Orbitally assembled ship fuel flow problem

[NFunky]

Hey all, sorry if this is the wrong sub-forum for this, but I'm not convinced it's a bug. It very well just be my lack of understanding of how these things work, so Gameplay seemed like the place to start looking for solutions.

I've built this big old interplanetary monster, mostly by bringing up modules in a shuttle and docking them together. It has three Nerv engines which were each individually docked to a small fuel tank and four fuel tank modules connected radially to the body. These four are not directly connected to the engine, but I figured I could just manually pump fuel from them when I needed it. However, I assumed that docking the three engines to a fuel tank would allow them to crossfeed and fire. They are each connected via docking ports to a tri-coupler, which is attached to the fuel tank. I thought this configuration would allow flow from the tank to the engines, but it doesn't seem to be.

Before I go any further, here's a picture of the thing:

https://www.dropbox.com/s/odrzo69c8eb5afc/Interplanetary%20Ship.GIF?dl=0

As you can see, there is fuel in the tank, all engines are staged and the throttle is at 100%, but no engine flames and Kerbal Engineer reports I have 0 dV. Am I missing something or was this just not a feasible idea to begin with? Could this be a crossfeed bug or something? Any way to fix it?

BTW, I had an issue with the last shuttle docking where I had to edit the save file to get around the "Won't Dock / Undock" bug thing, but I can't find any threads dealing with this issue specifically. I spent SO long and so many flights to build this thing, I'd be so sad if I had to just scrap it.

Edited August 10, 2015 by NFunky

[AlanP]

The only thing I can think of is crossfeed enabled on the docking port? I'll keep looking and thinking.

[Eric S]

The only crossfeed "bug" I know of involves the one-connector side of the tricoupler not being able to pull fuel from any of the three connectors on the three-side. It's not a bug, it's a design decision that some people may not agree with, and I'm fairly confident that it's not the problem here.

I've created a similar craft to test this kind of fuel feed, and it works fine for me in the most basic case (mk 1 capsule, fuel tank, tricoupler, docking port, docking port, stabilizer, Nerve). So either you've found a corner case in the fuel flow code, you accidentally turned off fuel flow somewhere, or I failed to recreate the issue properly. Did you dock the engines in the VAB, or after launch (and if in the VAB, why have them, out of curiosity?).

[RandomRyan]

try right clicking on the docking port and is the crossfeed enabled?

[NFunky]

Thanks guys,

I did make sure all three ports have crossfeed enabled. In fact, I tried disabling them and shutting down all the engines, then realoading and reactivating everything, but it didn't help.

I brought up each engine separately and docked them to the main ship.

I'm totally stumped. Everything looks like it should work, but it just doesn't.

[Kryxal]

I wonder ... if you attach a smallish tank to the engine directly, instead of using only the fuel in the bigger tank, does it work? If so, it would probably be counted as a bug or logic error...

[NFunky]

An interesting idea actually. I'll send the shuttle back up with another liquid fuel module, but this time, with the help of a klaw tug, I'll try sticking it between the engine and the tricoupler. That'll be the most direct feed I can manage in orbit, and if it doesn't work, I'll know it has to be a bug.

If it really is a bug... I'll be sad. And maybe I'll just dump all 3 engines into the atmosphere and send up new ones with little tanks of their own. It'd be real ghetto and require a lot of right-click fuel routing to fly, but at least all my work wouldn't have been for nothing.

If anybody has any other ideas that might help, I'd be extremely greatful.

[Reactordrone]

Possibly a stupid question but are you sure the engines have been activated? It is possible for engines to appear in the current staging without being active and they'll show no fuel available in that state.

[Red Iron Crown]

I would also double check that you haven't thrust limited the nukes to zero for some reason (I do this sometimes when an engine is a payload).

[Pecan]

My suspicion would fall on this tri-coupler, which I can't see.

You could attach the tank-side ports directly, with symmetry-3 so the coupler isn't needed. If it IS there though, presumably you've clipped it inside the tank itself and, possibly, to a different node inside the tank. This is what makes me wonder - is that coupler getting any fuel feed in the first place? Certainly, if it is, the 1->3 feed and transfer via the ports should work.

[NFunky]

I was wondering about that tricoupler. It is indeed clipped into the fuel tank, cause I didn't think that was too cheaty, but I did originally connect it to the bottom of the tank before sliding it in. I also checked the AttN lines for the tank, the tricoupler and the docking ports and they all seem to have the appropriate parent numbers.

I'm pretty positive I've got the engines all started up. Here's another picture that's a close up of the engine assembly. I've activated the right-click menus for the fuel tank, a docking node and the engine attached to it.

https://www.dropbox.com/s/362aiknzo6ymbnr/planetary%20ship%202.GIF?dl=0

Oh and thanks again for brainstorming on this guys. All your answers have helped kick my brain into gear. Still have no idea why it's not working though.

[Reactordrone]

It does look like you need to click on the activate engine button. If they were active you'd have a "shutdown engine" button there instead of an "activate engine".

[NFunky]

Oh man, egg on my face. Sorry about that. I had been turning them on and off to try to fix it. Here's a new pic with the right-click menus that has the engine activated.

https://www.dropbox.com/s/w00d1pc0nr9ylrl/planetary%20ship%203.GIF?dl=0

[NFunky]

It is with great pleasure that I have now been able to mark this answered. I figured it out! It looks like it was the DOCKING PORTS after all, and was almost identical to the fix for ports that won't undock. Seems the top node of the ports aren't specifically connected to their partners, so changing that allowed fuel to flow again!

Thanks for helping bouce ideas around guys. I'm so happy I've got this thing working again.

[Sharpy]

Ah, yep. My suspicion. At first I thought you surface-attached the ports to the tank, then I read about the tricoupler, and... you surface-attached the ports to the tricoupler