Killing relative velocity during rendezvous.

[Yuming]

I still can not dock. It is driving me crazy, and confining me to the kerbin system. Whenever I get an encounter around 5km (Yes, I'm terrible), I can get my relative speed down to 50 meters per second, but not any lower, and my target zips by. How do I get this lower? Am I doing the whole thing wrong and their is some "Special way of getting an encounter" other than slinging my orbit around? How an I even supposed to know where to point to kill my velocity?

[rryy]

First, you want to set your target as "target" in map view or by double clicking on it in normal view. Then, if you are close enough, you should see that the velocity shown above the navball is set to "target." This shows your relative velocity with the prograde and retrograde markers. Depending on the acceleration of your ship, you want to begin burning retrograde when you are about 2 km away. Then, once your relative velocity is zeroed out, to get closer, look for the pink circular marker on the navball. Point in that direction and then thrust that direction. Once you get very close, kill that relative velocity

[Flixxbeatz]

This might help you. It's on 8x speed since docking does take time.

[Rickenbacker]

If you target the ship you're trying to rendesvouz with, and make sure the navball is set to "target" mode, the prograde and retrograde markers show you which way you're going in relation to the target. So zeroing out relative velocity is then as easy as pointing at the retrograde marker and burning until you get down to zero, or as close as you care to.

[ctbram]

This is not working there is some kind of loveing physics bug.  I get 10m apart in target mode reduce relative velocity to ZERO that is ZERO.ZERO ZERO ZERO ZERO. and then then ship magi-loveingly starts accelerating.  the navball goes to 0.0 I turn off all thrust so the thrust is zero then the navball goes to 0.1 0.2 0.3... until I am moving away at meters per sec!!!!  Then I turn target retrograde AGAIN reduce relative velocity to 0.0 then kill ALL THRUST then the navball goes to 0.1 0.2 0.3.... all over again!!!!

A body in motion stays in motion unless acted on by an external force!!!!!!

With no thrust HOW THE BLOODY HELL am  I loveIJNG ACCELERATING from 0.0m/s?

I am sorry for swearing but I just spent three hours trying to make a Twitch video demonstrating orbital rendezvous and I  could not get the two ships to zero relative velocity and stay there!!!   It turned into a GAME of getting as close to zero then jump ships, Eva, and HOPE TO GOD you could get back to the rescue ship. That is NOT teaching someone how to rendezvous.,  That is demonstrating a game of Zorkon mobile3 jump hop skip duck eye-hand coordination GAME, not real physics!!!!

What is loveing causing this?  I was so frustrated I was screaming into the mic and ready to kick my puppy!!!!  How can this be fixed?  Do I have a mod problem?  A setting problem?

Because there is no loveing thing as free energy and once relative velocity is zero IT SHOULD loveING STAY ZERO!!!!  I would say the ships were trying to orbit each other but even the physics for that was wrong.  If so they should just circle around each other but they just accelerated infinitely away from each other with NBoo 

[Spricigo]

8 hours ago, ctbram said:

A body in motion stays in motion unless acted on by an external force!!!!!!

Yes, without an external force those crafts would be moving with constant velocity in a straight line.  Are they?

Given the game's limitations  (it is a game after all) the physics is quite accurate, what you experienced is what is supposed to happen. You just got confused because the non-inertial referential, gravity is acting upon the crafts the whole time.

 

[Hotel26]

25 minutes ago, Spricigo said:

Given the game's limitations  (it is a game after all) the physics is quite accurate, what you experienced is what is supposed to happen. You just got confused because the non-inertial referential, gravity is acting upon the crafts the whole time.

I don't want to sound picky, but while the above may be true, it doesn't explain well to someone who doesn't know what you do.

First consider this case: craft A and B are in the precise same orbit around a body, experiencing graviational acceleration, but separated by 100m.  Their relative speed will remain 0.  And distance, 100m.  Because they are in the precise same orbit, separated only by position within that orbit (Orbital Elements: True Anomaly).  (So gravity isn't the sufficient condition.)

