Question about fuel tanks

[Neil Kermstrong]

I am trying to make the best shuttle I can and it's going pretty well, but I have some problems: It always use the main fuel tank before everything else, and I want to make it so only the external tank is used until it decoupled. I could just transfer all the fuel from it to the main one, but for some reason there are some parts from the tank I cannot transfer fuel, I have no idea on why.

 

[Spricigo]

 

Set the Flow Priority in the tanks that you want to use first to a higher value and Enable Crossfeed in the decouple/docking port between the tank and the engines you want to use.

 

Those option appear on the Part Action Windows if you have Advanced Tweakables enabled. 

[Linkageless]

Whilst use of fuel priority is the best way IMHO, you can achieve more of less the desired effect with an 'External Fuel Duct'.

They will cleanly break when the tank is decoupled.  You can even use a combination, say between the tanks in a stage but have the fuel duct transfer fuel between stages.  Be sure to get the direction right!

[Spricigo]

4 hours ago, Linkageless said:

Whilst use of fuel priority is the best way IMHO, you can achieve more of less the desired effect with an 'External Fuel Duct'.

Don't confuse fuel flow with fuel priority.  External fuel ducts allow/control fuel flow but don't fix a messed up fuel priority. The later need changes in the tweakables or the staging sequence.

[Linkageless]

9 hours ago, Spricigo said:

Don't confuse fuel flow with fuel priority.

I'm not at all confused.  I think we can agree that differing priorities can have the same effect as fuel flow, ie - that certain tanks will be consumed first.

Having re-read the OP, I see there's still an outstanding mystery;

On 9/18/2020 at 11:21 PM, Neil Kermstrong said:

for some reason there are some parts from the tank I cannot transfer fuel

I can't account for this unless there's parts with no fuel crossfeed in between.  A screenshot or craft file would help with the diagnosis.

[Neil Kermstrong]

6 hours ago, Linkageless said:

I'm not at all confused.  I think we can agree that differing priorities can have the same effect as fuel flow, ie - that certain tanks will be consumed first.

Having re-read the OP, I see there's still an outstanding mystery;

I can't account for this unless there's parts with no fuel crossfeed in between.  A screenshot or craft file would help with the diagnosis.

When I reach the space, I try transferring the fuel from my (liquid fuel) boosters to the main tank, but for some reason nothing happen when I press "in", and when I press "out" the button becomes "stop" but no fuel is transferred.

I have crossfeed enabled, the decoupler isn't connected to the ones I am trying to transfer, is that the reason for it?

[Linkageless]

4 hours ago, Neil Kermstrong said:

When I reach the space, I try transferring the fuel from my (liquid fuel) boosters to the main tank, but for some reason nothing happen when I press "in", and when I press "out" the button becomes "stop" but no fuel is transferred.

Come to think of it, I've seen this occasionally, where the transfer doesn't work between two or more parts.  I have to deselect them, then re-select them to make it work.

Forgive me if you already know this, but it's worth mentioning in case it helps someone - be sure to select both the source and the destination tanks.  You can do this by holding the right shift as you right click the parts in Linux (I think it's right Alt in windoze and Mac) to get the part action menus.  If you need to you can pin some or all of these with the little pin icon.  At this point, pressing an 'in' button will pull the resource in from all the other selected parts, and 'out' the opposite.

5 hours ago, Neil Kermstrong said:

I have crossfeed enabled, the decoupler isn't connected to the ones I am trying to transfer, is that the reason for it?

Lets assume all the tanks you are transferring from are all connected together, the decoupler has crossfeed enabled (ie - it shows a button saying 'Disable crossfeed', which perhaps counter-intuitively means crossfeed *is* enabled).  We'll assume the decoupler is between those tanks and the tank you want to transfer to.  This setup should allow transfers any way you'd like.

