As you read this section, refer to figure 2-1. To understand how a rocket operates, visualize a closed container  that  contains  a  gas  under  pressure.  The pressure  of  the  gas  against  all  the  interior  surfaces  is equal  (view  A).  If  the  right  end  of  the  container  is removed  (view  B),  the  pressure  against  the  left  end  will cause the container to move to the left. In the rocket motor, gases produced by the burning propellant are confined to permit a buildup of pressure to sustain a driving force. The size of the opening is restricted  by  a  Venturi-type  nozzle  (view  C).  The Venturi-type   nozzle   decreases   the   turbulence   of escaping gases and increases the thrust. In this design, gas pressure inside the container provides about 70 percent of the force, and the escaping gases provide about 30 percent of the force necessary to move the container  forward. ROCKET COMPONENTS A complete round of service rocket ammunition consists  of  three  major  components-the  motor,  the Figure  2-1.—Principles  of  rocket  propulsion. warhead, and a fuze. A general description of these components is given in the following paragraphs. Motors The  rocket  motor  consists  of  components  that propel and stabilize the rocket in flight. Not all rocket motors are identical, but they do have certain common components. These components are the motor tube, propellant, inhibitors, stabilizing rod, igniter, and nozzle and  fin  assembly.  The  rocket  motors  discussed  in  the following paragraphs are for the 2.75-inch Mk 4 Mods, Mk 40 Mods, the 5.0-inch Mk 16 Mods, and Mk 71 Mods. MOTOR  TUBE.—  The  motor  tube  (fig.  2-2) supports  the  other  components  of  the  rocket.  Presently, all motor tubes are aluminum, threaded internally at the front  end  for  warhead  installation,  and  grooved  or threaded internally at the aft end for nozzle and fin assembly  installation. The Mk 4 Mods 1 through 6 and the Mk 40 Mod 0 have   a   nonintegral   (two-piece)   bulkhead   at   the forward  end  of  the  tube.  This  bulkhead  has  a  disc that blows out in case of accidental propellant ignition before  installation  of  the  warhead.  This  action neutralizes rocket thrust by allowing gas produced by  the  burning  propellant  to  escape  from  both  the forward  and  aft  ends  of  the  motor  tube,  making  it nonpropulsive.  It is still a fire hazard. All other Mark and  Mod  motors  discussed  in  this  chapter  have  tubes with integral bulkheads that do not rupture. In case of accidental   propellant   ignition,   these   motors   are propulsive, becoming a missile hazard as well as a fire hazard. PROPELLANTS.—   The  propellant  grain  (fig. 2-2)   contained   in   the   Navy’s   2.75-inch   and   the 5.0-inch  rocket  motors  is  an  internal  burning,  star perforation,  double-base  solid  propellant.  The  star perforation is designed to produce a nearly constant thrust level. The  Mk  66  rocket  motor  has  the  star  points machined off (conned) to reduce erosive burning. In addition this propellant grain is 5 inches longer than the Mk 4/40 propellant grain. INHIBITORS.—   Inhibitors  restrict  or  control burning on the propellant surface. In the 2.75-inch and the 5.0-inch motors, the propellant grains are inhibited at the forward and aft ends, as well as the entire outer diametral  surface.  The  forward  and  aft  end  inhibitors are molded plastic (ethyl cellulose) components bonded 2-2