NASA has scrubbed the first attempted launch of its Space Launch System (SLS) Moon rocket after running into multiple issues, one of which could not be solved in time.

The delay is bad news for the tens to hundreds of thousands of tourists who traveled to Cape Canaveral, Florida to witness the launch in person. Worse, by NASA’s own implicit admission, there’s a good chance the main problem SLS encountered could have already been dealt with and rectified in advance of the launch attempt if the space agency had finished testing the rocket earlier this summer.

Ultimately, that omission turned the first SLS launch attempt into more of a continuation of the rocket’s first four wet dress rehearsal (WDR) attempts, none of which ended as expected. NASA engineers will now have to decide how to proceed and whether the SLS rocket can be made ready in time for another launch attempt on September 2nd or 5th. If not, the next opportunity could be weeks away.

As far as SLS test operations go, the August 28/29th launch attempt was fairly ordinary, with the rocket running into multiple issues – a few minor, a few significant, and one identical to a previous problem. The first problem – a hydrogen leak near the SLS rocket’s base – came after a risk of lightning delayed the start of propellant loading by more than an hour. A very similar, if not identical, hydrogen fuel leak had already occurred during official wet dress rehearsal testing in April and July.

That leak was fixed on the fly by properly chilling all related systems, and propellant loading was eventually completed – albeit a few hours late thanks to inclement weather. Shortly after, there were reports of a crack that needed careful analysis. Only later did NASA specify that the suspected crack was in the rocket’s foam insulation rather than its structures, the latter of which could have been a catastrophic problem.

Around the same time, the true showstopper of the day occurred when NASA attempted to chill the SLS Core Stage’s four RS-25 engines, all of which flew several times aboard reusable Space Shuttle orbiters. Three engines performed (mostly) as expected, flowing a bit of liquid hydrogen fuel to cool themselves down, but one – engine #3 – was never able to make progress toward the optimal temperature needed for ignition (~5°C/~41°F). After hours of remote troubleshooting attempts, no progress had been made, and NASA ultimately decided to scrub the launch attempt at T-40 minutes to liftoff.

Over the course of four separate wet dress rehearsal attempts in April and June 2022, NASA was never able to test the core stage’s engine chill capabilities. In a post-scrub press conference, Jim Free – NASA’s Associate Administrator of the Exploration Systems Development Division – revealed that all four engines were warmer than intended, further confirming that skipping a fully nominal wet dress rehearsal was likely a mistake. Clear and present evidence aside, Free stated that he and other executives still believed skipping that test was the right decision, claiming that ending explicit WDR testing reduced the number of times the rocket needed to be moved on its transporter.

Making the situation even harder to explain, Artemis I Mission Manager Mike Sarafin revealed in the conference Q&A that Boeing had changed the design of parts of the SLS engine chill (bleed) system after the Core Stage finally conducted a nominal static fire test at Mississippi’s Stennis Space Center. Completed in March 2021, the SLS rocket then sat inside NASA’s Kennedy Space Center, Florida Vehicle Assembly Building (VAB) for a full year before attempting its first wet dress rehearsal tests at the launch pad.

The first round of three WDRs were not as smooth as NASA expected and instead uncovered three relatively small issues: a hydrogen leak, a single faulty upper stage valve, and problems with a ground supply of nitrogen gas. Those small issues led NASA to roll SLS back to the VAB for repairs, incurring a minimum multi-week delay that stretched into two months. SLS also failed to complete a fourth WDR attempt in July 2022, but NASA decided to overlook the rocket parts and phases of preflight operations that were never actually tested as planned, one of which was the engine chill system.

If NASA cannot fix the RS-25 chill system within the next few days, it will be forced to roll the entire rocket and mobile launch platform back to the VAB to – at a minimum – replace its flight termination system (FTS). The US Eastern Range requires that all rocket FTS systems be tested no more than 15 days before launch, and NASA was able to secure special permission for a gap of up to 25 days. However, because Boeing’s Core Stage design places the FTS system in a location that is reportedly inaccessible at the pad, the entire SLS rocket will need to roll back to the VAB to have its FTS systems “retested” after that period.

As a result, NASA’s SLS launch debut will be delayed by several weeks (at best) if it can’t recycle for another attempt on September 2nd or 5th. The next window runs from September 20th to October 4th, but the SLS rocket took 10 days to go from its latest rollout to first launch attempt – a figure that doesn’t include the time required to remove the rocket from the pad, roll it back to the VAB, and conduct any necessary repairs or tests while back in the bay. If NASA can’t fix the engine problem at the pad by September 3rd or 4th, the true delay could be more like 4-6 weeks.

With any luck, that won’t happen, but it’s clear that a lot of stress and discomfort could have been avoided if NASA had gone into its first launch attempt knowing that its SLS rocket was truly ready.