When  you  burn  fuel  in  a  container  that  has  an opening (or nozzle) at one end, expanding gases rush out of the nozzle at a high velocity, as shown in figure 6-2. Releasing internal pressure at the nozzle end of the container  leaves  an  unbalanced  pressure  at  the  other end.  The  released  pressure  moves  the  container  in the direction opposite to that of the escaping gases. This   is   the   basic   operating   principle   for   all   jet engines.   Obviously,   propulsion   depends   solely   on internal   conditions.   The   container   does   not   "push against" external air. In fact, a complete vacuum would produce even greater force. The   jet   propulsion   engine   operates   like   a   toy balloon.  Newton's  third  law  of  motion  explains  this operation. This law states "for every acting force there is  an  equal  and  opposite  reacting  force." Inflate  a balloon. The air pressure inside the balloon, which is stretching the skin, is greater than the pressure outside the  balloon.  If  the  stem  is  tied  closed,  the  inside  air pushes in all directions and the balloon will not move. Place the balloon in a vacuum and release the stem. The escaping air has nothing to push against, but the balloon will move in a direction away from the stem, just as it does in a normal atmosphere. Releasing the stem removes a section of skin on the side   of   the   balloon   against   which   the   air   has   been pushing.    On    the    side    directly    opposite    the    stem, however, the air continues to push on an equal area of skin. The continued push of air on this area causes the balloon to move in the direction away from the stem. The acting force that Newton's third law refers to is the acceleration of the escaping air from the rear of the balloon. The reaction to this acceleration is a force in the opposite direction. In addition, the amount of force acting on the balloon is the product of the mass of air being  accelerated  times  the  acceleration  of  that  air. Since the forces always occur in pairs, we can say that if a certain force is needed to accelerate a mass rearward, the   reaction   to   this   force   is   thrust   in   the   opposite direction (force = thrust, as shown in figure 6-3). RAMJET ENGINES The ramjet is often described as a flying stovepipe. It    is    the    simplest    of    all    power    plants    that    use atmospheric air to support combustion. A ramjet is an appropriately shaped duct, tapered at both  ends,  in  which  fuel  is  injected  and  burned  at  a constant pressure, as shown in figure 6-4. Except for the possibility of fuel pumps or other accessories, there are no moving parts. The   air   inlet   diffuser   of   the   ramjet   engine   is designed to convert the velocity energy of the entering air  into  static  pressure.  This  is  commonly  known  as ram. During the inlet process, fuel is injected into the airstream, where it is well mixed with the air so that it will burn readily. At about the point of highest pressure in  the  engine,  combustion  is  initiated  and  the  fuel-air mixture  is  burned.  The  gases  of  combustion  and  the heated  air  expand,  thus  air  is  ejected  from  the  exit nozzle  at  a  much  higher  velocity  than  it  had  when  it entered the engine. This change in the velocity of the entering and departing air results in the thrust. PULSEJET ENGINES The  pulsejet  engine  is  a  member  of  the  athodyd (aero-thermodynamic-duct)  family,  since  it  does  not have a compressor or a turbine. The pulsejet engine differs from the ramjet in that the  inlet  duct  is  sealed  with  a  disc  that  incorporates flapper valves. The purpose of the flapper valves is to provide    the    required    air    intake    system,    seal    the high-pressure  gases  in  the  combustion  chamber,  and prevent   their   escape   out   the   inlet   duct   during   the combustion cycle. A pulsejet engine consists essentially of a diffuser, an air valve bank (automatic or 6-2 Figure 6-2.—Principle of jet propulsion.