ISRO has mastered the multi-burn technology by successfully placing satellites into two different orbits. The advanced version of the ISRO’s workhorse PSLV-C35 during the launch on September 26^th hoisted 8 satellites into multiple orbits.

About PSLV-35

PSLV made its debut in 1993, over the years earned a formidable reputation for its consistent performance. PSLV vehicles capitalizes on the unique advantage of using a combination of solid rocket motors and liquid-fuelled engines. PSLV known to be reliable and effective launch vehicle so far had delivered payloads in straight mission and never attempted to accommodate flexible missions where payloads are deployed into different orbits. Lifting satellites to different orbits invariably requires multiple restarts. The current PSLV mission was designed accommodate this feature. Consequently, engineers reworked the design of the L-2-5 engines in the upper stage of PSLV responsible for multiple start. This was achieved by developing new propellant handling techniques so that propellant would settle prior to engine start-up so that engines would take only liquid propellants and no gas bubbles. Moreover, additional power margins are included in PS4 (Upper stage) for extended coast periods so that PSLV can conduct missions for long hours. Initial tests for restarting of the upper stage were conducted in December 2015. After its successful performance in initial tests, a second round of tests set stage for final flight were completed in June 2016. After a green signal from the Launch Authorisation Board (LAB), the flight was conducted on Monday. Multi-burn technology involves switching on and off of rocket engines endowing the launch vehicle the ability to deliver satellites to two different orbits.

In its longest missions conducted so far that lasted for 2hrs 15 minutes and 33 seconds, PSLV-C35 weighing 320 tons and measuring 44.4mts lifted off from the first launch pad of the Satish Dhawan Space Centre, SHAR, Sriharikota, carrying eight satellites of total weight 675 kg. The major Indian weather satellite SCATSAT-1 weighing 371kg was deployed into a Sun Synchronous Orbit (SSO) at an inclination of 98.1 degrees over the equator.  While the two other Indian satellites PRATHAM (10 Kg) developed by IIT B students and PISAT (5.25kg) constructed by PES University, Bangalore and five other foreign satellites were placed in a 689 Km Polar Orbit at 98.21-degree inclination. PRATHAM was aimed at estimating the Total Electron Count (TEC) and PISAT Nanosatellite was constructed for remote sensing applications.

On its 37^th mission PSLV-C35, after a flight of around 17 minutes, SCATSAT-1 separated from the PSLV fourth stage. After separation solar panels were deployed automatically and ISRO’s Telemetry, Tracking and Command Network (ISTRAC) and Bangalore took over the control of the satellite. SCATSAT is a continuity to the Ocean SAT-II Scatterometer (popularly termed as Oscat) launched in September 2009 to provide wind vector data products for weather forecasting, cyclone detection and tracking services to users. This Earth observation satellite carries a ku-band Scatterometer similar to OceanSat-II and has mission life of 5 years.  OScat was the first microwave remote sensing satellite. It earned global repute for contributing the areas of wind-retreival, weather-forecasting, cyclone-tracking and prediction. It accurately predicted Cyclone Phalin that hit Odisha and avoided loss of human life. The 17 minutes’ ascent for deployment of SCATSAT involved standard patterns of flight and was relatively hassle-free. But the PSLV still had the task of placing the seven other satellites in a different orbit.

Post SCATSAT deployment

The PSLV began to coast over the South Polar region and then ascended towards the Northern Hemisphere. After a gap of one hour 22 minutes, when the PSLV is in North polar region, the two engines of the PS4 were reignited and fired for 20 seconds. As a result, it entered into an elliptical orbit which is 725km from one side of the earth and 685 km from other side. Again after 50 min, when PS4 was coasting in the South Polar region, engines were fired for another 20 seconds. This second firing propelled the rocket to enter the circular orbit of 669 km altitude with an inclination of 98.21 degree. Within the next three minutes the dual launcher adapter separated from PSLV fourth stage and the first satellite to separate was ALSAT-1N followed by NLS-19, PRATHAM, PISAT, ALSAT-1B, ALSAT-2B and PATHFINDER-1.

Foreign Satellites

Other than PRATHAM and PISAT which are of Indian origin, rest of them belonged to international customers. ALSAT series satellites are from Algeria. The 103 kg, ALSAT 1B earth observation satellite aids in environment and disaster monitoring, ALSAT-2B weighing 117 kg is a high resolution remote sensing satellite with panchromatic and multispectral imaging capability and the 7kg ALSAT-1N is a Nanosatellite built by students for technology demonstration. NLS-19 is 8kg Canadian Nanosatellite used to perform experiments for reducing the space debris and tracking commercial aircraft. Finally, the PATHFINDER-1 of Black Sky Global of Seattle, is a 44kg American microsatellite used for commercial high resolution imaging. With this PSLV-C35 launch, ISRO has so far delivered 42 Indian payloads and 79 foreign satellites. As a precedent ISRO conducts four PSLV launch missions. This launch was fifth PSLV launch, a record for this year and seems to be in line with ISRO’s long term goals for PSLV of carrying out 6 to 8 launches a year to gradually shift the vehicle’s satellite services for commercial purposes. Europe’s Vega rocket clocking seven successful launches this year emerged as stiff competitor for PSLV. Vega operators are aiming to increase their launches to 10 per a year. Similarly, Khurinchev Space Centre of Moscow and its commercial wing, International Launch Services of Reston, Virginia with the announcement of two variant Proton Launch Vehicles for small satellites is also offering tough competition to ISRO. While ISRO undoubtedly has immense cost benefit ratio over the rest, they must indeed work really hard to stay ahead in the commercial satellite launching market.