What are all the design criteria considered when designing a payload fairing (some are payload volume, drag) ?
Some factors, not in order and probably incomplete:
- size and volume: payloads need to fit inside. For large launchers, a payload diameter of about 4.5 m is more or less standard, lengths are also standardized. For smaller launchers there is more variation.
- aerodynamics: the fairing has to be stable from launch to an altitude of 100 km, while the rocket accelerates. Air drag gives a certain load on the fairing that varies with speed and altitude.
- acoustics: the fairing needs to protect the payload from the noise at takeoff and ascent. Those are known, so you know how much noise level reduction you need, there are various ways to achieve that (foam panels, Helmholz resonators). These take up weight and volume.
- acceleration: the launcher accelerates at a certain rate, this gives a structural load on the fairing it has to withstand.
- rocket assembly: if the rocket is assembled horizontally, the fairing needs to withstand the sideways loading that occurs. For some rockets, the fairing is a structural component (it carries the payload during assembly), so that's another structural demand.
- the goal is a fairing that has minimal weight and is easy to build while still protecting the payload.