Why was Booster 9's 29 engine static fire ended 2.36 seconds early? [duplicate]

Question

Suggested by Darth Pseudonym in answer to my other similar question. Recently (August 7), Booster 9 conducted a static fire test where it shutdown after only 2.74 of the planned 5 seconds of burn duration. This is the big static fire, there not another one. I have found that this happened a bunch (example), but not why. So, what happened, and why was it ended 2.36 seconds early?

Answer 1

As with pretty much all questions about spacex, the answer is: We don't know. SpaceX is a private company, unlike a government agency, they are not required to publish their information, for example under something like the Freedom of Information Act. Also, they are privately held, not publicly traded at the stock exchange, so they don't fall under SEC disclosure rules for publicly traded companies either. And no property other than SpaceX's own was damaged (if at all), so there will be no FAA / NTSB / other law enforcement investigation.

Basically, they don't have to tell anybody anything, and they frequently don't.

Typically, the only sources of information we have are official statements from SpaceX on their website, their 𝕏 account, or from hosts during one of their live webcasts, as well as semi-official statements from SpaceX's CEO. At the moment, the only two statements we have are from SpaceX's official webcast of the static fire. One from an unknown engineer (it did not sound like the launch director) over the countdown net:

We reached a test duration of 2.74 s with 4 engines shutting down prematurely.

There is no other callout over the countdown net, so the only other statement we have is from the webcast host, John Insprucker, who has no additional information and just recaps the earlier callout:

We did hear the one callout that we got into the ignition of the 33 engines. It sounds like we had 4 that did shut down during the ignition, but we've not heard any other followup. We do know from other discussions the pad looks good, the launch vehicle looks good, and right now the team is going to plan for proceeding into the regular propellant offload on the methane and liquid oxygen propellants on Super Heavy.

This is the extent of the publicly available information. Anything else you may read, hear, or see, is, at least at this point in time, pure speculation by people who have the exact same information that you have. Including this:

According to SpaceX, the Starship system needs 30 working engines on the Super Heavy booster to lift off. If more than 3 engines shut down before liftoff, the launch is aborted. Assuming that SpaceX follows the motto "test as you fly", this would mean the vehicle's flight computer would automatically abort as soon as at least 4 engines shut down.

So, it is possible that the flight computer aborted the ignition sequence because the launch commit criteria were violated.

But again, this is just speculation on my part, and the only information I have available is what I quoted above.