I wrote a program to calculate to calculate the thrust and pressure of a 3/1 zinc-sulfur powder motor. The software provides the possibility to calculate the expected performance of a rocket motor filled with zinc-sulfur powder with following composition: 66,5 % zinc powder and 33.5 % sulfur.

The program can be downloaded from our Library. See at the end of the article for direct links.

- throat diameter (in meter): this is the smallest diameter of the rocket nozzle

- total propellant mass (in kg): it is the amount of zinc-sulfur propellant that went in the motor.

- Ae/At: this is the ratio of the outlet area of the nozzle to the throat area

- Max Thrust scale (in N): this is the expected maximum thrust from the motor. If choosen too low only part of the thrust curve will be available

- Max Pressure scale (in bar): the maxium expected pressure.

The green windows give output data of the calculation. 

- total impulse "Ns"

- specific impulse ot Isp "s: In reality about 4 to 5% of the total propellant mass will remain in the motor. Hence for flight calculations this should bet taken into account. Hence the "empty" motor will be with 4 to 5% of the propellant mass heavier than before the firing. On the other hand, the specific impulse will increase with the same percentage. So the Isp should be multiplied with 1,04 to 1,05 for flight calculations.

The programme is based upon 4 important relations which have been derived by VRO and NERO, based on a large number of static tests performed by both rocket amateur organisations :

- relation 1: between the maxumum thust and throat area (At). (NERO)

- relation 2: between the total duration and propellant mass and throat area (NERO);

- relation 2: between Ae/At and the specific impulse (VRO)

- relation 3: between the thrust coefficient Cf and the pressure in the motor (VRO);

Strong deviations from the calculations may occur under the following circumstances:

- too high powder density: in this case the chemical reaction will be slower and the time to the top will become longer;

- erosion of the nozzle throat: in this case the duration will be shorter and the maxium thrust higher;

- Addition of constituents to the powder which may decompose in ideal gases: in this case the pressure may be far bejond the expected pressure;

- Use of easy decomposable liners to protect the inner motor case: this case is similar to the former one: higher pressure will occur.

The program can be downloaded from these locations (including this manual)

Zinc Sulfur Powder Calculator (High Screen Resolution) 68.04 Kb

Zinc Sulfur Powder Calculator (Low Screen Resolution) 68.04 Kb

For general information about zinc-sulfur see www.vro.be