Space Tug and station wobble effect

[LoganMalone]

How do all of you handle transporting large fuel tanks, space stations, etc. with a space tug without generating excessive wobble even at under 5 meters per second speed?

I'm in the process of creating my first space station which I'm going to place in orbit around the Mun. I'm using a space tug to assemble the large parts of the station and transport to the Mun. I continue to experience excessive wobble between my space tug and the station parts (Ex. Rockomax Jumbo-64 Fuel Tank)where their Clamp-O-Tron Docking Ports meet. In addition to this, if I attempt to haul a Rokomax Jumbo-64 Fuel Tank from Kerbin to the Mun with my space tug, the mass of the fuel tank makes my Holman Transfer to the Mun fail where two begin to wobble and it causes the tug to loose it's trajectory during the transfer burn. I'm role-playing and am trying to all the manual work in near Kerbin orbit rather than just assemble the entire station in the VAB and sent up on a massive rocket. My space tug and the fuel tank is below.

Any advice would be greatly appreciated. Thank you.

http://i1310.photobucket.com/albums/s652/LoganMalone/screenshot2_zps6043e5ea.png' alt='screenshot2_zps6043e5ea.png'>

Edited September 9, 2014 by LoganMalone
Change Prefix to Answered

[RexKramer]

I like to use the large docking collar instead of the regular one for that size tank- makes it much less wobbly.

Also, if you need more tanks, build sideways (outward) instead of just making the ship longer. In general, longer ships are more wobbly than shorter ones of the same mass.

The configuration you had pictured actually looks reasonable- I would start by trying the larger docking collar (Clamp o Tron Sr.). Should hold a single Jumbo 64 no problem.

[RexKramer]

I just noticed your Orange tank has the regular Clamp o Trons all over it- so I'm guessing your current space program is geared around a single size Clamp O Tron. It may be a bit of work reconfiguring your space program, but seriously it's worth the effort switching to the Clamp O Tron Sr.s for the large diameter tanks.

[Cerberus738]

Pull your loads. Make sure to disable gimbal on the engines. They vector the opposite way you want when placed in front of the load. With that said make sure you have a way to turn your ship (RCS, torque...) since you can't use the engines to turn. Also be sure the load is well clear of the engine exhaust. They well destroy your ship and anything placed to close to the engine will block it and create 0 thrust.

Another thing you can do is place landing legs around your docking port so that when you extend them they stabilize the load you want to move.

Quantum struts mod allows you to add struts outside of the VAB with an EVA Kerbal.

Another suggestion is to assemble at your destination. Less mass means less wobble

[M4ck]

Quantum Struts is great for stabilizing docked ships, but it seems to be MIA, at least for me. I cannot find it anywhere. Is it still worked on? Links to the forum posts I had saved somewhere do not work anymore.

And for your tug: use larger docking port, or multiple smaller ones(like placed on a tricoupler). Also, for space tugs of this size, you may be better off using nuclear engines - that is, if you already have them.

Edited September 8, 2014 by M4ck

[Pecan]

Yes, the three things that have been suggested already:

1. Large docking port
   
2. Pull, don't push
   
3. Moar struts, possibly applied in-flight with KAS/Quantum
   

This design doesn't use more struts but can pull a whole station at once (final chapter of the tutorial in my signature):

Tractor Light Towing a Complete Station

[Geschosskopf]

How do all of you handle transporting large fuel tanks, space stations, etc. with a space tug without generating wobble

Several things affect the wobbliness of docking ports:

1. Diameter of port compared to diameter of station/ship module. The latter includes radial tanks, BTW, which increase the effective diameter of the module. In general, if the port is the same diameter as the module, it won't wobble unless the modules on either side (including any extra modules attached on the ends) is too massive.

2. As mentioned above, the amount of mass on each side of the port. Even with ports the same diameter as the module, the strength of the joint does have an upper limit.

3. Distance of docking port joint from CoM of assembly. Ships rotate about their CoM so the further the joint is from the CoM, the more torque applied to it.

4. Conflicting torque from reaction wheels in several modules. This usually is only a problem if the modules aren't all facing the same direction when docked. IOW, the modules are docked nose-to-nose instead of nose-to-tail or, even worse, perpendicular to each other. Here you can disable the torque of selected SAS units and command pods and leave just those oriented in the direction of travel. This is really only an issue, however, if all torque values are about equal. A 2.5m reaction wheel unit will overpower a 1.25m unit in most cases.

