A launch vehicle is a system used to launch satellites into stable orbits. It works on Newton's third law of motion, where the exhaust gases ejected from the rocket engine provide an equal and opposite reaction force that propels the launch vehicle upward. Launch vehicles are classified based on their payload capacity and number of stages. India's space agency ISRO has used several launch vehicles over the years of increasing capability, starting with the SLV, then the ASLV, and now predominantly uses the PSLV and GSLV.