Final thoughts on the CATO / overpressurization of the KILO 12G KNSB motor at the LRE 2017 launch:

• The motor had a very conservative port-to-throat area ratio of 1.5 to prevent velocity based erosive burning. This resulted in a higher than necessary,  Kn_average of 700 and thus higher Maximum Expected Operating Pressure (MEOP).
• Mass flux based erosive burning. Little has been described about this phenomenon and KNSB or “candy” propellants although it’s widely known among the APCP community. There is an informative article about both velocity and mass flux based erosive burning called: “Erosive burning design criteria – Charles Rogers”. By modifying the SRM 2014 excel file I was able to calculate the mass flux for the KILO 12G KNSB motor at:
  • KNSB coarse: 5,15gr/s-mm²
  • KNSB fine: 6,87gr/s-mm²
  • As an example, maximum mass flux for APCP at 55-96bar is approximately 1,41-2,11 gr/s-mm²
     Although my gut feeling says that KNSB and candy propellants in general are not very susceptible to mass flux based erosive burning, the above mentioned figures might be a bit on the aggressive side.
• It appears the “KNSB coarse” propellant burn rate in SRM 2014 software isn’t as well documented as the “KNSB fine” propellant. Although it provided accurate simulations on the KILO 6G motor, I suspect it isn’t properly characterized at a Kn of 700 / chamber pressure of 100bar. Keeping the same motor configuration but only changing the KNSB propellant from coarse to fine resulted in an increase of the MEOP from 98 to 130bar. Since the casing (theoretically) starts to yield at 110bar and bursts at 160bar. I suspect a higher than 110bar chamber pressure, shortly after ignition, caused the motor to overpressurize.
• Collapse of the bottom grain / casting tube due to the high G-forces causing nozzle blockage thereby overpressurizing the motor.
• (micro) Fractures in the propellant due to the <0°C temperatures at night thereby increasing the burning area.
• In any of the above mentioned cases, the safety margin on the design pressure and the yield pressure of the casing, without static testing, proved to be too small.
• As a note: the grains were some of the nicest grains I cast with a very high density (average density ratio of 0,983) due to the spring-loaded mold design. Motor assemble went without a hick-up, nothing was forced and the liner fitted perfectly with no buckling. With confidence I can say the failure is not related to any of these items. Furthermore the casing ruptured in a typical hoop failure manner. Initiation appeared to be about 2 grains from the forward closure side of the motor. Since the casing failed it is expected that there was a chamber pressure exceeding 150-160bar. The forward closure remained in place with no blow-by validating both the forward bulkhead retention, as well as the o-ring design.

I will machine another nearly identical motor, make some minor alterations and static test this motor in September 2017.