Just hours ago, SpaceX confirmed that Flight 9 is officially scheduled to liftoff on May 27th. In addition, they released a full statement regarding Flight 8 with specifics on what happened to the upper stage, which was lost during its engine burn.
What was initially thought to be an issue very similar to what happened on Flight 7 related to harmonic resonance, turns out to have been an individual Raptor engine.
Raptor Engine Failure
Looking back at the Flight 8 video, onboard cameras first showed a fire in the aft section of Starship. We then were shown a view from one of the flaps where an energetic event could been seen around the engines causing the vehicle to lose control and start spinning.
In SpaceX’s new statement they are quoted saying, “Starship’s upper stage flew along its expected trajectory following separation from the Super Heavy booster. Approximately five and a half minutes into its ascent burn, a flash was observed in the aft section of the vehicle near one of the center Raptor sea level engines followed by an energetic event that resulted in the loss of the engine. Immediately after, the remaining two center Raptor engines and one of the Raptor vacuum engines shut down and vehicle control authority was lost. Telemetry from the vehicle was last received approximately nine and a half minutes into the flight, or a little more than two minutes following the first flash observation, at which point all engines had shut down” they said.
This statement aligns with the image that was going around showing the engines where both a vacuum Raptor engine, or at the very least, a vacuum Raptor engine nozzle, and a sea level engine were missing. In their statement they point out how the issue originated from that single sea level raptor engine, and basically once it exploded, it likey shot shrapel in each direction, damaging and immediately shutting down some of the surrounding engines.
The company goes on to say, “Contact with Starship was lost prior to triggering any destruct rules for its Autonomous Flight Safety System, which was fully healthy when communication was lost. It is expected that the Autonomous Flight Safety System fired upon loss of communication, ensuring vehicle breakup following the mishap. The vehicle was observed to re-enter the atmosphere and break apart following the loss of communication” they said.
They then go into more detail about what caused the energetic event with the sea level Raptor engine. Here they are quoted saying, “The most probable root cause for the loss of Starship was identified as a hardware failure in one of the upper stage’s center Raptor engines that resulted in inadvertent propellant mixing and ignition. Extensive ground testing has taken place since the flight test to better understand the failure, including more than 100 long-duration Raptor firings at SpaceX’s McGregor test facility.
To address the issue on upcoming flights, engines on the Starship’s upper stage will receive additional preload on key joints, a new nitrogen purge system, and improvements to the propellant drain system. Future upgrades to Starship will introduce the Raptor 3 engine which will include additional reliability improvements to address the failure mechanism.”
Very importantly, they then say, “While the failure manifested at a similar point in the flight timeline as Starship’s seventh flight test, it is worth noting that the failures are distinctly different. The mitigations put in place after Starship’s seventh flight test to address harmonic response and flammability of the ship’s attic section worked as designed prior to the failure on Flight 8.”
This is quite a big deal as it suggests there isn’t some massive flaw with the core design of Starship V2. After flight 8, many believed that the harmonic resonance and design changes, including new downcomers for each vacuum Raptor engine, may have been the cause of Flight 7 and Flight 8’s demise. While the issue with the Raptor engine is not ideal, it seems like SpaceX is confident the vehicle is very capable as is and won’t require a significant redesign.
Focusing back on the statement, SpaceX said, “Starship flew within a designated launch corridor to safeguard the public both on the ground, on water, and in the air. Following the mishap, SpaceX teams immediately began coordination with the FAA, ATO (air traffic control) and other safety officials to implement pre-planned contingency responses. SpaceX worked closely with the Bahamian government and sent a team of experts to coordinate and execute clean-up efforts. All debris came down within the pre-planned Debris Response Area, and there were no hazardous materials present in the debris and no significant impacts expected to occur to marine species or water quality.”
“SpaceX led the investigation efforts with oversight from the FAA and participation from NASA, the National Transportation and Safety Board, and the United States Space Force. SpaceX submitted a mishap report to the FAA for review and received a flight safety determination from the FAA to enable its next flight of Starship” they said.
While the early loss of the upper stage was the focus of Flight 8, the booster also lost a few engines with multiple missing during the boostback burn as well as the landing burn. Even though the booster was still able to complete a successful catch, the company is trying to increase Raptor engine reliability.
SpaceX commented, “The Super Heavy booster then relit 11 of 13 planned Raptor engines and performed a boostback burn to return itself to the launch site. Once there, it relit 12 of the planned 13 engines for its landing burn, including one of the engines that did not start up for the boostback burn. The three center engines continued running to maneuver the booster to the launch and catch tower arms, resulting in the third successful catch of a Super Heavy booster.
“The most probable cause for engines not relighting during the boostback and landing burn phases was traced to torch ignition issues on the individual engines caused by thermal conditions local to the igniter. Post-flight testing was able to replicate the issue and engines on future flights will have additional insulation as mitigation” they said.
In other words, it seems that the sole culprit of issues with both the booster and upper stage during Flight 8 all had to do with the Raptor engines. Granted, there are 39 engines on Starship in total, and it’s worth noting that the booster can complete its mission without all of them. However, as the program continues to make progress, Raptor engine innovation and the push to Raptor 3 will be a big milestone for the company, and in theory, significantly improve the reliability of both the booster and Starship.
At the end of the statement, they finished by saying, “Starship is designed to fundamentally change and enhance humanity’s ability to reach space. This step change in capability won’t happen overnight and progress towards that goal won’t always come in leaps. But by putting hardware into a real-world environment as frequently as possible, while still maximizing controls for public safety, progress can be made to achieve the goal of flying a reliable, fully and rapidly reusable rocket.”
On the next flight, scheduled just 4 days from now on the 27th, we have quite a bit to look forward to. The Starship upper stage will again target multiple in-space objectives, including the deployment of eight Starlink simulators, similar in size to next-generation Starlink satellites. The Starlink simulators will be on the same suborbital trajectory as Starship and are expected to demise upon entry. A relight of a single Raptor engine while in space is also planned.
The flight test includes several experiments focused on enabling Starship’s upper stage to return to the launch site. A significant number of tiles have been removed from Starship to stress-test vulnerable areas across the vehicle during reentry. Multiple metallic tile options, including one with active cooling, will test alternative materials for protecting Starship during reentry. On the sides of the vehicle, functional catch fittings are installed and will test the fittings’ thermal and structural performance. The entire ship’s tile line also received a smoothed and tapered edge to address hot spots observed during reentry on Starship’s sixth flight test. Starship’s reentry profile is designed to intentionally stress the structural limits of the upper stage’s rear flaps while at the point of maximum entry dynamic pressure.
Even though Flight 9 will be the third flight of Starship V2, this variant still hasn’t had a chance to reenter the Earth’s atmosphere in one piece and gather data. On the last few flights, they have been trying to test different heat shield configurations among other tests, but the flights have been cut short. The goal for Flight 9 is to complete the flight profile and start providing some invaluable data on the new vehicle.
In one last quote they highlight, “Developmental testing by definition is unpredictable. But by putting hardware in a flight environment as frequently as possible, we’re able to quickly learn and execute design changes as we seek to bring Starship online as a fully and rapidly reusable vehicle.”
Conclusion
We now know that the issues that caused the early loss of Starship on Flight 7 and Flight 8 were different. It wasn’t harmonic resonance on Flight 8 but instead an anomaly with an individual sea level raptor engine, causing an energetic demise and the loss of surrounding engines. For the upcoming Flight 9, SpaceX has made some upgrades to the engines with the goal of completing the full flight profile.