So the better, and simple explanation, is that the two craft (in the user's case above) are in slightly different orbits -- hence their distance separation will vary.

Then helpfully suggest: "open the Map view and look at the orbits and see that they are visibly different, or failing that, that their apoapses and periapses differ."

Apologize for the above (especially after ten years long gone) but I suggest it makes 100% difference to the utility of the answer.

Edited December 4, 2023 by Hotel26

[Spricigo]

7 hours ago, Hotel26 said:

Apologize for the above (especially after ten years long gone) but I suggest it makes 100% difference to the utility of the answer.

Oh, I guess you got some valid points there.  No need for apologizes.

In any case, precision is limited both on your maneuvers and the displayed information. E.g. 0.0m/s may be 0.0000041592653589793... (or maybe as much as 0.0499).  So, while there is way to reduce that imprecision (matching orbits instead of only reduce relative speed,  using less powerful engines, doing the rendezvous at higher orbit, etc...), keep in mind that there will still be  some drifting happening just under your nose.

[Bej Kerman]

22 hours ago, ctbram said:

the navball goes to 0.0 I turn off all thrust so the thrust is zero then the navball goes to 0.1 0.2 0.3... until I am moving away at meters per sec!!!!  Then I turn target retrograde AGAIN reduce relative velocity to 0.0 then kill ALL THRUST then the navball goes to 0.1 0.2 0.3.... all over again!!!!

Because you and your target aren't attached? There's going to be a different level of gravity applied to two objects separated by a few dozen meters. The point of KSP is to simulate physics to a reasonable degree of accuracy and this is what it's doing, simulating two slightly different orbits.

Edited December 4, 2023 by Bej Kerman

[Kerbart]

Docking can be hard when you're not aware of the quirks. Yes, two objects can be close, and with a relative velocity of 0 — and for argument's sake, we can safely assume that's without rounding; they are truly motionless in relation to each other.

For the moment. Unless those object have exactly the same orbits they're going to drift apart. Both of them will climb to their apoapsis and descend to their periapsis. And if one them has a larger orbit, it's going to be slower eventually.

So, until you're docked there's always going to be relative movement. Make sure the navball is set to relative mode, and keep the relative velocity low; it's easy to overshoot.

[king of nowhere]

23 hours ago, ctbram said:

This is not working there is some kind of loveing physics bug.  I get 10m apart in target mode reduce relative velocity to ZERO that is ZERO.ZERO ZERO ZERO ZERO. and then then ship magi-loveingly starts accelerating.  the navball goes to 0.0 I turn off all thrust so the thrust is zero then the navball goes to 0.1 0.2 0.3... until I am moving away at meters per sec!!!!  Then I turn target retrograde AGAIN reduce relative velocity to 0.0 then kill ALL THRUST then the navball goes to 0.1 0.2 0.3.... all over again!!!!

others have already explained well why drifting happens and why it's so normal (really, I am surprised that you are knowledgeable enough to think of the two ships orbiting each other and realize the physics is wrong, but you still didn't realize how those ships, close as they are, they are still in slightly different orbits).

I will add that, for the purpose of docking, once you kill velocity you have enough time to dock. just don't take too much time, you can ignore drift if you dock fast enough. otherwise you can use rcs to correct for the drift.

[18Watt]

Two vessels in orbit with zero relative motion will almost always have different velocities.  So the zero relative motion will not stay that way forever.  There are two edge cases where you would be able to maintain zero relative motion.

The two edge cases are when the vessels are in identical orbits, and one vessel is behind or in front of the other.  For that to work, the orbits need to be perfectly circular as well, and you need to assume single-body gravity solutions are accurate, which is the case in stock KSP.

If the orbits are not perfectly circular, even being in identical orbits will not allow zero motion to be maintained, because the two vessels will experience acceleration at different times- the lead ship will sometimes pull away, at other times the trailing ship will catch up.

