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Space Tug...


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Ok so I have been wanting to build a pretty big space station around Kerbal. I would love to store some huge fuel tanks in space to have other craft later refuel for long interplanetary voyages. Since the Kerbals don't care about how much money I spend, I figure that's grand.

Now the problem is I would like to be able to use a sort of "Tug Boat" ship to move my modules around and link them up. I keep trying to build ships that will let me move laterally in space but so far all I do is rotate or roll. RCS's are giving me a fit. I suspect this is about a center of gravity issue and I don't see this problem going away.

If I were to get my "Tug Boat" balanced nicely, whenever I would connect to something in space I am quite sure that will screw up the COG up again and start this mess over.

Any suggestions? Are you aware of anyone or anything used to solve this issue?

Thanks and Launch Safe,

FlyMario

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for moving large fuel tanks in space, you'll need to have RCS thrusters on the payload itself as well, not just on the tug.

another thing to consider is having an (empty) command pod on your tug, so that you can use its rot power to steer better.

maybe you could post a screenshot of what you've tried so far and we'll take it from there.

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I'm currently working with a design to build a multi-part ship for interplanetary voyages. I've recently decided to go a different route on getting the fuel up there, but what I was working with was essentially a flying gas can; it was an OKTO probe with 4 PB-NUKs stuck radially on its sides attached to a Mk2 adapter, attached to an FL-R1 RCS Fuel Tank, attached to a Rockomax X200-32 Fuel Tank with Clamp-o-Trons at both ends (on the bottom of the fuel tank and one sitting on top of the probe). It had 12 RCS thrusters and that was it; four at the top, four at the bottom and four along the center of mass of the whole thing. A real simple design as a whole. Total mass was 22.66 tonnes, so you'd need a reasonably hefty rocket to get it into orbit but after that it could maneuver on RCS.

You might try something similar in this case; you need the RCS thrusters at both ends and along the center of mass to dock successfully. I tried the same design with just the center-line thrusters and the damn thing handled while a whale (no rotational torque, you see). Just put them on the ends and you haven't got good translational capabilities (the thing tries to turn one way or the other).

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Put some RCS radial thrusters on your payload to get the balance you need. You can always deactivate them later, if you don't want them to continue functioning.

The Bobcat FLY comes with RCS quads on extendable booms, though. You can move the quads forward and back as well as outward and inward, so you can move them into a position that surrounds your payload, so long as it's not overly large.

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Here's an idea, two tugs, one for the front and one for the back.

Both have RCS, fuel and engines (for easy forwards/backwards thrust, use an action group) and they can dock to each other when not working a payload.

You'd have to get into position and undock the one on the front so the payload can be docked though, but that shouldn't be too hard.

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Here's an idea, two tugs, one for the front and one for the back.

Both have RCS, fuel and engines (for easy forwards/backwards thrust, use an action group) and they can dock to each other when not working a payload.

You'd have to get into position and undock the one on the front so the payload can be docked though, but that shouldn't be too hard.

Or design the two tugs so that they have the option to dock to the sides of the payload at COM - then you have both ends free to plug it into something. :) Either way, getting them perfectly lined up along their mutual rotation axis would be difficult.

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I've been putting extra docking nodes on the sides of large station modules and using multiple tugs.

My tugs carry a lot of RCS, a little bit of fuel, and move with RCS or the little radial engines. Putting one on either side of a large payload and controlling from the docking point on that payload keeps things pretty balanced. The nice thing about the little RCS tugs is that they're pretty light if they're all RCS, and if you design a rocket to carry them you can deliver three or four to a station at a time.

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Or design the two tugs so that they have the option to dock to the sides of the payload at COM - then you have both ends free to plug it into something. :) Either way, getting them perfectly lined up along their mutual rotation axis would be difficult.

Put a single unextended communitron 16 to keep track of roll relative to each other.

I used space tugs to build the LKO Spacebase that featured in my Integrated Plan mission report/graphic fan fic. It is definitely hard to translate with the tug if your center of thrust is not in the same place as the total docked center of mass. Every time you thrust you set up a moment proportional to the distance between the center of mass and center of thrust (moment arm) which causes the whole thing to spin. If you don't want to relentlessly abuse the time acceleration to violate conservation of angular momentum my advice it to put at least one set of RCS quads about the center of mass of each docked module in question. My general advice for a Space Tug is to put an ASAS module on it, and also set up an action group to turn off RCS quads such that they are 'balanced' about the center of mass (in other words, when the tug is free flying, leave all of the thrusters open, when docked with another module turn off all the RCS thrusters except the ones in a ring about the center of mass). When docking make sure you're pointed in the direction opposite of the docking port you are going for then turn on the ASAS and use only RCS translation to complete the docking maneuver. It helps to pick a point of reference that doesn't precess as you orbit, otherwise you will be trying to track a moving target.

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I've had that very problem myself lately; still haven't come up with a good solution for it.

What really would be nice if the docking clamps had the 'green dots' every 45 degrees around the clamping side.

This would insure a consistent connection between two objects and allow an uniform looking structure.

(sorry about the 'green dots' thing, I dont really know a better way of describing it)

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My heavy tug consists of the following...

A command pod MkI-2 sat on top of an ASAS, a FL-R1 RCS Tank and a X200-32 Fuel Tank. On the nose is a tri-port docking adapter. From the main body it has 4 nacelles made from FL-T800 tanks each of which has an LV-909 facing forward and aft. The rearward engines are of course used for orbit changes etc and the forward engines can be used to 'brake' when carrying heavy loads if I find myself approaching too fast and RCS can't cope. They are of course mapped to action groups to kill one set and activate the other.

RCS is always a problem. If I'm launching the tug with it's load as I often do for small and medium pieces (5-100 tons) then I work out the center of mass and install a set of RCS at the correct distances both on the tug and the load (with a second set around the tug-only center of mass to give it some re-usability). I also have a tug up there that has about 12 rows of RCS along it. This one I use for picking up massive loads I've launched on their own... I can model the center of mass in the VAB and it should have a set that corresponds ok to the load. have seen a few other near-identical designs in people's pictures so this seems to be a common design people come to.

I currently have a massive 800ton load to move in space, and my heavy tug has proven inadequate. Going to try the design but with 6 nacelles and toroidals instead of Lv909's and see how that works.

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Just a tip to control rotation, use multiple docking nodes!

You can do it with stock using the 1-3 adapter, I use KSPX and use the large 1-2.

Here is a screenshot of my setup using KSPX... You can modify for pure stock if you wish.

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I use a 3 man command pod on my tug and often also on my orange station fuel tank. It seems to turn pretty well without using RCS.

If you're going to redock the tank, just in a different orientation, then you transfer all the fuel out of it before undocking and fill it back up after you re-dock.

Alternately, you could try to balance it a little bit by moving fuel in/out of your tug fuel tank

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Ion engines are a bad idea. They are too weak - to get any sort of reasonable thrust, you have to use many od them, which will raise both mass and part count. Believe me, i tried to build interplanetary ship propelled by ions, and it was more trouble than its worth. Ions are for probes, and you still need a lot of patience during flight. Use aerospikes or NERVA's, or install a mod with Hybrid Ion engines.

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