Complex sentence: The Advanced Launch System (ALS) was a joint USAF and NASA study from 1987-1990 that emerged in the post-Challenger periodfor launch vehicles that would be used to deploy the space-based elements of the Strategic Defense Initiative (SDI) program.
Split into simpler sentences: The Advanced Launch System (ALS) was a joint USAF and NASA study from 1987-1990 that emerged in the post-Challenger period. The program considered requirements and launch vehicles that would be used to deploy the space-based elements of the Strategic Defense Initiative (SDI) program.