Do fuel ducts have problems with decouplers?

[twilite20K]

I've found that in building some rockets, I haven't been able to link two fuel tanks with a decoupler between them. The fuel duct line always velcros itself directly to the decoupler. Now if I remove the decoupler, the line instantly links to the next fuel tank. But put a decoupler in the mix, and the ducts go directly to it.

Is this by design, or a bug? I've come up with interesting ways around this issue, but it would be nice to just connect a hose and link past the decoupler.

http://i.imgur.com/NnVbnce.jpg

Edited August 31, 2013 by twilite20K

[Ben Kerman]

It probably is because the decoupler blocks the line from the upper to the lower tanks.

Apart from that, from what's visible in the picture I don't get why you would put a decoupler there.

[capi3101]

I haven't had problems with fuel lines connecting to decouplers in 0.21 and I build asparagus fairly regularly, so it's likely to be a question of where you're attempting to make your connections. I usually steer mine well clear of the decouplers on principle.

I will caution you never to try to feed a fuel line through the center of a Hydraulic Detachment Manifold...it may look like it connected to the target tank, but I guarantee you the game will count it as stuck to the decoupler...

[twilite20K]

Since the duct is external, I'm not understanding how the decoupler is blocking anything. I've gotten around this by using the smaller TR-18D decoupler that doesn't fully extend outward, so the two tanks will link up via ducting. But this can't take a lot of stress, and really, I physically shouldn't have to do it, since external ducting should extend around the plane of any decoupler.

It seems to be a problem, since decouplers of course don't automatically pass fuel through, so a fuel line to a decoupler would be useless, (I am thinking anyway).

And I want to remove the engine and fuel tank once I'm in upper orbit, as the 2 big orange tanks are meant to connect to a fueling station.

(But I've had this issue with other designs where the decoupler in the middle of the stack blocks fuel ducts from tanks further up the stack. So this odd design shouldn't really be an issue. I am still learning of course.)

Edited August 30, 2013 by twilite20K

[Tiron]

Since the duct is external, I'm not understanding how the decoupler is blocking anything. I've gotten around this by using the smaller TR-18D decoupler that doesn't fully extend outward, so the two tanks will link up via ducting. But this can't take a lot of stress, and really, I physically shouldn't have to do it, since external ducting should extend around the plane of any decoupler.

It seems to be a problem, since decouplers of course don't automatically pass fuel through, so a fuel line to a decoupler would be useless, (I am thinking anyway).

And I want to remove the engine and fuel tank once I'm in upper orbit, as the 2 big orange tanks are meant to connect to a fueling station.

(But I've had this issue with other designs where the decoupler in the middle of the stack blocks fuel ducts from tanks further up the stack. So this odd design shouldn't really be an issue. I am still learning of course.)

It works like the struts: the duct is drawn in a straight line between the two attach points. If something gets in the way of that line, it attaches to that spot instead of where you clicked. It can't 'route around' anything.

[twilite20K]

The struts are buggy too. So...it's messed up then? I understand now that it works this way, but is it really supposed to work this way? The decoupler doesn't inturrupt the plane of the yellow fuel hose so it seems like a geometry bug, going for the nearest possible connecting joint instead of where it is intended to go.

What I want is automatic fuel flow, past the decoupler. And so far I have only figured out how to do that with fuel ducting. And not without trouble. Usually having to connect to another radially attached fuel tank instead of directly via the stack.

[Dispatcher]

The above posters give good information. I would only add that since fuel lines tend to snap to the next part instead of past it, I have only been able to get past this (no pun intended) by connecting a line from the tank to the part that is in between it and the target, and then connect a separate line from that in between part to the target. It works, but you end up with more parts, more drag, a small bit more mass and something that may not look pretty. I try to tuck these things under wings or otherwise where they are least noticeable (if possible).

[Tiron]

The above posters give good information. I would only add that since fuel lines tend to snap to the next part instead of past it, I have only been able to get past this (no pun intended) by connecting a line from the tank to the part that is in between it and the target, and then connect a separate line from that in between part to the target. It works, but you end up with more parts, more drag, a small bit more mass and something that may not look pretty. I try to tuck these things under wings or otherwise where they are least noticeable (if possible).

I'm pretty sure fuel ducts don't actually have mass or drag, much like struts. Also, you may not NEED the second fuel line if your target is connected to the part that intercepts the fuel line: Fuel lines attached to a part make it able to pass fuel, even if it normally isn't.

Attach some launch clamps and do a static test, it might actually work anyway.

[capi3101]

Push comes to shove and nothings working, try attaching a cubic octagonal strut (one of the little girder-looking things) to the central stack and running the fuel line to it. Cubic struts pass fuel (at least I think they do; barring that, use a BZ-52, I know those pass fuel but they weigh a lot more).

EDIT: Hang on - for some reason I thought you were talking about radial decouplers; didn't realize the problem was with a stack decoupler. That's what I get for not looking at your picture sooner.

Okay...try SCE to AUX.

So what you could do here is ditch the fuel line entirely and in the VAB set up an action group with the decoupler to "Enable Crossfeed". Activate the action group as soon as the physics engine releases your rocket. Alternatively, you could try the cubic octagonal strut trick I mentioned earlier. And worst case, try a senior docking port - you wouldn't need a corresponding port above it. Again, you'd need to set it up to decouple as an action group.

Might also try one of the large stack seperators to see if you get any improved results.

I do have to question why you would want to set your staging up in that particular way. As a rule an engine in the center stack provides your rocket with some stability (meaning it doesn't spin around like a damn blender when you go to launch the stupid thing). If the idea is to conserve fuel for a later engine, you could get the same effect by putting the big fuel tank below the decoupler - it'd give you a much simpler setup.

[M4ck]

You can get rid of the problem and have the exact same part in the end(even without the now unneeded fuel line) bu using a Sr docking port oriented the docking side up, instead of the decoupler. You can also set the "undock" action to an action group to not have to manually detach it when needed. It's a bit more wobbly, but works without the need for fuel lines.

[Galacticruler]

Put the fuel ducts ontop of cubic structs attached to both tanks, it tall enough to ignore the decoupler and short enough to barely notice.

[thereaverofdarkness]

It looks like it should come out from the side, move past the decoupler, and hen go back in, but the game considers your fuel line to go straight across without coming out. I have made similar setups and I don't use fuel lines, but instead just transfer the fuel mid-flight. To do this, just right click one tank, then hold alt and right-click the other tank. You can run the engine while its tank is filling, too!

[twilite20K]

I was able to work around this by using a Sr Dock instead of a decoupler. Which is great because now it's a giant dock for dual use. Thanks for the suggestion M4ck.

And thanks for the other responses guys. Phoenix Refueling Station is now in orbit. Now if I can just decouple the nuclear motor enabled control ship and re-dock it to the side of the twin fuel tanks, I can try to meet up with the other pieces of the station, already in orbit.

Orbit matching is freakin difficult.

[capi3101]

It isn't so bad; like everything else in the game, it just takes practice. Goes a lot faster if you can use LOX fuel but it is possible on RCS alone - and of course RCS makes the actual docking process a hell of a lot easier as a rule.