What is the primary difference between a J640 and a J320 high power rocket motor, assuming full 1280 Newton-second J motors?

The primary difference between a J640 and a J320 high power rocket motor lies in their burn rates and thrust profiles. The designation in these motor names typically reflects the thrust and performance characteristics, where the numerical part represents thrust in Newtons for a specific time duration.

A J640 motor is designed to deliver thrust at a higher rate over a shorter burn duration compared to a J320 motor. This implies that the J640 burns out faster because it produces a greater peak thrust output relative to its total impulse. Therefore, the statement that the J640 burns out twice as fast as the J320 is accurate, as it showcases the fundamental differences in the operational characteristics of these motors.

The other choices revolve around aspects such as suitability for flight types and thrust outputs, but they do not encapsulate the primary distinguishing feature related to burn rates as effectively.