If you want to minimize those effects, try performing the rendezvous at a greater distance from the CB (celestial body).  For example, those effects will be greatest in LKO, but will be far less (and easier to deal with) at a higher orbit- say out towards Minmus.  Even just changing from a 100km orbit to a 300km orbit will greatly minimize the effects of slightly different orbits.  It’s still there, but you have more time before the different velocities cause big changes to your relative motion.

One more note-  Even if you have two vessels, in identical, perfectly circular orbits, with one trailing the other, you will still need to actively apply force to keep them aligned.  That is because although you could theoretically maintain zero relative velocity in that edge case, the vessels will still maintain their attitudes unless the attitude of both vessels is actively controlled with a force of some sort.  If the vessels are aligned at one point in the orbit, then 90 degrees later they will be parallel to each other, unless you are actively managing the attitudes of both vessels.

[Superfluous J]

I'll add 2 things to this already very information-filled slew of responses:

1. Until you're comfortable docking, do it at higher orbits. 100km, 200km, heck go up to 1,000km. The higher you orbit, the slower you orbit AND the less dramatic the minute-to-minute change in your velocity.
2. This is one of the few things that is EASIER in real life compared to KSP. Even at LEO, ships docking with the ISS can dawdle and not see much relative movement because their "Low Earth Orbit" orbit is a pretty "High Kerbin Orbit"

[ctbram]

Where did you guys learn physics?  By your logic, two ships could never maintain station keeping!  Furthermore, once you have achieved 0.0 relative velocity even taking into account the gravitational effects of the ship's masses and they - "not" being in the "perfectly same orbit" ships do not rapidly accelerate towards each other and apart accelerating infinitely. 

So you have never rendezvoused with a ship and been able to hold at a stable distance?  LOL.  And can you explain why one time I can hold at a stable distance at 0.0 relative velocity and then in the very same save get to roughly the same point get to  0.0 relative velocity and then just accelerate past the ship infinitely with zero external force being applied and never be able to achieve station keeping?

I have a video of Physics working as it should and then with the bug I am describing here from the same save, rendezvousing with the same two ships.  Perhaps you are all just not clear on what I  am describing. The same two shiips should not behave one was (the correct way) one time and then completely differently another!

I think with a MS  in aerospace engineering from UofM in 1985 with a focus on control systems and 40-plus years of writing guidance control code I have a pretty damn good idea of how orbital mechanics and physics work and don't appreciate being talked down to with snarky comments questioning what I know about physics.

I do not want to go off and get snarky myself.  I will make a video and post it here showing the behavior I am describing so you can understand that this is a bug and improper physics simulation.

Edited December 5, 2023 by ctbram

[HebaruSan]

1 hour ago, ctbram said:

And can you explain why one time I can hold at a stable distance at 0.0 relative velocity and then in the very same save get to roughly the same point get to  0.0 relative velocity and then just accelerate past the ship infinitely with zero external force being applied and never be able to achieve station keeping?

Yes, it depends on the direction from which you approach the other ship. If you come at it from the prograde  or retrograde  directions, then you'll be at almost exactly the same altitude with the same inclination, so 0 relative velocity is a good indicator of identical or extremely similar orbits, and it will probably hold like that. If you approach from radial-in  or radial-out , then you're either higher or lower than the other ship, so the orbits will be different regardless of whether the relative velocity is 0, and you'll drift out of that arrangement.

The normal  and anti-normal  directions are special; here you can use the drift to your advantage! An approach from these directions starting from 0 relative velocity means your orbits are the same in everything except inclination, which means they intersect at ascending and descending nodes. So as you approach the next such intersection, the motion will be towards the other ship, and if you're patient, you can use it to complete the docking maneuver!

[Bej Kerman]

5 hours ago, ctbram said:

Where did you guys learn physics?  By your logic, two ships could never maintain station keeping!

Elaborate

5 hours ago, ctbram said:

Furthermore, once you have achieved 0.0 relative velocity even taking into account the gravitational effects of the ship's masses and they - "not" being in the "perfectly same orbit" ships do not rapidly accelerate towards each other and apart accelerating infinitely. 

You're forcing words into our mouths, nobody said they'd "accelerate infinitely". We said the exact opposite, that without any thrust they'd follow their own orbits.

