Unbalanced fuel consumption :: a Study

2024-10-29

Fellow Kerbonaut EnderGround asked for help on Reddit about a craft they built that was consuming fuel asymmetrically:

So basically I was trying to launch a 85 ish ton payload to solar orbit and I decided to use a 5m wide mid stage and a 7.5m lower stage with 9 mainsails, 2 extra 5m boosters with 9 vector engines each and I put fuel pipes on the boosters to pour their fuel into the core stage, but it seems to draw unevenly from my boosters and 1 side empties quicker than the other one. I tried removing and re adding the pipe but it didn't fix it.

I knew I had seen this before, but not on my own crafts. By pure luck, or by prioritising using my own add'ons over others (or both :P) I never faced this myself.

Then I remembered a situation where a modded KSP was breaking symmetry and screwing some parts' configurations and then something made "click" inside my tick dull skull!

And I made this:

Craft

This somewhat reproduces the user's craft overall configuration.

What I think it happened is that somehow the boosters had their symmetry broken, and one side got their sepratron with the cross-fuel enabled and the other not - see the green circle.

To test this theory, I duplicated the craft into a "bad" version where:

  1. I broke the symmetry using the paw exactly over the sepratron (see the green circle)
  2. I set the cross-fuel to enabled on one booster's sepratron
  3. I made sure the other booster had its sepratron's cross-fuel disable.

And then fired both to see if anything changes.

On the "good" craft, I got:

good craft

And both boosters had the exact same amount of fuel and oxidiser.

However, the "bad" craft gave me:

bad craft

And we found our asymmetry! With an extra fuel line active, one of the boosters consumed more LFO than the other!

Another way to have this misbehaviour would be adding the Fuel Lines after attaching the boosters, but without using symmetry - and then attaching one of them inverted (ie, feeding the booster with the main fuselage's fuel). In this (mis)configuration, one booster would end up feeding the other booster using the main stage as "tank in the middle".

In a way or another, the solution (if I'm right) is to detach one of the boosters using the sepratron as subassembly's root, then reattach it with symmetry active, and then reconfigure the cross-fuel and priorities. And remembering to attach the Fuel Lines also using symmetry, and from the right direction (from booster to the main fuselage).

Downloadables

2024-10-30

EnderGround sent me his craft.

Mod list:

Kerbal Engineer
Athmospheric sound enhancement
B9Part Switch
Community Resource pack/tech tree
Filter extensions
Click through blocker
XenonHallThrusters
Waterfall
WildBlueIndustries
WarpPlugin
TweakScale
TweakScaleCompanion
TimeControl
Trajectories
Trigger Tech
System Heat
Stockalike stations parts expansion redux (used parts from this mod for the craft)
SpaceDust
Photonsails
NearFutureSpacecraft/Solar/Rovers/Propulsion/LaunchVehicles/Exploration/Electrical/Construction/Aeronautics(used parts from probably launch vehicles and a solar panel)
MechJeb2
Kopernicus
ModularFlightIntegrator
ManeuverNodeEvolved
InterstellarFuelSwitch
FarFutureTechnologies
FShangarExtender
CryoTanks
DecayingRTGs
DecouplerShroud
DeployableEngines
DynamicBatteryStorage
DynoSystem
CryoEngines

This one has 8 twins boars instead of the dual LFO boosters he described above, but I tested her nevertheless and didn't found anything wrong on the fuel consumption. All twin boards exhausted their LFO at precisely the same time.

So I got rid of them, and shoved 2 absolutely, pornographically ludicrous boosters. (oh boy... hehehe). But found no problem neither.

My rig is a Mac Mini 6.2 - i7-3615QM CPU @ 2.30GHz, 16GB RAM and Intel HD 4000. Pretty crappy, slower than his but with twice the cores.

I didn't installed every single add'on he did. I installed only the ones needed to fire this craft, so perhaps one of the ones I didn't installed could be the culprit. CKAN file with the add'ons on my rig below.

If by reproducing my rig, the craft I provided consumes fuel asymmetrically, then I will start to believe that it's something environmental in his rig and, so, would recommend trying the stunt on this link.

Screenshots or didn't happened

Space-Station-2-000

Space-Station-2-001

Space-Station-2-002

Space-Station-2-003

Downloadables