> What is it you're doing as an entrepreneur you find so rewarding?
The aerospace industry rewards making minor improvements to conventional airplanes rather than building something novel. Entrepreneurship rewards building something new and "radically different", which is what he was looking for in the first place.
If the GP commenter was working at, for example, SpaceX (a large aerospace company), I can 100% guarantee their work would be more meaningful and rewarding than at basically any startup I've ever encountered. It often seems like it's a unique conceit of the HN community to think that most of the tech startups floating around here are really doing much more than established companies.
On the other hand, SV is littered with names of people who single-handedly turned unprofitable late-stage tech companies into major successes. There's less regulation, stakes are lower, capital requirements are lower, less of the available landscape has been claimed, and the industry lends itself well to the power of computation. It's a little bit like trying to get rich in 18th century England vs 18th century USA. Would it be fair for US farmers to claim that they are doing "radically different" work compared to their English counterparts? Perhaps the work looked similar ("put food on the table"), but the rules and opportunities were vastly different and created different outcomes for individuals (as in - the smartest/most fit/hardest working people got more out of their work).
I guess what I'm trying to say is that if you want to actually change the world, you're unlikely to do actually do it at a tech startup. You're instead likely to talk a lot about it while the real world-changing things are happening elsewhere. But you'll probably be happier at the startup because you get more of that magic combination of autonomy, teamwork, and quickly tangible results. I just personally get frustrated that almost every startup and its employees act like they're working on the biggest problems in the world when they just... aren't.
Edit: forgot to mention. I know two people who left ULA for similar reasons to you and went to SpaceX. They loved working at SpaceX in comparison to ULA because the level of company ambition, speed, and impact was so ridiculously higher. However, one of them left SpaceX to go to a smaller aerospace startup and the other is now at Raytheon for the superior work-life balance. I'm not sure what the lesson is there...
Elon Musk comes from the start-up world and he is hoping to get to Mars himself, so he is in a hurry. I would say if you can get a job there, you should. If you have not already, check out reddit/r/spacex for lots of interesting stuff about the company and what they are doing. You might want to apply to work there if you are looking for a fast paced aerospace company.
There is also the startup called Boom that is working on building a supersonic passenger plane. They have just finished a smaller human-piloted supersonic test plane to prove out their ideas and hope to have the commercial airplane in service by 2025.
These companies are both run by people that started in software and then moved to aerospace without any other real prior experience in the field. Maybe you can have a similar route with founding a successful software company and then, with that success, found an aerospace one of your own. Good luck.