A comment describing functioning of Retro Motors in between PS2 shutoff and PS3 ignition by Chandrasekaran Jayaraman
When the second stage completes its job the engine is shut down and the stage seperates from the vehicle - Now third stage has to take over- To minimise the jolt caused by the seperation and to keep control RETRO motors [Small solid motors with nozele pointing upward.]are fired for a clean seperation. There is software to control the pitching of PS3 by giving comands to the Flex nozele to counter the effect of the jolt. The jolt was more and it was beyond the scope of the programe but with in the scope of the Third Stage! and so the pitcing of PS3 was not correct resulting in the failure [Pitching down]. In the next launch the software was modified to handle the jolt and Retro motorperformance was ensured and the nulling of the second stage also was slightly altered. This brought out the rugedness of the PSLV and its ability cope with distrbences during the stage seperations. The mission was grand sucess -IRS P2 was put in the desired orbit using minimum fuel About 8. KG of the availble 30Kg and now it is a weather proof work horse and can launch not only remote sensing sats but also Geo synchronous Sats!