In a new NASA briefing, SpaceX vice president of build and flight reliability Hans Koenigsmann was able to explain in far more detail why a recent last-second Falcon 9 launch abort happened and how it wound up delaying the company’s first operational astronaut launch.

Now scheduled to lift off no earlier than (NET) 7:49 pm EST (00:49 UTC) on Saturday, November 14th, SpaceX’s Crew Dragon Crew-1 mission was originally expected to launch in late September, October 23rd, and October 31st. On October 2nd, however, a new Falcon 9 booster – sibling to Crew-1’s own new booster – automatically aborted its GPS III SV04 satellite launch attempt just two seconds before liftoff. The rare last-second abort was quickly blamed on “unexpected pressure rise in the turbomachinery gas generator” by CEO Elon Musk.

Likely built side-by-side with faulty GPS III SV04 Falcon 9 booster B1062 at SpaceX’s Hawthorne, California factory, Crew-1 Falcon 9 booster B1061 was almost immediately inspected to search for any commonality once the cause of the abort was better understood.

Just one week before the latest briefing, NASA human spaceflight program administrator and former Commercial Crew Program manager Kathy Lueders revealed in a statement on Twitter that SpaceX was still analyzing the cause of the abort but had already determined that at least one Crew-1 booster engine would need to be replaced, as well as one engine on Falcon 9 booster B1063.

Now, during NASA’s October 28th Crew-1 briefing, SpaceX’s Koenigsmann revealed that the company had ultimately decided to replace not one but two of Crew-1 booster B1061’s nine Merlin 1D engines. Thanks to Falcon 9’s namesake nine-engine booster design and SpaceX’s prolific rocket factory, that process was completed extraordinarily quickly, simply requiring the redirection of already qualified Merlin 1D engines from a fairly large pool. Based on Koenigsmann’s phrasing, SpaceX has already installed both replacement engines on the Crew-1 booster.

What, though, caused GPS III SV04’s launch abort and how did that affect Crew-1?

Rocket engine vs. “nail polish”

According to Koenigsmann, in the course of the rapid and complex mechanical and electrical ballet preceding Falcon 9 first stage ignition, the rocket’s autonomous flight computer observed that two of the GPS III SV04 booster’s nine Merlin 1D engines appeared to be running ahead of schedule, so to speak. The computer immediately halted the ignition process to avoid what could have otherwise been a “hard” (i.e. stressful or damaging) start. SpaceX quickly began inspecting the rocket within 24 hours but was unable to detect anything physically or electrically wrong with Falcon 9’s Merlin 1D engines and engine section.

Out of an abundance of caution, SpaceX removed both misbehaving engines and shipped them to its McGregor, Texas development and test facilities where – somewhat miraculously – the same premature startup behavior was replicated on the test stand. After a great deal of increasingly granular inspections, SpaceX finally narrowed the likely cause down to a tiny plumbing line feeding one of the engine’s gas generator relief valves. In a seemingly random subset of relatively new Merlin 1D engines, SpaceX eventually discovered that a supplier-provided relief valve line was sometimes clogged by a protective lacquer Koenigsmann likened to “red nail polish.”

Used to selectively exclude parts of the engine tubing during a surface finishing process known as anodization, the lacquer was either unsuccessfully removed on a random selection of engine parts or was accidentally channeled into a blockage by over-enthusiastic cleaning. Ultimately, for whatever, reason that miniscule blockage was enough to cause affected Merlin 1D engines to consistently attempt to ignite a tiny fraction of a second early.

Crucially, when SpaceX discovered the possible cause and cleaned out the blocked plumbing, each previously affected Merlin 1D engine performed perfectly, all but directly confirming both the cause and the cure for Falcon 9’s October 2nd abort.

Astronauts enter quarantine

In anticipation of SpaceX seemingly simple solution to the gas generator problem, NASA Commercial Crew Program manager Steve Stich revealed that SpaceX’s Crew-1 mission astronauts – Shannon Walker, Victor Glover, and Mike Hopkins, and JAXA (Japanese) astronaut Soichi Noguchi – had begun routine prelaunch quarantine procedures in anticipation of a November 14th launch.

Stich also offered a more specific Crew-1 schedule, beginning with an integrated Falcon 9 and Crew Dragon static fire test NET November 9th and a full dry dress rehearsal on November 11th before the first launch attempt on November 14th. Notably, thanks to coincidental orbital dynamics, a successful launch on November 14th would enable Crew Dragon to raise its orbit and rendezvous with the International Space Station a brisk eight and a half hours after liftoff – three times quicker than the more common 27.5-hour transit.

Stay tuned for updates as the mission’s launch date approaches.