Last month, USCRPL fired Earthshakiier (ESII), the space-shot motor designed for Domepiercer, which failed 0.552 seconds into the test. Following the anomaly, USCRPL immediately began the first step of our all-hands anomaly investigation by documenting the condition and location of all debris around the site. We then spent four weeks combing through the data, including strain gauge and load cell measurements. After data review and discussion, we have concluded that this failure was different from Earthshaker (ESI), which failed due to a forward end burn-through in Fall 2020. The most likely cause of the failure is the carbon fiber motorcase, which attempted to balance complex axial and hoop deformation to reduce the mass of the motorcase.
All personnel were in the Reaction Research Society’s bunker at the time of the explosion, and USCRPL conducted the firing with experienced pyro ops overseeing operations. The test was conducted with all necessary safety precautions for experimental motors and as such, posed no risk to observers and resulted in no injuries.
We first attempted to see if ESII’s failure shared any resemblance to the ESI failure in 2020. ESI suffered a forward-end burn-through, which was visible in the video recording of the test and supported by post-hardware inspection. ESI’s burn through was determined to be caused by deformation of the case creating a leak path at the forward end for hot combustion gasses to recirculate next to the case, eventually causing its failure. We also realized that the ESI motor’s integration procedure deviated from the intended design, exacerbating this failure mode. This was rectified before ESII.
ESI static fire with burn through visible at the forward end.
An inspection of the forward bulkhead and retention mechanism, as well as a review of the thermal video, indicate ESII did not suffer a burn-through. ESII’s forward bulkhead shows soot build-up on the aft side of the aft-most o-ring groove, while everything forward of that is clean, the expected result if combustion gasses stagnated at a sealing surface. There is also no evidence of high-velocity combustion gasses carving channels into the bulkhead. This implies that the o-rings successfully sealed the motor and that the failure differs from ESI
ESII forward bulkhead with clean line visible in first o-ring groove.
Thermal camera courtesy of FLIR.
A clean and straight seam is visible down the entire remaining length of the case. This is a tell-tale sign of a failure due to hoop stress. Additionally, data from the load cell at the forward end of the motor, as well as pressure backed out from strain gauge data, indicates that the motor’s chamber pressure was at the higher end of expected. This would be the second time we have observed motor overperformance with this propellant formula, with the first being on the flight of CTRL+V.
ESII’s case as it was found with the clean cut visible.
ESII’s moment of failure as captured by the overhead GoPro camera.
Before our next static fire, Earthshakiiiest (ESIII), we will incorporate what we learned into our analysis models and use them to properly envelope the environment the case experiences during firing. Additionally, we will be conducting a propellant characterization campaign in the coming months to make improvements to our propellant and record better data than we have for our current formula. In addition to the media featured in this update, we have also released the uncut pad footage here. Thank you to everyone for your support and encouragement following ESII, we can’t wait to see everyone at the test site again soon!