Why do I want to study Aerospace engineering? - SOP required - what to write?

I need to write an essay as to why I want to pursue aerospace engineering....the essay should be convincing and in about 2000 words...I also have to convince my dad with the same essay....the only thing is I don't know how and when to start because I don't have a clue about the subject...I have always been passionate about the subject and that's really it...

First, if you have been so passionate about this subject, you cannot claim that you don't have any idea about what it is. If you are interested in something, you naturally tend to know about it more and more and find information from various sources. So, first make sure that you still want to pursue this line. If you really do, first do some research to gain more knowledge about this subject. Ask yourself why you like this? Then note down the reasons and expand on them in your writing.

Duke Application about 'why engineering' - how to cut into the topic?

If you are applying to the Pratt School of Engineering as a first year applicant, please discuss why you want to study engineering and why you would like to study at Duke. (Please limit your response to no more than 150 words.)

For this essay... I have no idea how to cut into the topic...

If I were you, I would tackle this essay with a short anecdote followed by a deeper look into how it persuaded you to tackle engineering. If there is no real memorable experience that flung you into this field try writing about some core idea that motivated you. They just want 150 words so it doesn't need to be a life story.