Asparagus staging, why isn't this working correctly?

[johnnyhandsome]

I've built this Asparagus staged rocket, but it's behaving strangely, can you help me figure out what's going on?

Look at the album below. The arrows show the flow of fuel, with the blue arrows highlighting the problematic part. That stage burns less, so when it comes to staging, all the other tanks at that stage are empty, while those two are full.

This results in picture two, where the other four stages fall away as they should, empty. But these two stages still have fuel in them and continue flying upward, usually crashing together and raining debris down on the rest of the rocket. Which admittedly is cool, but not very helpful for going offworld.

Picture three is the fuel situation before separation. The blue arrow shows the fuel level that's in the two problematic rockets vs the red arrow showing what's in the other four stages, which are emptying much faster.

Edited to add album, doh!

<iframe class="imgur-album" width="100%" height="550" frameborder="0" src="//imgur.com/a/BMovW/embed"></iframe>

Here's a link to the craft file, which is stock. Anyone have any ideas what the problem is?

Edited August 7, 2014 by johnnyhandsome
adding album

[Padishar]

Look at the album below.

Errr, what album?

[johnnyhandsome]

Ok, how to add an Imgur album embed? I pasted the HTML code in after your message, but it's just displaying as code.

[NecroBones]

You need the "BMovW" portion from the URL and place it inside "imgur" tags like this:

<imgur>BMovW</imgur>

Except with square brackets instead of angle brackets. It will appear like this:

Javascript is disabled. View full album

[NecroBones]

I can't tell from the pictures, but I'd double-check the direction of the fuel lines on the two problematic boosters. If the line is facing the wrong way, then it will draw fuel from the two that are working correctly, and then the next one in, before using its own fuel.

If it's not that, then I'm not sure. Sometimes you can get strangeness like this, if your fuel lines go from one, to many, back to one. But this isn't the case here, as you have several just feeding into the last standard asparagus pair.

[NecroBones]

If it's not that, then I'm not sure. Sometimes you can get strangeness like this, if your fuel lines go from one, to many, back to one. But this isn't the case here, as you have several just feeding into the last standard asparagus pair.

OK, so I looked at the .craft, and this is the case. It's just acting strange due to the "many to one" arrangement, with tank prioritization.

Here's a possible fix: http://ksp.necrobones.com/temp/S3%20Four%20Booster%20IIalt%20Test.craft

What I did was change the fuel-line arrangements in those outer boosters, so that only one of them feeds inward, but amongst them, they use bi-directional fuel lines (which many people consider a no-no, but I use them successfully all the time). What you'll see is that the center of the three on each side uses its fuel last, but all of the engines remain active until the very end, and cut out together, as you want.

[johnnyhandsome]

Hey THANKS!!! At least I know I'm not crazy, things were set up "correctly".

But I still don't quite get what you did. Is this a work around for a bug or something? Fuel seems to be going all over place!

[NecroBones]

It's not a bug exactly, it's just a side-effect of how fuel-tank prioritization works. Theoretically, the inner engines should all draw on those outer tanks equally, since they're equidistant, but when they have multiple choices like that, sometimes it favors one set of tanks over another.

What I did basically makes the outer three pairs of boosters just share all of their fuel with each other in both directions (so that they all stay lit, even if their own tanks are empty), and limits the entry point into the inner asparagus to just one location, ensuring that the outer tanks are truly prioritized first.

Edited August 7, 2014 by NecroBones

[johnnyhandsome]

Cool, thanks for the explanation, it makes sense now.

[Kasuha]

Many to one is not a problem. If multiple fuel lines end in the same part, any engines that have access to that part should draw fuel from all these lines equally. I'll try to check your craft file when I get home.

By the way, here you can find a description how the fuel system works. If you manage to understand it, you may me able to troubleshoot all your future fuel drawing problems yourself.

[Kasuha]

Found the problem. This fuel line is attached to the strut nub rather than to the tank and it is therefore not transferring any fuel.

Your design is fine, as soon as I fixed the pipe it started working as intended.

[NecroBones]

Cool, thanks for the explanation, it makes sense now.

Glad to help.

Found the problem. This fuel line is attached to the strut nub rather than to the tank and it is therefore not transferring any fuel.

Your design is fine, as soon as I fixed the pipe it started working as intended.

http://i.imgur.com/Af3FDav.png

Hah! I missed that. Good catch.

The thing is, I've seen a similar behavior before with having many "leaf nodes" feeding into an inner asparagus stage before, and I didn't have fuel lines hitting struts. So it makes me wonder what else could have been the issue. That was a few versions ago, however.

Edited August 7, 2014 by NecroBones

[johnnyhandsome]

Found the problem. This fuel line is attached to the strut nub rather than to the tank and it is therefore not transferring any fuel.

Your design is fine, as soon as I fixed the pipe it started working as intended.

Oh. My. God.

THANK YOU!

[Kasuha]

The thing is, I've seen a similar behavior before with having many "leaf nodes" feeding into an inner asparagus stage before, and I didn't have fuel lines hitting struts. So it makes me wonder what else could have been the issue. That was a few versions ago, however.

The only thing that comes on mind (and what I was looking for initially in this case) is that a stack of tanks does not work as a single tank. If your lines were attached to different tanks in a stack, the game was drawing the fuel from the first set of pipes it encountered and did not bother following the rest of the stack.

However, I also know that fuel did not always work the way it works today. It did not change since I started playing in 0.21 but there definitely were some changes before that.

[NecroBones]

The only thing that comes on mind (and what I was looking for initially in this case) is that a stack of tanks does not work as a single tank. If your lines were attached to different tanks in a stack, the game was drawing the fuel from the first set of pipes it encountered and did not bother following the rest of the stack.

However, I also know that fuel did not always work the way it works today. It did not change since I started playing in 0.21 but there definitely were some changes before that.

I was still initially skeptical of "many to one" being the issue, except that I was recalling this previous design problem. I'm careful about feeding in to the same tanks rather than separate ones in a stack, so that wouldn't have been it either. It was just too long ago to remember more detail. But it would have probably been in 0.23, so the rules shouldn't have changed. I'll just chalk it up to bad memory at this point.

[Kasuha]

"many to one" is fine, "one to many" is fine as well. Even "many to one to many" is fine. Only "one to many to one" fails.

And sometimes you don't even notice that you've made such design because sometimes fuel finds unexpected ways.