Overview

This was the team's first P motor. It was designed to be around a P9000 but due to an unknown effect had a much shorter burn time and higher than expected thrust. Current theories for the burnrate-increasing effect include erosivity, or more complete aluminum combustion leading to much higher temperatures. The second theory is supported by the slight bulge in the case as well as higher residency time of half of the aluminum particles. After reviewing the BurnSim files used to design the motors, it appears that a software bug may have played a role in the motor's unexpectedly high thrust. BurnSim assumes that the highest mass flux will always be at the bottom grain, but in the case of the P motor's tapered core, it was actually at the end of the grains with the 1.72" cores. This almost certainly led to significant erosivity at this point and was probably responsible for the motor's high burn rate. More details can be found here.

Media

Link to Dropbox: here

Data

Raw Data: 237.TXT

ENG File: P18000.eng

Discussion

Design Delta Discussion

Observations

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

Unpredicted High Burn Rate

Why was the pressure so much higher than predicted? We think it may be because the longer motor caused more complete aluminum combustion that increased temperature/some other factor and led to an increase in the "a" value of the propellant. We tuned our characterization numbers in Burnsim and matched the actual thrust curve quite nicely with an "a" value of .026 (instead of the original .0169). Other possibilities are that this propellant doesn't fit our original burn rate law at high pressures, or that there was burning on the outside of the grains, although we think this is unlikely. If anybody has ideas, I'd love to hear them.

Nozzle Failure

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?

Where do we go now?