On July 11, 2024, the second stage of the Falcon 9 had a LOX (Liquid Oxygen) leak in the second stage and it couldn't perform SES-2 (the second upper stage burn), causing the Starlinks onboard to be in too low of an orbit and fall back to earth (source). But, Falcon 9 is a very reliable rocket with nearly 300 consecutive launches without significant issue. So, what was different that caused the leak? What happened?
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2$\begingroup$ That's a very good question that i doubt anybody can answer outside of spacex unless there's a report published $\endgroup$– Alan BirtlesCommented Jul 15 at 5:16
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6$\begingroup$ On Aviation, it is not allowed to ask questions about incidents before the final report has been published. Here on Space Exploration, there is no such rule, but it doesn't change the fact that the question can't be answered until a report is published. The only difference is that aviation accident reports by the NTSB are public, whereas space mishap investigations are not, so a report may never be available. SpaceX has been very open in the past, so that's unlikely, but it is a possibility. $\endgroup$– Jörg W MittagCommented Jul 15 at 7:40
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2$\begingroup$ I can't possibly know whether they will eventually publish a report, and yes I didn't see a report. If I had, I wouldn't be asking this question, since that would tell me the answer. But it could be that I didn't see, it isn't easily accessible, etc, hence I asked. @JörgWMittag $\endgroup$– StarshipCommented Jul 15 at 13:19
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$\begingroup$ Whether a report is published or not, I would expect some summary information to be revealed at some point. But the real problem is that it is just too soon. And unless they can track it quickly to a very specific problem (e.g., work logs specific to this particular second stage showing a specific issue that wasn't handled properly) it will likely be weeks, possibly months before SpaceX/govt. (a) determine the cause and (b) determine the fix (the fix can be anything from a little extra testing/certification on each 2nd stage, which is easy, to redesign of valves or other parts, etc.) $\endgroup$– manassehkatz-Moving 2 CodidactCommented Jul 15 at 17:27
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1 Answer
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What happened?
SpaceX has submitted the mishap investigation report to the Federal Aviation Administration. We have no access to this report, but SpaceX has posted a press release on their website with a high-level overview.
The leak
The leak occurred in an oxygen pressure sense line in the oxygen system of the Merlin Vacuum engine. The cause of the leak was engine vibration that led to a crack in the sense line. The sense line is secured by a clamp, but this clamp was too loose to constrain the sense line from vibrating.
The engine failure
The escaping oxygen super-cooled parts of the engine below their operating temperature. In particular, the system which delivers the pyrophoric mixture of Triethylaluminium and Triethylborane (aka TEA-TEB) used as ignition fluid to start the the engine could no longer function properly, which led to a hard start of the engine.
What is the fix?
Immediate
SpaceX has checked all sense lines and all clamps on all existing vehicles (not just upper stages but boosters and Dragons as well) and replaced some of them.
Short term
The sense line is not a safety-critical system. It is not used by the Autonomous Flight Termination System. The data can also be recovered from other sensors. Therefore, SpaceX chose to remove the sense line from future engines.
Long term
SpaceX will develop an alternative to the sense line.
[This was mentioned in a press conference with Steve Stich.]
Return to flight
The FAA has not yet accepted SpaceX's mishap investigation report. However, the FAA has issued a positive public safety determination, meaning the FAA agrees with SpaceX that there was and is no danger to public safety. Therefore, the FAA has reinstated the launch license with immediate effect, and SpaceX has already launched Starlink 10-9 on 2024-07-27T05:45Z with Starlink 10-4 and Starlink 9-4 currently planned about 24 hours after and about 3 hours apart.
Near future missions
There are currently a couple of high-profile missions on SpaceX's launch manifest for the immediate future:
- The Cygnus CRS NG-21 commercial resupply mission to the ISS, currently planned for no-earlier-than August 3rd.
- The Crew-9 commercial crew mission to the ISS, currently planned for no-earlier-than August 18th.
- The Arctic Satellite Broadband Mission mission for Space Norway that also carries payloads for the US Space Force, currently planned for no-earlier-than August.
- The Polaris Dawn mission, currently planned for no-earlier-than August.
