How much is too much to dock?

[mackadelik]

I've had an interesting experience trying to dock in space. 3 out of 4 of my modules went together with almost no anxiety or difficulty (2 on manual, 1 on Mech Jeb). The 4th part (identical to the third) has been a major pain! Its two long orange fuel tanks with docks on each side. The first one went in without a hitch (and a little help from MechJeb). The 2nd one I managed to spend over 2 hours trying to fit it into place. So my question is how long can an object be that can still easily dock with a station? I'm guessing two full orange fuel tanks stacked on top of each other vertically is what my issue is. Its like the tug just isn't able to accurately manuever in small increments. MechJeb is seriously having a problem with is one lol. I'm also guessing that dock on the side of a cylinder shaped structure is harder than a single point extended below a module... I'll have to play around with it. Maybe putting some actual RCS modules on the tanks themselves instead of up high on the Tug will help with stable burns... Maybe emptying those tanks too. Still amazed I landed the first pair of orange tanks on the first try!

Sorry for no pics while I'm at work. I could do a really crummy drawing upon request though haha.

Edited May 21, 2015 by mackadelik
Answered

[Slam_Jones]

What I usually end up doing (which has saved me a lot of headaches!) is to equip every module going up with a probe core, a solar panel or two (enough so it can keep electricity up), and some RCS for positioning.

This way, each module is basically it's own self-contained ship. It makes the whole process a lot easier IMO, as add, removing, or even re-arranging modules can be done without an addition 'tug' ship.

I personally haven't used MechJeb; I can't help you out with that part of it. I know that the Docking Port Alignment Indicator mod can also help get everything pointed the right way, though personally I learned to do it by hand and have stuck with it. If it's not your style, don't worry - to each their own.

Hope this helps!

Edited May 21, 2015 by Slam_Jones

[mackadelik]

Crummy drawings included : p

http://imgur.com/PJuypCo,XTKs047

http://imgur.com/PJuypCo,XTKs047#1

- - - Updated - - -

Thanks Slam! I might try this. I might also add tanks empty or try adding one tank that can dock with another tank so I still keep up my design... Although I think your suggestion will just save me the most headaches. After spending so much time trying to dock I might be able to have more luck by just taking a break and trying again tonight.

[justidutch]

The longest ship(s) I have docked to a station was (back in 0.90) a ship that was basically 8 of those biggest Kerbodyne gas tanks in a line.

I had a big ring station up in orbit that I stuck it to.

I sent four of those ships up, because I had that much fuel sitting up there of sending all the ring pieces up. The fourth one took probably an hour because I didn't get it exactly right initially, so I bumped into the piece I was trying to dock to. Although the bump was very slight (less than 0.2 m/s), it made everything start moving and wobbling, so the targeted dock was now moving. Even time-warping didn't really stop the moving and wobbling. But I finally got it, and decided that was enough. A 32-tank long section though, I am proud of that one!

I did have a lot of RCS on the ship. Both to get things pointed in the right direction, but even more importantly for lateral translations (I, J, K, L keys). Moving the ship up/down and left/right. Because the ship is so big and heavy, I was using those Vernor engines as the RCS. I have never used MechJeb, being of the mind that if I can't fly it myself, what's the point. But that is not a criticism at all, just me. So, I have no idea how well MJ can do those lateral translations and exact docking and stuff.

Edited May 21, 2015 by justidutch

[Kryten 2X4B 523P]

Hold a broom handle out at arm's length and try to turn on a lightswitch - thats basically what youre dealing with. All the control is at one end, and the tinest change at the control end brings about a large change at the far end - very difficult to exert any fine control over.

I generally do as Slam suggests - make each large component either a self contained vessel, or at the very least equip it with RCS thrusters so my tug can take over that duty.

[mackadelik]

Wow Justidutch, awesome station! I'll get back to work on that and play around with RCS placement and a little bit of my own manual manuevering. I wonder if two rings of RCS controlls will help (one at the top and one at the bottom).

Thanks Kryten! The broom handle explanation is perfect!

Edited May 21, 2015 by mackadelik

[justidutch]

For RCS, I'll make sure I have thrust in the X and Y axis on both ends of the ship. This allows you to get pointed in the right direction, and also for the lateral translations. But also you'll want the Z axis for fine-tuned adjustments to go forwards and backwards. I usually place those as close to the CoM as I can get them, though that's not strictly a necessity.

Good luck!

[Slam_Jones]

Wow Justidutch, awesome station! I'll get back to work on that and play around with RCS placement and a little bit of my own manual manuevering. I wonder if two rings of RCS controlls will help (one at the top and one at the bottom).

Thanks Kryten! The broom handle explanation is perfect!

As I understand it, you want your SAS right on your CoM, while you want RCS to be equally distant from the CoM. Basically one ring (4-symmetry) at the top, and one at the bottom. You want the CoM to be directly between them for the best control.

[mackadelik]

I must have really lucked out with the other modules lol. The first two are smaller and more manageable. The last 2 fuel arms are much longer and awkward. The first one I nailed it in approach out of luck and MechJeb docked it for me. Funny thing is that I have been adding more RCS and making larger tugs to retry docking, but I think I've actually been moving my RCS and SAS modules further away from the CoM, lol. I bet paying attention to the CoM with the RCS will really help with my docking issues. Maybe it will even help out MechJeb, haha.

[BigFatStupidHead]

It sounds like you might really benefit from RCS Build Aid. This will take the guesswork out of where you should be planting your RCS thrusters to avoid unwanted pitch and roll.

[moronwrocket]

One option, if you already have it in orbit, is to send up a small "sacrificial" RCS tug. Probe core, 2x docking ports, 4x RCS thrusters, a little monoprop (no LFO or LFO engine needed). It would stay in between your tanks and your station permanently, or if you're feeling ambitious you could get aligned and start your approach, detach the probe and pull it out of the way before the dock finishes. Be sure to strut between the two docking ports for stiffness if you plan to leave it.

Another would be to try grabbing with 2 or 4 KLAW tugs with RCS+mono, but that might be impossible to get accurate enough to control.

Third option would be to build a new tug, but give it long arms on struts that put the RCS thrusters closer to the CoM of the combined vessel. It still might have problems due to docking ports not being "stiff" enough.

Fourth option would be to install KAS/KIS and add RCS blocks to the tanks - you could pull them off your tug or somewhere else on your station.

If you're going to redesign, I think 1x4 ring of RCS right at the far end of your station part should be "good enough" to get you docked, the near end doesn't need any because the tug can provide it. If you have RCS build aid installed, you could mockup your tug + station part in the VAB and zero out the rotational torques during pitch/yaw by accurate RCS placement.

I did it with an RCS ring on one end of an oblong object (it only has radial mounting on one end), it was 'fun' due to the station blocking power for the panels so I had to bring in an RCS drone anyways, but now I know better ways to handle it. If I was to redo this, I would probably build a cage mounted with decouplers around the Shipyard using RCS mounted out there, like a flower, and strut it stiff.

For last few meters, disable your SAS, it can prevent a dock because it keeps the docking magnets from pulling your lighter part in.

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[mackadelik]

Thanks for the tips and the pics. I'll have to try these when I get home!

[mackadelik]

And thanks to everyone's help and some Center of Mass lessons on RCS placement... I finally have my simple yet very functional space station completed! Thanks again everyone! On a side note - Auto Dock with MechJeb works crazy fast when I have my RCS thrusters properly placed!

And Docked!

Top View of Space Station I

I think I'm about as happy as those Kerbinaughts feel!

Edited May 22, 2015 by mackadelik