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Discussion
Design Delta Discussion
Observations
•Erosive burning spike was observed in thrust curve, then progresses with normal BATES thrust profile until nozzle failed
•Nozzle cracked at phenolic interface
•Casting tube was likely ejected at +3 seconds around the same time the nozzle cracked, corresponding to the burnout of the aft grain
•Liner surrounding aft grain was ejected at +6 seconds
•Leak path at nozzle crack led to significant loss of pressure and thrust
Timeline
Here's a reconstruction of events based on video and physical evidence collected after the test:
•T-0: Motor ignites
•T+.5: Erosive burning spike settles down, but motor is now burning at a much higher pressure than expected
•T+.5 - 2.9: Thrust curve follows typical BATES profile
•T+2.9: Aft grain burns out, casting tube is ejected
•T+2.9: Interaction with casting tube event along with elevated thermal and mechanical stresses causes nozzle failure
•T+2.9-6.2: Motor continues to burn at lower pressure/thrust due to fewer grains, leakage at nozzle break
•T+6.2: Second grain from aft end burns out, unprotected liner at aft end is ejected due to high velocity gas passing over it
•T+7.8: Motor shuts down
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The nozzle cracked axially at around 2.9 s into the burn. It was being exposed to 1.8x the design pressure when it failed, but maybe there were other factors that led to it failing, such as the casting tube being ejected? This is what the nozzle design looked like. The diverging section of the graphite is insulated from the aluminum nozzle carrier by a section of 98mm phenolic liner, and the convergent section is insulated by a piece of phenolic casting tube.
What caused this?
•Increased Al combustion in high L/D motor raised temperature and interacted with other compounds to enter an uncharacterized combustion regime
•Nonlinearity in burn rate law (r = aP^n) unrelated to Al combustion
•Burning on outside of grains (inside of casting tubes)
Where do we go now?
•Option 1: Build the same motor again for SAC and reconsider the vehicle’s structure to handle higher loads
•Option 2: Redesign the motor for combustion in the “aggressive” regime, static test it, and then rebuild it for SAC
•Option 3: Redesign the motor for the “aggressive” regime, but don’t static test it before SAC
•Option 4: Descope to 4G motor
How much additional money would the team need to spend to complete each option?
•Option 1: $690 (not including structural redesign)
•Option 2: $1,137
•Option 3: $690
•Option 4: $0