ISRO's Polar Satellite Launch Vehicle (PSLV-C34) accomplished its 35th consecutive successful flight with the launch of 20 satellites in a single mission recently on June 22, 2016. Another landmark achievement of this mission was the successful restart of its upper stage liquid engines (PS4). This has demonstrated and validated the upper stage systems for the upcoming PSLV missions to enable injection of satellites into multiple orbits.

PSLV, the workhorse of ISRO, is capable of launching satellites into different types of orbits like Sun Synchronous Polar Orbit (SSPO), Low Earth Orbit (LEO) and Geosynchronous Transfer Orbit (GTO), as per the customer requirements. The value engineered PSLV has flexibility in the upper stage (PS4) configuration enabling cost effective launch of different class of satellites. So far, 113 satellites have been placed by PSLV in different orbits, which include India’s remote sensing and communication satellites, maiden lunar mission Chandrayaan-1, Mars Orbiter Mission (MOM) spacecraft, Indian regional navigational satellite constellation (NavIC), besides 74 foreign satellites.

Today, advancements in satellite technologies and large scale miniaturisation enable the realisation of small satellites with functionalities of earlier larger satellites. For the customers of small satellite launch service, employing a dedicated launcher is not an economic option due to larger costs. Hence, it is essential to pool multiple satellites in a single launcher to reduce the launch cost and save time. This trend poses challenges to the launcher for carrying multiple satellites into different orbits to effectively utilise vehicle payload capability. ISRO’s PSLV is a unique launcher capable of meeting this type of demands with minimal vehicle changes.

In order to achieve multiple orbits, two different options, namely, having separate propulsion module or employing upper stage restart, are available. PSLV has selected the second option-restart off PS4 engines twice with a long coasting between two restarts. Even though ISRO had enough experience in long duration satellite missions employing multiple restart of the satellite's Liquid Apogee Motor (LAM) engine, a long duration launch vehicle mission was attempted during the recent PSLV-C34 mission.

PS4 stage is powered by twin liquid engines, which demands maintenance of critical parameters within the limit for overall controllability during restart. In view of these constraints and the long mission duration of around 7600 seconds between two restarts at two widely spaced points in orbit, appropriate measures were taken with overall mission planning and management, augmentation of control requirements, manoeuvring and controlling the vehicle into different orbits, reconditioning of propulsion systems and propellant management under micro-gravity conditions.

Liquid Upper Stage of PSLV PSLV Upper Stage Engine

In order to build up confidence levels for the upcoming missions, the experimental restart of PS4 engine was carried out for the first time in PSLV-C29 mission using available propellant, after the accomplishment of primary mission objective. Based on the success in PSLV-C29, a full complement of PS4 engine restart, the same as the functional requirement in an upcoming mission, was accomplished in the recent PSLV-C34 mission. Two engine restarts were made very close to North and South Pole validating the thermal and power management.

With PS4 restart, the following parameters were validated:

Mission Planning and Overall Guidance algorithm for orbit change manoeuvres

Reconditioning of upper stage engine and stage systems for restart

Overall thermal management including fluid circuits, control components, actuators and on-board avionics systems

Additional powering requirements for on-board electronics

Ceramic servo accelerometer for better orbital parameters

The PS4 chamber pressure during both the restarts was as predicted and closely matched with first start. Overall controllability, navigation and guidance required for manoeuvring the upper stage into two different orbits were demonstrated. Thus, PSLV-C34 mission could successfully accomplish all the requirements, which gives full confidence for executing the main mission with two functional restarts of PS4 engines.