What do I need for successful Orbital Tug ?

[Mapoko]

Hello.

Until recently I assembled my stations like a caveman, putting engine on the back, dock then decouple the engine.

But recently I felt the need of reconfiguring my stations and w/o a tug and engines already decoupled and gone it turned into KAS winch madness.

This is what I thought of on top of my mind :

Basically, use the medium port for the small modules and it will not deviate the RCS power too much ? And for larger modules I will put the RCS down towards the payload to try to compensate.

I am aware I can make it with KAS & RCS blocks but it will take way more time and dedication to do so and also not solving the future.

The build is so basic, so I would like to know what more do I need. The LFO engines only go 1 way, but I can push the small ones and pull the large ones, then RCS maneuver, right ?

Edited April 14, 2015 by Mapoko
Changed to Answered.

[Hotblack Desiato]

my orbital tugs always look the same:

docking port on each side (maybe one claw and one port, if you ever have to move something without dockingport), a 1,25m probe core, 1-3 1,25m rcs tanks, battery, sas. 4 to 8 rcs-ports and solar panels.

this thing can happily push around everything, and doesn't consume any LFO. and it's so small that it usually fits on a rocket as secondary payload (a friend uses them even as counterweight for his spaceplanes... 50 tons in the back, and 2 tons attached to the nose-dockingport to balance the plane).

[TheXRuler]

Generally what I do is build my launcher with a returnable and recoverable second stage (use chutes and landing legs) bring the payload close to the station and then dock it with my docking tugs, pictures coming up in a second.

Sorry it took so long I'm not at my main rig so loading KSP took... "a while" -.-

I used to do things the same way as you do, however when I engaged in the Grand Orbital Space Station Challenge I faced a completely new problem. Launch weight was going to be subtracted from the score, so basically using a spaceplane was the way to go, since recovered weight would be subtracted from the original launch weight. So I came up with a new way to do things, I built docking tugs, solely for the purpose of docking the various modules to the station.

Anyways this is what my docking tugs look like.

I had two main criteria for them to fullfill.

1. Longterm autonomous operation (solar panels, batteries, a probe core [hidden inside the 1.25m -> 2.5m Adapter] and tons of RCS to keep the number of refueling runs to a minimum.

2. Low part count. I am using them to build a truly huge station and can't afford to waste any parts. Kept it down to 16 apiece

The tugs each hold 2400 units of monoprop and have a lot of torque due to the large SAS module. I also use two of them together, since many of my payloads are very long and very heavy making one tug on one end completely impractical.

I dock one tug to each side and then monouvre the payload into position in front of the designated docking port, kill all relative velocity and then slide the tug which is in the way out of the way and just push forward gently :3

Edited April 13, 2015 by TheXRuler
Pics up now

[Red Iron Crown]

All monoprop is the way to go for station building tugs. Throw a couple of the monoprop engines on there in case you need throttleable engines (you generally don't, though).

Sample pic, tug at lower docking port. Note the adapter from one size of docking port to another, makes the tug "universal":

[Mapoko]

Thank you all!

So basically I need to focus on high maneuverability, relatively good fuel supply and obviously low part count.

So adding a reaction wheel and relying only on RCS (need some practice) is probably a good idea.

I will launch 2 and possibly one with Klaw for all the maintenance needed.

Just 1 last question - all those have balanced RCS to themselves and not accounted for extra payload, because you will move both 5t and 25t ones for example, right ?

[Red Iron Crown]

Just 1 last question - all those have balanced RCS to themselves and not accounted for extra payload, because you will move both 5t and 25t ones for example, right ?

It's tricky and perhaps impossible to make one that is balanced for all possible loads. I try to do all translation in the fore/aft directions when using a tug.

Another option is to put RCS thrusters on your payloads and have them draw fuel from the tug.

[TheXRuler]

What I do to circumvent the issue of balance is to design the payload so that the CoM is roughly in the middle, then attach one tug to each end. That way you have equal RCS force on both ends and balance is no longer an issue. Barring that possibility I will pump fuel from one tug to the other in an attempt to better compensate for the unbalanced payload (remember to balance out fuel levels before disconnecting the tugs!!!). Another possibility is to disable one set of thrusters on one of the tugs (the one further away from the CoM) and balance RCS force that way.

Since each tug has not only eight RSC thrusters but also a reaction wheel the control authority is usually enough to compensate a slightly offset CoM, providing you use precision controls for translation (enabled by pressing caps lock).

[impyre]

It's tricky and perhaps impossible to make one that is balanced for all possible loads. I try to do all translation in the fore/aft directions when using a tug.

Another option is to put RCS thrusters on your payloads and have them draw fuel from the tug.

