TL;DR
make Rocket silo recipe dont clear the product slot.What ?
currently,Rocket silo do have ouput slot,however the output slot are not actually doing anything except delete item put in that slot,and which doesnt increatment the rocket part amount because rocket part increatmented as the recipe completed(include productivity).
Why ?
so we can have "assembling machine"that has a built in progress bar and inventory slot,which can be used as drilling the hole or farming or any special purpose if you want it to do.and in the currently system,I think vanilla rocket can be a recipe dont really produce anything(like a gas vent),and with icon shown as rocket part icon.