RCS placement for space tug

[CaptainKipard]

I've looked into this and I read somewhere that if you place balanced RCS on your tug and separately on the object being towed then that wont give you any unwanted torque when translating. Unfortunately this ignores the fact that the amount thrust has to be proportional for all RCS thrusters to the distance from the common COM when docked with the object you want to manipulate.

This is sorted out by fine control AFAIK, but with fine control you don't even have to be careful with RCS placement, plus fine control is slow as hell.

There must be a better way to control translation perfectly at full thrust. I've thought about adding a number of thrusters roughly proportional to the mass of each object and the tug, and just adding a strong reaction wheel, but that's still not ideal.

Edited March 6, 2014 by Cpt. Kipard

[kahlzun]

Are you talking about -any- ship you dock to, or ones in a known configuration?

If it's always the same design of ship, you can have extra RCS thrusters you toggle on and off to help with the torque, but you'd probably need some part (girder?) which extends out past the shared COM.

[CaptainKipard]

Any ship, yes. I always keep all my ships perfectly balanced for all permutations of wet, partially wet and dry masses, but problems start when you have to translate after docking with some other ship which on it's own is also perfectly balanced. Together they are not balanced.

Toggling thrusters is ok but still not more ideal than what I already tried, and it might get a little fiddly.

I've thought about a tug with a very long cradle at the end of which I'd put some RCS but that's also not ideal, and would only restrict the shape and size of the thing I'm able to move.

[SRV Ron]

If the object being moved doesn't contain thrusters, attempting to translate the tug and load becomes very RCS fuel inefficient as the SAS attempts to balance the effort.

[Geschosskopf]

Any ship, yes. I always keep all my ships perfectly balanced for all permutations of wet, partially wet and dry masses, but problems start when you have to translate after docking with some other ship which on it's own is also perfectly balanced. Together they are not balanced.

Toggling thrusters is ok but still not more ideal than what I already tried, and it might get a little fiddly.

I've thought about a tug with a very long cradle at the end of which I'd put some RCS but that's also not ideal, and would only restrict the shape and size of the thing I'm able to move.

I also build my individual modules perfectly balanced (thank you RCS Build Aid) and assemblies of them can be imbalanced. Therefore, I try to avoid this problem as much as possible, and minimize it when its unavoidable. The best way to avoid the problem is by never trying to translate an assembly, just individual modules. Thus, when assembling 3 or more modules, the previously assembled part is the passive ship and the incoming module is the active ship. I also try to design things so the need to dock completed assemblies together is eliminated as much as possible. When I have to do this, I minimize the balance issue by trying to make the assemblies as balanced as possible. This is fairly easy to do if you keep the design of the assemblies within reasonable size limits.

All this leads me to doing flotillas instead of single monster ships. For instance, when building a station at some other planet, I send each module separately and assemble them on location. Each station module goes out as a 3-module, linearly connected ship: payload, fuel, and engines. The payload and engines weigh about the same and the fuel goes in the middle. Such ships are usually balanced well enough that they translate with only a slight tendency to rotate, which is easily dealt with. If for some reason it's not well-balanced, no biggie because the station module by itself is perfectly balanced; the fuel and engines aren't going to be part of the station anyway. So once this gets to the destination, I can separate the station module and dock it no problems at all.

[CaptainKipard]

What I'm trying to do is design a large modular ship, with interchangeable engines (because I haven't gotten my nuclear ones yet), and a separate truss to accommodate an indefinite number of unpowered dockable fuel tanks to arbitrarily extend the range of the ship. For that I really need a tug capable of translating very heavy loads. I can do it just fine, it's just a matter of making something that moves quickly and accurately.

[Jouni]

If the object being moved doesn't contain thrusters, attempting to translate the tug and load becomes very RCS fuel inefficient as the SAS attempts to balance the effort.

I've done a lot of docking with utility tugs pushing or pulling large payloads without RCS. There is just one trick you need to know, and the whole thing becomes about as easy (or as hard) and as efficient as docking with balanced RCS. Unless you are doing something terribly wrong, even the most badly imbalanced monster is going to have balanced RCS on its main axis of thrust. So instead of translating in arbitrary directions, you just rotate the craft and translate on that axis, until you have almost reached the destination. Then you turn to face the docking port, thrust forward, and dock. The final corrections with imbalanced RCS may look interesting or even alarming, but all you really need is a bit of patience.

[m4rt14n]

You might need to use them in pairs..

[CaptainKipard]

... you just rotate the craft and translate on that axis, until you have almost reached the destination...

... The final corrections with imbalanced RCS may look interesting or even alarming...

This is a simple, and effective solution, thanks. Once I'm close enough I shouldn't have a problem just using fine control. It shouldn't take too much time then.

You might need to use them in pairs..

This is a little more complex and potentially more limited, but it's still very interesting, I'll have to try it.

[m4rt14n]

I read some old threads a while back about pairing tugs. Best if you can dock them on the sides. There is of course a problem of aligning them properly...

[CaptainKipard]

There is of course a problem of aligning them properly...

If your tugs are well balanced, then all you'd need to do in theory is place some multi-port attachment points in the right place on your ship section. RCS Build Aid makes this easy too.

[problemecium]

Actually OP, I've found through experience that what you heard was wrong. You can perfectly balance two things, but if they have different TMRs then when you dock them together you end up with a chunk of ship on one end that has more power than the chunk on the other end, so the whole thing goes off balance. The effect is proportional to the difference in masses / TMRs of the tug and the payload.

Imagine a ship made of two orange tanks, but one has 8 RCS blocks and the other has 4. It'll be unbalanced, right? That's what's going on.

That said, I still try to make perfectly balanced space tugs just so they're extra-easy to dock to the cargo and I can save my effort for the hauling.