Need help with tug design.

[annallia]

Keep in mind this is the tug I am currently using, the new one will have a remote core (the large plate one) rather than the cupola module.

I have to redesign the tug anyway because I forgot to save the other one so using this one as a reference as they both have similar problems.

My early tugs had thrust issues, the ones without thrust issues had fuel issues (no doubt due to my excessive use of engines in them) and the current generations have just the right mix of thrust and fuel usage.

Problem is when it comes to docking. Current design when not docked with anything is difficult to control. A small tap on say A translates into spinning nearly 40 degrees before I can counter thrust rather than the 5 or 10 that I wanted. This only happens when trying to turn it, going forward/backward doesn't seem to share this problem neither does maneuvering up/down. Would I be right in assuming that this is because the RCS thrusters are above the COM?

The other problem comes when trying to dock a station part, specifically an elongated part.

An example of one of the parts (and I have done it with longer parts without a problem in the past...) would be this one:

Ignore anything above the hitchhiker cans as that is just the interplanetary stage. This part I docked with, took it to the station, the problem came while trying to dock, having issues maneuvering the part suddenly it is waving all over the place. Was just fine changing its inclination and during the rendezvous with the station but something about the docking controls (yes I tried fine controls too) made it go banana's

Now my question becomes three fold.

1) Is this also related to the placement of my RCS ports?

2) Would adding more/ taking away a few ports help? It currently has 6.

3) Is there any reason to believe that adding an SAS module to the tug will help?

I got "some" time to work out the kinks. I got 140 some odd days before the next Duna window opens to design, test, and prep the new tug + 3 new station parts.

[Psycix]

Place the RCS ports around the center of mass to avoid rotation when translating and reverse. If you have fuel tanks far away from it, keep in mind it may shift as they drain!

If you need to maneuver using RCS both with and without a payload attached, then you could have two sets of RCS thrusters on the vehicle:

One near the center of mass of the tug, the other set towards the center of mass when a payload is attached, and toggle them using action groups. (unless you have RCS on the payload as well)

EDIT:

A SAS module would help negating small rotations that occur during translating, and also give the option of rotating without using RCS.

Edited August 15, 2013 by Psycix

[annallia]

Just a few points of clarification due to my sleep addled brain not making me be clear.

When I said that it translated into turning too far, I did not mean I was trying to do something like move the entire ship sideways and it turned, I mean when I am trying to say angle my nose down a few degrees it instead overshoots big time.

That said RCS near the new COM isn't feasible for a reusable tug. Some of these parts are going to be light weight, others are going to be quite heavy, some will be short others will be long. Some I will have the option (like the one above) of grabbing on the heavy end so the COM has very little shift (though it is all on one end in that scenario) others I will have to grab on the light end.

Breaking them up is an option however there are limits to that option. Those limits being the insane time that it would take, the other being part counts going through the roof.

[Psycix]

A possibility would be some sort of donut-shaped tug that holds the payload inside of it.

[Captain Sierra]

Your RCS thrusters are offcenter. In the current configuration, there is no way it will translate straight.

1)always place RCS in 4x symmetry to maximize thrust and efficiency and minimize part count.

2) always place two sets equidistant from the C0M or 1 set right on the CoM.

3) with tugs, try to keep in mind that your payload will have RCS but you need to dock to it first.

[Psycix]

Personally I'm a big fan of this mod: http://kerbalspaceprogram.com/rla-stockalike-0-1/

It offers 45* RCS ports. Instead of spewing RCS everywhere since most thrusters are partly aligned in SOME way to the translation, having 4 of these on exactly 45* makes everything perfectly aligned, and only those thrusters actually fire.

[ThePsuedoMonkey]

Try disabling the reaction wheels on the tug; having too much can still make it wobble. The payload will also wobble less with a Sr port if you can work that into your design, or you could try using the quantum struts plugin.

[annallia]

Your RCS thrusters are offcenter. In the current configuration, there is no way it will translate straight.

