Crossfeed mystery

[Loren Pechtel]

Somehow fuel is getting past a decoupler which has crossfeed turned off.

I have a major drag monster that I'm trying to send to Minmus.  A skycrane design, 4 rocket stacks pushing it.

Stage #1:  Mainsails under big orange tanks, there is a fuel tank adapter down to 1.25m.

At this point poodles take over.  Stage "#2" is drop tanks on the sides of the main tanks that feed the poodles.  The fuel priority is set so these drain before the main tanks and don't have to be hauled to the moon.  Stage #3 goes all the way.

The decouplers on top of the first stage have cross feed *off* but the fuel feed ignores this.  If the first stage tanks don't have a higher priority than the drop tanks they'll drain them early in the ascent, leaving it woefully short of fuel.  While obviously I can handle this with the priority setting why do I need to?

[Cpt Kerbalkrunch]

Are your orange tanks radially attached to a central fuselage? That might be how your fuel is sneaking past the decouplers. I'm assuming, of course, that you don't use any fuel mods. In any case, and as always, pics are helpful.

[Loren Pechtel]

26 minutes ago, Cpt Kerbalkrunch said:

Are your orange tanks radially attached to a central fuselage? That might be how your fuel is sneaking past the decouplers. I'm assuming, of course, that you don't use any fuel mods. In any case, and as always, pics are helpful.

No.  It's in effect 4 separate rocket stacks.  There are 4 first stages that come nowhere near each other.  The only connection is upwards through their decouplers.

Is there something in the configuration that could bypass the crossfeed rules?

[Cpt Kerbalkrunch]

13 minutes ago, Loren Pechtel said:

No.  It's in effect 4 separate rocket stacks.  There are 4 first stages that come nowhere near each other.  The only connection is upwards through their decouplers.

Is there something in the configuration that could bypass the crossfeed rules?

I've seen that before on a similar setup, but it was for an asteroid grabber I built. I'm guessing then that the tanks above the orange ones are the drop tanks for your Poodle stage. I think that's where you see fuel flow get funny sometimes. The tanks above don't have engines of their own, so l think the game is thinking they are going to be drop tanks for your Mainsails (even though the decouplers are facing, I'm assuming, the other way). I build designs like that where the top tanks really are the drop tanks for the bottom engines. It allows me to dump the weight without dumping my engines. Anyway, unless someone gives you a better answer, I'd just use the fuel flow priority setting like you've been doing.

And one last thought, with crossfeed disabled on the vertical decouplers, enable it on the decouplers attaching to your central tank with the Poodle, instead of just using flow priority. This might help the game realize those tanks are meant for the Poodle, not the Mainsails.

[Loren Pechtel]

38 minutes ago, Cpt Kerbalkrunch said:

I've seen that before on a similar setup, but it was for an asteroid grabber I built. I'm guessing then that the tanks above the orange ones are the drop tanks for your Poodle stage. I think that's where you see fuel flow get funny sometimes. The tanks above don't have engines of their own, so l think the game is thinking they are going to be drop tanks for your Mainsails (even though the decouplers are facing, I'm assuming, the other way). I build designs like that where the top tanks really are the drop tanks for the bottom engines. It allows me to dump the weight without dumping my engines. Anyway, unless someone gives you a better answer, I'd just use the fuel flow priority setting like you've been doing.

And one last thought, with crossfeed disabled on the vertical decouplers, enable it on the decouplers attaching to your central tank with the Poodle, instead of just using flow priority. This might help the game realize those tanks are meant for the Poodle, not the Mainsails.

No.  It goes:

 

Mainsail

Tank

Adapter Tank

Decoupler with crossfeed turned off

Poodle

Tank -- decoupler with feed enabled to tank & 2x fuel-holding nosecones to make it aerodynamic.

Skycrane (which I believe has some tankage)

Everything is behaving like the decoupler has crossfeed enabled.

[Cpt Kerbalkrunch]

I'm having trouble picturing how everything is connected then. Is there no central fuselage anywhere? And why would you need an adapter from the Mainsail to the Poodle? They're both 2.5m. A pic would really be helpful.

Also, I'm gonna guess that you probably have some form of fuel mod installed. Mods are always a good place to start looking when there's a problem.

Edited July 12, 2017 by Cpt Kerbalkrunch

[Loren Pechtel]

30 minutes ago, Cpt Kerbalkrunch said:

I'm having trouble picturing how everything is connected then. Is there no central fuselage anywhere? And why would you need an adapter from the Mainsail to the Poodle? They're both 2.5m. A pic would really be helpful.

Also, I'm gonna guess that you probably have some form of fuel mod installed. Mods are always a good place to start looking when there's a problem.

Oops, brain not in gear.

Terrier, not poodle.

And you're right, there's no central fuselage.  The top of the rocket is a skycrane part, I forget what mod it's from.  It has 5 connectors laid out in a plus shape--the central for slinging your payload, the outer 4 for the rockets that push it.  It's a fiery mess when you're pushing it through the atmosphere but it makes it so easy to land rovers on other worlds because the rockets are attached to the top.  Land, uncouple, fly up a bit then turn sideways and your booster is out of the way.

[Cpt Kerbalkrunch]

20 minutes ago, Loren Pechtel said:

Oops, brain not in gear.

Terrier, not poodle.

And you're right, there's no central fuselage.  The top of the rocket is a skycrane part, I forget what mod it's from.  It has 5 connectors laid out in a plus shape--the central for slinging your payload, the outer 4 for the rockets that push it.  It's a fiery mess when you're pushing it through the atmosphere but it makes it so easy to land rovers on other worlds because the rockets are attached to the top.  Land, uncouple, fly up a bit then turn sideways and your booster is out of the way.

Okay, now I can picture it. I'm thinking it's definitely one of your mods that's causing the issue. I would just use the fuel flow priority and be done with it.

[Snark]

4 hours ago, Loren Pechtel said:

I have a major drag monster that I'm trying to send to Minmus. <description with a lot of words>

3 hours ago, Loren Pechtel said:

No.  <description, with words>

2 hours ago, Loren Pechtel said:

No.  It goes: <description, with more words>

44 minutes ago, Loren Pechtel said:

Oops, brain not in gear.

Terrier, not poodle.

And you're right, <still more descriptive words>

Great to see a lot of careful description... but I gotta say, sure would be nice to see a screenshot or two.  Picture worth a thousand words, and all that.

Just sayin'. 

Have you tried turning on the fuel crossfeed display in the VAB?  Just wondering whether it might be educational.

[Loren Pechtel]

18 hours ago, Snark said:

Great to see a lot of careful description... but I gotta say, sure would be nice to see a screenshot or two.  Picture worth a thousand words, and all that.

Just sayin'. 

Have you tried turning on the fuel crossfeed display in the VAB?  Just wondering whether it might be educational.

It's supposed to burn all the orange fuel first.  Those boosters drop off, exposing the terriers.  They burn the fuel in those side tanks to circularize, then they burn the fuel in the tanks above them to take that science rover to Minmus.  (And possibly the Mun, I haven't checked if I have the fuel for that.)

Edited July 12, 2017 by Loren Pechtel