Characterizing 38mm PVC motors

A previous article described fabricating PVC motors using couplings and endcaps to secure the bulkheads. 

 

Figure 1:  PVC fitting used to retain grains in 2" diameter PVC motor with cork liner. 

 In an effort to make the motors more flyable, the motors have been redesigned without those elements.  

First attempts consisted of drilling a series of 4-6 radial holes and casting the rockite into the holes. This provide insufficient to retain the bulkheads in 4/6 tests, and none of the successful tests exceeded 150 psi. 

Figure 2:  Note the forward bulkhead and nozzle bulkheads are extruded out from the pvc. 

The next attempt was to cut a 0.25" section 1 1/4" PVC pipe, slit it and glue it into the end of the 1 1/2" pipe.  The Rockite was cast with this lip retaining it.  This was successful in 1/4 tests, but the successful test did hold 380 psi, and the other tests failed at above 300psi.   

Here is the data from the successful test:

 

Data:

• Peak Thrust 36lb +- 1 lb
  
• Average Thrust 20lb +- 1 lb
• Burn time 3.6 s +- .2 s
  
•  Delivered impulse 76 lb . s or 338 Ns
  

Based on the data, the total impulse of the motor was 338 Ns, or a minimum I motor.  This is considered success. 

A model of the motor was created in SRM2014;

Figure 3: Predicted Force vs time based on the grain geometry and nozzle throat from SRM2014

Figure 4:  Data from SRM 2014 spreadsheet

Conclusion:

• Average Thrust:  20lb vs 66lb
• Thrust Time: 3.6s vs 1.59 seconds
• Total Impulse 76 lb-sec vs 105 lb-sec

While the measured numbers are internally consistent, the longer burn time of the PVC motor compared to the model suggests that it was running at much lower pressure then anticipated based on the grain geometry and the nozzle throat.  This test was performed without a pressure sensor in place as the primary goal was to test the nozzle retention.  Future work suggest that using a pressure transducer would measure the actual case pressure. However, given that the goal is to design a motor that can propel a 4lb rocket, (and this motor currently weighs 1lb 11oz) we are currently above the minimum 3x1 thrust to weight ratio.