ISRO

Two key tests after release of 20 satellites

Print edition : July 22, 2016

K. Sivan, director, VSSC. Photo: M. Karunakaran

THE Indian Space Research Organisation’s Polar Satellite Launch Vehicle (PSLV-C34) mission on June 22 from Sriharikota, Andhra Pradesh, had a lot of “glamour”—it was to put 20 satellites into orbit. It placed all of them flawlessy in the same orbit and hogged the headlines, but what went unnoticed was the two audacious experiments ISRO performed with the PSLV’s fourth stage engine, the results of which will enable the same rocket to put multiple satellites into different orbits. This will make the PSLV even more cost-effective than it already is and attract a variety of customers who have satellites to be put into different orbits.

The PSLV-C34 mission was the 35th straight success for the rocket from 1994. Sixteen of these 35 lift-offs, including the latest, put multiple satellites into orbit, all of them into the same orbit each time. The June 22 mission was the first time that the PSLV injected 20 satellites into orbit, the highest number by a PSLV so far. The previous highest was on April 28, 2008, when PSLV-C9 deployed 10 satellites.

They included three Indian satellites—one of ISRO’s Cartosat-2 series and two student satellites, Sathyabamasat of Sathyabama University, Chennai, and Swayam of College of Engineering, Pune. The remaining 17 were from the United States, Canada, Germany and Indonesia. The mission was so precise that against the intended polar sun synchronous orbit of 505 km that each satellite was to go into, they were injected into an orbit of 507 km.

K. Sivan, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, which designed and developed the PSLV, said ISRO tested the waters on December 16, 2015, with “a small experiment”. On that day, after PSLV-C29’s fourth stage engine put six Singapore satellites into the same orbit, ISRO re-started the engine for four seconds. (The satellites are put into orbit after the PSLV’s fourth stage engine is cut off.) “This small experiment will enable us to put satellites into different orbits,” using a single vehicle, he had said then.

In simple terms, switching off the engine for some time and re-starting it will enable the stage to go into a different orbit for other satellites to be put into that orbit. On June 22, ISRO made bold to switch off the engine twice after all the 20 satellites were put into orbit, each time for as long as 50 minutes, and re-ignite it for five seconds. The two experiments turned out “beautifully”, the VSSC Director said.

Sivan explained: Eight minutes after blast-off, the rocket’s fourth-stage (the topmost stage) engine ignited. It burned for 16 minutes and 30 seconds. Then it was shut down. Over the next 10 minutes, 20 satellites were injected into the same orbit one after another. After each satellite was injected into orbit, the vehicle was re-oriented if required and the next satellite was put into orbit with a varying velocity so that the distance between the two satellites grew “monotonically” and there was no collision of satellites.

After the 20 satellites were put into orbit, PS-4 headed towards the South Pole and then travelled towards the North Pole in shut-down mode for 50 minutes. As it neared the North Pole, it was re-ignited for five seconds and again shut down for 50 minutes. As it came towards the South Pole, it was ignited again for five seconds.

In August 2016, a PSLV will put ISRO’s SCATSAT (for forecasting weather and detecting cyclones) and some satellites from abroad into different orbits.

T.S. Subramanian

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