Docking with Heavy/Massive Payloads

[wolfman29]

Hi everyone.

I recently built a pretty awesome space station (managed to lift all ~400t of it to orbit in one go, adding a few extra parts with KIS now that it's up there), and sometimes, I want to be able to dock large masses to my space station (for example - I want to dock my 500t fuel depot to my space station to refuel it). I have a few low-mass tugs for use on small things (gets things decently into place), and I recently put into orbit a pair of tugs with claws on their sides for use in pairing up and grabbing more massive payloads. However, I tried using this pair on my 500t fuel depot, and, lo and behold, my acceleration is near zero. There's no feasible way I would be able to dock this thing - I can't close the distance faster than they would drift apart due to orbital distances (right now, my game is paused with my space station ~500m away).

Here is the craft file for my half-tugs (they are identical):

https://dl.dropboxusercontent.com/u/50345936/Half-Tug.craft

Now, I've tried using Vernor engines, but they tend to increase part count to the point where the lag fest is ridiculous, and they also don't seem to be particularly effective.

Maybe this is an intractable problem - are there any good ways to make massive payloads maneuverable? The only other option I can think of is using tugs with 'Thuds' on them and use action groups to control the thrust (or something like that) which seems ridiculously difficult.

Thoughts? I'd like to be able to dock my massive payloads, otherwise what's the purpose of being able to lift 1500t into orbit!

Edited August 22, 2015 by wolfman29

[Alshain]

Without increasing part count? Nope. It's a matter of thrust to weight ratio, if you don't have enough TWR, you can't move it. Unfortunately our options on RCS are limited to 2 in stock so you have to double up on your RCS jets for bigger payloads. You can rotate it with more reaction wheel, that also is more parts.

The only other option is to dock it with a main engine. Yes, this can be done, but maneuvering is a bit annoying since you can only go one direction without turning (no translation).

If you know the general shape and size of your payload you can build the tug with arms that allow better RCS placement. Ideally you want some control at the center of mass.

Edited August 21, 2015 by Alshain

[wolfman29]

Hm. That's a shame. While I don't play exactly stock (I have KIS/KAS, Mechjeb, KER, etc.), I don't like playing with parts packs, nor do I like editing the parts to be better than they should be! How do you go about docking with a main engine? Never seen that done before, and I would have no idea how to do it. I guess you could use a main engine in the front and the back (maybe using big radials?) and use those for front/back?

Really, the biggest issue is translation. I usually don't have a huge issue with rotation.

EDIT: What if I made some custom action groups and just placed 6 main engines around a tug, having each action group shut down the others? Think that would help with translation?

Edited August 21, 2015 by wolfman29

[Belphegor]

maybe send 10 tugs with 'reliant' engines, dock 2 to every side of the station, one in front of the center of mass and one on the other side except for the back where your main engine is. this way you can rotate and move just like you would with RCS, only you won't be able to control it with WASD keys so you will have to turn engines on/off manually and use shift to increase thrust. if you give each one a small fuel tank you can even launch them all on the same rocket and just transfer fuel to them from the station once they are attached to it

quite the effort for an otherwise simple docking procedure but giving the mass of your station i can't think of anything else that wouldn't involve many many rcs ports

edit: you might be able to reduce tug count to 2-4 with the robotics mod since you can place the engines on a rotatable mount

Edited August 21, 2015 by Belphegor

[TheBedla]

Well you can reduce the requirements for translation with rotation of the passive ship (your station). When you get to a close rendezvous distance (say <400 m, the closer the better of course), simply switch vessels to the station, select the docking ship as target, select an appropriate docking port and select "control from here" and rotate so that it faces the target. If you have powerful fore and aft thrusters (RCS or otherwise), you can dock with barely and translation at all. If necessary, you can repeat the process once you get closer.

If you decide to go with Belphegor's suggestion, I think you might use the "actions on the fly" mod. Haven't used it, but it should allow you to assign action groups after docking with the tugs, and if you select your action group smartly, you should be able to translate pretty smoothly using your numpad. The functionality would be different than RCS, as the action groups would toggle the individual thrusters, but it should get the job done.

[Jouni]

KSP is a game about waiting. Docking large ships just requires a bit more waiting than usual.

I've had good results with one RCS thruster block for every 15-25 tonnes of mass. With half of the blocks firing at once, the acceleration is around 0.02-0.03 m/s^2, and you get decent changes in velocity with 5-10-second burns.

[Sharpy]

If you're playing with KIS/KAS, put these vernors on the depot and suffer the framerate drop, then remove them once docking is complete.

[Pecan]

Rendezvous and close to within 100m using main engines. After that you don't want more than 0.1 - 0.2 m/s relative velocity anyway.

It just takes time - but if you're insisting on 400/500t monsters, you've got to live with it.

[Mystique]

Does this count as a massive ships for docking?

I found out that I forgot to put RCS thrusters only when I started docking. It was even more interesting taking into account just a regular docking port (not Sr) that was buried deep enough withing that rocket monstrosity, so any deviation from course would have caused a collision. Also it was really slow to turn, reaction wheels were not actually enough. Main idea was to get around 200-300 m, kill all relative speed with engines, switch to target ship, rotate it towards piloted ship (and kill all rotation), then point really precise at docking port and gently push with main engines. Took just 1 try, but those were quite intense moments))

For all further docking of heavy things I designed small remotely controlled tugboat (tank+engine+RCS tank+whole bunch of RCS thrusters), it's really nice thing, especially taking into account that it's located and the end of object for docking which makes rotation with RCS really efficient.

