Need rudimentary help about fuel flow

[FloppyRocket]

Hi,

I wasn't able to find some basic information about fuel flow on the Forums.

 Previously I've always used fuel ducts for asparagus-staged rockets, which I pretty much understand. But I'm looking at using decouplers and fuel priority, because I find the UI for attaching the fuel ducts pretty clunky and prone to deleting my whole rocket stack when I try to attach anything to the middle of a large asparagus stack.

The image has a simple test ship for fuel flow.

Advanced Tweakables is on. The decoupler has crossfeed enabled. Fuel Delivery Overlay is checked.

Let's call the tank under the capsule the "main" tank, the tank to the left is the "aux" tank.

I did not set the "p#" fuel priority numbers, they were automatically assigned.

The green arrows (and the animation) point from the main tank to the aux tank, and also from each tank down to its engine.

This seems to indicate that fuel is going to flow from the main tank to the aux tank. - and that therefore the main tank will empty first. But when I launch it, the opposite happens - the aux tank empties. That's consistent with what the "p#" values say - the highest priority number us used first, and 0 is higher than -10.

So, here's the questions:

1) I'm confused where those "p#" values came from - I did not assign them.

2) The direction of the green arrows seems misleading. Or am I just interpreting it wrong? Or is the flow diagram buggy?

3) In the main tank info box, the "Flow Priority" row has these symbols:

• button labeled '-' 
• value 10
• value (0)
• button labeled '+'
• button labeled '0'

What do these things do? I pretty much understand the + and - buttons, but I don't  know what the (0) value means, and the '0' button doesn't seem to set anything to zero.

This UI is non-obvious.

4) Why does the yellow node in the aux tank have some embedded red hoops?

5) For a larger asparagus-staged rocket, If you have multiple tanks with the same "p#" value, will they all drain at the same rate?

6) Does it matter in what order you build and attach each tank, or can you set the priorities later?

7) If you have multiple tanks stacked on top of each other, do you have to set the "p#" for each tank separately? Do they all need to have unique "p#' values? For a big rocket with lots of stacked tanks, setting them individually would be very tedious and error-prone.

8) Trial-and-error seems to indicate that you can only use the symmetry tools for placing the decouplers and enabling fuel flow - but you can't use symmetry tools for placing any of the tanks or setting the fuel priority values. Is this the way it's supposed to work?

9) Should I just go back to using fuel ducts?

Thanks for any replies

 

[GRS]

It said that your thing has the main tank drained first, never really tried that so i never seen that type of Overlay, currently playing 1.5, maybe you should try stuff until you reversed the fuel flow.

[Foxster]

It is working as designed, the difference between the flow through decouplers and ducts is that ducts only flow one way, the direction of the arrow on the duct. Decouplers flow both ways. 

Crossfeed on decouplers is kinda OK if you have just drop tanks. If you have engines in the outer stack though (as in your example) then things can get messy. The outer tank with the higher priority gets used first but when empty the outer engines gets fed by the inner tank, which usually is not the desired behaviour as you'd want the outer engine to stop running and you'd stage that stack off. 

An enhancement request was made to be able to set the flow direction of de-couplers. In the meantime it's perhaps best to stick with ducts. As well as being one way, they do have the additional advantage of making fuel flow visually more obvious. 

When I have more time I'll try to answer some of your other questions. 

Edited March 13, 2019 by Foxster

[Superfluous J]

4 hours ago, FloppyRocket said:

This UI is non-obvious.

You can say that again. I don't even look at the green stuff, personally. Just the numbers, and the actual tanks in flight.

4 hours ago, FloppyRocket said:

1) I'm confused where those "p#" values came from - I did not assign them.

The game did, when you set up your staging. The game sets up your rocket so that any tanks that can cross-feed through a decoupler will empty first, so you can toss them. This is based entirely on the root part of the vessel. The game will try to help you shed things that are separated from the root part by a decoupler. In the case of your ship, the outer tank should drain first because 0 is higher than -10. When it does THE ENGINE WILL KEEP FIRING because it can still pull fuel from the center tank, but you can still toss it because that entire assembly has no fuel.

