Is a wobble-free vessel connected by docking ports possible?

[Smurfy987]

I've about had it with wobble on interplanetary vessels that have a lander. I just can't seem to get a wobble free setup.

I've tried multiple docking ports, but it's a nightmare to dock. This isn't fun - I don't want to have to spend 30 minutes getting absolutely perfect alignment.

I've tried the clamp-o-tron senior - much harder to dock than the regular clamp-o-tron, and it doesn't work that much better. Still crazy amounts of wobble.

At this point, I'm willing to try the most cheaty modded part available. I don't care, I just want the two vessels welded together as if they were made in the VAB that way.

This is completely realistic, real rocket scientists would design a solution for this if the need came up. Wobble is in no way required by engineering or physics - and it's undamped wobble, or at least poorly damped, which is unrealistic. Real docking ports have dampers in them to soak up relative motion.

[Sovek]

With Kerbal Attacthment System or Quantum Struts, yes.

[barrenwaste]

struts, think of them like interstellar duct tape. They fix everything.

[Armchair Rocket Scientist]

struts, think of them like interstellar duct tape. They fix everything.

The problem is you can't strut together two ships that you just docked in orbit, unless you're using one of the mods Sovek suggested.

OP, I don't know if Kerbal Joint reinforcement stiffens docking ports, but it might. Also, try downloading a docking alignment mod to help with multiple docking.

[Jouni]

It's kind of realistic that the stock docking ports are wobbly. After all, they are just flat rings, so using them to form rigid connections between heavy modules would just break them. A rigid connection would have to be deeper: maybe we should have heavy docking ports the size of FL-T100 and X200-8 fuel tanks.

There are some ways to cope with wobbling landers:

- Try to keep lander fuel tanks empty when possible.

- Disable the reaction wheels in the lander when it's docked.

- Accept that turning an interplanetary ship around may take a couple of minutes. Switch SAS off before starting the turn, keep the turning rate slow enough, and stop the turn by using manual SAS intermittently.

- If your transfer burns take so long that you have to use physics warp, consider adding more engines instead.

[MaverickSawyer]

KJR does help, but it does not eliminate the wobble entirely. More than 2 ports in a row, no matter what size, start to have wobble issues. The 2.5 meter ports are a little more forgiving, but they will wobble if you aren't careful.

[SSSPutnik]

Real docking ports don't usually have to hold whilst a ship is under thrust, or if they do, the mass is usually way lower.

There are mods, all the way up welding the craft into a single lump.

[Landge]

KAS is nice because it's radial clamping ports are reusable. But they can only be attached during EVA.

If you want to stay stock use multiple ports. This I did back in 0.20 used 1 Clamp-o-tron Sr and 4 Jr's.

1st time I made it to Eeloo.

It looks ungainly but it kept my vessel stable. I only used stock struts across the docking ports during the initial launch on Kerbin.

From one extreme to the other...

Here's a docking sequence of a mission in progress to send a rescue vessel with fuel to a stranded orbiter and lander on Moho.

Here a Kerbalnaut is attaching KAS struts to secure the tri-tank adapter to the vessel.

Links connected.

A tug (far left) has opened the tri-tank adapter so the 1st of 3 fuel tanks can dock with the rescue ship.

!st tank has docked to the center port and the tug closes in to dock with the tank.

The end clamp is docked perpendicular temporarily so the tug doesn't drift off while I launch and dock the other 2 tanks. The last tank is about to dock.

The rescue vessel with end cap in place.

5 separate launches were made. The rescue rover vehicle, the tug with the tri-tank adapter clamps, and 3 fuel tanks.

Edited February 19, 2014 by Landge

[problemecium]

In a word, multi-docking. If you don't like KAS, you can easily make a stable stock ship by using multiple docking ports... you just have to develop the skill of getting them all lined up at once

EDIT: Missed the part where you said you tried that. But I still recommend it, it's a worthy skill to have when you do have it.

[Geschosskopf]

I've about had it with wobble on interplanetary vessels that have a lander. I just can't seem to get a wobble free setup

....

At this point, I'm willing to try the most cheaty modded part available. I don't care, I just want the two vessels welded together as if they were made in the VAB that way...

I keep hearing people complain about wobbly docking ports and I keep not having the problem myself. I just use stock docking ports, no KAS struts, no quantum struts, no joint reinforcement mod, no multiple docking ports for the same joint, no nothing. Mostly I use Sr. ports but sometimes standard. Anyway, they always hold perfectly still for me except for 1 time I had bad wobbles. I determined the cause, removed it, and have had no further problems. Ever.

