Question about Tug design.

[Chocki]

I am building an orbital fuel station. The core is up, and now I am doing early testing and design on a tug. The main objective for the tug will be to dock with a fuel tank attached to a lifter, then move the tank to the station and dock it with the station in a 125k orbit. Leaving the lifter behind (for its own retro burn to dispose).

The tug currently has 1 2.5m atomic motor (300 @ 800 isp), and 2 1.25 atomic motors (60 @ 800 isp) for any heavy pushes it may need to do. (I would like it to double as a station mover if feasible.)

My question is in regards to the rcs thrusters. I currently have 8 (mounted 4x symmetrical) equidistant fron the center of the unloaded tugs mass.

Would it be better (as in easier to control) to mount rcs thrusters on I beams extended foward from the tug, allowing for equidistant rcs while carrying its load?

The load is just a single 2.5m orange fuel tank with docking struts. Future payloads will include rcs fuel, and possibly a lander. My idea is to have the extended rcs thrusters on the tug, so payloads can be designed to keep the tug c.o.m. around the same point.

The c.o.m for the loaded tug, is close to the front (docking point), and the tug itself ways around 40 tons.

Will placing those extended rcs ports make a noticeable difference? Are they really needed? The station the tanks will attach to is perfectly balanced as long as I dock tanks symmetrically. And until I dock the tug on it's rear end.

[dr_jt]

Kerbals are not well understood

Edited June 6, 2013 by dr_jt

[Zephram Kerman]

I ended up doing the opposite. My tug has balanced RCS ports, like you described. Whatever modules it installs always have their own RCS arrangements too, so that the combination is still balanced. That's a lot simpler than guessing an offset distance for modules you haven't even thought of yet.

[K^2]

Precisely. If each ship has balanced RCS, when they dock, the combined ship has balanced RCS. I can demonstrate that with equations, but I doubt any of you actually want to see these.

[sal_vager]

Oh go on K^2, there are a lot of members who like the science aspect

And you could repost it in the science labs!

[Chocki]

I thought about giving the payloads rcs, but that means adding rcs fuel to every payload. My station (Citgo Station I) core already has 2 750 unit rcs tanks, any more would be wasted. My goal, is to desgin 2 types of fuel payloads, 1 being the large orange liquid tanks, the other being a half rcs, half liquid that equals the same weight as the orange tank described above.

As much as future payloads haven't been made, I am willing to abide by tolerances that are set by my tug.

Honesty, I am worried about 2 things.

Will those 3 motors be enough for an interplanetary push? Allowing the tug to double as a station delivery method.

Will a small offset of rcs on the tug, during payload delivery, be a big issue for a docking newb?

When payloads are attached, and tug docked, K2 (with math powers that I trust) figures the rcs balance should be fine for the complete station.

[Noticeably FAT]

No need to add RCS to the payload, the payload thrusters should share fuel from the tug (just make sure your tug carries plenty. I would assign all the payload RCS thrusters to a common action group so you can shut them down once they are on the station. If you don't you'll end up with the situation I have, that is you either manually disable the ones you don't need on the station or you have dozens of unnecessary thrusters firing.

[maltesh]

Will those 3 motors be enough for an interplanetary push?

Once in orbit, any amount of thrust can take you interplanetary, as long as you have sufficient delta-V capability in your fuel storage. Very low thrust can make the trip more complicated to achieve, though.

I ran a similar Tugbot and Tanker design to the one you mention in the OP (though with fewer NTRS) to Moho last week.

Unfortunately, said design didn't have much fuel after arrival. But that's Moho for you.

[leax256]

Scott manely has pointed out that it is better(more stable) to use the tug in a pulling action rather than a push.