Instrumented rocket motor service life program

Abstract

The paper describes a Solid Propellant Rocket Motor Service Life program conducted at the Aeronautical and Maritime Research Laboratory1 DSTO in Australia, which combined a nonlinear viscoelastic analysis of an Australian Pictor motor and used miniature embedded stress transducers to confirm analysis or define the actual stresses in the rocket motor during thermal cycling, cooling, storage and motor processing conditions. The measured stresses during solid propellant rocket motor processing, cure, cooldown, thermal cycling and accelerated and ambient ageing were obtained for comparison with linear elasticlviscoelastic and nonlinear viscoelastic finite element analysis. Measurements are in reasonable agreement with nonlinear viscoelastic analysis for most motor loading conditions. measurements. Pictor is a 5.5 inch diameter motor which is partially case bonded (Fig. 1). Four of these motors were instrumented with miniature SensoMetrics normal stress sensors-'4 which were installed near regions of critical stress predicted by finite element analysis using the linear ealstic code NASTRANIPATRAN and the viscoelastic code, STRAND 6.1. STRAND 6.1 is a linear elasticlnonlinear viscoelastic finite element code developed by G+D Computing in Sydney, Australia for stress analysis of the Pictor motor. Material properties (Table 1) for the unaged Pictor propellant, inhibitor, adhesive, and EPDM insulation and propellant/inhibitor bond were evaluated over the temperature range -40 to 60DC. Material properties of the aged motors were planned to be evaluated at the end of the environmental program. Rocket Motor Instrumentation Introduction Most solid propellant rocket motor service life programs have been comprised of experimentally based methods such as qualification of testing and surveillance by scheduled material property tests'-3. The lack of analytical tools and confirmation with embedded stress transducer technology has led to gross errors in service life estimates. Solid propellants exhibit significant nonlinear viscoelastic response which challenges even the best finite element analysis and experimental stress analysis t e ~ h n i ~ u e s ~ ' ~ . Service life analysis remains further complicated by uncertainties in material ageing characteristics and failure criterion. This report presents a first step in the use of new miniature embedded stress transducer technology and recent developments in nonlinear constitutive models. The Australian Pictor motor was selected for evaluating these new analytical and experimental The Pictor motor (Fig. 1) is an end burning design. It is filled with a non-aluminized HTPBIAP propellant, inhibited with a tapered thickness beaker (inhibitor) of adiprenejTMP. This inhibited charge is case bonded at the head end with an epikotelversamid adhesive. The motor has a maraging steel case and is thermally insulated with EPDM which is spun cast in the case. There is a small air gap in the the side wall of the motor between the inhibitor charge and the insulated case. These motors were instrumented with SensoMetrics miniature stress sensors (Model # 602003). These sensors (5 mm diameter by 1.5 mm thick) were used with 5 milliamp constant current excitation. The sensors were flush mounted in the inhibitor tube at peak stress locations (Fig. 1) identified by early linear elastic analysis. The locations are at the inhibitor to propellant interface before the propellant is cast into To whom correspondence should be addressed. Copyright @ 1995 by the Commonwealth of Australia. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.