[1.1.2] Orbital Utility Vehicle v1.2.4

[nli2work]

59 minutes ago, Jimbodiah said:

When I disable the pod's RCS and add stock thrusters to CoM, then RCS works like a charm. Having RCS at the front and rear of a tug (stock parts) to create a lever when a load is present still gives me a stable ship while docking without the load on it, so I'm thinking it's a balancing issue on this tug (thrust power vs position).

I'll try the thrust slider to see if it helps in the meantime.

As far as jitter I get the same issue with stock RCS or otherwise. It's rooted in stock SAS module, RCS exacerbates it.

if you're docked to a larger object and all the RCS is on the tug then yes, there will be lever action when you try to translate with RCS IKJL controls. RCS has to be more or less evenly distributed around the COM the combined vessel for optimal level of control. SAS on the otherhand isn't position dependent.

Edited March 12, 2016 by nli2work

[ComatoseJedi]

This is going to sound like a very stupid question: But how is the pusher adapter supposed to work? Is it supposed to dock with other docking ports? Grapple any object? Or both? 

[nli2work]

7 hours ago, ComatoseJedi said:

This is going to sound like a very stupid question: But how is the pusher adapter supposed to work? Is it supposed to dock with other docking ports? Grapple any object? Or both? 

I might have broken that. It's supposed to be a docking port. 

[Neutrinovore]

12 minutes ago, nli2work said:

I might have broken that. It's supposed to be a docking port. 

If anyone's interested in my 2 cents, I think that it would work beautifully if it had BOTH functions.    I don't think that it would be necessary to change the model, just add a grappling MODULE to the existing part.  That way, it would function as a size 1 (1.25 meter) docking port all the time, but then if one arms the grappler function (which would be what are now just what you have labeled as "docking clamps"), well, now you can grab on to ANYTHING.  I think that would be pretty useful, myself.  Anyway, fwiw.

Also, just a quick report for nli2work:  Using RCSBuildAid, the RCS thrusters on the.... well, actually, I think that it's most parts in this pack that have RCS thrusters... anyway, most of the centers-of-thrust are just a bit off kilter in left/right and up/down axes.  For some reason, and as far as I can tell, you've got the forward/backward thrust axis perfectly aligned with the center of mass.  BTW, this is most evident on the 'core' part, the one with the grabber arms, mainly because the RCS thrusters on that are the most powerful.  Anyhow, this isn't a huge issue, it's not like they're so far out of whack that there's hundreds of KnM of torque, but you might want to go back and look at the part models, specifically in regard to the location and orientation of the rcs thrust transforms.

Otherwise, I think everything's cool with this mod pack now.  Later!   

[nli2work]

Good call on using RCSBuildAid! I assumed RCS on a single part automatically adjusted thrust to maintain proper orientation while translating. Obviously I'm wrong. I'll definitely check all the thruster placements again.

After some testing I decided to leave the RCS thrusters mostly as they are. Slightly reduced Adaptor's RCS thrust and adjusted it's CoM. It's impossible to eliminate torque for all possible attachments. With standard command pod the torque is low enough to be handled by SAS without any jitter. Until Squad fixes the SAS overcompensating problem we'll have to live with it. Best thing for now is turn off unnecessary SAS and RCS until needed.

On the upside, I originally assumed Grapple Node and Docking Node couldn't reside on the same part, but I was wrong. Now the Adaptor functions as Multi-Dock® compatible with Size1,2, and 3 ports; and additionally as grapple node!

 

Edited March 13, 2016 by nli2work

[Neutrinovore]

On ‎3‎/‎12‎/‎2016 at 10:54 PM, nli2work said:

Good call on using RCSBuildAid! I assumed RCS on a single part automatically adjusted thrust to maintain proper orientation while translating. Obviously I'm wrong. I'll definitely check all the thruster placements again.

After some testing I decided to leave the RCS thrusters mostly as they are. Slightly reduced Adaptor's RCS thrust and adjusted it's CoM. It's impossible to eliminate torque for all possible attachments. With standard command pod the torque is low enough to be handled by SAS without any jitter. Until Squad fixes the SAS overcompensating problem we'll have to live with it. Best thing for now is turn off unnecessary SAS and RCS until needed.

On the upside, I originally assumed Grapple Node and Docking Node couldn't reside on the same part, but I was wrong. Now the Adaptor functions as Multi-Dock® compatible with Size1,2, and 3 ports; and additionally as grapple node!

