Slag accumulation occurs in large solid rocket motors having submerged nozzles. Large solid motors use aluminised solid propellant. During combustion, aluminium forms aluminium oxide (Al 2 O 3 ). This alumina or slag is trapped within the submerged region of the nozzle by the recirculating gas streams. The entrapped slag affects the motor performance by reducing delivered specific impulse, augmenting insulator erosion in the submerged region and influencing the vehicle dynamics. The objective of this work is to experimentally determine the slag accumulation characteristics inside solid rocket motors when fired in horizontal and vertical mode. To determine the slag accumulation and also to compare the slag quantity, solid motor static tests were conducted in horizontal and vertical mode and slag quantity was determined. This paper presents the details of the test scheme, test motor, slag evaluation method, results of the test and comparison of slag in the tests conducted in horizontal and vertical modes. These tests provided valuable inputs on slag accumulation between the horizontal and vertical modes of testing. The results indicated higher slag in the vertically fired test motor than one fired horizontally. Nomenclature Al 2 O 3 = Aluminium Oxide (slag) D = Diameter HTPB = Hydroxy Terminated Polyutadiene IR = Inhibition resin L = Length SRB = Solid rocket booster µ = Microns

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