propulsive landings. That plan was scrapped a while ago, but some legacy design components existed in the system. This was not expected to be a problem, but they learned some new materials science stuff that was shared with NASA and other aerospace firms. Since propulsive landing is no longer needed, they redesigned that part of the system to prevent the failure from happening again.

The following is from an article on spaceflightnow.com

A faulty check valve inside the propulsion system allowed nitrogen tetroxide oxidizer to enter high-pressure helium tubes during ground processing before the attempted static fire test in April.

The helium system is used to quickly pressurize the propulsion system, allowing the SuperDraco thrusters to fire up during a launch emergency.

When the abort system began pressurizing on the April test, nitrogen tetroxide that had leaked into the helium pressurization system was driven back into the check valve, which is made of titanium.

“Imagine a lot of pressure driving back a slug of liquid (that) has significant force, and that basically destroyed the check valve and caused an explosion,” said Hans Koenigsmann, SpaceX’s vice president of build and flight reliability, in a July press briefing.

“We found out that … when the pressure is high, and you drive a slug with a lot of energy into a titanium component, that you can have this rather violent reaction,” Koenigsmann said.

The violent result was surprising. Engineers did not expect titanium, a material commonly used for decades on space vehicles around the world, could react so explosively in such an environment.

SpaceX added a burst disk to prevent propellant from leaking into the high-pressure lines before ignition.