The thrust curve of a zinc-sulfur powder rocket motor has a typical shape, which cannot be changed. The area of the nozzle throat (At) will determine how long the motor will burn, and this can vary between a few tents of a second to several seconds (maximum demonstrated in ZAS10-2 is 4s). Together with the exit area (Ae) it will determine the maximum thrust and the specific impulse.

The relation between the burning time (or rather the working time, because the chemical reaction only takes a fraction of a second) and the throat area for 3/1 powders are the following:

Bt = Mb * 3.7 10-5 / At

Example:

If At = 0.0003 m² and Mb = 10 kg , then Bt = 10* 3.7 10-5 / 0.0003 = 1.233 s

So the motor will burn for 1.233 s.

Bt is expressed in seconds, Mb is the total amount of propellant expressed in kg, If in the same example the nozzle throat is made smaller, say half the area, then the burning time will double: 2.467 s