Somewhere in the difficulty settings there's a 'Fuel transfers obey crossfeed rules', it's worth seeing how that is set.  If you have a part that doesn't normally allow crossfeed in between (eg certain structural panels), then turning this setting off will allow you to work around that.  It's not something I like to do, but whatever brings you joy is the right thing.  From memory, that difficulty setting still doesn't work round the crossfeed settings on docking ports & decouplers.

[Spricigo]

14 hours ago, Linkageless said:

I'm not at all confused.  I think we can agree that differing priorities can have the same effect as fuel flow, ie - that certain tanks will be consumed first.

Maybe we can agree but I'd rather not, since that is not the effect of fuel flow.

Consider a craft with a single engine and multiple tanks. For a for a given tank we may ask:

• Is the engine able to draw fuel from this tank? (fuel flow)
• The engine will draw fuel from this tank after/before other tank in the craft? (fuel priority)

 

Now, we can agre (and I assume that is what you meant in your previous comment) is that we can either control the flow with crossfeed or external fuel ducts (the later allowing restriction to only one way flow). But, agian, the engine will just draw from the with higher priority first,if there is no flow from the tank to the engine that fuel will not be used at all.

Edited September 22, 2020 by Spricigo

[Spricigo]

8 hours ago, Neil Kermstrong said:

I have crossfeed enabled, the decoupler isn't connected to the ones I am trying to transfer, is that the reason for it?

That is not supposed to be the issue. Decoupler will either allow or block the flow for all parts in "the other side".

Anyways, that is a Kerbal Forum, give us MOAR DETAILS!!! Providing a picture is better but at least tell us what exactly you have between the tanks you want to tranfer fuel.  

Edited September 22, 2020 by Spricigo

[Neil Kermstrong]

4 minutes ago, Spricigo said:

That is not supposed to be the issue. Decoupler will either allow or block the flow for all parts in "the other side".

Anyways, that is a Kerbal Forum, give us MOAR DETAILS!!! Providing a picture is better but at least tell us what exactly you have between the tanks you want to tranfer fuel.  

It's just 2 hydraulic radial decouplers connecting the shuttle and the external tank, and in the external tank 4 HLD connecting both the boosters to it, and there are no things like batteries or anything between each tank, it's just pure fuel

[Spricigo]

9 minutes ago, Neil Kermstrong said:

HLD

I'd pleased to meet that guy. Right now I have no idea who he is.

[Neil Kermstrong]

9 minutes ago, Spricigo said:

I'd pleased to meet that guy. Right now I have no idea who he is.

I meant HRD (Hydraulic Radial Decouplers) lmao

Just noticed they are actually called Hydraulic Detachment Manifold

Edited September 22, 2020 by Neil Kermstrong
HDM

[Spricigo]

If you have crossfeed enabled in each and all decouplers, that setup is supposed to work.

 

Not much ideas left...I think I read something about a bug with fuel transfer recently. Let's ask @bewing if he know something about that.

[bewing]

I've been doing a lot of reading for fun, and haven't played 1.10 yet. But I do know there is a fuel transfer bug. It's my understanding that restricting transfers to be between only two tanks is a workaround.

https://bugs.kerbalspaceprogram.com/issues/25881

 

[Grogs]

11 hours ago, Neil Kermstrong said:

It's just 2 hydraulic radial decouplers connecting the shuttle and the external tank, and in the external tank 4 HLD connecting both the boosters to it, and there are no things like batteries or anything between each tank, it's just pure fuel

Maybe I'm misunderstanding, but it sounds like you're trying to use multiple decouplers on each connection. If so, KSP will only connect at one decoupler and the other remains unattached - basically a useless part doing nothing except adding a bit of drag. That could be the problem if the unattached decoupler is the one with crossfeed enabled.

 

If it's not that, then I'm thinking fuel transfer bug. To fix it, I close all the part action menus (right click menus) on the tanks, time warp briefly, and then try again. That usually works for me, but some people report that they have to change ships or go to the tracking station to fix it. No matter what fix I use, I'm only able to transfer to/from an individual tank one time before it stops working again.