Complex sentence: On an orbital mission, rocket engines were ignited 4 second before lift-off and the launch vehicle held to the ground by clamps and was released when a sufficient thrust was build up at lift-off (A).
Split into simpler sentences: On an orbital mission, rocket engines were ignited 4 seconds before lift-off. The launch vehicle was held to the ground by clamps and then released when sufficient thrust was built up at lift-off (A).