Grain geometry selection and configuration is one of the key steps towards design of solid propellant rocket motors towards meeting performance and other stated objectives. Monolithic metallic mandrels are in extensive use for propellant casting of both small rocket motors as well as for large boosters of launch vehicles. While the monolithic mandrels are easy to design, fabricate and use they also largely ensure safety during decoring operations. However, they bring a limitation to the grain designer in that the mandrel lateral dimensions should be smaller than the motor mouth opening to facilitate its removal from the motor after casting and curing operations. Hence, a large body of experiments have been reported with this limitation. Although collapsible casting mandrel designs were made use of in large motors and also patented, their use in experimental research motors is absent based on available open literature.
This research proposal brings out an extensive literature review on the theoretical studies and experiments conducted by various agencies worldwide on solid propellant rocket motors to understand the effect of grain geometry on the stability of the motor. The primary focus of such work has been to understand the underlying physical mechanisms responsible for combustion instability both through modelling and experimental approaches. Experiments have been conducted on subscale and full-scale motors at various centers to examine the effect of pressure of motor operation, motor length or frequency, effect of additives in propellant and grain geometry on the stability behaviour of solid rocket motors.

The grain geometries tested include combinations of star shape and fins with circular port and some motors were also pulsed during burn. It has been noted that grain geometries which offer a closure of port area towards the aft end of the motor, such as the forward star (i.e. star port at head end of motor followed by cylindrical port towards nozzle end), favour stability. As noted above, experiments were restricted to geometric shapes of the grain which could be realized through monolithic solid mandrels.
Positioning of fins or other large surfaces at the middle of the motor poses a problem that the mandrel cannot be decored after casting from either end. This necessitates usage of a collapsible mandrel where the mandrel can be extracted after casting. Towards this, recently a wax mandrel technology is developed in the country. Work has been carried out to cast ballistic evaluation motors using this technology and successful firing has been demonstrated. It is proposed to make use of this technology to cast experimental motors with central fins.
The objective of the current work is to configure motors with aft-finocyl, forward-finocyl and central fin grain geometries utilizing the collapsible wax mandrel technology and test them. It is further planned to pulse each motor during burn to study the relative stability characteristics of these grain configurations.