5 hours ago, ctbram said:

So you have never rendezvoused with a ship and been able to hold at a stable distance?  LOL.  And can you explain why one time I can hold at a stable distance at 0.0 relative velocity and then in the very same save get to roughly the same point get to  0.0 relative velocity and then just accelerate past the ship infinitely with zero external force being applied and never be able to achieve station keeping?

The most fundamental principle of docking is that you and your target follow different orbits. If you're in a higher orbit than your target, you wait for it to catch up below you then you burn into a lower orbit. That doesn't magically stop happening just because you're within a few hundred meters.

5 hours ago, ctbram said:

I have a video of Physics working as it should and then with the bug I am describing here from the same save, rendezvousing with the same two ships.  Perhaps you are all just not clear on what I  am describing. The same two shiips should not behave one was (the correct way) one time and then completely differently another!

Feel free to show us.

5 hours ago, ctbram said:

I think with a MS  in aerospace engineering from UofM in 1985 with a focus on control systems and 40-plus years of writing guidance control code I have a pretty damn good idea of how orbital mechanics and physics work and don't appreciate being talked down to with snarky comments questioning what I know about physics.

I doubt that. Your first post in this thread was you complaining that two vessels in different orbits didn't remain effectively attached to each other.

5 hours ago, ctbram said:

I do not want to go off and get snarky myself.

Mail that to 5 hours ago when you were still writing that first paragraph  

5 hours ago, ctbram said:

I will make a video and post it here showing the behavior I am describing so you can understand that this is a bug and improper physics simulation.

So, a vessel in a 200x200km orbit and another vessel in a 205x205km orbit should NOT drift apart under normal physics?

[Kerbart]

6 hours ago, ctbram said:

Where did you guys learn physics?  By your logic, two ships could never maintain station keeping! 

If by "station keeping" you mean without any extra input, yes. Station keeping is a lot of work, and it's all gone if you're away for three or four orbits.

Quote

Furthermore, once you have achieved 0.0 relative velocity even taking into account the gravitational effects of the ship's masses and they - "not" being in the "perfectly same orbit" ships do not rapidly accelerate towards each other and apart accelerating infinitely.

They do. Unless your orbit is perfectly circular, the vessels will have a higher velocit at periapsis and a lower velocity at apoapsis. I'm sure you're familiar with the vis-viva equation. And unless their apoapsis and periapsis perfectly overlap, that'll be at different points in time. so they will have different velocities, drift apart, come closer, as long as they are in orbit - in principle indefinitely.

 

Edited December 5, 2023 by Kerbart

[Dakitess]

Aside of the physic explanation, this is why you basically never want to cancel your relative speed. This is the EXACT reason why people following step by step tutorial don't understand why they are drifting around their target, warping, cancelling speed, going forward at 2m/s and 2km distance, warping, drifting, and so on : you're never travelling in straight line, you can only consider it when traveling at a good speed to distance ratio, otherwise the orbital drift will be even more important than the forward speed you gave to your craft...

I don't understand how / why people keep canceling speed rather than doing the retrograde-active-redirection, when you pull / push it toward the target indicator, keeping all your momentum, only reducing it partially when approaching. It's easy, very practical, very relevant, way more efficient RCS / Elec wise, etc.

Don't cancel your speed at 3km to give it back 20s after. And you do it, fine, it works, but only if you push toward the target a sufficient speed regarding your distance. 2km ? 10m/s is fine, this is plenty time to react. Any less will make you drift...

[Bej Kerman]

On 12/3/2023 at 5:29 PM, ctbram said:

A body in motion stays in motion unless acted on by an external force!!!!!!

And you are acted on by an external force, gravity.

[Kerbart]

2 hours ago, Dakitess said:

I don't understand how / why people keep canceling speed rather than doing the retrograde-active-redirection, when you pull / push it toward the target indicator, keeping all your momentum, only reducing it partially when approaching. It's easy, very practical, very relevant, way more efficient RCS / Elec wise, etc.

