Fuel Lines misbehaving (Asparagus staging broken)

[Farsyte]

I tend to use Asparagus staging (using fuel lines to keep inner tanks topped up, staging away outer engine/tank sets) and it seems like this is only partially functional in KSP-2: it appears that some fuel flows but not enough to keep inner tanks topped up. Adding more fuel lines does not change the results.

I did a walk through of the situation, showing what I expect (using KSP-1) and what is actually happening in KSP-2.

I have sent this in via the launcher feedback. I did not post here before because I thought I saw it already covered - but checking today, I only see mentions of Asparagus not being reliable in KSP-2; the fuel flow issues I see may or may not be related.

I am hopeful that the team working on the performance issues around fuel flow have already nailed this, but here's a video of the symptom so you can see exactly what is troubling me. I think I managed to convince myself this is not merely a display bug ...

 

 

[sir_frost]

asparagus staging is indeed broken the problem is that there exist 3 separate systems for fuel transfer.
we have fuel ducts: these should function in such a way that fuel from the transmitting end is uses instead of the receiving end.
then we have fuel crossfeed in ksp 1 at least this ment that fuel could flow through a part, and all parts containing fuel had a fuel flow priority setting. the tanks with the highest number drained first.
this was no issue to worry about since this numbering system was linked to the staging. in KSP 2 this system is nonexistent and all fuel drain evenly between tanks with crossfeed.
the third system is the manual fuel transfer. this is at least working in KSP 2. whenever you do asparagus staging you need to manually transfer the fuel.

the best solution is to reimplement the numbering system for fuel drain. then we let fuel ducts act like one way crossfeed.
and we let the staging sequence be the base for the numbering in a way that tanks being dropped first should drain first (this would be the standard numbering but we need to be able to renumber them for whatever reason)

in most of my ksp 1 builds where i use asparagus staging i have stopped using fuel ducts and instead use the more modern crossfeed system. it is more user friendly since you place parts, then enable crossfeed and it simply works.                

[Farsyte]

You know, I never attempted to use the crossfeed system, in favor of always laying out explicit ducting. Will have to try it.

quick update to save anyone time who is following in my footsteps: the radial decoupler I use in KSP-1 (TT-38k, the first one we get) briefly supported fuel crossfeed but that was removed, in January 2012, because fuel ducts made it obsolete ... Yeah, I found out when digging around after being unable to get it to work, because the TT-38k still has an option to enable and disable crossfeed.

Moved over to KSP-2 and ... well, didn't find crossfeed and fuel tank priority options, but I may not have looked hard enough.

I'll just wait for Patch Two before I start launching my veggies.

Edited March 18, 2023 by Farsyte
followup: KSP-1 tt-38k does not support fuel crossfeed

[ArmchairGravy]

Yeah, I'm using onion staging, but the final tank ends up being about 1/3 empty instead of completely full, and suddenly I don't have enough dv to do the mission. It's better than it was because the landing stage isn't getting sucked dry any more, but there are definite issues with fuel feed in onion/asparagus stacks.

[sir_frost]

On 3/17/2023 at 2:49 AM, Farsyte said:

You know, I never attempted to use the crossfeed system, in favor of always laying out explicit ducting. Will have to try it.

quick update to save anyone time who is following in my footsteps: the radial decoupler I use in KSP-1 (TT-38k, the first one we get) briefly supported fuel crossfeed but that was removed, in January 2012, because fuel ducts made it obsolete ... Yeah, I found out when digging around after being unable to get it to work, because the TT-38k still has an option to enable and disable crossfeed.

Moved over to KSP-2 and ... well, didn't find crossfeed and fuel tank priority options, but I may not have looked hard enough.

I'll just wait for Patch Two before I start launching my veggies.

you are wrong about the decouplers in KSP 1 
fuel crossfeed require a upgrade of the RnD building to work
when that is done all decouplers have crossfeed as an option in flight or in the editor.

fuel lines in KSP 1 was made obsolete when they introduced the fuel tank priorities, because crossfeed does the same thing with no extra part.   

