We all have some amount of free time, even while going through flight school and fleet tours etc. If you want to spend your free time at home inventing a better mousetrap, rather than going surfing, skiing, drinking or whatever, then there's no reason you can't take up an interest on the side. As long as you get your quals early/on time and answer the phone when opso needs you to fly, evals need to be chopped, short notice det needs another body (i.e. squadron needs and job comes first). I have a friend who has spent the last 10 years developing, patenting and marketing a new invention that's likely to make him a fair bit of money. But, it's taken a looong time because every once in a while he had to put it on the shelf for months at a time. If you're ok with that, then go for it.