Complex sentence: Initially the AAP office in Washington was an off-shoot of the Apollo "X" bureau, also known as the "Apollo Extension Series", that was developing technology concepts for mission proposals based on the Saturn IB and Saturn V boosters.
Split into simpler sentences: Initially the AAP office in Washington was an off-shoot of the Apollo "X" bureau, also known as the "Apollo Extension Series". AES was developing technology concepts for mission proposals based on the Saturn IB and Saturn V boosters.