Docking ports do not have a motor in them that kicks in and makes your ship wobble for no reason. The reason you see wobbles at docking ports is because the whole ship is wobbling, and this motion is just amplified by the flexible docking port joint. So if you're getting wobbly ports, it's because you made a wobbly ship as a whole. If you don't build wobbly ships, you won't have wobbly docking ports, so don't need any of the "solutions" to the "problem".

Here's an example of a typical interplanetary ship I build as it is utterly refusing to wobble:

This ship is entirely stock. The main body of this ship is 3 sections held together with 2 joints of Sr. ports, with the lander on the nose on a standard port. You can see the main joints easily because this was in career mode before I'd unlocked large-diameter probe cores and batteries, so the big ports are sitting on 1.25m parts. As you can see, it's got a Mobile Lab in the middle so you can tell this is from 0.23. Here it's aerocapturing at Duna and despite all the joints and their dubious foundations, no wobbles whatsoever, either here or on the way there. In fact, once it got back to Kerbin and I offloaded the Kerbals and science, I lithobraked this ship and it didn't wobble then, either.

The above is pretty typical of my ships. I don't tend to make them any bigger than this. If I need more stuff, I just send multiple ships instead of trying to cram everything into 1 ginormous ship. Also note that it's a pusher, not a puller. I never build pullers because, even though KSP doesn't model radiant heat, I pretend it's there. So the pusher/puller issue is moot. Wobbles are caused by something else.

Now, here's the 1 and only time I've ever had wobble:

Here we have an SSTO seaplane docked by a standard port to a big, heavy transfer tug on the end of a long I-beam. The SSTO is flying backwards. Under certain conditions, this wobbled so much it snapped the docking port joint. But under other conditions, it didn't wobble at all.

I attribute my general lack of wobbliness to several factors that I take pains to maintain during ship constructions:

1. Not building anything insanely huge. The more weight you put behind docking ports, and the further away this weight is, the more torque it applies to the joint.
   
2. Maintaining strict control over balance and symmetry in the rocket; no off-center masses either in construction or from fuel use in flight.
   
3. Maintaining strict control over thrust balance; no off-center thrust ever.
   
4. Not using engine gimbals if at all possible. Engine gimbals, especially with high-thrust engines, will make ships wobble even if they don't have docking ports, because they have no fine control. So they slam back and forth against their stops, constantly over-correcting for the previous overcorrection, and setting up oscillations all through the ship.
   

The 1 wobbly ship I made violated some of these rules. Airplanes in general usually ain't balanced due to wheels only on 1 side, and it's worse when you hang big pontoons under them. By careful placement of other parts, I ALMOST balanced this one but not quite enough. Second, the masses on both sides of the docking port were relatively big and relatively far from the joint, so they applied a lot of torque. And this was made worse by the engine gimbals of the LV-Ns. Normally, LV-N gimbals are no big deal because they don't have much thrust even in groups of 8. However, in this case, when combined with the other problems, it definitely contributed to wobbliness and failure. I locked the gimbals and it reduced, but didn't quite eliminate, the wobbles. But while this was acceptable in space, it failed in aerobraking. So in the end, I scrapped the separate tug, built a stack decoupler and transfer stage on the back of the SSTO, and launched it with a vertical rocket.

So the bottom line is, if you've got wobbly docking ports, it's because you built an imbalanced ship and/or made it insanely big and/or let gimbals go nuts on you. IOW, your ship itself is inherently wobbly. Don't blame the docking ports, blame your overall design and construction methods. I don't mean to sound harsh with that but it is what it is. And seeking "solutions" to the "problem" in mods does nothing to cure the root cause.

[Jouni]

So the bottom line is, if you've got wobbly docking ports, it's because you built an imbalanced ship and/or made it insanely big and/or let gimbals go nuts on you. IOW, your ship itself is inherently wobbly. Don't blame the docking ports, blame your overall design and construction methods. I don't mean to sound harsh with that but it is what it is. And seeking "solutions" to the "problem" in mods does nothing to cure the root cause.

In my experience, the main cause of wobbly interplanetary ships is that SAS is quite stupid. You can easily make a reasonably-sized, well-balanced ship to wobble, even with its engines shut down, if you

1) start turning

2) with SAS on,

3) and you have a heavy lander

4) with its reaction wheels operational

5) docked far from the center of mass

6) using a standard docking port.

[Yasmy]

An option to consider is to make your drive section attach to the front and pull the rest of the vehicle.

Put your engines out on arms so they don't roast the payload you drag behind.

Payloads dragged behind are ideally self-stabilizing. Payloads pushed from behind are ideally unstable to wobbling.

[Rodyle]

An option to consider is to make your drive section attach to the front and pull the rest of the vehicle.

Put your engines out on arms so they don't roast the payload you drag behind.

Payloads dragged behind are ideally self-stabilizing. Payloads pushed from behind are ideally unstable to wobbling.

This, basically. For anything large, pull, don't push.

