The S139 (S for Solid, 139 for weight of 139 tonnes) is a Solid Rocket Booster manufactured by the Indian Space Research Organisation at the Satish Dhawan Space Centre in the SPROB facility.
[1] The rocket motor was first developed for use in the Polar Satellite Launch Vehicle.
It uses hydroxyl-terminated polybutadiene (HTPB) as a propellant.
[2] It is also used as a core stage in Geosynchronous Satellite Launch Vehicle MKII rocket.
[3] It is also the part of a variant of the Unified Launch Vehicle which is under development.