NASA's new moon rocket sprang another dangerous fuel leak Saturday, forcing launch controllers to call off their second attempt to send a crew capsule into lunar orbit with test dummies.
The first attempt earlier in the week was also marred by escaping hydrogen, but those leaks were elsewhere on the 322-foot (98-meter) rocket, the most powerful ever built by NASA.
Launch director Charlie Blackwell-Thompson and her team tried to plug Saturday's leak the way they did the last time: stopping and restarting the flow of super-cold liquid hydrogen in hopes of removing the gap around a seal in the supply line. They tried that twice, in fact, and also flushed helium through the line. But the leak persisted.
Blackwell-Thompson finally halted the countdown after three to four hours of futile effort.
More than 400,000 people have flocked to Florida's Space Coast for the launch, according to the Orlando Sentinel newspaper.
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To bungle one rocket launch might be regarded as a misfortune, but to botch two is beginning to look like carelessness.
Nasa suffered the indignity of scrubbing a second attempt to get back to the Moon on Saturday, after springing yet another fuel leak in its Space Launch System (SLS) rocket.
Leaks have dogged the $93 billion Artemis programme since dummy fuelling runs earlier in the year, with no solution seemingly in sight.
There had been problems even before the fatal leak was discovered, after liquid oxygen fuel stopped flowing shortly after the countdown began.
The SLS rocket - the most powerful ever built - needs nearly one million gallons of liquid hydrogen and oxygen but has suffered ongoing fuelling and engine issues.
The next launch window is on Monday, but Nasa does not seem close to solving the myriad of problems facing the programme.
The #Artemis I mission to the Moon has been postponed. Teams attempted to fix an issue related to a leak in the hardware transferring fuel into the rocket, but were unsuccessful. Join NASA leaders later today for a news conference. Check for updates: https://t.co/6LVDrA1toy pic.twitter.com/LgXnjCy40u
During tanking of the #Artemis I mission, a leak developed in the supply side of the 8-inch quick disconnect while attempting to transfer fuel to the rocket. Attempts to fix it so far have been unsuccessful. Stand by for updates. https://t.co/6LVDrA1toy
Charlie-Blackwell-Thompson, the Artemis launch director, has said she wants to wait before scrubbing today's launch.
It's unclear what more the teams can do to fix the problems today, and there surely cannot be enough time to launch now.
The cooling issue was the final nail in the coffin for Monday's launch attempt, after a series of challenges.
"This is all new technology," said Nelson. "That's why we're doing a test flight. We're going to stress it and test it in a way that we would never do with humans on top of it but that's the purpose of a test flight.
"I'm very confident and if there are what we call 'anomalies,' in other words mistakes or unexpected events, that's part of the test flight."
When at the pad, I took a moment to soak in the fact that I'm about to see the largest, most powerful rocket of my lifetime (for now). I've been waiting over 8 years for this one! #Artemis1 will be incredible and I've got a good feeling about Saturday's attempt! pic.twitter.com/6x7ezc4WnH
Nasa chief Bill Nelson earlier said he was "very confident" that the launch of the groundbreaking Artemis 1 moon rocket will go ahead successfully on Saturday afternoon after the first attempt was foiled by engineering problems.
He told BBC Radio 4 that the rocket had been checked "from gizzard to gizzard" and that they are "absolutely sure" the engine cooling is functioning, adding that the problems may have been caused by faulty indications from a sensor.
We are delighted that you have joined us for our live coverage of Nasa's second attempt at the Artemis 1 launch. Problems with Engine 3 meant that the rocket could not set off on its first attempt on August 29. We hope that the sequel effort is a little better!