Docking Ports and Acceleration

[keyscapeunit]

Hi all,

I am playing through science mode and have a question about docking and acceleration.

I'm currently using the small docking ports.  I docked a ship to the side of another ship around Minimus at a perpendicular angle and tried to tow it back to Kerbal.  The docked ship (the smaller ship attached to the side of the main ship perpendicularly) immediately swung into the accelerating ship and caused all kinds of havoc.  It makes sense to me why this would happen but I'm curious if I were to align the ships (i.e. nose to nose) would I have the same issue?  Ultimately I would like to understand how to dock a tug and push a ship somewhere.

Edited December 9, 2015 by keyscapeunit

[Kerbart]

The short answer: physics.

The long answer: the center of mass of the docked ship is not aligned with the axle of thrust. This will cause torque, which causes the docked ship to twist.

Think of holding a car battery right in front of your chest. Doable, right? Now stretch your arms horizontally in front of you and try to hold the same car battery. If you can, impressive, but likely, you cannot. And yet, it's the same battery! The "turning force" is called torque and is proportional to the distance of the pivot point. If you push or pull directly in line with the center of mass there will be no torque, and no twisting. Offset the force (or the mass) and things will start to twist and turns. Welcome to the world of engineering

Edited December 9, 2015 by Kerbart
fixed typo

[Pronoes]

Short answer - no.

From what you describe, it sounds as if you docked your "tow ship" into the side of the "mothership". I don't understand if the tow ship swung into the mothership? That shouldn't happen. The only way I can imagine something similar is if the mothership was long, the docking port wasn't in centre of mass, and/or your tow ship had too much thrust at full acceleration. That would be like firing a missile into a piece of string.

Generally, any item is stronger across it's length than it's breadth. 

[Venusgate]

Under normal circumstances, no, it will not cause troubles if you dock in line.

Unusual circumstances:
TWR of 10+
CoM of the docking vessel is not centered over the docking port axis

The docking ports form a pretty solid attachment, but it is at least half as weak a connection as a straight up editor build (like an engine on a tank)

[keyscapeunit]

Thanks all, that makes perfect sense.  Just wanted to be sure docking ports would work with my longer term objectives.  Appreciate the help!

[Geschosskopf]

2 hours ago, keyscapeunit said:

Thanks all, that makes perfect sense.  Just wanted to be sure docking ports would work with my longer term objectives.  Appreciate the help!

Docking ports in general aren't as solid a joint as an actual structural node connection.  Thus, you can have trouble even with docking stuff inline if you overload the docking ports.

In general, docking ports like to connect things that aren't significantly wider than they are themselves, and aren't obscenely long, either.  IOW, a 1.25m docking port should mostly be used to connect 1.25m modules, etc.  You CAN have some radially attached stuff out beyond the docking port's diameter but don't go overboard there.  The radial stuff should be the same or smaller diameter as the port and also shouldn't be hugely massive compared to the central stack with the docking port.

These are just general rules of thumb and actual results will vary based on the designs of individual ships and docked modules.  Harmonics and all that.  You need to experiment to get a general feel for what you can get away with.  But basically, if you violate these rules of thumb too much, your combined rocket will behave like a piece of spaghetti and flex badly at the docking ports when under thrust, making it difficult to keep it pointed in the right direction during burns.  Some folks "solve' this by putting outrigger engines at the front end of combined rockets, which tends to reduce the wobbles, but which is only possible because KSP totally ignores the radiant heat from the exhaust flames.   I find it better to just not build things that invite the wobbles and keep the engines at the back where Gawd intended :).

 

[KerikBalm]

Docking connections are weak... generally, you always want to have them in line with the thrust.

Example:

In that case there are 4 nukes pushing through two pairs of 2.5m docking ports... and it works fine.. I wouldn't recommend pushing through the 1.25m connection though (that was just a rendevous, with the ship on the right action as a station/depot.

Pulling works even better...

But you can get away with side mounted docking ports provided the acceleration is low, and the docked thing is small.

The lander on the right side here (with space for a kerbal)... its docking connection probably can't withstand much acceleration at all (not to mention the center of mass issue)... while the probe lander on the left is small enough that the .625m docking connection can handle a decent acceleration.

Although that is a station.. and pushing through two pairs of 0.625m docking ports before it even gets to the spaceplane (the battery and 2 pairs o 1x6 panels was an add-on... and then the FL-T200 tank with a 1.25m docking port and 2 side 0.625m docking ports was another add-on)... those 0.625m connections can't handle that even though they are inline.

[Snark]

Stuff slung on the sides will have problems.  If you're pushing something in front of you, then within limits you should be okay as long as all mass distribution is perfectly symmetrical around the axis of thrust.

There can still be a tendency to flop around, which is determined by a few factors:

• If the thing you're pushing is heavy and has a CoM that's a long way forward of the docking port by which you're pushing it:  that makes flopping worse.
• The wider your docking port, the stiffer the connection.  The "junior" sized ports are very weak and floppy, you'll have problems with them unless the thing you're pushing is a very small ship.
• The higher the acceleration, the worse the flopping gets.  You can have a design that works just fine when accelerating at low thrust, but runs into diffulty at higher thrust.

If you're amenable to using mods:  Kerbal Attachment System has some nice deployable struts you can use to stiffen docking connections.  Basically, it's a simple, small, fairly low-tech part that's a "strut socket" which you can attach to your ships at design time.  You'd use them like this:  1. Put sockets on both ships at design time in the VAB.  2. Dock them.  3. Send out a kerbal on EVA and go over to where they're docked.  Right click on a socket on ship A, choose "Link".  Right click on a socket on ship B, choose "Link".  You now have a strut connecting the two ships, besides the docking port itself.  A few such struts, strategically placed, can do wonders.  To unlink them, just right-click and choose "Unlink".  I like it because not only does it solve the floppy-docking-port problem, but it also gives my kerbals something interesting to do on EVA.