Last test of RS-25 engine number 0528 was performed on July 14 and as it was announced, it was terminated due the problems with one of the connectors part of the Advanced Health Management System.
This test, just as the previous one, was performed at NASA engine testing facility in Stennis, Mississippi on A-1 test stand. It started on early afternoon local time, at 19:10 GMT, on July 29, 2016, just two weeks after last fire test. During these weeks specialists and engineers from Stennis and Aerojet Rocketdyne were able to locate problem. In fact it was unveiled, that problem was not connected with part of the engine, but it was result of technical problems with test stand. Specialists from Syncom Space Services, main contractor for Stennis facility testing equipment, worked hard and managed to fix problem; still they were present during test performed on Friday. Finally, test begun and last for whole 650 seconds from 19:10 GMT to 19:21 GMT, just as it was planned for test two weeks ago. 0528 is second engine, which is tested to check all improvements necessary for adopting RS-25 from Space Shuttle to SLS;test was important to confirm that everything is correct and problem from two weeks was nothing more than broken connector or problem with test stand. Jim Paulsen, vice president for NASA programs at Aerojet Rocketdyne, described shortly why tests are so important and what kind of changes were made in construction of RS-25:
"During flight, the RS-25 engines will endure more heat, pressure and thrust on SLS than on the space shuttle. Four RS-25 engines will fly at the bottom of the SLS core stage with each operating at 109 percent power level, as opposed to three on the space shuttle operating at 104.5 percent power level. The solid rocket booster motors will also be closer in position to the RS-25 engine cluster this time around, and the taller core stage will result in higher propellant inlet pressure on the engine system. All of this adds up to multiple ground tests to ensure astronaut safety."
It is worth to remind that RS-25 is proven and reliable construction, but it was designed to work in different conditions that SLS. All the changes were necessary to adopt it to SLS and in case of any problems whole program could be delayed.