Are docking ports strong as normal connection of same size?

[omelaw]

As docking port sr is 2.5m, are their connection as strong as between other 2.5 parts? or weeker?

how about normal (1.25m) and 0.625m docking port jr?

Edited February 29, 2016 by omelaw

[Snark]

Yes or no, depending on what you mean by "strength."

The stiffness of a docking connection is a lot lower-- it's "floppy," easy to bend. Bigger docking ports are stiffer than smaller ones, but they're less stiff than a regular joint.

However, as far as I can tell, the tensile strength (resistance to being pulled apart) is as strong as a regular joint.

Edited February 29, 2016 by Snark

[seaces]

Does the fact that I got last night on my minmus station docked huge fuel tanker made out of rockomax fuel tanks connected without any issues using jr. ports answers your question can you relay on smaller ones in terms of strength? Only thing is that is was a bit of wobbling around until it settled more then I think it would be with normal sized ones I suppose.

[Snark]

35 minutes ago, seaces said:

Does the fact that I got last night on my minmus station docked huge fuel tanker made out of rockomax fuel tanks connected without any issues using jr. ports answers your question can you relay on smaller ones in terms of strength? Only thing is that is was a bit of wobbling around until it settled more then I think it would be with normal sized ones I suppose.

Actually, I would contend that no, it doesn't really answer the question. 

The fact is that docking ports are a lot less stiff than normal connections.  The tiny 0.625m ports in particular are extraordinarily wobbly.  This is a simple fact of KSP, there's not really anything to debate about it.

Whether that lack of stiffness matters or not totally depends on the player's situation and what the player is trying to accomplish.

There are some situations where stiffness doesn't matter much.  For example, if all you're doing is building a static station, where all it has to do is just hang there motionless in space, then stiffness doesn't matter a lot, if you don't mind a bit of swaying now and then.  Sometimes SAS resonance can cause runaway wobble and hilarity ensues, but just turning off SAS lets things settle down.  From your description, @seaces, I would guess that this covers your situation.

On the other hand, there are other scenarios where the stiffness really matters.  (I assume that this is the OP's use case, or he wouldn't have been asking about it.)

For example, a common use of docking ports is for "tugs" or motherships, where one ship is moving another one under thrust.  Stiffness really matters there, especially if the ship doing the thrusting is pushing the other ship (which in my experience is the most common case).  The floppiness of docking ports really matters there, and has a major impact on ship design:  it affects the size of docking port choice (the bigger the better), the physical arrangement of the ships involved (try to keep them short on the longitudinal axis to minimize moment of inertia relative to the docking-port joint), and the ability to daisy-chain multiple items in a line (don't do it if you have a "pusher" tug, it's a recipe for tears).  The floppiness of docking ports substantially reduces the practicality of building large ships in orbit by assembling them out of docked sections.

If you're in a situation where docking-port floppiness is causing problems for you, there are various things you can do to try to mitigate the problem:

• Use bigger docking ports for better stiffness.
• For tugs, consider a design that pulls the load (which is an inherently stable configuration) rather than pushes it (which is inherently unstable).
• It's possible to attain stiffer docking-port connections by carefully designing ships so that you make multiple simultaneous connections (e.g. instead of connecting with one docking port, you use, say, three ports arranged in a triangle).  This is doable, and some people have had good results with it.  (I tend to stay away from it myself, mainly because I don't like the looks, it makes for ship designs whose appearance I don't care for, and I find the docking experience to be more finicky than I'd like.)
• Another option for stiffening connections, if you're willing to use mods:  Kerbal Attachment System has a really nice part available, "strut sockets".  These are small, light, radially-attachable parts that you can put on your ships.  You can send out a kerbal on EVA and click on them to establish a strut connection between two ports.  So you can get a nice stiff docking-port connection by making sure you've got some strut sockets around the port on each of the two connected ships.  Dock the ships, then send out a kerbal to reinforce the joint with some well-placed struts.  (I've frequently used this technique for assembling big ships in orbit, and I really like it:  works great, has minimal visual impact on the design, and it gives my EVA kerbals something fun and "astronauty" to do.)
• If you're pushing rather than pulling, ship design matters.  Two ships docked together will wobble less if there's a big mass disparity between them (i.e. one's a lot less massive than the other).  You'll also get less wobble if you can reduce the moment of inertia around the docking-port joint, i.e. keep your ships squat and short on the longitudinal axis, try to keep the CoM as close to the docking port as possible.