1)always place RCS in 4x symmetry to maximize thrust and efficiency and minimize part count.

2) always place two sets equidistant from the C0M or 1 set right on the CoM.

3) with tugs, try to keep in mind that your payload will have RCS but you need to dock to it first.

I will try putting a few on each end, but my payloads will not have RCS with the exception of the payload delivering RCS tanks. I hate the look of RCS thrusters and simply don't tolerate them on stations except on the tanks for them. I realize that is making things a little harder on myself but that is why I am here looking for a workaround.

Personally I'm a big fan of this mod: http://kerbalspaceprogram.com/rla-stockalike-0-1/

It offers 45* RCS ports. Instead of spewing RCS everywhere since most thrusters are partly aligned in SOME way to the translation, having 4 of these on exactly 45* makes everything perfectly aligned, and only those thrusters actually fire.

I have too many mods as it is. But I will look into it because it sounds interesting.

A possibility would be some sort of donut-shaped tug that holds the payload inside of it.

That would be difficult for a multipurpose tug. Not every piece of the station is going to be a long tube type shape, some will be wide. I would have no objections if this was in Kerbal orbit, but the tugs work will be done in Duna orbit where I have no quick way of getting a specially crafted to the part tug quickly (ok no quick way without cheating). I have a half dozen manned and more than a dozen unmanned tug designs for Kerbal, but they are specialized to specific types of payloads. I am trying to design a GP tug that I can use anywhere I am building a station which will eventually be everywhere.

Try disabling the reaction wheels on the tug; having too much can still make it wobble. The payload will also wobble less with a Sr port if you can work that into your design, or you could try using the quantum struts plugin.

I know it will wobble less with a Sr port but that doesn't work well with the functionality of my fleet and the station, most of my ships use standard ports because they simply aren't large enough to use a Sr port. That said has quantum struts been fixed for .21? Last I heard it was causing crashes.

Edited August 15, 2013 by annallia

[sojourner]

I never liked the style of quantum struts so I use Docking struts instead. you can find them as part of this pack and they work in .21:

http://kerbalspaceprogram.com/attachment-pack/

I just pulled the docking strut part and the .dll and installed them, avoiding all of the other parts.

[Captain Sierra]

Okay, here is another idea to avoid your appearance issue. I've done this once to prevent RCS spamming on a station so it wouldnt destroy itself (but that was a 1-time use probe tug). Build the tug to the payload (only really works if your payloads are the same diameter always) and build a framework of girders (quantum struts advised to prevent flexing, but there are other ways) that cover either 2 sides (easier to dock to payload) or that cover 4 (more strut options, better stength). Try to imagine a forklift. Your tug should have these two arms coming off it in radial symmetry that straddle the payload and these arms should have RCS on either side to allow translation.

This is hard to build and even harder to dock without payload. It is advised to have a set of RCS thrusters linked to the tug CoM and then to toggle the rest off when docking to the payload initially.

Note: This works best if your tug and payload are the same diameter. It still works if the payload is smaller but designing a 2.5m tug for a 3.75m payload with this design is hard, very hard.

[sojourner]

Here's a couple images of my tug. It has a docking port Sr. plus the adaptor docked to it allows me to also grab thing with a standard docking port. If I don't need the standard docking port I just leave that part docked to my station and disconnect the Sr. The half spherical tank is monoprop, with the only ;iquid fuel being from that small tank below it. More than enough for moving stuff around near the station, but not really useful for bringing big items from other orbits. The engine is a 40isp nuke from the fatman series.

http://i479.photobucket.com/albums/rr153/seattle221/Kerbal/screenshot12.png~original

http://i479.photobucket.com/albums/rr153/seattle221/Kerbal/screenshot11.png~original

[Psycix]

Just constructed this. The 45* RCS thrusters can be turned on and off using 5 action groups in order to shift the center of RCS thrust back and forth as needed.

Also contains lights, a big battery, a large reaction wheel and 4 KAS winches to stabilize or pull in payloads.