That is not a cruise missile. A cruise missile flies below 1000. Why do you think they are sub sonic, air density. It would cost far to much fuel to go Mach 1. Why do you think this projectile flies so high. Same problem as with the SR 71, friction. If it would just go to airliner altitude, 35,000 feet, it would burn up a hypersonic speeds and lets assume it would not burn up, or getting destroyed by the aerodynamic pressure, it would need insane amounts of fuel to get to Hypersonic speeds and maintain it. That sucker flies a ballistic curve and when it runs out of fuel and the engine stops, it immediately decelerates, like hitting a brick wall. When it hits the ground it will be supersonic, friction of the air. Physics.