a convergent-divergent type of nozzle.  [Figure 3-47]  The  advantage of a convergent-divergent nozzle is greatest at  high Mach numbers because of the resulting higher pressure  ratio across the engine exhaust nozzle. To ensure that a constant weight or volume of a gas flows  past any given point after sonic velocity is reached, the rear  part of a supersonic exhaust duct is enlarged to accommodate  the additional weight or volume of a gas that flows at  supersonic rates. If this is not done, the nozzle does not operate  efficiently. This is the divergent section of the exhaust duct. When a divergent duct is used in combination with a  conventional exhaust duct, it is called a convergent-divergent  exhaust duct. In the convergent-divergent, or C-D nozzle, the  convergent section is designed to handle the gases while they  remain subsonic, and to deliver the gases to the throat of the  nozzle just as they attain sonic velocity. The divergent section  handles the gases, further increasing their velocity, after they  emerge from the throat and become supersonic. As the gas  flows from the throat of the nozzle, it becomes supersonic  (Mach 1 and above) and then passes into the divergent section  of the nozzle. Since it is supersonic, it continues to increase  in velocity. This type of nozzle is generally used on very  high speed aerospace vehicles. As the exhaust gases exit the rear of the engine, they flow  into the exhaust nozzle.  [Figure 3-46]  The very first part of  the exhaust nozzle and the exhaust plug form a divergent duct  to reduce turbulence in the airflow, then the exhaust gases  flow into the convergent component of the exhaust nozzle  where the flow is restricted by a smaller outlet opening. Since  this forms a convergent duct, the gas velocity is increased  providing increased thrust. The restriction of the opening of  the outlet of the exhaust nozzle is limited by two factors. If  the nozzle opening is too big, thrust is being wasted. If it is  too little, the flow is choked in the other components of the  engine. In other words, the exhaust nozzle acts as an orifice,  the size of which determines the density and velocity of the  gases as they emerge from the engine. This is critical to  thrust performance. Adjusting the area of the exhaust nozzle  changes both the engine performance and the exhaust gas  temperature. When the velocity of the exhaust gases at the  nozzle opening becomes Mach 1, the flow passes only at this  speed—it does not increase or decrease. Sufficient flow to  maintain Mach 1 at the nozzle opening and have extra flow  (flow that is being restricted by the opening) creates what  is called a choked nozzle. The extra flow builds up pressure  in the nozzle, which is sometimes called pressure thrust. A  differential in pressure exists between the inside of the nozzle  and the ambient air. By multiplying this difference in pressure  times the area of the nozzle opening, pressure thrust can be  calculated. Many engines cannot develop pressure thrust  because most of the energy is used to drive turbines that turn  propellers, large fans, or helicopter rotors. As aircraft have increased in gross weights with higher  landing airspeeds, the problem of stopping an aircraft after  landing has greatly increased. In many instances, the aircraft  brakes can no longer be relied upon solely to slow the aircraft  within a reasonable distance, immediately after touchdown.  Most thrust reverser systems can be divided into two  categories: mechanical-blockage and aerodynamic-blockage. Whenever the engine pressure ratio is high enough to produce  exhaust gas velocities which might exceed Mach 1 at the  engine exhaust nozzle, more thrust can be gained by using  Figure 3-47.  A convergent-divergent nozzle can be used to help  produce more thrust when exhaust gas velocities are greater than  Mach 1. Figure 3-46.  Exhaust gases exit the rear of the engine through the  exhaust nozzle. 3-28 Convergent-Divergent Exhaust Nozzle Thrust Reversers Convergent Exhaust Nozzle Tail cone Exhaust nozzle Gas attains sonic velocity Exhaust nozzle Subsonic convergent section Supersonic divergent section