by Bill Claybaugh, RRS.ORG
EDITOR’S NOTE: This is a continuation of the reporting from the 10-16-2021 flight of the 6-inch rocket design, built and flown by RRS member, Bill Claybaugh.
Post-Flight Motor Inspection
Recovery of the spent motor hardware allowed a detailed disassembly and inspection of the parts. This revealed several useful observations:
The recovered Motor Tube showed a dent just above the fins that was deep enough to have caused a pressure failure if it had been present while the motor was operating; we thus conclude that the dent occurred during or post impact.
Inspection of the Forward Bulkhead showed it to be in good condition with no evidence of any gas leaks above the two O-rings. The bottom of the Bulkhead showed some damage to the fiberglass heat shield from the ground impact of the rocket but showed plenty of fiberglass heat shield remaining after the about eight second burn. The “nose” of the ignitor assembly remained in place in contrast to previous tests where this part had shattered upon ignition; the change to a steel “gun barrel” liner for the initiator appears to have resolved this issue.
The four fins were intact and largely undamaged; they appear suitable for reuse in future flight vehicles. Checking with a 0.002” feeler gauge showed there was no gap between the “nose” of any of the fins and the motor tube. A further check using backlighting confirmed that there were no visible gaps between the fins and the motor tube at any location along the fin edges.
The graphite nozzle insert had broken free of its aluminum shell on impact; it was damaged at the exit end and is not suitable for reuse. The aluminum shell showed signs of erosion at the very top of the nozzle. This area was covered by a ring-shaped fiberglass heat shield that was not present upon disassembly. This suggests that the heat shield was fully consumed by hot gas erosion during motor operation; a thicker heat shield is evidently appropriate in future nozzles.
The titanium nozzle extension was undamaged and is suitable for reuse in future nozzles of the same design.
The internal “Fin Can” showed some evidence of blow by of the O-ring that normally sits between the Fin Can and the phenolic liner at the base of the propellant grain. No hot gas erosion was evident in the aluminum structure or in the O-ring, but soot was found on the downstream side of the O-ring. If this O-ring were breeched, hot gas could—in principle—circulate between the liner and the motor wall; thus, this is a potentially significant issue. Mitigating against circulation is the use of high temperature grease between the liner and the motor wall. There was no evidence of any soot or hot gas circulation along the interior of the motor wall. Likewise, there was no evidence of any hot gas leak between the fin can and the motor wall. With minor refurbishment, the fin can does appear suitable for reuse excepting the potential change to two O-rings between the liner and the fin can.
The propellant grain liner was partially consumed at the forward and bottom ends where the liner is exposed to hot gas for the full eight second duration of the burn. There was no evidence of any hot gas contact with the motor tube wall and we thus conclude that the existing liner is of sufficient thickness to handle the current eight second burn time.
Based on this inspection it appears some minor redesign of the nozzle top heat shield is required. It may likewise be prudent to replace the single O-ring used between the internal Fin Can and the phenolic liner with two O-rings. The rest of the vehicle hardware appears to be in good shape and does not seem to require any design changes.
The lack of gap between the fins and the motor wall appears to rule out the possibility of part of the belly-band having become trapped on one of the fins and causing the unexplained turn to the Northeast. The cause of that turn remains a mystery.