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. The rocket motor was first developed for use in the Polar Satellite Launch Vehicle. Later it was utilised in the GSLV MKII. It uses hydroxyl-terminated polybutadiene (HTPB) as a propellant. It has a maximum thrust of 4800 kN.
The S139 Booster has been used in 2 major ISRO rockets.
It is used in the PSLV as its core stage since 1993.
It is also used as a core stage in Geosynchronous Satellite Launch Vehicle MKII rocket.
It is also the part of a variant of the Unified Launch Vehicle which is under development.