DRDO is developing different Solid Rocket Motors for increasing the range of Missiles. Throttle control based Solid Rocket Motor which is under R&D. Hybrid (Solid + Liquid) Rocket Motor (already developed by ISRO). 3 pulse Solid Motor development has started after dual pulse motor (under prototype fabrication).

Conceptually, solid rocket motors (or SRMs) are simple devices with very few moving parts. An electrical signal is sent to the igniter which creates hot gases which ignite the main propellant grain (see image below). The propellant contains both fuel and oxidizer; therefore these devices can operate in the vacuum of space. Thrust is developed as the high thermal energy of the combustion gases is converted to kinetic energy in the exhaust. The simplicity of SRMs make them an attractive choice for many rocket propulsion applications. Because there are few structural components, the SRM is efficient in that the vast majority of its weight is actually usable propellant. SRMs can be ignited in a moments notice and don't require tanking of liquids prior to operation. On the downside, their efficiency (specific impulse) is generally lower than liquid systems, and they cannot be readily throttled. Once ignited, the motor will burn to extinction unless special provisions are included to terminate thrust during the middle of a firing.