[Nich]

I use a single orange tank with a nuke to be my transfer vehicle. It will take a couple trips or you can do something similar with a single 3.5m tank. Makes life a lot easier

[DancesWithSquirrels]

Moar tugs - a probe core, batteries, lots of RCS/vernors, and a Klaw to put them where needed.

[wolfman29]

Hmm. So thanks for all the replies - wasn't expecting this many! I think the solutions that may work the best for me are either attaching MANY Vernors to the payload and then removing them (my space station, when I initially brought it up, required 120 Vernors to keep it stable during flight, so I'm no stranger to removing mass amounts of Vernors) or more tugs. I'm just not that good at docking that I would feel comfortable docking with main engines only. Mystique - I currently have 4 tugs that I have been using, but they're not enough for the large loads I intend to move.

As for using RCS thrusters as before, as per Jouni - that may work. I just need a way to rendezvous with the depot first before I can start pulling it! I'll keep you all updated with what I figure out.

[Sharpy]

Once past the atmosphere you really don't need that many vernors. The standard set of two per direction (for a total of 12) is enough for quite large payloads. If you want more oomph, double that, 4 per direction (for a total of 24) and it will be a plenty for anything you could have put in orbit yourself. I was turning a class E asteroid using 4 vernors per direction, and while slowly, it worked.

[FancyMouse]

My experience is - attach Vernors for rotation, and use engine to move. Avoid translation if possible. Then be more patient when docking - it works for me at least

[DancesWithSquirrels]

I use Vernors like 4 or 5-way RCS thruster blocks - I put cubic struts on the side of my ship along the C0G, with Vernors on the outer 4 or 5 faces. Four of the 4-way Vernor blocks and I have three axis rotation and translation.

[wolfman29]

Unfortunately, I'm not exactly sure how I go about docking without translation. It's almost impossible to get exactly 0 m/s rel. vel., and the moment you activate the vernors, your rel. vel. changes, usually by about 0.1 m/s or so.

Sharpy - the issue is not necessarily controlling/turning, but translation.

As for solutions - I found that I guess I just need to be better at docking! I managed to dock a 400t fuel payload to my space station this morning (took about an hour) using a rendezvous rocket (a single mainsail attached to some separate fuel) and one of my tugs with 8 RCS blocks. The issue was that I usually use mechjeb's RCS Balancer with Smart Translation, which I guess because the center of mass was so far from the RCS blocks, had a really hard time doing any translation. So I turned it off and did it entirely by hand and it got there. Of course, the rendezvous rocket put in most of the work, getting about 10 meters away from the docking port and killing relative velocity. From there, I just had to meet up with it with my tug and it wasn't too bad!

[FancyMouse]

Unfortunately, I'm not exactly sure how I go about docking without translation.

Try learn docking without RCS at all not on a ship of your size of course, just a normal ship, and you'll get the idea.

[Norcalplanner]

A few observations:

1. MechJeb is awesome, particularly SmartASS. It makes cancelling relative velocity with the target really easy. Don't use the automatic docking or the RCS balancer thing - it will use way too much monoprop. Have each craft target the other craft's docking port, "control from here", and then have each craft point at the other using SmartASS. Use a tiny burst from the main engine (thrust limit the engine before trying this if you need to) to close at 0.3 m/s or so. As long as you have good enough reaction wheels, you can still be a little bit off and dock. Each ship will pirouette around the other, and should still connect. If you're coming straight in, turn SmartASS off on the target craft first at 10m out, then turn it off on the docking craft when it's 2m out and let the docking port magnets take over. If you keep SmartASS active on both craft with a straight in approach (instead of the pirouette) then your docking ports will likely end up doing a dance that can break things.

2. For docking really big craft, each should have its own RCS blocks and monoprop supply. Play with the fuel levels in the tanks in the VAB to approximate how much fuel will be left at the time of docking, then put two rings of 4 RCS blocks near the center of the craft, on either side of the CoM with that fuel load. I've used this setup to dock very large craft together without any problems. Having a tug with off-center RCS thrust is a recipe for a slow-motion nightmare with larger craft or station modules. I'm also not a big fan of the claw - I mainly use docking ports on my tugs, combined with KJR for a strong connection.

3. Space may be hard, but docking does get easier over time. Keep at it and you'll find things start to become second nature.

[Sharpy]

I use Vernors like 4 or 5-way RCS thruster blocks - I put cubic struts on the side of my ship along the C0G, with Vernors on the outer 4 or 5 faces. Four of the 4-way Vernor blocks and I have three axis rotation and translation.

Instead of placing them like thruster blocks, place them in 4x symmetry around two ends of your vessel for sideways translation + pitch/yaw, plus either single or pairs of them directed prograde/retrograde. Roll through reaction wheels.

As for RCS-less docking - stop a way from the port, facing its face - roughly straight ahead. As prograde direction is "pulled towards" direction of your burn, pull it to overlap the target marker, then aim your nose (center of navball) at that marker when about to dock.

But it's cumbersome, especially without retrograde RCS that allows you to slow down if you gained too much forward speed (and push the prograde marker away instead of pulling it to the center).

[Pecan]

...I found that I guess I just need to be better at docking! ... it wasn't too bad!

And now you are better :-)

Many congratulations, before your post becomes lost in amongst the rest.