"Crossfeed Capable" Hydraulic Detachment Manifold? How do I get fuel to flow through?

[Machinus]

This connector says it can crossfeed, but the fuel doesn't flow. How do I get it to work?

[Taki117]

If you have a fuel tank attached to either side of the decoupler you can transfer fuel manually between them through the decoupler, I am unsure if you have a fuel tank attached to an engine if the engine will fire (Will require testing) however, it will not automatically pull fuel from an attached tank.

[KvickFlygarn87]

That means that fuel will flow through it. However, fuel will not flow radially, so as for the HDM it means you can flow fuel (with fuel lines) into one side and out on the other (once again with fuel lines)

[Murph]

The crossfeed in part descriptions is only for manual fuel transfers, I believe. Automatic fuel feed to engines is moderately complex. Some parts do feed fuel, others don't. The way parts are attached also influences it  e.g. some will feed stacked but not radially. The solution is the same in all cases if you have a part that isn't feeding fuel where you need it to go  add an external fuel line, but also keep fuel lines to a minimum, as too many can cause a different set of problems.

[Kasuha]

The crossfeed capability is kinda pointless for this part because it is always mounted radially and the fuel scan does not pass to radially mounted parts.

By the way, crossfeed capability has nothing to do with manual fuel transfers.

To get fuel past any radial decoupler, you need to use fuel pipe.

[Taki117]

T...

By the way, crossfeed capability has nothing to do with manual fuel transfers.

...

I stand corrected, thank you.

[Rhomphaia]

The crossfeed capability is kinda pointless for this part because it is always mounted radially and the fuel scan does not pass to radially mounted parts.

By the way, crossfeed capability has nothing to do with manual fuel transfers.

To get fuel past any radial decoupler, you need to use fuel pipe.

Except for when it does.

Fuel will flow happily radially from parent to child and since those manifolds are crossfeed capable it will flow through them.

Won't flow the other way from child to parent though and fuel lines will take priority.

[Kasuha]

Except for when it does.

Well, I did not say the fuel scan cannot pass from the radially mounted part to the part on which it is radially mounted. But your example is still surprising and I gotta try it when I get home. I am pretty sure, though, that the child/parent relation is unrelated to this. Fuel can "flow" any direction through the ship structure as long as conditions for fuel scan to pass the connection are met. And one of these conditions usually is that the scan only goes through the two "main" attach points present on almost all parts.

The only way this is related to parent/child relation of parts is that (unless you use mods) you can never mount parent radially on its child.

Edited December 13, 2013 by Kasuha

[phunk]

Fuel can "flow" any direction through the ship structure as long as conditions for fuel scan to pass the connection are met.

Actually, direction matters with some parts, like the quad adapters. If you put 4 engines on one tank with a quad adapter, fuel will flow fine, but if you have 4 tanks and 1 engine, it will not flow that direction.

[Kasuha]

Actually, direction matters with some parts, like the quad adapters. If you put 4 engines on one tank with a quad adapter, fuel will flow fine, but if you have 4 tanks and 1 engine, it will not flow that direction.

That's meant by second half of my sentence.

There are other special cases. For instance in the HubMax part, the scan goes through all six of its attach ports. That was quite a surprise to me by then.

[Rhomphaia]

Parent child relationship is related, consider the following

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Reversing the above situation so that the rocket is on the parent and the fuel is in the child and no fuel flow.

Fuel will prefer to flow down the stack though.

[Rhomphaia]

Actually, direction matters with some parts, like the quad adapters. If you put 4 engines on one tank with a quad adapter, fuel will flow fine, but if you have 4 tanks and 1 engine, it will not flow that direction.

Not in this case though, It can be seen in directly radially attached tanks without the manifold. try mixing up jet fuselages and LOx tanks in various ways.

[Kasuha]

Parent child relationship is related, consider the following

That's not about parent-child relation, it's about radial mounting. The decoupler is radially mounted on the fuel tank, so the fuel scan (going from the engine) can go from the decoupler to the tank, but not the other way. The nacelle apparently has can forward the fuel scan towards things to which it is mounted by its side - which is not true for the fuel tank. That's property of the nacelle, not of the parent-child relation.

The nacelle and e.g. the girder segment can forward the fuel scan to part to which it is surface-mounted. That's all. And it's property of the part. And I believe that it doesn't depend on which of the two parts is the parent and which is the child. Because that can change, e.g. during docking.

Notice that being mounted by surface to something is a very different kind of relation that having something surface-mounted on it.

Finally, notice that not all parts have that property. Also notice that the fuel is in my example clearly drawn from a child node.

[Rhomphaia]

Surface attaching B to A means B is a child of A

All of the parts there with fuel flow are children of the girders, which draw fuel through the attachment node which I showed earlier takes precedence

None of the parts there where fuel doesn't flow are crossfeed capable.

[Kasuha]

And I believe that it doesn't depend on which of the two parts is the parent and which is the child.

Okay I take that back. The nacelle only draws fuel from the decoupler (or whatever it is attached to) if it is its child. I docked two identical structures and as soon as I disabled crossfeed through the docking port, one of engines lost access to fuel.

[Kasuha]

... and finally after a few experiments, it looks like every part which can be surface-mounted and is crossfeed capable allows fuel scan from child to parent, except fuel tanks. I have not tested all parts but I tested a lot of them. Even SRBs can draw fuel from the tank to which they are attached if you mount an engine to them.

So you were right.

[Rhomphaia]

... and finally after a few experiments, it looks like every part which can be surface-mounted and is crossfeed capable allows fuel scan from child to parent, except fuel tanks.

Fuel tanks can as well, but only if you close off the flow.

Probably to prevent loops.

[Kasuha]

Probably to prevent loops.

Given the rest of the implementation, I believe it is to prevent outer engines draining fuel from the middle tank and starving the middle engine in case of this kind of construction:

Edit: but that may still happen if you do something like this:

Edited December 14, 2013 by Kasuha

[Kasuha]

Fuel will prefer to flow down the stack though.

It's not just down the stack. It may be up the stack as well. Or branch out. And although the image below does not prove it, it doesn't depend on which is parent and which is child.

That makes three levels of priority:

- first go fuel pipes

- second is attach point traversal

- third is child to parent radial mounting -or- drawing the fuel from itself, if it is an (active) fuel tank

Edited December 14, 2013 by Kasuha