Quote

2) The direction of the green arrows seems misleading. Or am I just interpreting it wrong? Or is the flow diagram buggy?

Yes. Probably. Likely. I really don't know because like I said I don't pay attention to them.

Quote

3) In the main tank info box, the "Flow Priority" row has these symbols:

• button labeled '-' 
• value 10
• value (0)
• button labeled '+'
• button labeled '0'

What do these things do? I pretty much understand the + and - buttons, but I don't  know what the (0) value means, and the '0' button doesn't seem to set anything to zero.

• - lowers priority
• 10 is the current priority
• (0) is the deviation of that current priority from what the game thinks is "correct." It's actually almost always right if all you care about is dropping spent stages.
• + raises priority.
• 0 ... I don't acutally know. i suspect it's a reset but I don't actually remember even seeing it

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4) Why does the yellow node in the aux tank have some embedded red hoops?

No clue. Don't use it.

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5) For a larger asparagus-staged rocket, If you have multiple tanks with the same "p#" value, will they all drain at the same rate?

Yes.

Quote

6) Does it matter in what order you build and attach each tank, or can you set the priorities later?

Not the order you place them, the order you set them in staging. And yes, with those +/- keys.

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7) If you have multiple tanks stacked on top of each other, do you have to set the "p#" for each tank separately? Do they all need to have unique "p#' values? For a big rocket with lots of stacked tanks, setting them individually would be very tedious and error-prone.

You have to do it manually if you want it, and I frequently do. I like setting up early rockets so the fuel drains from the lower tanks first, so the upper tanks keep the center of mass forward. It's just about the single best way with a low-tech rocket to keep your engine from dominating the center of mass.

However with big high-tech rockets I almost never bother as the game always gets it right. You can leave all the tanks in a stage the same number and they'll work just fine, so long as you don't care about the rocket's center of mass.

Quote

8) Trial-and-error seems to indicate that you can only use the symmetry tools for placing the decouplers and enabling fuel flow - but you can't use symmetry tools for placing any of the tanks or setting the fuel priority values. Is this the way it's supposed to work?

Not sure what you mean here. If I attach 2 tanks radially and symmetrically, and change one's priority, the other's changes.

Quote

9) Should I just go back to using fuel ducts?

I can't say that for sure. I am actually a pretty big proponent of the new fuel flow system and even I use fuel ducts sometimes when I either can't be bothered or it would be too much work.

Oh, one more thing, I'd like to clear up a misconception:

4 hours ago, FloppyRocket said:

This seems to indicate that fuel is going to flow from the main tank to the aux tank.

This does not happen. You can imagine it happening in your head, but the game does not model this sort of behavior AT ALL. Well, it does when you manually pump fuel around your ship but it doesn't happen when burning engines.

ALL that happens when you burn your engines is the game finds the tank with the highest priority that still has fuel, and uses that. The fuel basically teleports from the tank to the engine. When that tank is empty, the engine finds the next one in the priority list, until it cannot find any more tanks. Then it stops. if it finds 10 tanks with the same priority, it takes from them evenly, but that's the only real exception to the above rules.

And, of course, it won't look past things that don't allow fuel flow.

This is why a side engine will keep burning even though all of its tanks are empty, and it means you have to watch them to know when to stage.

Edited March 11, 2019 by 5thHorseman

[Zhetaan]

@FloppyRocket:

I'll try to elaborate on some of the things that @5thHorseman didn't have much to tell you:

On 3/11/2019 at 2:00 AM, FloppyRocket said:

2) The direction of the green arrows seems misleading. Or am I just interpreting it wrong? Or is the flow diagram buggy?

Both.  The arrows are correct in that they correctly show that once the aux tank is empty, the engine attached to it will continue to draw fuel from the main tank.  However, it does not correctly show that the main engine will also draw from the aux tank.  Part of the problem is that the fuel flow has to reconcile multiple tanks with multiple consumers, and there is no straightforward way to show that with one directional arrow.  I do not know whether that could be fixed if you placed your main engine the same distance from the root part in terms of the tree structure of the vessel (meaning add a second main fuel tank separated from the first by a crossfeed-enabled decoupler, and put the engine on that), but either way, I find that flow arrows that are inaccurate some of the time are untrustworthy all of the time.

