SpaceX launched their Falcon 9 v1.1 rocket from Launch Complex 40 at 10:21 a.m. EDT on June 28th, 2015. This was meant to be SpaceX’s 7th resupply mission to the International Space Station. But 139 seconds into the flight, the vehicle began to disintegrate. This resulted in the loss of the rocket and the Cargo Dragon capsule. What happened?
Everything up until the failure had proceeded smoothly. The launch had been delayed a couple of times for various reasons but finally launched on June 28th. This would have been the third test of landing a booster on the droneship, Of Course I Still Love You. Along with general supplies for the station, the first International Docking Adapter was located in the trunk of Dragon. These adapters would be used for the new Crew Dragon and Boeing Starliner capsules.
Liftoff | 10:21:11 AM EDT, June 28th, 2015
The launch appeared to be nominal, however, a white cloud began to appear around 10:22:30 a.m. EDT. As the vehicle pressed onward, still producing this vapor, the Dragon capsule broke off and tumbled away. SpaceX continued to receive telemetry from the capsule until it eventually impacted the ocean. It was determined that had the capsule been programmed with an abort mode, the parachutes could have been deployed, and the capsule recovered. But what actually caused this failure? An Independent Review Team was set up to find out.
CRS-7 accident teams form
SpaceX quickly established an Accident Investigation Team (AIT) made up of SpaceX and representatives from NASA, the FAA, and the US Air Force. An Independent Review Team (IRT) was also created in the following months that was lead by NASA’s Launch Services Program. The IRT operated independently from the AIT to evaluate the events that occurred during flight and identify what lead to the failure. The IRT would also work with SpaceX to ensure that any recommended corrective actions be taken.
SpaceX AIT failure scenario
The SpaceX AIT determined their “primary failure scenario” to be the following: A composite overwrapped pressure vessel (COPV) that contained pressurized, gaseous helium used to keep the fuel tanks at flight pressures had become detached. This was caused by a strut that secured the COPV to the tank breaking. This COPV then shot upward to the top of the Stage 2 liquid oxygen tank and rupturing it in the process.
NASA LSP IRT failure scenarios
The NASA LSP IRT investigated alternative failure scenarios, ending with 2 prevailing theories. Both of these could have either happened before the COPV breaking free or an additional issue. These theories also deal with a specific part of the second stage plumbing. The liquid oxygen tank sits above the RP-1 tank, and there is a transfer tube that runs through the RP-1 tank. This transfer tube is made of concentric tubes that lead to the engine. The gap between these tubes is called the annulus.
- #1: RP-1 that is warmer than the liquid oxygen leaks into the annulus of the transfer tube. This causes liquid oxygen to spew or geyser.
- #2: Liquid oxygen leaks into the annulus of the transfer tube, and the thermal energy transferred by the RP-1 surrounding the tube could cause gasification and eventual geysering.
It was determined that there would not be enough thermal energy transferred by leaked RP-1 in scenario #1 to cause gasification of the liquid oxygen, but models indicated that scenario #2 could be probable. Testing was conducted at the Marshall Spaceflight Center but it was determined that the available thermal energy would not cause liquid oxygen gasification within the failure time.
SpaceX’s AIT concluded that a material failure in the COPV strut leads to the strut being incapable of withstanding flight loads and then caused the subsequent failure. The NASA LSP IRT’s findings were consistent with this conclusion, except for the material failure.
The IRT agreed that the direct and immediate causes of the failure were a liberated COPV and liquid oxygen tank rupture. The IRT could also not dismiss the “material failure” as a probable cause, but they did not view it as a “most probable” cause. They viewed it in line with Rod End Manufacturing Damage, Rod End Strut Mis-installation, or Rod End Collateral Damage. However, the IRT discovered that SpaceX was using an industrial-grade strut rod end rather than an aerospace-grade.
The following is what they believe to be the most credible cause for the CRS-7 failure.
The implementation was done without adequate screening or testing of the
industrial grade part, without regard to the manufacturer’s recommendations for a 4:1 factor of
safety when using their industrial-grade part in an application, and without proper modeling or
adequate load testing of the part under predicted flight conditions. This design error is directly
related to the Falcon 9 CRS-7 launch failure as a “credible” cause.
The IRT made a list of their findings and recommendations to SpaceX.
In the end, Jason-3 flew in January 2016 with all the findings either corrected or mitigated. This would also be the final flight of Falcon 9 v1.1. SpaceX has only had 1 catastrophic failure since CRS-7, that being the AMOS-6 anomaly that occurred during ground testing. Since CRS-7, the Falcon 9 has launched over 100 times without another mission lost during a flight.
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