Confusing fuel delivery

[miklkit]

I'm building a new rocket and want the twin boar and two mainsails to light off together and have the mainsail tanks fill the twin boar tanks until they are empty and drop off.
The fuel delivery system has a glitch that I do not know how to fix. What am I doing wrong?
https://steamcommunity.com/sharedfiles/filedetails/?id=2000770215

[Vectura]

Set the side tanks to higher priority and they’ll drain first, even if the overlay is broken. Give it a quick test flight to confirm that.

[Azunai]

the default behavior (if you don't mess around with priorities) would be for all 3 engines to suck the outer tanks dry before they even touch the inner tanks.

all you have to do is enable crossfeed on the decouplers. you don't need those yellow fuel hose for such a simple setup and you don't have to mess around with the priority system.

[miklkit]

Eh? I put the hoses on because that is what the experts do. Monkey see, monkey do.

The decouplers refused to work for months but recently did start working so I'm still learning how to use them. Will enable crossfeed later.

[Azunai]

the fuel lines date back to a time before the game even had that fuel flow overlay and the priority settings and all that jazz. don't think there are many applications in recent game versions where you have to use fuel lines to get the job done.

i guess you might even be able to set up an asparagus staging sequence without using fuel lines nowadays.

[Vectura]

You can easily set up the asparagus. The difference is, the engines on the empty tanks won't stop firing- you have to keep an eye on the fuel levels. Upside is that you don't spend even a second carrying dead engine weight, only the tanks while empty before you jettison them.

[miklkit]

I've been gone all day and will try some more later.
I did make one test run and all three tanks drained evenly and when the decouplers blew the tanks stayed put. FUBAR!

[Azunai]

guess the fastest way to fix that would be to delete the lower stage and the side boosters and rebuild them from scratch. guess the tanks may have been attached to the center tank instead of the decouplers. that also explains why the lower stage shows a single deltaV value and isn't split up into 2 different values for the boosters and the sustainer stage

[miklkit]

I've tried rebuilding it and still don't have it correct. The first attempt with just the decouplers crossfeed enabled had the fuel flow from the left stack flowing in the wrong direction.
https://steamcommunity.com/sharedfiles/filedetails/?id=2001819776

So I added a single fuel line to correct that. The rocket worked perfectly except that all fuel tanks drained evenly. The decouplers worked correctly and the empties fell away well and then the main fell away. Progress!

[Vectura]

Did it ACTUALLY have the fuel flowing in the wrong direction? I find the fuel overlay is often entirely incorrect, it doesn't show the effects of priorities.

[Azunai]

kinda hard to say from the image if anything is wrong. the decouplers are wrong, so the game can't properly calculate the deltaV for the boosters and sustainer stage.

other than that, the setup looks correct, though.

did you actually check which fuel tanks get drained during ascent? when i build something like that, the game automatically drains the outer tanks first. if you actually check their contents, the outer tanks should be empty by the time the staging sequence at the left side shows 0 m/s left for the lowest (booster) stage. at that point the main tanks are still full and haven't been touched at all.

[miklkit]

I have been trying every combination. The hydraulic decouplers do not work at all but the others do work as decouplers. It doesn't matter if they are set to crossfeed or not. All three engines drain their own fuel and the run out at the same time. I have tried no fuel lines as well 1,2,3, and 4 fuel lines. Nothing works.

I gave up and tried to go with solid fuel boosters but they will not attach to the decouplers. Oh well, it still works well enough to get to the Mun.

[MechBFP]

Originally posted by miklkit:

I have been trying every combination. The hydraulic decouplers do not work at all but the others do work as decouplers. It doesn't matter if they are set to crossfeed or not. All three engines drain their own fuel and the run out at the same time. I have tried no fuel lines as well 1,2,3, and 4 fuel lines. Nothing works.

I gave up and tried to go with solid fuel boosters but they will not attach to the decouplers. Oh well, it still works well enough to get to the Mun.

I literally used those decoupler a dozen times yesterday. They work fine. You must be accidentally attaching the boosters to the tanks instead of the decoupler.

[Manwith Noname]

Depends on what version of KSP. Attaching to decouplers went screwy in one of the releases.

As for fuel flow. I assume this is what you are trying to achieve?

https://steamcommunity.com/sharedfiles/filedetails/?id=2002021683

[miklkit]

Originally posted by MechBFP:

Originally posted by miklkit:

I have been trying every combination. The hydraulic decouplers do not work at all but the others do work as decouplers. It doesn't matter if they are set to crossfeed or not. All three engines drain their own fuel and the run out at the same time. I have tried no fuel lines as well 1,2,3, and 4 fuel lines. Nothing works.

I gave up and tried to go with solid fuel boosters but they will not attach to the decouplers. Oh well, it still works well enough to get to the Mun.

I literally used those decoupler a dozen times yesterday. They work fine. You must be accidentally attaching the boosters to the tanks instead of the decoupler.

The decouplers working is a vicious rumor. The 38 and 77 will work once in a great while, but if anything is changed they quit working again. In this screenie the decouplers worked but the fuel flow did not. When I detached them to try an idea they quit working and now non of them work.
https://steamcommunity.com/sharedfiles/filedetails/?id=2002180893

When the fuel tanks refuse to attach to the decouplers then the only thing they can attach to is the main tank.

[MechBFP]

Originally posted by miklkit:

Originally posted by MechBFP:

I literally used those decoupler a dozen times yesterday. They work fine. You must be accidentally attaching the boosters to the tanks instead of the decoupler.

The decouplers working is a vicious rumor. The 38 and 77 will work once in a great while, but if anything is changed they quit working again. In this screenie the decouplers worked but the fuel flow did not. When I detached them to try an idea they quit working and now non of them work.
https://steamcommunity.com/sharedfiles/filedetails/?id=2002180893

When the fuel tanks refuse to attach to the decouplers then the only thing they can attach to is the main tank.

You have mods screwing things up. Do a complete uninstall from steam, then manual delete the entire KSP folder (backup your saves first if required) and then reinstall from steam.