No worry about headspace during primary fermentation. The CO2 given off keeps oxygen away. As long as airlock is in place no oxygen getting in. When you rack to secondary is when you want to eliminate headspace.
Reality is an illusion that occurs due to the lack of alcohol.
Give a man a beer, he'll drink for the day.Teach a man to brew, he'll be drunk the rest of his life.
I have 8 carboys, 8 cornies, 5-1 gal jugs, 200 wine bottles, 10 cases of beer bottles and a nice assortment of flip tops....My goal is to keep them at least 50% occupied