[Geschosskopf]

7 hours ago, seaces said:

Does the fact that I got last night on my minmus station docked huge fuel tanker made out of rockomax fuel tanks connected without any issues using jr. ports answers your question can you relay on smaller ones in terms of strength? Only thing is that is was a bit of wobbling around until it settled more then I think it would be with normal sized ones I suppose.

In general, to avoid wobbles, you should use docking ports that are the same diameter as the modules/ships you are connecting, and those ships/modules should not have a whole lot of radially attached stuff on them that would make their total diameter bigger.  Also, the modules should not be excessively long, even if they are the same diameter as the docking port.

Basically, docking ports have some lateral flex built into them.  This is isn't much of a bother with the 2.5m ports, provided you don't use them to connect monstrous modules, because that port is the same diameter as the rocket.  But the 1.25m and 0.625m ports are actually narrower than their corresponding rocket ports.  Thus, they make stress concentrations as well as flexible joints.  Anyway, when the ship yaws or pitches, the momentum of the parts farther towards the end of the ship causes the docking port joint to flex, especially with the 2 smaller sizes due to the stress concentration  Thus, the wider, heavier, and longer the connected modules, the more bending moment they apply to the port.  In extreme cases, the bending can be so much that the docking fails and the modules become detached.

Now, this is more of a problem for ships that will be under high thrust.  These really need to follow the rules of thumb above about being mindful of part-to-port diameters and not making modules excessively long or wide.  And unless the ship is very small (say an Apollo-style lander and CSM), mobile ships should really only use the 2.5m port.  With stations that are just going to sit there, you can get away with a bit more, but it's risky.  This is because any flexibility in the station's joints can be applied when physics loads as you approach it, and this can cause adjacent modules to clip into each other.  When parts are clipped and then physics is applied, the Kraken attacks.  The station starts vibrating madly and shaking itself to pieces to the accompaniment of explosions.  So even with stations, it pays to keep the ports the same size as the parts, and also to keep the modules spaced far enough apart that they can't clip into each other.

[Plusck]

It seems this is a real "your mileage may vary" question.

I've had a lot of success using the 1.25m port almost exclusively, but then again almost all of the ships that use them are powered by a single LV-N. So, as long as your vessels are properly balanced, you can put just about anything you want on the other end of the docking port as long as your thrust is about at the LV-N level. Also, since the LV-N has no gimbal, SAS is safe to use indefinitely with such a setup : no kraken, no undampened wobble, even when the drive section is all on its own behind a docking port and you have a lab, lander, ISRU, drill, ore and LFOx refuelling tanks on the forward side of the port.

And of course, if the thing you're pushing is fairly light (your standard 3km/s minimal 1-man science lander, for example) you can stick just about any engine on the back.

And as a test the other day, I had twin Aerospikes pushing a full Rockomax 32 tank for over several thousand m/s dv on 4x physics warp without any problems. Rock solid, but then I was extremely careful about aligning thrust with the port when I was building the contraption.
 

So of course, docking ports are not as solid as most other connections. For the 1.25m port, you absolutely need to strut them when taking off from Kerbin with anything over about 2 tonnes above them.

And I suppose you should also be careful about clearance between modules and also how you leave your stations: if the station won't stay perfectly still when you do a microwarp, you absolutely mustn't go off and do something else until it's sorted and it can drop out of warp without budging... but then that's more a general rule for KSP as a whole.