What is the primary difference between a J640 and a J320 high power rocket motor?

The primary difference between a J640 and a J320 high power rocket motor relates to their performance characteristics, specifically in terms of their burn time and thrust profile. The J640 motor indeed has a specific impulse that translates into higher total impulse due to its energy output over its burn duration. In essence, while both motors fall within the 'J' classification, the numerical values (640 vs. 320) indicate that the J640 produces a larger amount of thrust over a shorter period.

The J640 burns out twice as fast as the J320, meaning it provides a higher thrust for a shorter duration. This characteristic can lead to a more rapid acceleration of a rocket, achieving higher altitude more quickly, which is beneficial in certain flight scenarios. Understanding these differences helps in selecting the right motor for specific missions according to weight and desired flight profile.

In contrast, the other choices don't accurately reflect the key differences in the motor specifications or performance, focusing instead on aspects like duration of burn or average thrust without addressing the comparative thrust-to-time ratio which is critical in high power rocketry.