ISRO Human Space Flight Project (HSP) Update: PS4 stage of the PSLV to be modified to act as a service module

ISRO is modifying the PS4 stage of its workhorse launcher PSLV to play a new role of service module for human space missions. ISRO's orbital vehicle consists of two main modules,  one is the crew module and the other is service module. The crew module is used for crew habitation during a space mission and which also returns the crew safely to earth. Service module contains the various systems necessary for the space operation like the propulsion, life support systems for the crew module, solar arrays, etc. 

Fig. 1 PSLV PS4 stage and Orbital Vehicle.

ISRO has judiciously decided to modify the PS4 stage of the PSLV into a service module as its sub-systems like propulsion are tested and proven in various PSLV missions. Many of these sub-systems will be reused for the Orbital Vehicle's service module, thus saving resources and development time, and also minimising the risks involved. 

PS4 Stage

PS4 stage of PSLV injects the satellites into the orbit. It is a liquid propulsion system that uses hypergolic propellants. It has two pressure fed, regeneratively cooled engines of 7 kN thrust. The stage is attitude stabilised and steered through engine gimballing that provides control moments about pitch, yaw and roll axes. For non-powered phases, control is through bi-propellant reaction control thrusters. 

Fig. 2 PSLV PS4 stage with equipment bay. 

Fig. 3  RCS thrusters on PS4.