These missions are for very risk-averse and conservative customers like NASA and the US Space Force. I have the feeling that SpaceX is trying to squeeze in as many Starlink missions as possible to re-build customer confidence. This might work especially well considering that Starlink missions are pushing the performance margins of the Falcon 9 launch system much more than a CRS or ISS Crew mission would – for example, the second engine relight, when the mishap occurred, isn't even done for missions to the ISS. They might also get in Transporter 11, which will typically have multiple second engine relights.
What about crewed flights?
SpaceX currently has two crewed flights on the manifest in the near future:
- The Crew-9 commercial crew mission to the ISS, currently planned for no-earlier-than August 18th.
- The Polaris Dawn mission, currently planned for no-earlier-than August.
NASA and SpaceX are in the comfortable position that currently all docking ports on the ISS are occupied, so Crew-9 cannot happen regardless. This leaves SpaceX more time to build customer confidence.
NASA's preference seems to be to bring home Starliner first, so that Crew-8 and Crew-9 can do a direct handover. However, since there is no return date for Starliner yet, there is also no hurry in getting Crew-9 off the ground. NASA has asked SpaceX to prioritize Crew-9 over Polaris Dawn, so there is also no hurry to launch that mission.
It is important to keep in mind:
- The mishap happened on the second engine burn. With ISS missions, there is no second engine burn. (However, to be fair, the leak already developed during the first engine burn.)
- The second stage was not destroyed. It was fully controllable, and it even deployed the Starlink satellites and performed its passivation procedure. The second stage sent telemetry the entire time, until it burned up in the atmosphere. It just couldn't start the engine.
- From a mission perspective, all that happened was that the payload was deployed into a too-low orbit and reentered prematurely. If this had been a Crew Dragon capsule, all that would have happened is that the astronauts would have spent 45 minutes in space instead of 6 months.
- Even with this mishap, Falcon 9 is still almost 3x above the required NASA safety threshold.
- If SpaceX can quickly ramp back up to their pre-mishap flight rate, By August 18th SpaceX may have had as many as 8 flights already to prove out any fixes they made.
answered Jul 27 at 16:28
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1$\begingroup$ The one lingering repercussion being that crew flights are not yet reinstated. The Polaris Dawn mission originally scheduled for July 31st has been moved to mid to late August. The Crew 9 mission is still moving forward with a NET launch date of August 18th, but that is subject to how quickly the investigation is concluded and the correction accepted and approved for crewed flights. $\endgroup$ Commented Jul 27 at 19:04
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$\begingroup$ Crew-9 requires a docking port which will not be available until either Crew-8 or Starliner CFT return. It is my understanding that the vehicle schedule on the ISS is what's driving the schedule for Crew-9 and that NASA asked SpaceX to push Polaris Dawn until that's sorted out to not create any additional constraints. But unfortunately, I did not watch that press conference with Steve Stich, where most of this was unpacked and explained as far as I know. $\endgroup$ Commented Jul 27 at 19:14
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$\begingroup$ NASA would prefer a direct crew handover from Crew-8 to Crew-9, which would mean Starliner CFT is the one to vacate the docking port first. However, NASA and Boeing are still waiting on further tests before setting a date for the undocking. So, it seems, the holdup is currently on Starliner. Of course that doesn't mean that SpaceX and NASA are not also using that time to dot all their is and cross all their ts. $\endgroup$ Commented Jul 27 at 19:16
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2$\begingroup$ I think what is being underplayed in the briefings is that we have no idea how long it will take for crew flights on Falcon 9 to be cleared. Approving uncrewed flights came relatively quickly because the only requirement was proving that there was no risk to public safety. Putting on my PR hat (if I had one), there is already ongoing publicity about how the CFT crew is remaining on ISS until the thruster investigation is completed. So why add to the news cycle (until you have to) a story about how U.S. astronauts are currently grounded and the Crew-8 astronauts have an indefinite return date. $\endgroup$ Commented Jul 27 at 19:40
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4$\begingroup$ I believe for the original certification, NASA required nine successive successful flights without making any changes to the vehicle. The mind boggling thing is: with their pre-mishap flight rate and a little bit of luck with the weather, SpaceX could theoretically pull that off by August 18th. $\endgroup$ Commented Jul 27 at 20:14