Complex sentence: Once close to the target the second chamber of the engine would accelerate the missile to Mach 3, over the target the engine would cut and the missile would free-fall before detonating its warhead as a air-burst.
Split into simpler sentences: Once close to the target the second chamber of the engine would accelerate the missile to Mach 3. Over the target the engine would cut-out and the missile would free-fall before detonating its warhead as a air-burst.