well yes and no, its not an utility sprite, rocket silos just have two progress bars, however you can modify on a per-entity basis how much "extra parts" can be stored, which effectively means how many rockets you can fit you just don't see the fancy progress bar.
i'm doing that in this mod of mine, happened to land on this mod since one of my users requested compatibility for it, mine currently gives a +1 rocket boost per quality.level but you should be able to adapt the code for your own needs just fine, note that the number doesn't have to be a full rocket it just has to be a number of parts so you can boost (assuming the vanilla rocket needs 50 parts per rocket) in 2% increments.
https://mods.factorio.com/mod/rocket-silos-can-buffer-more-rockets