The aircraft-design and aerospace-engineering tags have seen a decent amount of use so far. I created aerospace-engineering1 as a more general term that could be applied to rockets and spacecraft (and, or course, spaceplanes).
At the moment, though, they're being used rather interchangeably, as this question seems to show.
How should we differentiate between the two?
For what it's worth, here are the tag wiki excerpts:
For questions about how certain features of an aircraft affect its performance and function, such as engine type or wing configuration
Aerospace engineering is the primary branch of engineering concerned with the research, design, development, construction, and testing of aircraft and spacecraft.
Here's a paragraph from the Wikipedia page on aerospace engineering:
Aeronautical engineering was the original term for the field. As flight technology advanced to include craft operating in outer space, the broader term "aerospace engineering" has largely replaced it in common usage. Aerospace engineering, particularly the astronautics branch, is often referred to colloquially as "rocket science", such as in popular culture.
1I love being able to say, "I created aerospace engineering"!