Identical Fuel Tanks Draining at Different Rates with NO Fuel Lines

[VivaNOLA]

This makes no sense. I have a central rocket comprised of two stacked orange tanks with a Mainsail engine below. This configuration has been radially duplicated and attached so that I end up with a fairly typical 6-way array on the outside, so all 7 tank/engine units are identical. I usually test these locked on the platform to make sure they all burn as expected before I go in and set my asparagus staging fuel lines, and when I did so this time I noticed that two of the outer rockets are burning at a different rate than the others. Again, no cross feeding fuel lines exist - these are all isolated burners. When the first tank runs dry, the remaining six tanks are burning at the same rate, but as soon as that tank empties, the tank to it's immediate right starts draining slower than the remaining 5. Weird. Any ideas? Here's a short video walkthrough of the issue:

Thanks in advance for any insights.

Hales

[Vanamonde]

After the problem occurs, hit F3 to see a report of anything that might have broken due to the weight of the rocket on the pad. I suspect the upper tank of the engine that cuts out early still has fuel in it, which is not getting to the engine below because it believes it has no connection. If so, put some launch clamps on the upper orange tanks to help distribute the weight.

Or, it could be something else entirely.

[Rusty6899]

That seems pretty bizarre. It seems like the engine that is burning quickly is only using up one of the Jumbo-64 tanks, and after burning out, the other tank above it is being switched to the engine next to it. I do think I caught a fuel duct at 3.25 in the video for a split second, but that could have been in a later stage. I couldn't really tell.

[sojourner]

.craft file?

[700NitroXpress]

I believe the problem is that the orange tanks have a faulty connection between the the two tanks themselves. Meaning that the top tank and bottom tank are becoming disjointed, but are still attached to the craft with struts. This seems to happen when you stack orange tanks on top of each other. One possible solution you might try is using slack tank staging. Put the extra tanks on the outside of the engines and drop the spent fuel tanks with normal asparagus staging and that should fix this issue.

Here's an example.

The orange tanks on the outside drop as they get depleted so you're not carrying the excess weight with you. Also, in a stress test experiment that I did, I found that the orange tanks have weaker connections when stacked together than the double grey tanks. So I strongly believe that having the tanks stacked is causing the problem due to a faulty connection of the tanks to each other.

[VivaNOLA]

Thanks so much for the responses. @Rusty6899 - that was indeed a fuel line, but on the landing craft. I've removed the whole landing craft from the stack and problem persists. @sojourner, here is the .craft file: Download . @Vanamonde, here is the Flight Results after all engines stop (don't see any issues): Image

Hales

[Deadweasel]

Check the fuel levels in your tanks after a test burn. I'll bet you see fuel remaining in one or more of the top halves of those stacks, indicating the top half got disconnected from the stack. I had this happen a few times on a similar design I was using, but it was a little more blatantly obvious as the top tank would shuffle off slightly to one side (being kept in place by the acceleration of the ship).

I eventually just re-tooled the lifter to get rid of the orange tanks and used some KW Rocketry tanks and engines, and wound up with a lifter that had a LOT more delta-v and improved cargo capability to boot.

That said, part packs aren't for everybody, but there is a mod you can use to help sidestep the quirky nature of those orange tanks if you're a vanilla player. Check out http://forum.kerbalspaceprogram.com/threads/38577-0-22-UbioZur-Welding-Ltd-2-0-Playtest-3-Now-In-Game-Tool

It's still a WIP, but it was initially designed for the main vanilla tanks, so it should help you get around any potential issues caused by stacking those tanks, by allowing you to create a new single part instead.

Just a thought?

[VivaNOLA]

Thanks for all the info. As a couple of you suggested, the culprit turned out to be those orange tanks. For whatever reason they just don't seem to maintain a consistent seal when stacked. I rebuilt the rig using three of the smaller grey tanks rather than two of the larger orange ones and all is well.

[sojourner]

You might checkout the joint reinforcement addon. It's pretty sweet and will reduce strut use quite a bit.