5. The presence of struts reinforcing the joint by connecting the modules to each other directly. Obviously, using struts will either eliminate wobble or reduce it to a manageable minimum. Here you have several options. If the modules only need rigidity en route to a destination but won't move together ever again once there, you can use normal struts installed in the VAB. Undocking the 1st time will break them, and thereafter you don't need them. Or you can use the Quantum Strut mod, which magically creates and breaks struts every time you dock and undock. Or, if that's too gamey for you, you can use KAS struts installed and removed by EVA Kerbals.

-------------------------------

Anyway, the ship in your pic is doomed to wobble because 1) you're using 1.25m ports on 2.5m modules, and those modules are relatively massive anyway. However, a 2.5m port at the joint will probably solve the problem for you. I've used many ships of that general configuration with 2.5m ports and haven't found the need for any strutting. For example:

This ship was made of 3 modules: The engines on the back, the orange tank in the middle, and the Mobile Lab and 4-way hub on the front. The 2 joints were simply 2.5m ports without any struts. The masses of the modules were within the capabilities of these ports, all the SAS modules (1 in each module) pointed the same way so there was no need to disable any of them, and there wasn't too much radial mass even with 4x LVNs.

Anyway, despite being a pusher, not a puller, and not having any struts reinforcing the docking ports, it had essentially zero wobble, was perfectly controllable both thrusting and rotating, and was strong enough to survive aerocapture at both Duna (shown here) and back at Kerbin.

So like I said, you're entire problem is using a 1.25m port to connect 2.5m modules. Swap it out and you'll be fine.

[Claw]

I know other people have said add struts. Realize that struts do help, but you also don't have to turn your station into a jungle of with struts everywhere. The above two stations are a good example. How you design is also important. Also, I've started building stations by using claw tugs. They can be finicky though, and sometimes turn your station inside out.

Here is a station about the same size, and the only struts in use are around the docking arms.

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And here's the bigger version, also note that the only struts are near the docking arms. There are some hidden struts on this one, but those are mainly to deal with closing branches of the part tree.

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Cheers,

~Claw

[tsotha]

I haven't tried it yet, but the docking tutorials say you can get more strength in a junction by docking more than one port at a time, i.e. if you want two orange tanks end-to-end you can use a triangle of C-Srs for maximum strength.

They also say it makes the actual docking operation quite a bit harder.

[Pecan]

...use a triangle of C-Srs for maximum strength.

That would be standard-sized docking ports on a tri-coupler. Trying to arrange a triangle of large ones will require a fair bit of structural construction, which is likely to introduce its own problems, as well as being very wide.

[Laie]

That would be standard-sized docking ports on a tri-coupler. Trying to arrange a triangle of large ones will require a fair bit of structural construction, which is likely to introduce its own problems, as well as being very wide.

Ports on a tri-coupler aren't much better than a single large port. I've experimented with that lately and the most important factor seems to be distance between ports.

This vesselwas just as stable as if it had been built in one piece.

[LoganMalone]

I really like the tugs you all designed. I'm honestly trying to not over-engineer things, however it's a nasty habit, LOL. All is well so far after I went back to the drawing board and re-engineered the space tug. No wobble is being produced as long as I pull the loads. Thanks for the suggestions and advice. I needed it badly. Below is my new design which is very functional and a bit over-engineered in regards to power, however it's working out well so far. Notice the four EAS-4 Strut Connectors holding down the Large Advanced S.A.S Module. Without the struts, the Clamp-O-Tron Sr. Docking Port will rip right off the end. I dont know if it's a game bug or a simple original design flaw on my part.

Edited September 9, 2014 by LoganMalone

[Geschosskopf]

No wobble is being produced as long as I pull the loads.

A word about tractors instead of pushers.... Understand that in the real world, radiant heat from the nearby rocket exhaust would blow up the central tanks. KSP's heat transfer mechanics, however, are even less realistic than its aerodynamics, so we can get away with tractors at least for the time being. Still, something to keep in the back of your head, in case this bit of realism is important to you. And who knows. Maybe one day KSP's heat transfer mechanism will get an overhaul and tractors will no longer be possible. In which case, being good at designing pusher tugs will keep the change from being painful.

Notice the four EAS-4 Strut Connectors holding down the Large Advanced S.A.S Module. Without the struts, the Clamp-O-Tron Sr. Docking Port will rip right off the end. I dont know if it's a game bug or a simple original design flaw on my part.

Probably has to do with leaving the gimbals unlocked on the Poodles. Something I forgot to mention previously... In general, gimbals aren't a good idea for multi-part ships. This is because they change the thrust axis to be at an angle to all the stack joints of the rocket. This causes both torque and shear.