 

Awesome, thank you!    Yeah, no, I realize that once you start bolting stuff on to a given part the center of mass goes kerflooey, I was talking about the part's NATIVE CoM to CoT discrepancy.  If that's off to start with, then there is literally no way to zero-out all of the torque in all three axes.  But, thanks for taking a look and fixing it, and also thanks for adding the grappling functionality, as well as the multi-dock function.  That's not dependent on a third party plugin, is it?  Oh well, I'll download the latest version and update anyway, so I'll go dig into the configs to see.

Again, thanks greatly for responding so fast to feedback.  Later! 

[nli2work]

Yeah CoM shifts soon as you start adding stuff. the only direction that can be 0-ed out 90% of the time is Forward/Backward since most heavy parts are stacked together and all their CoMs are aligned along this axis.

multi-dock is stock function now. Size 0, 1, 2 are stock docking node types. Maybe I can get MM to make it true universal by adjusting the node type to include mode dock ports... kinda of a stretch goal I guess. Currently only issue I didn't expect is both Grapple and Docking node are activated at the same time. I'll have that split out to separate functions next iteration so there's no unexpected grappling while trying to dock proper.

[fr33soul]

plz do a repository for CKAN!

[Phoenix1583]

Absolutely fantastic mod! It's a ball in KSP!

[gunt3rgam3r]

Love the mod...just a little feedback on your latest changes.  I have been playing with the docking port on the front of the "pusher".  I think you should commit to a single size for the part, and not enable the claw for it.  I have not been able to get the docking port to dock reliably with any of the sizes...in almost every case the link is a little off-center and as a result you get an un-centered center of thrust.  

I have solved this myself by just adding a stock 1.25 part to the top of the pusher, but that makes the winch less useful, and looks a little dumb.  

Again thanks so much for the mod it is great!

[Neutrinovore]

4 hours ago, gunt3rgam3r said:

Love the mod...just a little feedback on your latest changes.  I have been playing with the docking port on the front of the "pusher".  I think you should commit to a single size for the part, and not enable the claw for it.  I have not been able to get the docking port to dock reliably with any of the sizes...in almost every case the link is a little off-center and as a result you get an un-centered center of thrust.  

I have solved this myself by just adding a stock 1.25 part to the top of the pusher, but that makes the winch less useful, and looks a little dumb.  

Again thanks so much for the mod it is great!

Wait... are you talking about the 'core' part, the one with the fold-out arms and the powerful RCS thrusters that has stack nodes at both ends, or the 'docking adapter' part, the one where you attach it so that the cockpit is in the middle of it and has a small service bay at the rear with the doors you can open to put stuff in?  Because the first one is JUST a grappler, it doesn't have any built in 'docking' port code.  But the second one, the docking adapter, has BOTH grappler functions AND 'universal' docking port code that will truly 'dock' with ports from 1.25 meter to 3.75 meters in diameter.  In the case of that part, you don't want to use the 'Arm Grappler' function if you want to actually dock to a docking port, all you need to do is select 'Control from here' and then target the docking port to which you want to dock, then use the method of your choice (manual docking, NavyFish's Docking Port Alignment Indicator, or MechJeb's Docking AP) to dock.

Anyway, sorry to butt in, just trying to help clarify things.    Have fun!

[akron]

This is absolutely fantastic! Great work there

[nli2work]

8 hours ago, gunt3rgam3r said:

Love the mod...just a little feedback on your latest changes.  I have been playing with the docking port on the front of the "pusher".  I think you should commit to a single size for the part, and not enable the claw for it.  I have not been able to get the docking port to dock reliably with any of the sizes...in almost every case the link is a little off-center and as a result you get an un-centered center of thrust.  

I have solved this myself by just adding a stock 1.25 part to the top of the pusher, but that makes the winch less useful, and looks a little dumb.  

Again thanks so much for the mod it is great!

Yeah it was very confusing initially since I set both the grappler and docking port to the same node. I've split them into separate nodes, so unless you set them both activated, there shouldn't be any interference between the two. I've added lights to indicate which mode is active. Docking Mode shows two ports and arrows pointed at each other. Grapple node shows wavy lines pointed at the port on one side. Unfortunately there's no ability to set those two to Either/Or mode.