Don't cancel your speed at 3km to give it back 20s after. And you do it, fine, it works, but only if you push toward the target a sufficient speed regarding your distance. 2km ? 10m/s is fine, this is plenty time to react. Any less will make you drift...

In fairness, I think the situation referred to is in docking proximity, not during rendezvous although your comment is spot on in regards to that. I think Jimorian on Twitch called it "pushing the marble" by keeping the retrograde relative speed vector on top of the target and depending on how aggressively you speed up/kill speed it can shave off 100 m/s DV or more, easily.

As for approach speed during final stages in rendezvous I stick to the maritime rule "never approach something faster than you're willing to crash into it"

[Kerbart]

9 hours ago, ctbram said:

 I will make a video and post it here showing the behavior I am describing so you can understand that this is a bug and improper physics simulation.

Please do, apparently we're talking about different things here. If possible do include the basic orbital parameters of both vessels—a/p and inclination because that will make it easier to understand what's going on and will exclude "obvious questions" when we're evaluating the video.

[GluttonyReaper]

8 hours ago, ctbram said:

Furthermore, once you have achieved 0.0 relative velocity even taking into account the gravitational effects of the ship's masses and they - "not" being in the "perfectly same orbit" ships do not rapidly accelerate towards each other and apart accelerating infinitely. 

As others have explained, this can appear to happen if the two ships have orbits with slightly different altitudes. Consider: you have two ships (Ship 1 & Ship 2) right next to each other, with Ship 1 in a perfectly circular orbit, and Ship 2 just slightly lower, but with the same relative velocity. Lower orbits need a higher velocity to be circular (gravity and all that). Ship 2 has the same velocity as Ship 1, but is in a lower orbit... so it doesn't have enough velocity to stay circular. Instead, Ship 2 has to lose altitude to convert some of that gravitational energy into extra velocity, effectively accelerating from the perspective of Ship 1, both radially and prograde. And because the entire orbit of Ship 2 is within the orbit of Ship 1, it's going to have a shorter orbital period, and the two ships will very quickly fall out of phase, so that effect doesn't just cancel out unfortunately.

This absolutely does happen in real life (IIRC, NASA effectively uses this effect in reverse to slowly approach the ISS), but the effects are hugely amplified in KSP because Kerbin is so small, so orbits are shorter and the curvature more extreme.

9 hours ago, ctbram said:

And can you explain why one time I can hold at a stable distance at 0.0 relative velocity and then in the very same save get to roughly the same point get to  0.0 relative velocity and then just accelerate past the ship infinitely with zero external force being applied and never be able to achieve station keeping?

That said, if it's behaving inconsistently, then there might be something buggy going on. It's hard to say without video of what's happening.

[Bej Kerman]

30 minutes ago, GluttonyReaper said:

10 hours ago, ctbram said:

And can you explain why one time I can hold at a stable distance at 0.0 relative velocity and then in the very same save get to roughly the same point get to  0.0 relative velocity and then just accelerate past the ship infinitely with zero external force being applied and never be able to achieve station keeping?

That said, if it's behaving inconsistently, then there might be something buggy going on. It's hard to say without video of what's happening.

ctbram was describing the vessels drifting apart slowly as opposed to a massive acceleration, so it's definitely not a bug.

[GluttonyReaper]

Just now, Bej Kerman said:

ctbram was describing the vessels drifting apart slowly as opposed to a massive acceleration, so it's definitely not a bug.

Now that I think about it, there's an easy way to check - if there is any anomalous acceleration, you would be able to see the orbit changing on the map screen. If it's just natural drift, it'll still happen even when everything's on rails.

[Bej Kerman]

3 minutes ago, GluttonyReaper said:

Now that I think about it, there's an easy way to check - if there is any anomalous acceleration, you would be able to see the orbit changing on the map screen. If it's just natural drift, it'll still happen even when everything's on rails.

@ctbram CC

If your drift is actually anomalous, this'll happen. When you're timewarping, your orbits are locked in place and it's impossible to exert force upon a craft. If the drift happens in timewarp then it's definitely just the physics you're claiming is broken.