together into one object.

The notion that "If two objects affected by some phenomenon went two different speeds when unattached to each other, then they'd have to be going two different speeds if attached to each other as one object, which is impossible, and that's why they have to go the same speed when not attached to each other" is a fallacious argument. Once you attach them together all that would mean is that there's tension pulling at the attachment point and if the attachment was strong enough to handle it, they would be forced to go the same speed even if they wouldn't when separated.

There is no need to make it complicated. The case of two identical objects in a vacuum is sufficient to break the Aristotlean argument--if it fails this simplest possible case (and it does), then who cares what it predicts about cardboard and lead in a wind tunnel?

[Dunbaratu]

There is no need to make it complicated. The case of two identical objects in a vacuum is sufficient to break the Aristotlean argument--if it fails this simplest possible case (and it does), then who cares what it predicts about cardboard and lead in a wind tunnel?

The point is that the explanation, while simpler, is a fallacy. The classic argument that the reason gravity must make all things fall the same is because of the contradiction that two objects attached together would simultaneously fall at the same speed and yet at different speeds if it was any other way is utterly false. It happens to arrive at the right conclusion but for all the wrong reasons. It's a bit like saying "cars are animals that like the taste of gasoline. You have to feed it to them or they get ornery and refuse to move". While it might teach someone that they need to put fuel in the car to make it go, it's still a faulty explanation that it's a bad idea to use.

The crux of the "fall at the same speed but different speeds at the same time" argument is the incorrect notion that movement based on gravity happens in a sort of "on rails" way. If it's properly expressed as a force that pulls on things rather than a magical forced deterministic path that things follow, then it's easy to see that objects attached together could still remain at the same speed if the force trying to pull them apart wasn't strong enough to break the attachment.

What matters is the experimentation. The fact that when you try to drop the two objects they don't in fact show any signs of being pulled apart (i.e. a string between them remains slack) is the reason. Not the mere thought experiment based on the false notion that objects can't remain stuck together if gravity pulled them differently. Of course they could remain stuck together, but then it would be like a tug-of-war between them.

[Starwhip]

If you ask me, the RCS is being activated by SAS, and it's slowly deorbiting you. Or you accidentally clipped parts while docking and made a Kraken Drive. Or both.

Look at the resources tab. (It's in the upper right corner; click the "resources" button to bring it up) If the one called "monopropellant" is dropping, then it's the RCS. Otherwise, it's a bug.

[John FX]

kerbatron343, the best explanation I can think of is that you're just seeing random jitter in how KSP calculates orbits (in which case yes, you can safely ignore it). I thought maybe if you always docked station parts from the prograde direction, the accumulated momentum of each module would have pushed you backward a bit, but that would only be a few m/s change. I'm pretty sure a 1 km drop in periapsis requires at least a few dozen m/s.

EDIT: Just did some tests; a few m/s could totally have the effect you described.

This was my first thought, slight motion changes caused by docking. It`s a very very small speed difference to go fro 99.8 to 98.9km...

It`s the simplest solution and fits very well with the observed events.

Occams razor suggests this is the actual situation.

[Aphobius]

The point is that the explanation, while simpler, is a fallacy. The classic argument that the reason gravity must make all things fall the same is because of the contradiction that two objects attached together would simultaneously fall at the same speed and yet at different speeds if it was any other way is utterly false. It happens to arrive at the right conclusion but for all the wrong reasons. It's a bit like saying "cars are animals that like the taste of gasoline. You have to feed it to them or they get ornery and refuse to move". While it might teach someone that they need to put fuel in the car to make it go, it's still a faulty explanation that it's a bad idea to use.

The crux of the "fall at the same speed but different speeds at the same time" argument is the incorrect notion that movement based on gravity happens in a sort of "on rails" way. If it's properly expressed as a force that pulls on things rather than a magical forced deterministic path that things follow, then it's easy to see that objects attached together could still remain at the same speed if the force trying to pull them apart wasn't strong enough to break the attachment.

What matters is the experimentation. The fact that when you try to drop the two objects they don't in fact show any signs of being pulled apart (i.e. a string between them remains slack) is the reason. Not the mere thought experiment based on the false notion that objects can't remain stuck together if gravity pulled them differently. Of course they could remain stuck together, but then it would be like a tug-of-war between them.

What the hell are you talking about? "The classic argument"? In kindergarten or where? xD

The current physics don't work with such simple "arguments" anymore.

You have the Newton's law of universal gravitation (F = (G*m1*m2)/r^2), you have the second law of motion (F = ma). That puerile "argument" is really necessary?

Newton's laws can be considered as puerile as your "classic argument" compared to quantum gravity, but in the context of KSP they are good enough.

[stoani96]

I am a complete noob, but I am assuming that the heavier it gets, the more that gravity has an effect on it? Therefore it gets pulled by earth's gravity slightly faster than when it was lighter?

without reading any other posts here:

Go back to school!!!

There are two forces (simplified) on each thing in orbit:

gravity and centrifugal force

if both are equal, you have and perfectly round orbit

if not, the orbit is elliptical and maybe even "hits" the surface of the object you are orbiting.

formula for gravity: F = m * g

zentrifugal force: F = m * v * v / R

like you can see: in both formulas is the mass. if the mass increases the gravity increases too (we all know that), but also the zentrifugal force increases to the same amount!