Basic physics question for orbital docking

[GungaDin]

I'm at the point in my career where I can maneuver myself into a rendezvous but have not yet tried to dock.  I just managed to squeeze out enough science to get RCS thrusters which I imagine are vital to docking.  Anyway here is my question:

If I have gotten close to my target and my closing rate is minimal or zero, can I forget that I am in orbit with my target and just treat it as if we were floating weightless?  In other words is there a difference between docking in completely free space as opposed to docking in low Earth orbit?   If I start to apply thrust and creep forward do I need to worry about rising up as a result of my speed increase or is that so minimal that  can ignore it?

[VoidSquid]

45 minutes ago, GungaDin said:

which I imagine are vital to docking

Yes, as they allow you dorsal/ventral/starboard/port and reverse movement, not just forward like a rocket engine.

45 minutes ago, GungaDin said:

is there a difference between docking in completely free space as opposed to docking in low Earth orbit?

Not much, but yes: orbital drift. The two orbits are almost but not completely identical, hence, even if you one moment have zero relative velocity, a few moments later it will increase a bit. Not much, of course, depends of how close to each other the orbits and the vessels are, how long you take for the docking process, how high the orbits are (the higher, the less orbital drift).

If you get close enough to your target when beginning the docking procedure (say 100m), you shouldn't have any issues as you can easily compensate for the orbital drift with your RCS, and the docking itself will take only a couple of minutes, not that much orbital drift during such short period.

Tips/suggestions:

Get yourself these mods:

and

The first one provides you an aid in the VAB where to place the RCS thrusters to minimize rotation when you move dorsal/ventral/starboard/port, the latter is an excellent visual aid for the docking process itself. 
I also recommend, if you're using reaction wheels, to disable the rotational components (yaw/pitch/roll) of the RCS thrusters, would be not only a waste of monoprop, but that way you can control rotation by WASDQE, translation by HNIJKL, without one control set affecting the other, much easier and cleaner steering.

 

P.S.: regarding the mods, they work fine with KSP 1.7.3, no idea if they are already updated for 1.8, I'm still playing KSP 1.7.3 and will continue to do for the foreseeable future. 

Счастливой стыковки, товарищ!
(Happy docking, comrade!)

Edited October 28, 2019 by VoidSquid

[Kerbart]

12 minutes ago, GungaDin said:

I'm at the point in my career where I can maneuver myself into a rendezvous but have not yet tried to dock.  I just managed to squeeze out enough science to get RCS thrusters which I imagine are vital to docking.  Anyway here is my question:

If I have gotten close to my target and my closing rate is minimal or zero, can I forget that I am in orbit with my target and just treat it as if we were floating weightless?  In other words is there a difference between docking in completely free space as opposed to docking in low Earth orbit?   If I start to apply thrust and creep forward do I need to worry about rising up as a result of my speed increase or is that so minimal that  can ignore it?

Within reason, yes, you can treat docking as if you're not orbiting. Orbital mechanics aren't even an issue for the least 5 km—what most of us will refer to as rendez-vous; it's only a factor when you're trying to match orbits and positions. Generally speaking* when map view is helpful, orbital mechanics matter, if it makes more sense to navigate by camera or navball, it doesn't.

 

* Rule of thumb. I'm sure there will be dozens of exceptions pointed out to me.

[linuxgurugamer]

10 minutes ago, Kerbart said:

Within reason, yes, you can treat docking as if you're not orbiting. Orbital mechanics aren't even an issue for the least 5 km—what most of us will refer to as rendez-vous; it's only a factor when you're trying to match orbits and positions. Generally speaking* when map view is helpful, orbital mechanics matter, if it makes more sense to navigate by camera or navball, it doesn't.

I'll disagree.  If you are in a low orbit around Kerbin, for example, orbital mechanics most definitely come into play.  If you are very far out, then not so much.  And if in solar orbit, not really at all due to the relatively extreme distance from the sun

[Kerbart]

5 minutes ago, linuxgurugamer said:

I'll disagree.  If you are in a low orbit around Kerbin, for example, orbital mechanics most definitely come into play.  If you are very far out, then not so much.  And if in solar orbit, not really at all due to the relatively extreme distance from the sun

For docking? We're talking about docking ports 10m removed from each other. This is not where "I have to burn away from the target, to get a lower orbit, to get closer" applies. That's how I read the question, at least.