[Farsyte]

This is all KSP-1 stuff. I'm hoping that the situation laid out below did not flow into KSP-2. Since I don't see fuel tank priorities in KSP-2, it seems like KSP-2 Asparagus configurations will need fuel ducts. If I see fuel tank priorities in KSP-2 I will certainly try to use them instead!

TLDR: cross-feed and tank priorities would obsolete fuel ducting, except that cross-feed in TT-38k and TT-70 is broken in KSP-1. I did not check other radial decouplers, since Asparagus staging is most critical to me in early career when few parts are available (it makes hitting Mun and Minmus massively easier).

More explicitly before more "you need to unlock it" and "you need to enable it" and "did you check the fuel feed overlay" ... ( As a software engineer, I spent a lot of time writing "did you turn it off and on again" style replies. we can skip that part of diagnosis, thanks. )

With cross-feed unlocked in R&D, and the option visible, and the option turned on in each mounted decoupler, and fuel priorities set for the tanks, and the fuel overlay showing fuel should flow where I want it to, I launch and observe all tanks in the asparagus staging set-up are being drawn down at the same rate. Historically I have depended on a  seven-engine  configuration using TT-38k, FL-T800 and FL-T400 tanks, and a mix of Reliant and Swivel engines.

I just crosschecked with the TT-70 obtaining the same result. Options set, overlay in VAB shows it should flow, but tanks drain evenly in flight.

So. At this moment, when it comes to Asparagus configurations in KSP-1, fuel ducts are needed and not actually obsolete.

https://wiki.kerbalspaceprogram.com/wiki/TT-38K_Radial_Decoupler

lists the following changes relating to fuel ducting, radial decouplers, and cross feed:

> 0.9 - August 12, 2011 - (undocumented) Radial decouplers can now cross-feed fuel
>0.13.1 - January 11, 2012 - Removed parent-wise fuel cross-feed from Radial Decouplers. Fuel Lines made that obsolete

https://wiki.kerbalspaceprogram.com/wiki/TT-70_Radial_Decoupler

Size aside, the TT-70 functions identically to the TT-38K Radial Decoupler

> 0.18 - 1 December 2012 - Initial Release

https://wiki.kerbalspaceprogram.com/wiki/FTX-2_External_Fuel_Duct

has wording that agrees with what you say, and what I remembered, but see below:

Fuel lines used to be a vital part of asparagus staging designs allowing for use of all engines available where fuel is first drained from external fuel tanks allowing them to be jettisoned (severing the FTX-2 External Fuel Duct) when they become empty. By the time the last external fuel tanks are jettisoned, the central or main fuel tanks left on the vessel will be at a full fuel level. Nowadays, most of this is done by setting the flow priorities appropriately, and fuel lines are no longer necessary for asparagus staging.

relevant change log entry:

>0.13 - December 16, 2011 - initial release

 

So why do I still use fuel ducts? Because fuel cross-feed in the TT-38k radial decoupler is broken.

Yes, I've upgraded R&D. I see the option in each decoupler placed, and I've turned it on (it defaults to off). I have double checked that the tanks have their priorities set.

Despite all indications in the UI that it should work, all of my tanks are draining equally; no fuel is being cross-fed, in any direction, between any of the tanks in my asparagus launcher.

 

[TomKerbal]

On 3/10/2023 at 5:08 PM, Farsyte said:

I tend to use Asparagus staging (using fuel lines to keep inner tanks topped up, staging away outer engine/tank sets) and it seems like this is only partially functional in KSP-2: it appears that some fuel flows but not enough to keep inner tanks topped up. Adding more fuel lines does not change the results.

I did a walk through of the situation, showing what I expect (using KSP-1) and what is actually happening in KSP-2.

I have sent this in via the launcher feedback. I did not post here before because I thought I saw it already covered - but checking today, I only see mentions of Asparagus not being reliable in KSP-2; the fuel flow issues I see may or may not be related.

