50th High Performance Motor Case (HPS3) for Third Stage of PSLV Realised

India’s workhorse launcher Polar Satellite Launch Vehicle (PSLV), one of the most reliable launch vehicles, has 38 consecutively successful flights so far. It has been in service for more than two decades and has launched various satellites including historic missions like Chandrayaan-1, Mars Orbiter Mission, Space Capsule Recovery Experiment and Indian Regional Navigation Satellite System (NavIC). With its three configurations, PSLV has proved its multi-payload, multi-mission, multi-orbit capability in a single launch and its geosynchronous launch capability. PSLV has launched 226 satellites as of February 2017.

The PSLV has four stages with solid and liquid propulsion systems alternately. PSLV uses a solid motor in its third stage (PS3), which is of 2 m diameter and 4850 litre capacity. PS3 motor case weighing 325 kg was initially realised by filament winding process using Aramid/Epoxy for the construction for the shell and Carbon/Aramid/Epoxy hybrid construction for the skirt extension. Four numbers of PS3 motor cases were burst tested and gave more insight into the processing and design of composite motor case. It was observed that polar winding potential was underutilised due to non-uniform stress distribution and unequal opening, high boss opening ratio and fast-indexed multi star pattern in polar winding. Hence the design and processing parameters of PS3 motor case were optimised to augment the payload capacity of PSLV to meet the future requirements of ISRO.

High Performance motor case (HPS3) is the improved version of PS3 motor case. An optimum design for this was evolved by selecting appropriate different profiles for both the domes, introducing dome reinforcements and adopting modified helical winding with wide band in multi axis filament winding machine. 

The motor case is designed for a maximum operating pressure of 60.8 bar using Aramid/Epoxy for the construction of shell. The motor case consists of helical hoop layers and Aramid fabric for dome reinforcements. Carbon/Aramid/Epoxy hybrid construction for the skirt extension to attach with the fourth stage of PSLV is designed for taking the structural tensile load of 460 kN and a compressive load of 215 kN. A design factor of 1.25 is applied over the flight loads. Aluminium alloy end fittings are provided for the assembly of igniter and nozzle on either side. After rocasin insulation lay-ups, 7500 kg of solid propellant is cast. The design was qualified by proof pressure test, structural qualification level load test, burst testing of the case, cyclic proof pressure test with bucket flange and motor level static firing.

Each HPS3 motor case is accepted after fabrication based on the successful completion of hydro-proof pressure test at 1.05 times MEOP (Maximum Expected Operating Pressure), structural tension test at 1.1 times flight load and structural compression test at 1.1 times flight loads with internal pressure at MEOP. Considering the porous nature of the motor case, rubber bladder is used in hydro-pressure test.

The achieved HPS3 motor case weight is 285 kg and the demonstrated performance factor, which is the indicative of efficiency of design, is 14 km. Tests carried out have proven the adequacy of the design and fabrication technology. This high performance motor case was inducted from PSLV-C4 mission onwards and the performance of the case has been satisfactory.

Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, the lead centre for the launch vehicle systems, is responsible for the design and development of high performance solid motors. The Composites Entity of VSSC has realised 50 numbers of HPS3 motor cases so far. Recently, the 50th case for PSLV-C38 mission was flagged off to SDSC SHAR from VSSC.