[gunt3rgam3r]

Looks like I need to update the mod... I probably have an older version...thx for all the quick work

[Buster Charlie]

ckan?

[nli2work]

will get ckan metafile set up this weekend. I thought it was automatic with spacedock. 

[Buster Charlie]

edit to be more clear: 

 

The engines get higher delta-v if I only have Liquid fuel when in LFO mode, which makes me assume they're not using the oxygen, but the Grabber core has Mono/EC and Liquid/oxy/EC mode. I assume this is a bug, so I manually edited the cfg myself to store 360 liquid (which seems to be the fuel switcher ratio (LF+O *.9 = LF only?)

 

Anyway so now I have another issue. I don't have action groups unlocked so I manually engaged the engines, but only one of the engines engaged. Now it says can't start engine while stowed, but I can toggle it extended and back and it still says it's stowed.

 

 

PS> This mod looks FREAKING AWESOME! Not just complaining, this will probably replace my cobbled together orbital tugs!

 

 

PPS, I used RCS balancer to build the tug with no torque (besides the reaction wheel), when I'm carrying a payload it just FREAKS out, so i'm not talking about I can't translate very well, I mean, It just whips around like crazy? I tried reducing the RCS thrust to 25%, still freaks out. Very sad.

Edited March 18, 2016 by Buster Charlie

[Neutrinovore]

3 hours ago, Buster Charlie said:

PPS, I used RCS balancer to build the tug with no torque (besides the reaction wheel), when I'm carrying a payload it just FREAKS out, so i'm not talking about I can't translate very well, I mean, It just whips around like crazy? I tried reducing the RCS thrust to 25%, still freaks out. Very sad.

Yep, I was also experiencing the same thing just last night/this morning.  Here's what I found:  In the SPH/VAB, I tweak the torque of the CORE part, the one with the big fold-out grappling arms, as high as it will go, to double the original value, but then DISABLE the torque wheels so that they're not active on launch, and just use the default torque values that come with the command pod or drone, or whatever other command part you're using, those shouldn't be too strong for just the little tug by itself.  Now, when you're flying around in space, if you're not pushing around a big payload you can tweak down the RCS thrust on the core part to 10% or less, so that it's not too powerful.  But when you grab a big heavy payload, you need to activate the torque on the core grappler so as to counteract the WAY-off-center thrust, and gradually ramp up the RCS thruster power slider until you have enough power without it 'freaking out' and being way over-powered.  Then you just need to take it slow and easy with your maneuvering inputs, if you're flying manually.  If you're using MechJeb's Docking AP, set the docking speed to NO MORE than 0.5 meters per second, probably even slower if you've got something heavy and awkward, and just let it take its time.

Anyway, gotta go, hopefully these tips help.  If not, well, they're worth what you paid for them. 

Lol, later! 

[nli2work]

4 hours ago, Buster Charlie said:

edit to be more clear: 

 

The engines get higher delta-v if I only have Liquid fuel when in LFO mode, which makes me assume they're not using the oxygen, but the Grabber core has Mono/EC and Liquid/oxy/EC mode. I assume this is a bug, so I manually edited the cfg myself to store 360 liquid (which seems to be the fuel switcher ratio (LF+O *.9 = LF only?)

 

Anyway so now I have another issue. I don't have action groups unlocked so I manually engaged the engines, but only one of the engines engaged. Now it says can't start engine while stowed, but I can toggle it extended and back and it still says it's stowed.

 

 

PS> This mod looks FREAKING AWESOME! Not just complaining, this will probably replace my cobbled together orbital tugs!

 

 

PPS, I used RCS balancer to build the tug with no torque (besides the reaction wheel), when I'm carrying a payload it just FREAKS out, so i'm not talking about I can't translate very well, I mean, It just whips around like crazy? I tried reducing the RCS thrust to 25%, still freaks out. Very sad.

Damn, I had a nagging feeling I was forgetting something! I set the engine to LF only, like the stock nuke engine, but forgot to set the grabber tank to LF only... it's still LF/Ox or MOno. krakens... Will add LF to the fuel switch options, and set the default to Mono instead of LF/Ox

Regarding craft whipping about... can you send me a craft file or save game? or a screen cap of the vessel, or more specifics so I can replicate the situation? Is this happen only with MechJeb? 

