See BTech in Aerospace Engineering is a four year course and if you go for MTech after your BTech then its of two years. So upto your Masters its around 6 years. And if you go for PhD then it usually is of 5 years. So in total from BTech to PhD you will need 11 years of study. Now if you are sure that you want to go for research or higher education then I would recommend you to go for Integrated BTech MTech course through which you save 1 years i.e., the BTech and MTech will complete in 5 years. So the total period of education will be of 10 years. Its always not required that you need to go for PhD through fellowships only. If you go into ISRO or other research centers then you will get enough scope to do PhD from the department itself. SO while doing job you can also get your doctorate.

To know more about Aerospace Engineering visit the link below:

https://www.careers360.com/courses/aerospace-engineering-course

I hope this answer helps. All the very best for your future endeavors!