Neato. You made me step up my game, though; I have now managed it on two Oscar-Bs.     My innovation here was to action-group the crossfeed through the decoupler. This let me get a tank and a half of fuel to the Spark and half a tank to a single Spider engine that I center-mounted on the second stage, while keeping the upper tank as full as possible until needed, for aerodynamic stability. I also moved the battery to the lower stag
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