Dear,

You need to have a postgraduate degree in aerospace engineering in order to have an overview about the field. Mostly all colleges have the basic eligibility as a postgraduate degree. Usually, students who are interested in research, enrol for a Master’s program then convert to a PhD. But a direct admission to PhD after Bachelor’s degree is also an option. There are certain criteria which the applicant needs to meet in order to apply for a Direct PhD admission after a Bachelor’s (B.Tech or B.E.) and admission is tough. You can visit this link for details about top colleges and ther criteria.

https://engineering.careers360.com/colleges/list-of-phd-in-aeronautical-engineering-colleges-in-india

Hope this helps!