There's an issue with mixing Grapple Node in the adaptor unfortunately, might be best to separate it out eventually. The node never really disengages, just how the stock module works, which is why the stock grappler is built the way it is... if it wasn't inside of the big container it'll just stick to anything that gets close enough. So in the adapter you have to click "Release" each time you disengage/engage the grappler. :/

I thinkin it might be better the make the drone core welded onto an adaptor and make the whole thing 0.625 sized. retain the RCS thrust and the grappler so you can stick it anywhere that's convenient. It's really the only way to manipulate heavy objects... you have to attach a bunch of RCS around the mass for somewhat balanced controls or you just end up spinning around the massive object when you try to translate it... no amount of MechJeb is going to help you.

Edited March 19, 2016 by nli2work

[gunt3rgam3r]

4 hours ago, Buster Charlie said:

edit to be more clear: 

 

The engines get higher delta-v if I only have Liquid fuel when in LFO mode, which makes me assume they're not using the oxygen, but the Grabber core has Mono/EC and Liquid/oxy/EC mode. I assume this is a bug, so I manually edited the cfg myself to store 360 liquid (which seems to be the fuel switcher ratio (LF+O *.9 = LF only?)

 

Anyway so now I have another issue. I don't have action groups unlocked so I manually engaged the engines, but only one of the engines engaged. Now it says can't start engine while stowed, but I can toggle it extended and back and it still says it's stowed.

 

 

PS> This mod looks FREAKING AWESOME! Not just complaining, this will probably replace my cobbled together orbital tugs!

 

 

PPS, I used RCS balancer to build the tug with no torque (besides the reaction wheel), when I'm carrying a payload it just FREAKS out, so i'm not talking about I can't translate very well, I mean, It just whips around like crazy? I tried reducing the RCS thrust to 25%, still freaks out. Very sad.

 

I also had my craft "freak out" and spin around. I was only using one part at the time,  it was the docking port with the storage Bay.    I figured I clipped something in the bay when I squeezed in two batteries and a couple solar panels. I re-built the craft and that solved the problem... If it happens again I'll send the craft file... 

 

BTW my save is heavily modded

[nli2work]

I'm seeing the RCS gets disabled when the grappler attaches to something? that's the same for everyone else too? Seems to depend where you grab... some places doesn't disable the RCS other places do.

Edited March 19, 2016 by nli2work

[Buster Charlie]

11 hours ago, Neutrinovore said:

Yep, I was also experiencing the same thing just last night/this morning.  Here's what I found:  In the SPH/VAB, I tweak the torque of the CORE part, the one with the big fold-out grappling arms, as high as it will go, to double the original value, but then DISABLE the torque wheels so that they're not active on launch, and just use the default torque values that come with the command pod or drone, or whatever other command part you're using, those shouldn't be too strong for just the little tug by itself.  Now, when you're flying around in space, if you're not pushing around a big payload you can tweak down the RCS thrust on the core part to 10% or less, so that it's not too powerful.  But when you grab a big heavy payload, you need to activate the torque on the core grappler so as to counteract the WAY-off-center thrust, and gradually ramp up the RCS thruster power slider until you have enough power without it 'freaking out' and being way over-powered.  Then you just need to take it slow and easy with your maneuvering inputs, if you're flying manually.  If you're using MechJeb's Docking AP, set the docking speed to NO MORE than 0.5 meters per second, probably even slower if you've got something heavy and awkward, and just let it take its time.

Anyway, gotta go, hopefully these tips help.  If not, well, they're worth what you paid for them. 

Lol, later! 

I Don't have a mod that lets me tweak the torque value, I used to have one but felt it was a bit cheaty to be able to double the torque with no weight costs (I know no cheat in a SP game but to me it seemed too easy) I'll try disably some of the reaction wheels and RCS and see if it works..

[Buster Charlie]

11 hours ago, nli2work said:

Damn, I had a nagging feeling I was forgetting something! I set the engine to LF only, like the stock nuke engine, but forgot to set the grabber tank to LF only... it's still LF/Ox or MOno. krakens... Will add LF to the fuel switch options, and set the default to Mono instead of LF/Ox

Regarding craft whipping about... can you send me a craft file or save game? or a screen cap of the vessel, or more specifics so I can replicate the situation? Is this happen only with MechJeb? 

