Senior docking port or quadro-port?

[Tokay Gris]

I am currently building a huge transfer vessel with high TWR (last count: 28 nuclear engines). It gets into orbit with a little trick (a turnaround at apogee).

(Delta-V is not so bad. Just short of 7000 m/s without payload)

So, but how do I attach the payload?

Experiences in the past showed that a senior docking port, especially with substantial payload weight, got me pretty frightening wobbling and oszillations.

A test with six senior docking ports arranged in a hexagonal fashion an theoretically identical ports on the payload showed that it is so far impossible to dock all six. Usually just one docks, the rest are a bit offset.

Another test with three similar spaced docking ports also did not succeed. (It seems the diameter of a hitchhiker capsule is different from a fuel tank. Just by a tiny bit. But in that test, the three ports didn't even attach. Not even one.)

So, what I haven't tested yet is a tricoupler or a quadrocoupler (the actual building parts in the VAB).

Is one of those better than a senior docking port? Or will I get the same wobbling?

Hm... Ok. It seems I have to actually test that.

But maybe somebody knows beforehand....

EDIT: Just to show this monster:

Edited December 11, 2013 by Tokay Gris

[Taki117]

Oh jeez, that is a monster. You would probably actually get MORE wobble out of a tri-or quad-coupled connection since only 1 of the ports actually docks (The others just appear to dock) what you could do to reduce wobble is to make a puller tug (Where the cargo is below the center of thrust) which will reduce wobble.

[JimmyAgent007]

I did the tri-port method, before we had the quad, and it seemed to work rather well, when one docked where it was supposed too the others did as well. I was then able to push the whole station that had 4 large fuel tanks tri docked to the center station, from kerbin to the mun. It bent back while it was going on but made it there intact, 4 ports would be better, but any large construction with that kind of docking would have a lot of parts. so count up the total parts and if thats lower than what your computer can handle then use the quad or tri dock. but if you are docking a bunch of separate quads at once then you will prolly run into the same issues but havent tried that bit just yet.

i can confirm that all the ports did indeed dock, none were left just looking like they did

[Tokay Gris]

Oh jeez, that is a monster. You would probably actually get MORE wobble out of a tri-or quad-coupled connection since only 1 of the ports actually docks (The others just appear to dock)

No, actually I know that this works. If perfectly aligned all ports do dock.

what you could do to reduce wobble is to make a puller tug (Where the cargo is below the center of thrust) which will reduce wobble.

I tried that. And still am, but that is a totally different design. And since the engines need to be on the outside, it increases the part count immensely.

[phunk]

6 in a hex pattern is not impossible to dock. It requires more precision than one, but it's not impossible (as long as they are in the exact same pattern on both ships).

My last big ship was held together with just 3 large rings between each section, and that held together just fine through Jool aerobraking. So 6 might be overkill.

[Stargate525]

Well, for me, it comes down to part count. A single large port is two parts. A quad port is ten.

However, it does let you more accurately control rotation, if that's important.

[Turvok]

Also read somewhere in here on the forums that you can insert a small Docking port inside the larger ones for a lot more strength. So that might also be a viable option. Just remember if you try that to map the ports to keys otherwise you can't right click on the inside ones to unlock them.

[Tokay Gris]

6 in a hex pattern is not impossible to dock. It requires more precision than one, but it's not impossible (as long as they are in the exact same pattern on both ships).

I am trying that right now. It should be the exact same pattern. But I will know afterwards.

If it doesn't work, I just end up with another bunch of fuel tanks in orbit. Which is not so bad....

[RadHazard]

I know this isn't related to your question, but what's the TWR of that monster (with payload attached)? If it approaches or exceeds 1, you'd be better off using LV-909s or some other kind of engine. LV-Ns only provide superior dV overall when you have a TWR somewhere less than one, due to how low their natural (i.e. without payload) TWR is. You can use the tool I have in my sig (or any in-game tool that can tell you your dV, for that matter) to see this for yourself.

As for multi-port docking, I have seen Whackjob dock several monsters with 6 Sr. ports in a hex configuration, so it is possible to do. I've not done it myself, so I can't say how difficult it would be.

EDIT: I actually ran the numbers for what I guess your craft is (7 X200-32 tanks, 28 LV-Ns, a few extra tons to compensate for monoprop/batteries/docking ports/etc.), and it actually looks like it'd have a TWR of 0.4 with a 200 ton payload attached. This gets it 2900 dV, compared to 1600 dV with the LV-Ns replaced with LV-909s, so it looks good to me. Carry on.

Edited December 11, 2013 by RadHazard

[Kasuha]

Tri- or quadcoupler can only replace one Sr. docking port, but it is a nightmare to dock right. So I don't understand what is the problem. If Sr. docking port doesn't provide enough structural integrity, strut it to whatever it lies on. Because the docking itself is very strong, the thing which usually fails is the other side of the port.

[Tokay Gris]

I know this isn't related to your question, but what's the TWR of that monster (with payload attached)?

Without payload it is around 0.7. Or something like that.

If it approaches or exceeds 1, you'd be better off using LV-909s or some other kind of engine.

It is possible that I could get more Delta-V out of it if I used less engines with higher TWR and lower ISP, but I am not sure.

EDIT: I actually ran the numbers for what I guess your craft is (7 X200-32 tanks, 28 LV-Ns, a few extra tons to compensate for monoprop/batteries/docking ports/etc.), and it actually looks like it'd have a TWR of 0.4 with a 200 ton payload attached. This gets it 2900 dV, compared to 1600 dV with the LV-Ns replaced with LV-909s, so it looks good to me. Carry on.

That sound like a lot. But is probably right.

I just noticed that my docking ports were not exactly aligned, so I build an MK2 of that beast. (Was a structural problem)

Will tell you the actual numbers once I actually have a payload attached. Which I am doing right now.

[thereaverofdarkness]

I would recommend the quantum struts addon. It'll let you strut the ship after you have docked.

[Decent Weasel]

I'm not sure if this would help, but after terrifying wobbling trying to ship an SSTO to Laythe, and a horrible bugsplosion using Quantum Struts, I experimented with a system that towed the payload behind the port, with the engines arranged radially. It worked quite well for me and I was just using a standard clamp-o-tron.