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The effects of bypass air on the combustion of highly loaded boron carbide/HTPB fuel grains in a solid‐fuel ramjet motor were investigated experimentally. It was found that combustion efficiency can be significantly increased by employing high bypass rations with low dump momentum. The results indicated that high bypass dump momentum causes the particles to collide with the wall and extinguish. Increasing pressure increased efficiency, apparently through the resulting increased residence time. Increased overall equivalence ratio (grain length) also was found to be beneficial, the result of a smaller percentage of larger particles which are generated in the recirculation zone. Some of the reduction in combustion efficiency was determined to be due to incomplete combustion of the HTPB rather than just to the incomplete burning of the boron carbide particles.

Particle size distributions were measured in the chamber, nozzle, and plume of a subscale solid propellant rocket motor. A significant reduction in the mean size of the aluminum/aluminum oxide particles occurred within the motor chamber. The mass fraction of small particles (<2 ft) at the nozzle entrance was less than 10%. Also, most particles were smaller than 50 ft, although a few as large as 85 ft were present. In the converging and throat portions of the nozzle it appeared that particle breakup dominated over collision coalescence. Collision coalescence was observed to occur more dominantly in the supersonic nozzle flow. At the nozzle exit the particle mean size was usually less than 2.5 ft. The particle size distributions were bimodal or trimodal, with the larger particles concentrated near the plume centerline. Because of the short nozzle residence times it is not known whether or not these results are also applicable to full-scale motors. The mean size of the small A1 2 O 3 particles in the plume edges was less than 0.5 ft in diam, with an index of refraction of 1.64 ± 0.04 (apparently y-Al 2 O 3 ), independent of propellant composition, motor operating conditions and nozzle geometry.

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