Rocket science is simpler.
It relies on fixed formulas and predictable physical laws and conditions. By contrast, as a public speaking instructor you will deal with human beings learning an ancient art form through which the course of history has been fashioned and refashioned. People have been teaching others to speak publicly for 5,000 years or more. Therefore, as you prepare to teach, you join an ancient tradition in the academy. You may find yourself teaching public speaking with or without broad speaking experience yourself, with or without teaching experience in a college classroom, and with or without a deep theoretical foundation in the field of communication.