Indian Space Research Organisation (ISRO) uses solid motors of 139 ton capacity for PSLV/GSLV flights and 200 ton capacity for the GSLV Mk III flights as part of their first stage. These motors are produced in the indigenously developed Solid Propellant Plants of ISRO at Satish Dhawan Space Centre SHAR, Srihairkota.

Propellant mixing is one of the critical processes in the solid propellant manufacturing where different solid ingredients are mixed with binder in a vertical planetary mixer to achieve required homogeneity and viscosity. The propellant slurry is highly viscous semi-solid and less than 5% co-efficient of variation (COV) needs to be achieved for the homogeneity.

ISRO was importing planetary mixers of 2.5 ton capacity during 1980's. Initially, M/s. Central Manufacturing Technology Institute (CMTI), Bengaluru, had developed the 2.5 ton capacity mixer as per the requirements of ISRO. So far, six mixers were built by M/s CMTI and supplied to ISRO. Subsequently, M/s.CMTI, has developed the 4.5 ton mixer by suitably scaling up the design parameters of 2.5 ton vertical mixer.

The 4.5 ton Mixer consists of two outer agitators and one centre agitator. The planetary motion to the outer agitators guarantees that the whole volume of the bowl is swept without leaving any dead zones. The lateral surface of the mixer blades is helical thus ensuring good pumping capability.

The major challenges for mixer development are: a) Uniform homogenisation of mixing with less than 5% variation; b)Sizing of the agitators with 6 to 7 mm gap; c) Low speed and high torque hydro motor to drive the mixer for mixing high viscous fluids; d) Operating speeds for agitators varying from 2 to 8 RPM; e) Selection and sizing the gear box for achieving the reduction ratios.

The mixer system is facilitated by other subsystems like feeding system, propellant slurry temperature control system, PLC based control system, compressed air system for operating the bowl, spill tray collection system for collecting the remnant propellant on the agitators and bowl lid lifting system.

Safety is the prime criteria for the mixers deployed for the mixing of hazardous, explosive material like solid propellant. The mixer has been designed and provided with over load protection devices in gearboxes to protect from the overload during charging of powders and unusual viscosity build up during mixing operations. The mixer is also provided with automatic bowl drop mechanism in bowl lift hydraulic circuit to relieve the unusual pressure build up during mixing operations.

Very elaborate process and quality plans have been prepared for the realisation of the components. Many technical challenges were faced during the realisation of some of the critical components. All the critical elements like mixer blades, mixer central agitator shaft for mounting the mixer central agitator, mixer bowls (stainless steel mixer bowl of 2050 mm x 1350 mm depth and shell thickness of 35 mm, weighing 9 tons), etc., are realised with dedicated team effort of SDSC SHAR and CMTI Engineers.

After realisation of the subsystems, M/s. CMTI successfully completed shop floor integration of all subsystems and demonstrated the basic functions of the vertical mixer. The vertical mixer was then moved and commissioned at SDSC SHAR, Sriharikota. A detailed test and evaluation procedure was established and executed towards final commissioning of the first indigenously developed 4.5 ton vertical mixer. Chairman ISRO inaugurated this facility recently.

The realisation of this critical technological challenging high capacity vertical mixer has paved way for increasing the throughput of the plant and also reducing variations in processing, thus improving the overall product quality. This has given a major impetus to the indigenisation efforts of ISRO in space activities.