Complex sentence: Operating an air-breathing jet engine at velocities of up to Mach 5.5 poses numerous engineering problems; several previous engines proposed by other designers have worked well as jet engines, but performed poorly as rockets.
Split into simpler sentences: Operating an air-breathing jet engine at velocities of up to Mach 5.5 poses numerous engineering problems. Several previous engines proposed by other designers worked well as jet engines, but performed poorly as rockets.