Workers at McDornel-Douglas install the Saturn IB S-IVB (second) stage for the Apollo-Soyuz mission into the company's S-IVB assembly and checkout tower in Huntington Beach, California. The Saturn IB launch vehicle was developed by the Marshall Space Flight Center (MSFC) as an interim vehicle in its "building block" approach to Saturn rocket development. This vehicle utilized the Saturn I technology to further develop and refine the capabilities of a larger booster and the Apollo spacecraft required for the manned lunar missions. The S-IVB stage, later used as the third stage of the Saturn V launch vehicle, was powered by a single J-2 engine initially capable of 200,000 pounds of thrust.