Flat-Earthers have a fixed set of unfounded assertions they cycle through when confronting the rational. One of the most used is the ‘Rockets can’t work in space because there is no air for them to push agains!’ claim.
The following video will help you respond to them in a clear, concise way. There are even TWO experiments you can have them try out themselves, so they know you’re not just spewing illuminati-NASA-reptoid disinformation.