Whenever the inserters aren't loading, it runs a 2 second countdown which increments a selector at the end which will cycle around to the next lighter item to load. If the item fits, the inserter will grab it and stop/reset the countdown, if it doesn't 2 seconds later it will try the next item. When the rocket launches (is empty) it resets (and holds) the selector to the heaviest item just for good measure. All I need to do is put items into the chest remotely. This should maximize the space utilization in a single rocket.