This. I always try to treat each module as if it had its own internal RCS source. This way, when something connects with RCS they are fairly well balanced. It won't be precise, but usually close enough. Combined with a torque source, it's good enough to take care of most anything. As long as the amount of RCS TWR is kept fairly constant, things should stay relatively balanced.

EDIT: FYI, so long as RCS is placed symmetrically and has no rotational offset there's no need to calculate the RCS TWR for each direction. Total thrust/weight will give you just as useful a ratio... it doesn't matter whether it's being divided by three or four or more (so long as *all* vessels that will be connected have the same symmetry mode). If you're connecting one vehicle with 8-fold symm RCS to one with 4-fold symm rcs, you need to account for the difference in TWR. Again, none of this is *exact*, but it's usually good enough that getting it more closely matched doesn't end up making any noticeable difference (especially when torque is present from other sources).

Edited April 13, 2015 by impyre

[Mapoko]

Putting RCS on payload increases part count. I can remove them with KAS, but if I need to move them again I need to reattach them, not to mention having to align my tug docking, so I have the RCS blocks aligned to tug's.

Basically kind of a mess.

I will launch modified version of the tug on my OP and try RCS movement practice. I admit sometimes I get lost on what side is "up" and when I want to move something left it goes right

@Crown, the adapter design is genius, true.

@XRuler What are those parts on the side of your tug ?

[impyre]

Putting RCS on payload increases part count. I can remove them with KAS, but if I need to move them again I need to reattach them, not to mention having to align my tug docking, so I have the RCS blocks aligned to tug's.

Basically kind of a mess.

I will launch modified version of the tug on my OP and try RCS movement practice. I admit sometimes I get lost on what side is "up" and when I want to move something left it goes right

@Crown, the adapter design is genius, true.

@XRuler What are those parts on the side of your tug ?

The boxy looking pieces on xruler's craft are the larger K&W batteries.

[Warzouz]

It largely depends on what you need your tug to do. I use a "Station Tug" which had RCS only but is able to deorbit some stuff and move landers and stations parts around. It rarely quit the vicinity of the station. It has RCS rocket engines and RCS packs. A probe core and a SAS wheel. Batteries and fixed solar panels. Basically a cylinder with 1 docking ring at each end. It refuels on the station (it was sent to Jool with a very big tank of RCS)

Then I use a Tug (I have 2) to carry landers to their destination (specific for Tylo). They are built as regular ships and have docking rings at each end and LVN.

[impyre]

Honestly, I never bother with tugs (at least not in the Kerbin system). I simply stick to an expandable structure with the largest parts on the core. IE: 2.5m parts that have docking port sr. on both ends (disconnect payload by "undocking" once it's in place). This is pretty easy and simple. If you ever need to rearrange, you'd need a tug (or use a vehicle that you've just used as a launch vehicle for another part). Personally, I like to use a modular approach and design modules that all have the same length. I usually end up attaching them in a cubic-shaped structure for added rigidity/attitude control. Stations/ships built this way can be difficult to disassemble (especially if you use multi-point docking).

[Mapoko]

Thank you.

I am aware I practically do not need a tug for assembly of my KLO station or even Mun one, but I somehow managed to go to pretty much every celestial body (and back) avoiding RCS usage.

So my plan is to launch a tug or 2, reconfigure my KLO station and meanwhile get used to RCS only maneuvering.

I will need tugs mostly for reconfiguring my station, so they will not really leave the vicinity of their home station.

Maybe later I will have a tug that will haul whole sections from LKO to Mun base for example, but by that time I should have pretty good experience and idea of what I need.

Thank you all very much for giving me all the info I needed to start up!

[TheXRuler]

I forgot to mention in my first post, the main reason for having docking tugs is when you are using a cargo SSTO to get things to orbit, since it is possible, but rather impractical, very slow and maddeningly fiddly to try docking a 330 ton spaceplane with a 60 ton payload. Also the wings/nose/tailfinns tend to get in the way and since your the payload is inside the cargo bay it would be necessary to devise some system of getting the payload out of the cargo bay without decoupling it from the spaceplane, ince sticking monoprop tanks, a probe and RCS thrusters on every payload means wasted payload capacity and more parts (at least 10 per payload, not counting batteries and solar panels).

In general a disposable system is perfectly fine.

Also, don't use monoprop to go interplanetary, it is horiffically inefficient (low ISP). Use it to build multiple launch interplanetary ships if anything.

Edit: I just realized you probably already know this, since you have been to most bodies.

[micr0wave]

For balancing i used a different approach.

i used 2 tugs

with claws

and even heavy things were easy to control

[Mapoko]

Yes, the plan is to make a space plane and haul them to the station, then go back with the plane.

