On Saturday, September 3, 2022, NASA made a second attempt to kickoff their Artemis moon program but the embattled Space Launch System rocket remains tethered to Earth by technical problems still being assessed by the agency.

The first launch attempt was scheduled for August 29th with hundreds of reporters, VIPs, and politicians in attendance at Kennedy Space Center. Tens of thousands of onlookers gathered on Florida's Space Coast to catch the gigantic rocket's liftoff. The weather looked good as the Artemis I's Launch Director, Charlie Blackwell-Thompson, gave a GO to begin loading fuel onto SLS.

Before the super cold liquid propellant can start to flow, the liquid oxygen (LOX) and hydrogen propellant lines are chilled down to prevent a thermal shock to the system. In under 6 hours, over 2 million liters (537,000 gallons) of liquid hydrogen and 742,000 liters (196,000 gallons) of liquid oxygen are loaded into the rocket's core stage step by step.

First, the propellant is slowly filled to thermally condition the tanks until the temperature and pressure are stable. This is known as the slow fill. This is followed by the fast fill operation, where the tank is filled at a quicker pump speed.

As the teams transitioned from slow to fast-fill liquid hydrogen operations, a hydrogen leak was discovered from the mating surface of the Mobile Launch Tower’s quick disconnect arm. The quick disconnect arm helps load the propellant in the core stage and disconnects safely once the rocket lifts off. 

Since the leak wasn’t a substantial one, teams continued with the LH2 fast fill while troubleshooting the issue. It was later ratified as the launch controllers manually adjusted the propellant flow rate.

As propellant fill neared completion in the rocket's core stage, the mission management gave a GO to begin cryogenic load on the ICPS second stage. However, during its LOX load, engineers encountered yet another issue with one of four RS-25 engines on the core stage. Readings from the onboard temperature sensor showed that engine 3 was not properly conditioned for launch since it repeatedly failed the bleed test. RS-25 engines are required to be chilled at approximately -251 degrees Celsius (-420 degrees Fahrenheit) to handle the injection of super chilled liquid hydrogen and oxygen propellants. 

Following the scrub, NASA’s program manager for SLS, John Honeycutt, explained that the engineering teams believe that the engine was actually cooled down to the required level but it was not properly measured by a faulty temperature sensor. "The way the sensor is behaving does not line up with the physics of the situation,” he said.

The sensor onboard engine 3 can’t be replaced while the rocket is still on the launch pad. To facilitate that, NASA would have to roll back the SLS back to the VAB, delaying the launch to at least October. 

From a physics standpoint, igniting super-chilled propellants in a warmer-than-anticipated engine would likely severely damage the RS-25 engine's turbopump, at a minimum. This would cause a lot of delays to an already delayed program, but Honeycutt said that he’s confident that liquid hydrogen is flowing into engine 3 and this was corroborated by other sensors including the pressure measurements that indicated that the engine was in an environment that would’ve properly cooled it down.

"We will be looking at all of the other data we have and [will] use it to make an informed decision.”

Engineers continued to access the data after the scrub. Mission managers met on Tuesday, August 30th, to discuss findings and formulate a plan to address the issues that arose during the first attempt. The new plan called for a modified propellant loading procedure, similar to one followed during the Green run at NASA’s Stennis Space Center in Mississippi, and a flight rationale that would allow the rocket to launch without getting good data from the temperature sensor on engine 3.

This modified procedure called for an earlier chill-down test of the engines during the liquid hydrogen’s fast fill phase, allowing additional time to appropriately cool the engines needed for the launch. Teams also worked to fix the leak on the quick disconnect arm by replacing the components responsible.

These steps were enough to satisfy the launch officials and after convening with the meteorologists at the US space Force Space Launch Delta 45, NASA targeted Saturday, September 3rd for their next launch attempt.

With an updated plan and a better understanding of the issues which arose during the first attempt, Blackwell officially gave a GO for cryogenic operations for the second time. The fueling operations were going very smoothly until launch engineers again detected a liquid hydrogen leak from the 8-inch diameter line located in the quick disconnect arm and stopped the propellant flow to troubleshoot. 

To resolve it, controllers first attempted to warm up the quick disconnect to achieve a proper seal but this didn’t work as they encountered another leak as soon as they resumed the propellant flow. Next, they tried increasing the pressure on a ground transfer line using helium to reseal the leak, but this attempt also went in vain as the leak remained. For the third time, the propellant loading was stopped. Engineers attempted to warm up the quick disconnect to reset the seal but this attempt was also unsuccessful.

Liquid hydrogen loading had plagued NASA’s second launch attempt to launch their rocket to the Moon and after trying and failing three times to seal the leak, the launch attempt was called off. Engineers continued to gather additional data to understand the nature of the leak and how much wasn’t detected during the first attempt.

NASA is familiar with liquid hydrogen leaks as they occurred frequently during the space shuttle era. On average, Shuttle launches were scrubbed nearly once every launch attempt. Some were scrubbed as many as five times before finally lifting off. The complex fueling process was almost always the problem.

Being extremely flammable, NASA has a tight tolerance for hydrogen leakage. Anything above a 4 percent concentration of H2 is considered a fire hazard. 

"We were seeing in excess of that by two or three times," Mike Sarafin, NASA's Artemis I Mission Manager, said of the second attempt's hydrogen leak. "It was pretty clear we weren’t going to be able to work our way through it. Every time we saw a leak, it pretty quickly exceeded our flammability limits.”

The cause of the leak is still being determined but SLS engineers believe it may be due to an errant valve being opened, which might’ve happened during the process of chilling down the rocket before loading LH2. A command — along with a sequence of other commands — might’ve been sent to open a wrong valve. 

Over the next several days, teams will establish an area to access the leak and decide whether to replace the seal on the launch pad or roll it back to the Vehicle Assembly Building (VAB). If the team decides to replace the hardware at the pad, they might have an option to perform a partial fueling test to determine the integrity of the fix while keeping the vehicle on the pad ahead of the next launch attempt.

It seems more and more probable that NASA will roll the SLS and the Orion spacecraft back to the VAB. Orbital dynamics of the Earth and the Moon dictate the launch windows of Artemis 1. Currently, the opportunity exists between September 19th and October 4th, but launching during this window would require a waiver from the US Space Force’s Eastern Range.

This issue here is the batteries of the Flight Termination System, which can destroy the rocket in a controlled manner if it goes off trajectory. FTS is powered independently of the rocket and its batteries are rated for 25 days, a requirement set by the range. If NASA wants to launch during this launch period, they’ll have to extend the rating to 40 days.

If the SLS is rolled back to the VAB — necessary to service the FTS or perform work at the launch pad — the next launch opportunity will fall between October 17 and October 31. Download the Supercluster app for more information on Artemis 1 and for updates on the mission's launch date and time.