[ComradeGoat]

You've said you don't like multi docking, but there are ways to make it easier. I have a spaceplane cradle which I use multi docking to make a single rigid vehicle. It has one clamp o tron and one JR, with a pairing on my planes (got the idea from someone else on here). To dock, I align both vehicles to point precisely north and then just bring the plane in under the carrier. As the ports align, the magnets take over. Pointing them both north before starting is the key: it makes sure the docking ports will be aligned for the multi dock, and using two different sizes means the wrong ports won't be attracted to each other.

Good luck!

[Kasuha]

Pull instead of pushing. And make sure you have enough torque parts distributed evenly around the ship. It will still wobble but it won't be a problem.

[Kruleworld]

Maybe we need some kind of hard Lock to use for thrust type attachments?

Something like a small scale of the Enterprise D's reattachable saucer section.

[sjwt]

Thanks to an update in unity we are getting a massive improvement in how joints can be setup in KSP, see this post for more details..

[Red Iron Crown]

Try to keep lander fuel tanks empty when possible.

I hate doing this. It's wasteful of delta-V to cart along extra tankage.

Those recommending pull instead of push have the right idea, just remember to disable the gimbals on the engines if they are so equipped.

[Mystique]

I usually use 2-4 KAS struts to make my station or huge crafts deadly stable, but yesterday there was a strange glitch - after loading of small refueling station (using Davon supply mod; really very simple station - just fuel tanks and bunch of docking ports) 4 struts that were attached to stock 6-port hub were just torn off and became debris O_O Even though station itself wasn't moving/wobbling even a bit - I was really precise even when it was assembled. So not even KAS is totally reliable, even though it worked well for my 150-300 ton interplanetary missions with biofuel station onboard.

[Jouni]

I hate doing this. It's wasteful of delta-V to cart along extra tankage.

I didn't recommend to launch the ship with lander fuel tanks empty. What I meant is that a few minutes into the mission, you have most likely burned more fuel than fits into the lander, so you should transfer fuel from the lander to the main fuel tanks.

[Red Iron Crown]

I didn't recommend to launch the ship with lander fuel tanks empty. What I meant is that a few minutes into the mission, you have most likely burned more fuel than fits into the lander, so you should transfer fuel from the lander to the main fuel tanks.

You mean after your interplanetary transfer burn? Maybe I should try this.

[Kryxal]

If they're bother centerline mass, during should work well enough, and keep the throttle down before that maybe?

[Geschosskopf]

In my experience, the main cause of wobbly interplanetary ships is that SAS is quite stupid. You can easily make a reasonably-sized, well-balanced ship to wobble, even with its engines shut down, if you

1) start turning

2) with SAS on,

3) and you have a heavy lander

4) with its reaction wheels operational

5) docked far from the center of mass

6) using a standard docking port.

A ship flexing during reorientation is a different issue from wobbling under forward thrust. Wobbling under thrust is a result of the ship as a whole being dynamically unstable due to imbalances. Flexing during reorientation is the result of insufficient strength for the lateral forces and moments applied. A ship that flexes when turning may be wobble-free under thrust, although if it's going to wobble anyway, the flexibility of its structure may amplify the wobble in the same way that docking ports can. But both issues have the same root cause: design and/or construction deficiencies.

For instance, a "heavy lander on a standard docking port". The term "heavy" is subjective and relative so we have to define it. Let's assume here it's built around a 2.5m capsule. If that's true, then yes, it will flex if sitting on a standard 1.25m docking port, especially if the structure on the other side of the port is also 2.5m. But this should be an obvious outcome. If you have a cylinder of constant diameter except for a narrow neck somewhere in its length, that narrow place is a stress concentration so is where it's going to bend. Avoid this by using 2.5m docking ports so the whole thing is the same diameter.

In general, the resistance of docking ports to flexing during reorientation is scaled to the weights and torques of parts of the same diameter. Sure, you can overload this by making huge assemblies, but as long as you keep things within reason, you'll have little or no torque.

[DerekL1963]

It's kind of realistic that the stock docking ports are wobbly. After all, they are just flat rings, so using them to form rigid connections between heavy modules would just break them. A rigid connection would have to be deeper

You do know the Saturn V was joined with nothing more than essentially flat rings? The ISS too. Rigid connections don't need to be deep, they just need to be properly designed. (Or in the case of KSP, properly simulated.)

[Anton P. Nym]

I didn't recommend to launch the ship with lander fuel tanks empty. What I meant is that a few minutes into the mission, you have most likely burned more fuel than fits into the lander, so you should transfer fuel from the lander to the main fuel tanks.

These days I tend to treat the lander as an external tank for the transfer stage, and then replenish it after attaining orbit; greatly saves wear and tear on the docking ports (and my nerves).

-- Steve