26 is formed in the body 12 just forward of the area into which the gases 24 are injected into the airstream this configuration causes a recirculation zone just behind step 26 and when the gases 24 are injected into this region combustion efficiency is noticeably increased significant improvement of the ignition and combustion processes for external burning has been thus achieved by injecting the fuel rich exhaust of the solid propellant motor into the recirculation zone behind the downstream facing step in one example with subsonic exhaust and the step configuration it was possible to greatly improve the combustion at a combustion chamber temperature of 1600 u00b0 k which is well below 2200 u00b0 k the temperature of 80 ap 20 htpb propellant used as a base line without the step the combustion extinguished a short distance downstream on the other hand with a quarter inch step for example intense combustion continued with increasing mixing length with supersonic primary exhaust velocity and the quarter inch step ignition and intense combustion of the fuel rich reaction products were possible at 2200 u00b0 k primary chamber temperature under the same circumstances without the step no ignition occurred in the airstream the fig2 device generally indicated at 100 is similar in all aspects to the device of fig1 except that the products of combustion 124 are injected into the airstream at an angle achieved by the angular positioning of the port or ports 122 in the projectile illustrated generally at 200 in fig3 chamber 118 does not communicate directly through the injection port means 224 because of a.