Overview

*propellant mass estimated by delivered impulse prior to anomaly, improved estimate to come after failure analysis

Unfortunately, a failure of the liner at the splice point led to propellant gases burning through the middle of the case at approximately 3.1 seconds into the burn.

Media

Dropbox Link: https://www.dropbox.com/sh/l7inp5sku5ggu6s/AADc6fPjA6Nl4wnU-S-KvGWla?dl=0

Data

Raw Data: cut.TXT

ENG File:  P2.eng

Discussion

The burn began nominally and continued for approximately three seconds, however, at the three second mark, the case ruptured in a large fireball that engulfed the test stand. The failure led to a rapid loss of thrust and damaged the test stand and several sensors in the surrounding area.

Upon inspection of the motor after the anomaly, it was almost immediately clear what the source of failure was. The motor split at approximately the same location at which the liner was spliced.

Failure theory:

During liner splicing in the past, Gorilla Glue was applied to the middle grain that contained the splice joint. The two halves of the liner were slid on opposite ends of the grain and pushed together, meeting at the halfway mark in the grain. RTV was applied to the seam and a piece of clear packing tape was applied at the seam to facilitate insertion of the grain assembly into the liner. There was typically a bulge of RTV that was covered by tape at the splice point, as shown below.

In preparation for this static fire, the liner halves were pushed together on the middle grain until they were two inches apart, and then the RTV was applied and the tubes pushed together. This may have caused a problem because the resulting mixture of Gorilla Glue and RTV may have either not cured, or cured with dubious thermal properties that were compromised when hot gases seeped to the middle grain during the burn. Further, the majority of the RTV on the seam was scraped off the splice joint because it was thought that there was enough RTV already in the seam to form an adequate seal. This proved not to be the case, however. From the post-test footage, one can see the case discolor and bulge at the splice point moments before the failure, showing that the liner splice likely failed instantaneously upon ignition and that the case was exposed to combustion gases for the entire three seconds leading up to failure. This is evident from the cooled aluminum drips on the test stand base that indicated that much of the material at the splice point was melting before the case ruptured.

Splice for failed static fire, OW-152-8G#2

Splice for successful static fire, OW-152-8G #1

Resolution:

In the future, the team will research liner splicing methods more thoroughly before attempting another liner splice in the fall. We are reaching out to other schools that have attempted liner splices before to gain more insight on the process can be performed more reliably in the future.