Complex sentence: The purpose of the MR-1 mission was to qualify the Mercury spacecraft and the Mercury-Redstone booster for the sub-orbital Mercury mission, and to test the booster's automatic inflight abort sensing system in "open-loop" mode.
Split into simpler sentences: The purpose of the MR-1 mission was to qualify the Mercury spacecraft and the Mercury-Redstone booster for the sub-orbital Mercury mission. The mission would also test the Mercury-Redstone booster's automatic inflight abort sensing system, which would be operating in "open-loop" mode.