ISRO puts 104 satellites into orbit in a single launch, in itself an achievement, but the real challenge was to release them in a predetermined sequence and time frame, ensuring that they did not collide. By T.S. SUBRAMANIAN
AT a press conference in Sriharikota after the Polar Satellite Launch Vehicle (PSLV-C37) put 104 satellites into orbit on February 15, A.S. Kiran Kumar, Chairman, Indian Space Research Organisation (ISRO), emphasised that the mission was not aimed at creating a record. “It is not about setting records. It is primarily about improving our capability to maximise our returns,” he said. He explained how the decision to put 104 satellites into orbit came about. “We had excess capacity [in the PSLV). This was effectively used to put small satellites into orbit. This is a timely effort. We are trying to maximise our resources.”
The four-stage PSLV, which has had 38 successful missions in a row, including the one on February 15, can put satellites totally weighing 1,350 kilograms into orbit. The PSLV-C37’s primary aim was to put a Cartosat-2 series satellite weighing 714 kg into orbit. Besides, it was to put into orbit two ISRO nanosatellites, INS-1A weighing 8.4 kg and INS-1B weighing 9.7 kg. So, the vehicle had “excess capacity”, that is, it could carry more satellites into orbit.
When the United States, Israel, Kazakhstan, the Netherlands, Switzerland and the United Arab Emirates approached Antrix, the commercial wing of the Department of Space, for putting their nanosatellites into orbit, ISRO readily agreed. It meant revenue generation—Antrix would charge these countries for putting their satellites into orbit.
That was how the PSLV-C37 carried 96 nanosatellites from the U.S. and one each from Israel, Kazakhstan, the Netherlands, Switzerland and the UAE. These 101 nanosatellites together weighed 645.9 kg. Thus, the total weight of all the satellites carried on board the PSLV-C37 was 1,378 kg.
Kiran Kumar gave a perspective to the mission: “Today, more and more small satellites are being realised. Many times they have to wait for a launch vehicle [to put them into orbit]…. So, we put into orbit as many small satellites as possible from this…. The real challenge was to put them into orbit within the time frame demanded by the customers.”
If the media in general celebrated the success of the mission in putting 104 satellites into orbit from a single vehicle, some online columnists derided it as needless “chest-thumping”. But what actually mattered was the complexity involved in putting so many satellites into orbit in a sequence and ensuring that they did not collide.
ISRO’s top brass explained how they did it. K. Sivan, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, said, “Even to separate one satellite from the vehicle is complex, to put 104 satellites into orbit was not an easy job.” All the 104 satellites had to be put into orbit in just 600-odd seconds before the vehicle reached the South Pole, Sivan said. After India’s Cartosat-2 series satellite and INS-1A and INS-1B were put into orbit one after another, the remaining 101 were shot into orbit in “sets” or in pairs separated at a 180 degree angle, he said.
After a smooth countdown, the 44.4-metre-tall PSLV-C37 XL version, weighing 320 tonnes and equipped with more powerful strap-on booster motors than the normal version, lifted off at 9.28 a.m. from the first launch pad at the Satish Dhawan Space Centre at Sriharikota.
The fourth stage of the vehicle broke free about 16 minutes and 48 seconds after lift-off and Cartosat-2 was injected into orbit at 17 minutes and 30 seconds from blast-off. Then INS-1A and INS-1B separated at 17 minutes and 39.80 seconds and 17 minutes and 40.30 seconds respectively.
The first pair of nanosatellites separated at 18 minutes and 32.80 seconds and the last pair was put into orbit at 28 minutes and 42.80 seconds from lift-off. Thus, in a span of 10 minutes, all the 101 nanosatellites were fired from the vehicle’s fourth stage in a sequence, ensuring that they did not collide. At about 29 minutes into the mission came the announcement from the Mission Control Centre: “Separation of 104 satellites confirmed.”
According to Kiran Kumar, the challenge lay in accommodating so much of mass [of the satellites] in the fourth stage of the vehicle and releasing them in a manner that they would not collide not only at their time of release but during their life in orbit. So, a novel sequencing system was adopted. The 101 nanosatellites were housed in quad packs, each accommodating four nanosatellites. The quad packs are box-like structures and once they are ejected from the vehicle’s fourth stage, a door slides open in each pack and the four nanosatellites are fired into orbit in pairs in opposite directions. The gap between two pairs of satellites being put into orbit was about four to 12 seconds. “This depended on the direction and the velocity with which each of the satellites was put into orbit,” said Sivan.
