What is the primary requirement for a rocket motor ignitor?

The primary requirement for a rocket motor ignitor is its ability to transfer sufficient heat to the propellant. This capability is crucial because, for combustion to occur in a rocket motor, the ignitor must initiate the reaction by producing enough heat to raise the temperature of the propellant to its ignition point.

Using the right type of ignitor ensures that the propellant ignites reliably and efficiently, which is critical in achieving the desired thrust and performance for the rocket. Factors such as waterproofing, weight, and sound are considerations in specific contexts but do not relate to the fundamental function of the ignitor in the ignition process of the rocket motor.