And yes. I will most likely use Nukes for the station to station tugs. I meant that for the "housemaids"

[radonek]

My 2 cents:

* good tug can serve you a long time, so its worth it to put some thought to it. Also its easy to replace making it a good testing field for funny ideas.

* Easiest way to "solve" balance is to just have a lot of SAS force. A big gyro ring is a must, more are better. If CoM is off in a big way, you can disable rcs ports, but tug should be able to eat up some difference.

* Have a enough batteries to power all that SAS power. Having solar panels sticking out while maneuvring around things is not fun.

* Do not forget to add lights so you can work on dark side too.

* It's handy to have decent fuel and monoprop tankage on board so you can top-off fuel in smaller ships or move few buckets of fuel somewhere.

* My tugs are based around a single LV-N. Not only are they able to push cargo around orbits and to moon/minmus, but heavy engine and fuel tanks act as counterbalance. It can be made to quite a compact design.

* Sooner or later you will bump into something, so it should be reasonably rugged.

[Mapoko]

How do I build/fly my RCS, so I can have good translation ?

I mean

[] -> []

[] -> []

not

[] -> / /

[] ->/ /

Does it have to do with the camera or RCS placement ?

I want to achieve the EVA like controls.

EDIT : The RCS Thrusters are placed evenly. Or do I need radial boosters / 5 way blocks ?

Edited April 14, 2015 by Mapoko

[jlcarneiro]

All monoprop is the way to go for station building tugs. Throw a couple of the monoprop engines on there in case you need throttleable engines (you generally don't, though).

Sample pic, tug at lower docking port. Note the adapter from one size of docking port to another, makes the tug "universal":

https://dl.dropboxusercontent.com/u/61004449/KSP/MRS/screenshot341.png

Hey, Red Iron Crown, what mod gives lights to docking ports?

[radonek]

I can think of few improvements:

One big battery bank is plenty while extra SAS is always handy.

Tanks are farther apart, helps to move CoM around by pumping monoprop between them.

Same goes about RCS blocks - farther they are from CoM, more torque you can squeeze off them.

Small photovolt panels should be enough most of the time and are not sticking out as much. Also not prone to "forget to deploy" fail.

Monoprop engines are below docking ring level, less likely to get struck by accident. (Would be event better at waist level, but this looks cool to me :-) )

As for maintaining balance: if your cargo is passive, you turn off RCS blocks on opposite end, so that remaining RCS is as close to CoM as possible. If cargo have its own RCS, you turn off whichever ports comes handy. Also note that while you cant see CoM in flight, camera is centered on it.

[Red Iron Crown]

Hey, Red Iron Crown, what mod gives lights to docking ports?

Modular Rocket Systems, they're called docking helpers.

[Mapoko]

-snip-

Also note that while you cant see CoM in flight, camera is centered on it.

That is so valuable info !

Most of my payloads are passive, including this monstrosity (I know it is easier to just attach stuff to it, rather than moving it around, but the plan is to have 2) :

This is what I ended up with thanks to radonek :

Added a winch system for odd payloads or unplanned situations. 2 in each side in case I need 2 tethers to balance it out.

Does the Kerbonaut change the CoM when on the seat ?

Containers can be (re)moved if I do not need to use the winches, so that reduces the torque to 0. It is balanced w/o the containers. (or Kerbal)

Also a cute Klaw:Docking port tool I can leave attached somewhere and pick it up when needed.

Also : to achieve EVA-like movement in space I need to disable the reaction wheels first, right ?

Also switch to docking mode ?

And this will be the garage once the tug is done with it's stuff.

Could not fit it into mk3, because of winches, so I am using Talisar's Cargo Transportation Solutions (WIP) . I already have dock like this docked to my station.

EDIT : Looking back at page 1 I think I forgot to consider the cute Port Sr>normal adapter, but I will put one on the second one. Planning to use 2 anyway. Maybe the other one will not have KAS as well. Diversity.

EDIT 2 : Wonder how much of it can I use [0.90] UbioZur Welding Ltd. Continued 2.1.1 (2015-01-14) on to reduce the part count, because near station it is always a concern.

Edited April 15, 2015 by Mapoko

[Stone Blue]

EDIT 2 : Wonder how much of it can I use [0.90] UbioZur Welding Ltd. Continued 2.1.1 (2015-01-14) on to reduce the part count, because near station it is always a concern.

IIRC, Do NOT use on docking ports, engines, and any parts that animate.

[jlcarneiro]

Modular Rocket Systems, they're called docking helpers.

Thanks!

[Mapoko]

IIRC, Do NOT use on docking ports, engines, and any parts that animate.

So basically, no RCS, Winches, Ports, Lightings?, Engines, Klaw, KIS Containers.

Reducing part count from 45 to 38 or something.

Guess that will not work. At least for this ship.