On 3/11/2019 at 2:00 AM, FloppyRocket said:

3) In the main tank info box, the "Flow Priority" row has these symbols:

• button labeled '-' 
• value 10
• value (0)
• button labeled '+'
• button labeled '0'

What do these things do? I pretty much understand the + and - buttons, but I don't  know what the (0) value means, and the '0' button doesn't seem to set anything to zero.

This UI is non-obvious.

The (0) value is the deviation from the game's guessed priority, as @5thHorseman said.  The '0' button resets the fuel priority to that guessed value, not zero.  In other words, it resets the deviation from the game's guess; it does not set the priority to zero.

On 3/11/2019 at 2:00 AM, FloppyRocket said:

4) Why does the yellow node in the aux tank have some embedded red hoops?

That's a very good question.  I'll ping @bewing; if he doesn't know the answer himself, he probably knows who does.

On 3/11/2019 at 2:00 AM, FloppyRocket said:

7) If you have multiple tanks stacked on top of each other, do you have to set the "p#" for each tank separately? Do they all need to have unique "p#' values? For a big rocket with lots of stacked tanks, setting them individually would be very tedious and error-prone.

You do not have to have unique priority values.  However, all tanks of the same priority will drain evenly, which you may not want.  Fuel ducts can override this behaviour.  Generally, all tanks in a given contiguous stack (meaning tanks node-attached to other tanks) will have the same priority.  Priority changes occur when you insert a decoupler.  There are reasons for manually changing the priority of stacked tanks; for example, I frequently set the upper tank to a lower p# value so my stack drains from the bottom up for better handling and mass control.  But there's no requirement to do so.

On 3/11/2019 at 2:00 AM, FloppyRocket said:

9) Should I just go back to using fuel ducts?

I use fuel ducts for asparagus staging because fuel ducts are absolutely one-way; when the aux tanks empty, the aux engines shut down.  However, I also use fuel priority without ducts for preferential draining of drop tanks.  For example, my early Minmus landers often have radial fuel tanks with landing legs attached, but there's also a central tank with a Spark attached to it.  When I run out of fuel in the radial tanks, I stage them away and use what's left to return to Kerbin.  Priority settings save parts in this, but the launchers that send those landers to Minmus use ducts.

I also use priority without ducts for aeroplanes; one of the main reasons for introducing this drain-evenly fuel system was that it kept people from needing cobwebs of draggy fuel ducts to keep their planes balanced in flight.

However, note that they didn't remove ducts from the game when they introduced flow priority.  There is still plenty of room for them; use what is most comfortable for you.

Edited March 12, 2019 by Zhetaan

[bewing]

41 minutes ago, Zhetaan said:

That's a very good question.  I'll ping @bewing; if he doesn't know the answer himself, he probably knows who does.

The diagram is drawn based on a particular "consumer" or "source" of fuel. The green nodes are consumers, the yellow nodes are sources. The arrows show the ways that fuel can flow in and out of the particular node that you selected.

You open the diagram by selecting a particular engine or tank's context menu -- the red hoops show which node/part was the one you chose when you turned on the diagram (ie. the base node of the diagram).

So in FloppyRocket's case, he opened the fuel flow diagram by right clicking on the discardable fuel tank on the left -- and then he opened the context menu for the main tank afterward (which you can also tell because the flow diagram button is unclicked).

Edited March 11, 2019 by bewing

[FloppyRocket]

14 hours ago, bewing said:

The diagram is drawn based on a particular "consumer" or "source" of fuel. The green nodes are consumers, the yellow nodes are sources. The arrows show the ways that fuel can flow in and out of the particular node that you selected.

You open the diagram by selecting a particular engine or tank's context menu -- the red hoops show which node/part was the one you chose when you turned on the diagram (ie. the base node of the diagram).

So in FloppyRocket's case, he opened the fuel flow diagram by right clicking on the discardable fuel tank on the left -- and then he opened the context menu for the main tank afterward (which you can also tell because the flow diagram button is unclicked).

