Low pressure temperature performance of KNSb propellant

Tests were performed on the temperature effect on burn rate for KNSb as follows:

Four grains were prepared as follows:

• 3" Diameter grain
  
• 5.5" long 
• 1" Core
• 3.5" Diameter casting tube
•  1040 grams propellant +- 50 grams
• Humidity controlled to < 10%
  

The grains were placed into a low pressure test motor

• .85" nozzle throat - .02"
  
• 98mm phenolic liner
• KN 65 

Single Grain motor from Rick Maschek

Notes:

• Four tests were performed with one failing due to an improperly seated snap ring.   
• Force data was measured directly and the pressure was obtained basd on the nozzle throat and Cf value for the motor.
  

Test 1: Grain Temperature 55oF +- 5

 

 

 Data:

• Peak Pressure 6.7 PSI +-7
  
• Average Pressure 5.5 PSI +- 6
  
• Peak Force 5 lbs +- 3
  
• Average Force  4 +- 2 
• Burn Time: 18.3s +-  1.8s

 Note:  The low pressure of this burn makes it difficult to estimate burn time.  The original estimate was on the order of 16 seconds.

 

Test 2: Grain Temperature 122oF +- 5

 

 

 Data: NA

 Note:  The nozzle snap ring failed 0.6s +- 2. after ignition.

  

 Test 3:   Grain temperature 122 oF +- 5

 Data:

• Peak Pressure 45 PSI +-10
  
• Average Pressure 27 PSI +- 8
  
• Peak Force 35.5 lbs +- 4
  
• Average Force  20.8 +- 2 
• Burn Time: 5.1s +-  .5

 Test 4:   Grain temperature 75 oF +- 10

 Data:

• Peak Pressure 27 PSI +-10
  
• Average Pressure 17.5 PSI +- 10
  
• Peak Force 21 lbs +- 3
  
• Average Force  13.6 +- 2 
• Burn Time: 6.8s +-  .2
  

 

Burn Time vs Temperature

 

With large amateur motors  safety factors are smaller for bulkheads, fasteners, case rupture etc due to the scale of the project.  I have experienced five sequential failures in large KNSb experimental motors.  The last two 8inch tests were performed in temperatures on the order of 120oF (Mojave Desert in July and August)

Further testing will be performed at higher pressure (KN200) to determine if the temperature effect persists.