Some experimental results on the L-star instability of metallized composite propellants

Abstract

Experimental results are reported on the Lstar instability characteristics of three AP I composite propellants. The metal content of the propellants is 2%, 16%, and 16%. Chuffing, bulk mode oscillations, and time-independent combustion are observed with all three of these propellants. The stability boundary, defined as the boundary between time-independent and unstable combustion, is found to be well defined for two of the propellants in agreement with recognized trends available in the literature on other propellants. The frequency of bulk mode oscillations is presented as a function of the chamber characteristic length. One of the propellants tested has shown bulk mode instability at as high a pres sure as 217 psia. All of these tests were performed in a stainless steel L'~ motor with convenient, interchangeable stainless steel nozzles. The troublesome blockage of the small metal nozzles by the aluminum oxide slag was Overcome in most of these tests by the application of viscous silicone oil on the nozzle surface before each run. This technique is being pursued further, with plans for the inclusion of silicone compounds in propellant formulation, to reduce heat transfer to the inert nozzle.

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