Cool, thanks for the description. It makes sense, but is brilliantly non-intuitive without some form of documentation. I wasn't able to find any. Maybe a good topic for a tutorial on the Wiki.

@5thHorseman

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 When it does THE ENGINE WILL KEEP FIRING because it can still pull fuel from the center tank, but you can still toss it because that entire assembly has no fuel.

Thanks for all your answers.

Wow, that explains a few other "Huh?" things I noticed when test flying. I could not figure out why an engine would keep running when the tank it is connected to reads empty. I forgot to include that one on my list of questions. I was nervous about staging-off an engine that was still running, and figured I had set up the priorities wrong.

Fuel ducts seem simple and intuitive by comparison. Pity that it's so tough to get the end connectors to stick where I want them to go.

@Zhetaan, Thanks much for your replies, the added information was very helpful.

@bewing Speaking of the Wiki, I'd be happy to contribute an article about this, if it's still being maintained.

Edited March 12, 2019 by FloppyRocket

[FloppyRocket]

Thanks for all your replies.

After testing a couple of things, it looks like my concern about not using the symmetry controls was incorrect. Since it sets the priority values based on the staging setup, you can pretty much just leave it to sort out the details.

With the fuel priority system, the fuel gauges in the staging menu (left side of the view screen) seem kinda useless. Even when only one pair of tanks are going empty, all of the gauges are draining identically. So to know when to stage-off an empty set of tanks, I have to keep RMB-clicking on the next tanks that I expect to go empty, so i can stage them off when they hit zero. You can't tell when to stage by loss of an engine, because the engines keep firing.

In contrast, when using fuel ducts the left-side gauges clearly show  when a tank is about to run empty, without having to click on any of the tanks.

I'm off to try and make it work on a larger rocket (two layers of 6-stalk asparagus around a center core).

 

 

[Superfluous J]

2 hours ago, FloppyRocket said:

So to know when to stage-off an empty set of tanks, I have to keep RMB-clicking on the next tanks that I expect to go empty, so i can stage them off when they hit zero.

While this is a totally valid way to do it (and frankly it's the way I still do it even after discovering this next thing) there is a better way. Unlike KER, the stock dV system knows that you'd want to drop those tanks and engines. So, as long as your staging and fuel priorities are the default asparagus setup, and you didn't confuse the game with docking ports, you can confidently stage away when the current stage reads 0m/s.

[Zhetaan]

9 hours ago, FloppyRocket said:

Cool, thanks for the description. It makes sense, but is brilliantly non-intuitive without some form of documentation. I wasn't able to find any. Maybe a good topic for a tutorial on the Wiki.

If you should write your article, then please post a link to it in the Tutorials sub-forum, as well; I think I can safely speak for everyone when I say we'd all appreciate it.

[FloppyRocket]

19 hours ago, 5thHorseman said:

While this is a totally valid way to do it (and frankly it's the way I still do it even after discovering this next thing) there is a better way. Unlike KER, the stock dV system knows that you'd want to drop those tanks and engines. So, as long as your staging and fuel priorities are the default asparagus setup, and you didn't confuse the game with docking ports, you can confidently stage away when the current stage reads 0m/s.

Well, that makes good sense - just use the dv values instead of the bar graphs. I totally overlooked that as an alternate.

On 3/11/2019 at 12:55 AM, Foxster said:

It is working as designed, the difference between the flow through decouplers and ducts is that ducts only flow one way, the direction of the arrow on the duct. Decouplers flow both ways. 

Crossfeed on ducts is kinda OK if you have just drop tanks. If you have engines in the outer stack though (as in your example) then things can get messy. The outer tank with the higher priority gets used first but when empty the outer engines gets fed by the inner tank, which usually is not the desired behaviour as you'd want the outer engine to stop running and you'd stage that stack off. 

An enhancement request was made to be able to set the flow direction of de-couplers. In the meantime it's perhaps best to stick with ducts. As well as being one way, they do have the additional advantage of making fuel flow visually more obvious. 

When I have more time I'll try to answer some of your other questions. 

Yes, the two-way flow through decouplers had me pretty confused.