Nice size for mini jet engines. As a lad, we built a small mounted turbo jet, and could only get it down to about 120mm. Lots of lathe work, many hours of machining, and lucky enough to have access to a supply of alloy. (one of our buddy's dad's worked at an aircraft engine shop and we got blocks of aluminum/chromium/titanium) To build what they have here is many dollars and many more hours of work. Controls must also be a bitch. And I wouldn't want to be the one to crash it into the dirt, which is bound to happen at least once.
Quite an accomplishment.
