Need an nth opinion regarding fuel and inertia

[DerGolgo]

Okay, situation: Been trying to carry my vastly oversized Duna lander to, well, Duna. The mission objective: do it in about as silly a manner as possible, more or less.
The conundrum: This is about my fourth ship I have build for the purpose, the third fundamentally different design. I thought I could be clever by arranging fuel-tanks in a train-like manner, so that I could just run the engines from the one at the and and drop that once it's empty. AGX and very many click-clicky sessions on the flow-priority buttons, the old Drainex fuel meter from SmartParts, that should sort it.
'xcept it don't.
As you can see in the video, the fuel drainage is ridiculous. LH2 fuel is drawn from the tank with flow priority 0, while oxidizer gets drawn from buggerifIknowwhere. And the tank at the end, with flow priority 90, parts with not a drop.

Explanation I have for it: inertia is, of course, shoving the fuel away from my engines here, and the fuel pumps can't keep up.
Only solution I could think off, open TAC Fuel Balancer, set the tank I wanted to drain to "transfer out", and set ALL other tanks to "tranfer in", set the transfer rate to 100 and try dialing the throttle back until the fuel tanks can keep up ... has failed. I thought of it while writing this and just tried it, no joy.

So, have I indeed seen the error of my ways, is Dunacraft III - The Cryo Strikes Back just an idiotic design that I really should have used hyperedit to test before spending hours assembling the damn thing in orbit?
Or is there ANY way of making this work?
Like I said, I wanted to do it in as silly a manner as possible. Pretty sure there are ways sillier ways of doing it. But this one, hauling that trainload of disposable tanks, I kinda like that. And, fully expecting that, as usual, I only get things right when I try and work out what I got wrong earlier, am already mentally designing Dunacraft IV - The Last Cryo. But I really kinda like this one ... any advise is welcome!
 

 

[Corona688]

I can't tell for sure with a high-res game crammed into a low-res video, but it looks like your bottom tanks have a higher fuel flow priority than your top ones.  I think the game burns the lowest-numbered tanks first.

I think you can alter fuel flow priority in flight.  Checking.

[edit] Yes!  You can alter fuel flow priority in-flight.  Set your bottom tank to zero, the tank above it to 1, etc, etc, etc, and they should burn in the order you want.

Edited December 8, 2017 by Corona688

[DerGolgo]

It has been my experience that the game burned high priority tanks first. Been using the flow priority feature for some time.
I'll try what you suggest, as it is no effort to load the game and fiddle the priorities around. But I am quite certain that, whenever I used the feature in the past, higher priority tanks got burned first.

[Corona688]

You are correct, I got it backwards.  I was probably confused by all the the negative numbers the game auto-assigns for some reason.

Anyway, that's how you do it.

Edited December 8, 2017 by Corona688

[DerGolgo]

1 minute ago, Corona688 said:

Anyway, that's how you do it.

I know that it is how one should do it. And it is how I tried it. The tank at the bottom, the one that should drain first, has flow priority 90. The one near the top, that actually drains, has flow priority 0. Any of the tanks in between, with flow priorities, 20, 30, 40, etc., don't drain, either. So that's not it.

[Corona688]

Again, I can't really see your spacecraft, so I'm limited to wild guessing here.  The negative numbers fooled me, could they have fooled you too?  What if that's -20, -30, etc?

I do know the game doesn't care one bit about ullage or inertia.  Which is why you can climb your way into space by transferring fuel.

The root part may matter, as well, so if you built your craft upside-down or something, try reversing the polarities anyway.  It works in star trek.

Edited December 9, 2017 by Corona688

[swjr-swis]

1 hour ago, DerGolgo said:

Or is there ANY way of making this work?

You make no mention at all about crossfeed settings. Decouplers default to crossfeed disabled, which means that they form a barrier between every disposable tank and the rest of the craft. Have you checked if all the decouplers in that train are set to enable crossfeed? Otherwise no matter how you set the flow priority, they will never get used.

1 hour ago, DerGolgo said:

while oxidizer gets drawn from buggerifIknowwhere

It's very hard to tell what engines you are using with that low video quality, but there are engines that do not use oxidizer at all. Or, due again to the crossfeed setting not leaving it any other option, it may be using oxidizer from any other tanks that are accessible... like the orange tanks of the Duna lander.

[DerGolgo]

I've checked all crossfeed settings. When the flow priorities checked out, that was the first that had come to mind. I have manually inspected every single part. Every crossfeed is on.
Engines are LH2-O, "Sarnus-HE2JT-250K "Tohces-P" Cryogenic Aerospike, inflated with tweakscale. Bipropellant, 93.75% LH2, 6.25% O. Indeed, there are engines that will only draw hydrogen from the tanks. The atomic engines, for instance. Those tend to have a red flame, though.

I've been playing KSP for some time, I am quite sure that any flaws in operation or execution of the design can be ruled out. I believe it either is a fundamentally flawed design, one that just will not work in game. Or it requires special design considerations that would be pretty specific for this sort of problem.
 

Edited December 9, 2017 by DerGolgo

[Aegolius13]

The stock game goes not model inertia vs fuel flow. If a flow path obeys crossfeed and fuel priority rules,  it will flow regardless of what the skip is doing.   E.g., no village.  Though some mods might do stuff like this. 

It's very odd the game is treating LH2 and oxidizer differently.   This makes me wonder if it's related to an LH2-only tank somewhere on the ship. I'd check fuel levels,  crossfeed and priority for all such tanks. 

PS - I have done similar fuel tails before and they worked fine.   Nothing about the concept should run afoul of stock rules.   Seems like a mod bug or conflict is also possible. 

Edited December 9, 2017 by Aegolius13
Saw you were using LH2, not LF

[DerGolgo]

1 minute ago, Aegolius13 said:

It's very odd the game is treating LF and oxidizer differently.   This makes me wonder if it's related to an LF-only tank somewhere on the ship. I'd check fuel levels,  crossfeed and priority for all such tanks. 

No LF-only tanks. Just the main tanks, with LH2 and O, and smaller tanks with LFO for the RCS, and the lander, laden with LFO.
I have checked and set the flow priorities for all LH2O tanks, with the exception of the two tanks at the top, that have the engines clustered around them. Those were set with a -10 default, and I just left them thus.

I just took another look, just to see that I had in fact set all Crosfeeds to "On".
Thing I only just found was in the decouplers. I have three packs of three drop-tanks, with a docking port at either end and two decouplers within the trio.
Those decouplers, as it turns out, have two crossfeed settings. One at the bottom, "Crossfeed: On". But just above them, I only just found "Enable crossfeed". I'm baldy visually impaired, so while I probably noticed there was a word beginning with the letter E there, I couldn't see the rest. And since I had found the Crossfeed setting below it, I didn't look.
Just fixed those crossfeeds and the fuel flows from where it should, problem solved. THANK YOU!!

[DerGolgo]

I have also sussed out where I had the dumb regarding the oxidizer. Never noticed this before.
As it turns out, when draining any fuel from a bipropellant tank with priority x, it won't drain the necessary oxidizer alongside. Rather, it treats the priorities for any oxidizer separately from anything that's due to be oxidized.
If a tank of liquid hydrogen and oxidizer has a priority of 20, and no other tank with liquid hydrogen has a higher priority, that's where the hydrogen will come from. But if there's an LFO tank with a priority of 21, while not draining any of the liquid fuel from there, that is where it will take the oxidizer. Quite perplexing, confusing, and othering, really. Particularly when I set those priorities before launching the individual components of the craft, yet the game resets them all once those components are docket together. This I have observed.

Edited December 9, 2017 by DerGolgo