Complex sentence: The Air Force approved an 18-month program acceleration in March 1958 that rescheduled the first flight to December 1961, but was announced in the fall of 1958 that this acceleration would not be possible due to lack of funding.
Split into simpler sentences: The Air Force approved an 18-month program acceleration in March 1958 that rescheduled the first flight to December 1961. Then in the fall of 1958 it was announced that this acceleration would not be possible due to lack of funding.