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Bad "Twin Boar" fuel flow with very specific setup


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So this one's a bit weird, because it doesn't happen unless you have some very specific design elements.

The key elements:

1)  A pair of KR-1x2 engines on side separators of a central fuel tank with an engine.

2)  At least one additional tank on top of the KR-1x2 engines.

3)  Fuel pipes from the KR-1x2 engines or the extra top tanks to the central fuel tank.

4)  SRB's on side separators attached to the KR-1x2 engines.

5)  SRB's burn separately on first stage, then the LF+O engines.

A picture of an example ship that should be easy to recreate by sight is attached.  Here's what happens when you launch the ship:

1)  The SRB's lift off normally.

2)  After staging the SRB's away, one of the tanks on top of the KR-1x2 engines drains faster than the other.

3)  After the top tanks are empty, the KR-1x2 tanks themselves seem to drain normally, but their levels are by then uneven, causing a large yaw force.

As you can see in the image, the fuel delivery paths look messed up as well, though that's still true with the SRB's removed, which results in no fuel flow problem.

If you put all engines in the first stage, the fuel flows normally.

I tried to reproduce the problem with other engines in the same configuration, but was unable to.

Any thoughts?


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That is bizarre.

Set up exactly as you have it, I saw the same behavior. I was able to get it to drain symetrically by connecting the fuel lines from the tanks above the Twin-Boars rather than from the Twin-Boars themselves. That causes the top tanks to drain first, then the Twin-Boars, then the middle tank.

In that setup, the top tanks default to flow priority 20 and the Twin-Boars are priority 10. By increasing the priority of the Twin-Boars to match that of the top tanks (20), I was able to get them all to drain at once, and symetrically (i. e. the way I would have expected the your setup to behave in the first place). The resulting setup looked like this:


Hope this helps!

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