Toggle fuel transfer through fuel lines via action groups.

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As the title suggests; the ability to toggle fuel flow through fuel lines off and on via the action groups. There are some situations where this would be beneficial and it seems like a simple addition that could enable more complex rocket designs.

[TheAquired]

Totally agree. And it "should" be a fairly easy thing to implement.

[UbioZur]

I can't says for the implementation, however I like the idea.

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I got the idea while trying to replicate the Delta IV heavy:

The spacecraft has three identical engines on the boost stage and the same size fuel tanks, but the centre engine is throttled down (to maybe 50%?) relative to the side boosters, hence the side boosters drain fuel quicker. Once they are out of fuel, they fall away and the centre engine throttles up and burns its remaining fuel.

Since we can't throttle engines independently, a work around would either be to add more fuel to the centre booster, changing the rockets design significantly, or have fuel lines feeding from the side boosters to the centre. However, when the side boosters jettisoned, I wouldn't want a full centre tank, as it'd weigh too much for that one engine. So if I had the ability to stop fuel flow via the fuel lines midburn of the boost stage, the centre booster would be light enough to lift once the side boosters were jettison.

Anyway, that's just one example where this feature would come in handy.