There's an issue with mixing Grapple Node in the adaptor unfortunately, might be best to separate it out eventually. The node never really disengages, just how the stock module works, which is why the stock grappler is built the way it is... if it wasn't inside of the big container it'll just stick to anything that gets close enough. So in the adapter you have to click "Release" each time you disengage/engage the grappler. :/

I thinkin it might be better the make the drone core welded onto an adaptor and make the whole thing 0.625 sized. retain the RCS thrust and the grappler so you can stick it anywhere that's convenient. It's really the only way to manipulate heavy objects... you have to attach a bunch of RCS around the mass for somewhat balanced controls or you just end up spinning around the massive object when you try to translate it... no amount of MechJeb is going to help you.

 

I might tweak the design a bit this was my first version. My idea was drone core (I use life support, I might make a manned version just for a single crew transfer option) with the docking port pusher in front so it could dock to my space station port, grabber unit mounted in reverse so I could pull heavy loads (seems to wobble less than pushing)  You can see I placed the two engines so the empty craft will have zero translation torque when empty but I realize this is a futile effort since It'll create a tremendous amount of torque once attached to a long heavy load. I'm wondering if it would be possible to define some preset configs with action groups. I have a procedural test weight, I could figure out some common load sizes and maybe use a action group mod to set the RCS value based on a few common size ranges...

Now I am using mechjeb but even with manual docking mode the ship was spazzing out and basically wobbling everywhere. I don't normally have this issues with other tugs i've built, but maybe this is just extra powered and I'm not used to it.

 

BTW those delta-v numbers are with me manually converting the core to have 360 of liquid only fuel.

 

I'm intrigued by the propellant nuke, as i've built a few mono-only tugs to simplify logistics (Including a fun monopropellant-electric arcjet mod engine) but while I can see it having a lower ISP if the monopropellant is a heavier fuel, wouldn't it have more thrust (kinda like a trimodal NTR that uses oxygen as an 'afterburner'?) I'm only suggesting this because if it's lower thrust and lower ISP I'm not sure i'd ever use it, but if it was say lower ISP but higher thrust as a tradeoff, and maybe generated more heat so you can't run it as much, maybe that would make it more appealing. But it is your mod so have at it! 

 

BTW I got the engines to deploy and work I think I just reloaded it from desktop and it was active  when I loaded into the ship, but just FYI about the 'deployed' bug.

 

PS> I still want to say this is about the most exciting 'specific purpose' mod i've downloading, it looks excellent and it serves a very useful purpose for me with a very low part count compared to stock so I want you to keep developing this so much! 

Edited March 19, 2016 by Buster Charlie

[nli2work]

I did some testing on the launch pad with gravity disabled. same setup as your pic minus the nuke engines. All SAS and RCS were on for the grabber arm and command pod, and there wasn't any nonstop jittering. Tried to move 5x largest stock 3m fuel tanks around, was pretty much impossible with only the tug. Now I'm seriously thinking of some 0.625m RCS drones, single part with graber, remote, RCS and RTG with little bit of Mono. Something compact enough that you could stick 6 to 8 around a central stem inside a 1.25m volume. That was my original method around orbit, except each drone was a dozen parts and it took a long time to dock 5 drones around a fuel tank before I can dock the thing to the main station in orbit, plus I have to add half a dozen docking port jrs on every piece I add to the station. 

I see what you're sayin about Mono fuel option. Honestly I don't know squat about ISP and Fuel density, I just set it up the way I did because stock tanks of similar volume seem to hold less units of Mono than LF/Ox... I'm probably wrong about that too. Since Mono is essentially LF/Ox in a single fuel, it'd make sense to have it higher thrust but lower ISP than LF only. I think LF w/ Ox afterburner is something that's not in stock? Far as I know stock modules only allows dual modes?

Did you open the service bay on the drone by any chance when things flip out hog wild? The adaptor should be rotated 90 degrees if using with the drone so the bay doors don't clip into the adaptor arms.

[Buster Charlie]

Okay more reporting.

 

It may have to do with mechjeb to be honest, but none of my other ships have this issue with mechjeb, so I'm not sure if it's due to the intergrated RCS vs add on RCS. 

I tried turning off reaction wheels, i've tried  lowering RCS thrust. But what happens if I use the stock SAS to point Target + your tug will aim at the target and I can do manual translation with RCS (note I use mechjeb to turn off RCS for rotation so I can't confirm it wouldnt freak out with RCS rotation)

If I use mechjeb smart ASS to point at the target (and I try and handle translation manually) it freaks out spraying RCS everywhere and wobbling. However rendezvous autopilot and other mechjeb functions work flawlessly, only the more fine target alignment issues seem to be causing trouble.