PSLV-C37 Mission Director B. Jayakumar called it “a highly mission-intensive launch” with “a lot of complex manoeuvring” of the fourth stage.
M. Annadurai, Director, ISRO Satellite Centre, Bengaluru, which built the Cartosat-2 series satellite and INS-1A and INS-1B, said ISRO had put multiple satellites into orbit using a single vehicle in earlier missions and “the same was multiplied now by a number” and the satellites had to be separated at the correct intervals. “From the launch vehicle point of view, it is the mass that matters. From the separation point of view, only the number [of satellites] goes up. They should not collide. For that there is a proper orientation of the vehicle,” he said. The satellites were ejected at different timings, orientation and velocity. Annadurai added, “We separate the satellites in a very controlled way.”
An article called “The unique triumph of PSLV-C37” on the ISRO website said:
“The large number of satellites in this mission demanded adopting innovative approaches in satellite accommodation and mission design.
“The next requirement was managing the safe separation of these large number of satellites within the constraints of limited visibility duration of ground stations and maintaining a safe distance between the separated satellites over a longer period of time.
“This was managed by designing a unique sequencing and timing [system] for separating the satellites and with complex manoeuvring of the PS4 stage to which the satellites were attached. The separation sequence, direction and timing were finalised based on extensive studies to ensure safe distances among the 105 objects (including the PS4 stage) in orbit….
“The next major requirement was to ensure reaching the separation command from the launcher to respective satellites, honouring the pre-defined sequence, which involves a complex electrical wiring scheme. Any error in the wiring may result in the release of the wrong satellite, leading to an undesirable situation of collision between them.
“Another feature in this mission was capturing all the separation events of the vehicle stages and the 104 satellites using a comprehensive video imaging system onboard.
“Meticulous planning was done at the launch complex, the SDSC at Sriharikota for assembling and handling of all satellites’ preparation….”
The imagery sent by the Cartosat-2 series satellite will be used for mapping applications, monitoring coastal land use and regulating the use, road networking, creation of land-use maps and detecting changes on land such as man-made features and so on.
Building INS-1A and INS-1B at the ISRO Satellite Centre was an important initiative because the satellites were very small and each carried several payloads. “This is a remarkable achievement,” said Annadurai.
The INS bus could be used by universities and industries to build their own satellites. The bus would provide an opportunity to universities and research and development laboratories to carry their innovative payloads.
Next, GSLV-MK III
With this innovative mission behind it, ISRO is now looking forward to the maiden lift-off of its gigantic Geosynchronous Satellite Launch Vehicle, the GSLV-Mark III, in the second half of April 2017. It weighs 640 tonnes and is the heaviest and most formidable launch vehicle built by ISRO so far. The three-stage vehicle is 44 metres tall and features an indigenously designed and built cryogenic stage, which will be powered by 25 tonnes of liquid oxygen and liquid hydrogen. The rocket’s configuration comprises two solid strap-on booster stages called S-200, each with a capacity to hold 200 tonnes of solid propellants; a liquid core stage called L-110, which will hold 110 tonnes of liquid propellants; and the cryogenic upper stage called C-25, which uses 25 tonnes of cryogenic propellants.
P.V. Venkitakrishnan, Director, ISRO Propulsion Complex (IPRC), Mahendragiri near Nagercoil, Tamil Nadu, was upbeat on February 17 after the cryogenic stage, C-25, fired for its full flight duration of 640 seconds at the IPRC complex. This is a major milestone before the GSLV-MK III lift-off from Sriharikota. After the successful firing, which began at 5 p.m., Venkitakrishnan told Frontline from Mahendragiri: “This was a qualification test, a stage test. We will analyse the results of the test and if no fine-tuning is required, the flight cryo stage will move to Sriharikota in two weeks and the GSLV-MK III launch will take place in the second half of April. The two strap-on booster stages and the liquid core stage have already reached Sriharikota.”
The GSLV-MK III will put into orbit a communication satellite called GSAT-19E, weighing about 3.2 tonnes and built by ISRO Satellite Centre.
Said Venkitakrishnan: “GSLV-MK III is fully designed and developed in India. It is a 100 per cent Indian product.”