The results of efforts to establish a laboratory-scale setup for hot-fire testing of amodular hybrid rocketmotor andnexperimental evaluation of the effect of several additives on the regression rates of hydroxyl-terminated–npolybutadiene-based fuels are presented. Supercharged nitrous oxide was used as the oxidizer in all tests. The mainnobjectives of the reported test-programphasewere the design and buildup of the experimental facility and to sort out,nby hot-fire tests, various fuel compositions with respect to the fuel regression rate and combustion–extinction abilitynat oxidizer shutoff. The cartridge-loaded fuel grains had an initial circular port diameter of 42 mm, a length ofn400 mm, and a web thickness of 15 mm. Five fuel composition families with various additive contents of finenammoniumperchlorate, polystyrene, and a burning-rate catalystwere tested.The fuel regression rate increasedwithnthe increase in the ammonium perchlorate portion, but combustion extinguishment was not possible with highnammonium perchlorate content. A significant enhancement of the regression rate of up to 3:5 = at an averagenoxidizer mass flux of about 140 = 2nu0001 (0:2 = 2nu0001 ) was obtained with the addition of both fine ammoniumnperchlorate and large particles of polystyrene. Full combustion extinguishment and multipulse operation with anninsignificant effect on the regression rate was demonstrated by the use of large particles of polystyrene additive andnno addition of ammonium perchlorate.
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