From: Alan Minyard a

NASA even built a "zero-G" test rig to try out the landers controls.
It had a lift engine that could be throttled to exactly balance out
the weight of the rig, so the thrusted operated at "zero-G"

Al Minyard

I think they had more than one. I recall a pilot punching out of one just
before it crashed.

Dan, U.S. Air Force, retired