Complex sentence: On January 27,1967, Grissom, White and Chaffee were participating in a "plugs-out" countdown demonstration test at Cape Kennedy in preparation for the planned February 21 launch, when a fire broke out in the cabin, killing all three men.
Split into simpler sentences: On January 27,1967, Grissom, White and Chaffee were participating in a "plugs-out" countdown demonstration test at Cape Kennedy in preparation for the planned February 21 launch. During the test, a fire broke out in the cabin, killing all three men.