If you select the wrong ship, or change your mind part-way through progressive construction, there should be a way to cancel the build. Returning some percentage of the rocket parts used to that point would probably be reasonable.
I endorse. Also endorse cancellation penalty, perhaps as much as 50% of progress at moment of cancellation. So it's cheap to fix an oops-pushed-wrong-button, expensive to change your mind at the last minute.
If you select the wrong ship, or change your mind part-way through progressive construction, there should be a way to cancel the build. Returning some percentage of the rocket parts used to that point would probably be reasonable.