For the second time in five days, NASA on Saturday halted a countdown in progress and postponed a planned attempt to launch the debut test flight of its giant, next-generation rocket on Saturday as another dangerous fuel leak forced launch controllers to call off the launch that was 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 98-meter (322-foot) 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.