Mechjeb says: fuel? what fuel?

[Wallace]

I've been designing a long ship with front engines and large fuel tanks towed at the rear, but mechjeb refused to acknowledge the fuel at the back providing delta V.

I checked the parts in between and they all say crossfeed capable but mechjeb just sees the back fuel as dead weight, and puts the dV down.

Screenies

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The fuel should flow up the central column, everything says crossfeed capable, (the long thin bit with the docking ports is a stretchy tanks rcs tank, everything else in the column is stock), I don't get what's going on. Is my fuel really blocked from getting to the engines or is mechjeb just being a dummy? Is there any way to make mechjeb acknowledge the fuel?

[Zeblote]

Use the external fuel ducts to connect them together

[Kerbart]

Is my fuel really blocked from getting to the engines or is mechjeb just being a dummy?

I doubt that the fuel is really blocked but that doesn't make MJ a dummy. I am thoughâ€â€I didn't realize MJ could give that information in the VAB! But I digress...

First of all I see docking ports. I've never seen fuel automatically being carried through docking ports, so there's that. Then there's the complex design with engines up top; there is a good chance that MJ simply is not able to guess your intentions, especially since the staging tree (again, those docking ports!) doesn't give it any clues where you want to utilize that fuel. But that doesn't make mechjeb particularly “dumb,†I don’t think anybody expects clairvoyancy to be built into MJ.

Of course you can always calculate delta-V the way your grandparents did: with pen & paper and a (preferably HP RPN-style) calculator.

[Wallace]

I find that fuel flows through the ports by default, you have to switch them to make them crossfeed disabled. I've successfully made a backwards rocket before, and the fuel flowed up the body just fine, but I didn't check the dV with mechjeb while in the lab with those ones. I guess mechjeb must work out where fuel goes it's own way, I always just assumed it got that info from the game engine.

I can't add fuel lines past the docking ports because this is actually going to to be broken into sections to launch it. I guess I'll just have to work it out the proper way, I was just wondering if mechjeb knew something i didn't and one of those parts would really block the fuel flow. I guess I could put it on the pad and see what happens, if it doesn't fall apart that is.

[Starwaster]

MJ is temperamental about these things to say the least. I've never seen it this bad except for non-stock decouplers.. (which gives it fits because it relies on identifying known modules by name)

Not sure from these pictures why it's giving you so much trouble though.

However, try taking it out onto the launch pad. Depending on the cause it can help. It might even help you when you exit the pad back to the VAB.

Might.

[Captain Sierra]

Hack gravity and launch clamps can help with launchpad testing. Then just burn, physical time warp, and run outa gas. Proceed to check fuel tanks.