This model is in 1/700 scale. It includes two B-52 bomber Motherships, both equiped with either a NASA X-43A or Boeing X-51 WaveRider aircraft. There are four wires under the Motherships connecting the sprue which can be cut to show the two seperate models.
According to Wikipedia: "A scramjet (supersonic combusting ramjet) is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to forcefully compress the incoming air before combustion (hence ramjet), but a ramjet decelerates the air to subsonic velocities before combustion, while airflow in a scramjet is supersonic throughout the entire engine. This allows the scramjet to operate efficiently at extremely high speeds: theoretical projections place the top speed of a scramjet between Mach 12 (8,400 mph; 14,000 km/h) and Mach 24 (16,000 mph; 25,000 km/h)."