Curious. On the whole i'd like mechjeb to work with this because I can dock manually, I just find it more fun to focus on planning and building then too much hands on flying.

 

On the fuel issue, my understanding is the whole thermal rocket ISP equation is divided by the molecular mass of the propellant so the smaller the mass, the less your diving the isp by? 

(detailed fancy stuff here http://www.projectrho.com/public_html/rocket/engines.php)

So IRL liquid hydrogen (for all it's faults) provides the lowest mass, especially when I hear it disassociates at high temperatures into elemental hydrogen. 

So obviously liquidfuel is not liquid hydrogen, because it runs normal rockets and jet engines it's probably closer to kerosene, so obviously it wouldn't provide the same performance but squad wanted to KISS.

(BTW I've suggest the author of kerbal atomics make a liquid hydrogen patch for your mod so I can use your engines/tug with his hydrogen engine system, you may want to talk to him about that if interested)

Back to monopropellant: Now you have LOX-Augmented Nuclear Thermal Rocket (LANTR) http://www.projectrho.com/public_html/rocket/enginelist.php#lantr This is not a stock feature but I suspect it would be close to what would happen if you did use a hypergolic fuel in an NTR (Fuel density realism aside) From a gameplay perspective it makes sense, you trade fuel efficiency for thrust. Weight and cost of the system aside, my opinion is that if this is a hybrid mode between NTR and a pure monopropellant engine, monopropellant engines are already seeming like baby NTR (low thrust high ISP, but lower overall) my vote would be treating it like a LANTR (even if it is not 100% realistic) because it would give you an excise to make it unique between the two alternatives.

 

On the idea RCS Drones.

 

Maybe the simplest opption would be for a single disk shaped drone that uses the same low profile clamper as the pusher gravity grabber.  It would dock with the main tug during transit, refueling and recharging off the main tug parasitic style. To keep it light weight and small it pretty much has just enough electric charge to undock, rcs fly to the opposite end of the target, orientate (maybe the drone ring has a indicator light, along with the main tug, to help optically align both systems to make sure they're lined up) and attach to the opposite end of the payload. Once the payload, drone, and tug are docked they should share electric charge and monopropellant, this also means if the drone had sufficenctly powerful RCS it should balance out the load and work in unison with the tug, with only one part to dock and align vs trying to park lots of little drones in specific locations on the target ship.

 

As long the the drone could provide a large amount of RCS thrust, and be dialed down, you should be able to balance the torque.

 

I know what you're saying, what about docking?

 

A few options:

 

1) The drone RCS ring being only one piece and possibly fairly cheap, can actually have a docking grabber on both sides and actually act as a disposable permanent universal docking adapter.  Maybe the station can carry a stack of spares but this is not ideal so that brings me to point 2...

 

2) The payload is lined up with the target, the tug gets on an intercept course, everything is lined up and perfect and you're closing in at 1ms or so, you undock the drone ring, which being fairly light will either be pushed along or maybe even be pushed out of the way, if not you switch to it quickly, translate sideways a tiny bit to clear the payload,and switch back to the main tug to finish the docking, it shouldn't drift too much if you've got everything going right.

3) The Drone ring has two grabbers as in option 1, you fully dock the station ->drone -> Payload -> tug. Then you have a toggle to disable the tug drone grabber/undock it/make it inert, so it will detach from both station and payload, translate it sideways and maybe the docking ports will then kick in with their magnets and engage?

4) Variation on number 2. If there is a way to use the decoupler force, or solid rocket booster, or something that triggers when decoupled, that is re-usable. You've got the ship line up, payload about to dock, but instead of manually switching to the drone ring and decoupling it and flying it out of the way, you can bind an action group or staging stack  that when activated will decouple the ring drone, and fire a sideways mounted thruster automatically. It doesn't burn very long, it's not very powerful, just enough to kick it sideways when it detaches and move it out of the way at the last second before docking. Since it's a weak short thrust, it won't have traveled far by the time you dock, so when you dock you switch vehicles and fly it home to the tug. 

 

 

 

edit: Services bays closed during the whole mission.

Edited March 19, 2016 by Buster Charlie