I've docked hundreds of times and I never take orbital mechanics into account at close quarters. Never.

Edited October 28, 2019 by Kerbart

[linuxgurugamer]

1 minute ago, Kerbart said:

For docking? We're talking about docking ports 10m removed from each other. This is not where "I have to burn away from the target, to get a lower orbit, to get closer" applies. That's how I read the question, at least.

I've docked hundreds of times and I never take orbital mechanics into account at close quarters. Never.

Depends on orientation.  Even 10m apart, if separated vertically from the planet, and in a low kerbin orbit, if you are approaching slowly, you can see the difference

[GungaDin]

43 minutes ago, linuxgurugamer said:

Depends on orientation.  Even 10m apart, if separated vertically from the planet, and in a low kerbin orbit, if you are approaching slowly, you can see the difference

OK I get it now.  I'm assuming that I come it at the same altitude and I'm close and relative speed low then I can 'wing' it.  I did install a "docking port alignment" mod to help with this so I'll give it a go tonight.  Thanks to all.

[linuxgurugamer]

7 minutes ago, GungaDin said:

OK I get it now.  I'm assuming that I come it at the same altitude and I'm close and relative speed low then I can 'wing' it.  I did install a "docking port alignment" mod to help with this so I'll give it a go tonight.  Thanks to all.

With those assumptions, you should be fine

[Linkageless]

Docking without any RCS is very possible, but much much easier if the vessel approaching is compact and has good reaction wheel control. Reducing the thrust limiter on your main engine helps.

Low orbits will almost certainly shift you out of line if you are making a slow approach over a longer distance.  Even in a high orbit, don't expect to stay perfect alignment if you leave two adjacent vessels for a period of time.

[GungaDin]

1 hour ago, Linkageless said:

Docking without any RCS is very possible, but much much easier if the vessel approaching is compact and has good reaction wheel control. Reducing the thrust limiter on your main engine helps.

Low orbits will almost certainly shift you out of line if you are making a slow approach over a longer distance.  Even in a high orbit, don't expect to stay perfect alignment if you leave two adjacent vessels for a period of time.

Well I have tried this but if all I have is my big rear engine then the only way to slow down is to spin around 180 right?  And to adjust left right swing 90 degrees each time... or am i missing something?

[paul23]

Well if you switch the navball mode to "target" it's just as simple as making sure you kill your relative speed by firing pro/retrograde. (when in target mode). Then you just fire towards the target (thus making sure prograde marker overlaps the pink icon). And then turn you ship so your docking port is facing forward... Approaching with like 1 m/s.

 

If you miss, just return and do above again.

 

If you start from (say) less than 100 meter distance you'll be good.

[Linkageless]

46 minutes ago, GungaDin said:

if all I have is my big rear engine then the only way to slow down is to spin around 180 right?  And to adjust left right swing 90 degrees each time...

Correct. Hence the need to be compact with good reaction wheels.  Tiny adjustments are needed, so I would usually bring the thrust limiter on the engine right down.  This is how I initially learnt to dock, before I even knew the RCS did anything more than attitude control.

Compact and agile aren't even definite requirements.  I have docked my 4xLV-N powered asteroid harvester hauling a 2000t asteroid to my high kerbin orbit station with only 2 advanced reaction wheels.  It required patience and plenty of quicksaves.  The Kraken loved this.  I later sent up 4 vernor propelled + reaction wheel + Klaw RCS drones to make life a LOT easier.   Also, sticking a Klaw on the end of my station lets me get away with gently shoving the asteroid at it.

In short, you'd be mad to rely on engine+reaction wheel only manoeuvres for anything larger than a single lander can-size runabout.  It can be done, but the RCS translation controls make life so much easier.

[VoidSquid]

1 hour ago, GungaDin said:

or am i missing something?

Nope