I am hopeful that the team working on the performance issues around fuel flow have already nailed this, but here's a video of the symptom so you can see exactly what is troubling me. I think I managed to convince myself this is not merely a display bug ...

 

 

Thank you for your great presentation !

[sir_frost]

22 hours ago, Farsyte said:

So why do I still use fuel ducts? Because fuel cross-feed in the TT-38k radial decoupler is broken.

Yes, I've upgraded R&D. I see the option in each decoupler placed, and I've turned it on (it defaults to off). I have double checked that the tanks have their priorities set.

Despite all indications in the UI that it should work, all of my tanks are draining equally; no fuel is being cross-fed, in any direction, between any of the tanks in my asparagus launcher.

in my testing this works as intended. i just launched a craft in ksp 1 using the TT-38 radial decouplers
the tanks with the highest fuel priority number drain first and then they sequentially drain in fuel priority order until the last tank is empty.

the main difference with using fuel ducts in this case would be that the engines automatically cut off when the fuel in their tank is depleted.

if you do not experience this expected behavior in KSP 1 i suspect it is a bug on your behalf. 

my hope is that the Developers of KSP 2 implement an identical fuel crossfeed system as KSP 1. 

i add a spoiler that contain my KSP 1 mission working as intended. 

Spoiler

this is my craft before crosfeed is turned on

this is my craft with crossfeed enabled
please note that the P values correspond with the staging que.
and thus 2 tanks will drain as a pair on opposite sides of the rocket.
then the other 2 attached radial tanks will start to drain

also note the delta v change.  

here you see the craft on the launchpad 

here the first set of tanks have started to drain

here i have dethatched the first boosters and when i did the second set of boosters had full fuel tanks
note that you have to keep track of this in the staging sequence (or by monitoring fuel levels in tanks), because otherwise the entire ship will be drained before the engines stop.
the easy way is to stage when the current stage reach 0 delta v   

to conclude this example of how crossfeed work as intended in KSP 1
a simple to use system like this that require little setup and for me at least works every time, would be a great addition to KSP 2.    

   

[Farsyte]

When I get a chance, I'll rerun trying to match your configuration, and see if I can isolate which change in particular causes the crossfeed to stop. Might be a while (too many irons in the fire!).

It will probably be something really stupidly simple. I have this sixth sense of when a "headdesk" moment is coming ...

The fuel tank priority coupled with cross-feed is definitely preferable to running ducts around.

 

[EDIT after reproducing Sir Frost's configuration]

Yeah, it *was* something stupidly simple. All this is still KSP-1, of course.

1. Yes, The tanks that are connected by cross-feed do indeed drain strictly in priority order. This is visible if you actually show the fuel levels in each tank.

2. When the first pair of tanks is empty, the cross-feed and fuel-duct configurations have a critical difference: the engines attached to the drained tanks still have access to the fuel in the others. This leads to

3. The staging display shows fuel available to the engine, not fuel in the tank stack it is attached to. Even after a decade, this one is still learning how to read the User Interface.

4. Since the engines in the drained stages still have access to fuel, the engines are not marked as "flamed out" and they still provide their full thrust to the available thrust totals.

My personal conclusions:

A. I like the idea of using cross-feed and tank priorities (but note, this does not exactly match the fuel duct use case).

B. I am going to need a more complicated auto-stager since I can't use "flameout" or "availablethrust" to detect that staging is a good idea right now. I'll have to identify which tanks are jettisoned by each staging event, and trigger staging when those tanks are empty (probably by watching the vessel mass total).

C. Until I work out the new autostager, I'm personally stuck with fuel ducts, as I do a lot of mission automation. [k-OS fan]

Edited April 7, 2023 by Farsyte

[sir_frost]

On 4/7/2023 at 7:33 PM, Farsyte said:

Until I work out the new autostager, I'm personally stuck with fuel ducts, as I do a lot of mission automation. [k-OS fan]

you might be able to read the delta v of the stages. (I am not sure how available this is for k-OS) but it is information displayed on screen.