India's heaviest rocket till date GSLV Mk-III

When India fires the high thrust cryogenic engine CE-20 to launch GSLV Mk III in its first experimental flight from Sriharikota on Monday, it will propel ISRO's biggest dream albeit about 13 years later than it was originally planned. ISRO would have used the CE-20 powered GSLV Mk III in 2003 if not for the US sanction and a foisted case. The launch, a culmination of a long and arduous journey spanning more than three decades, will set the ball rolling for ISRO's future projects including Chandrayaan-2 and the manned mission.

It will also be a first step towards setting its foot in the global heavy payload market.

"If there was no sanction, we would have operationalised GSLV Mk-II in 1999. By 2003-2004, we would have launched what we would be witnessing on Monday," said former ISRO scientist Nambi Narayanan, who was the project director for the development of cryogenic engine in the early 1990s.

Even as the Indian space agency struggled over the years to get a cryogenic engine for its heavy launch vehicles, the delay also helped it in mastering the technology. A cryogenic engine involves a tricky job of using liquid hydrogen at -253ºC and oxygen at -183ºC as fuel and oxidiser. Only the US, Russia, China, Japan and the European Space Agency have achieved this feat.

ISRO began work to build indigenous cryogenic engine in the 1970s, though it gained momentum after Russia denied transfer of technology . But it was delayed as the space agency had to focus on their immediate requirements including development of Vikas engine, which now powers both PSLV and GSLV .

"With limited resources, manpower and budget, we had to give priority to immediate requirements including projects like SLV , ASLV and PSLV . It was the same team working for all," said K Sivan, director, Vikram Sarabhai Space Centre. In the late 1980s the erstwhile Soviet Union offered three engines and a technology transfer at a reasonable price. Talks with Japan, US and Europe to borrow cryogenic technology had failed due to prohibitive costs. But after the collapse of the USSR in 1991, Russia backtracked on its pact.

Finally when Russia sent seven KVD-1 engines to India after redrafting the contract post the US sanctions, it was a mix of enthusiasm, confusion and uncertainty , as it was only a supply of hardware and not a technology transfer. "Our technology related questioned were not going to be answered. We were neither able to continue nor able to drop the project," Narayanan recalled. But soon Narayanan, who was in the thick of things, was arrested on charges of espionage, which further affected the team's morale.

ISRO launched a project to build a cryogenic engine in 1994 and the knowledge their engineers acquired through pilot projects like the development of a 12-tonne thrust engine, one-tonne and seven-tonne engines in the 1980s came in handy . According to Isro scientists, the engine CE-7.5 can be called an indigenous version, working on a staged combustion cycle, with Russian design.

While work to develop a high thrust CE-20 engine began in 2002, the technical issues of its predecessor delayed the project. On April 15, 2010, the engine failed 800milliseconds after ignition during the launch of GSLV-D3 carrying GSAT-4 satellite. Isro used one of the last two Russian engines for their next launch, but the liquid fuel boosters failed. Another attempt at launch using an indigenous cryogenic engine on August 18, 2013 had to be aborted.

"When we started work on C25 stage, we faced problems with the CUS (cryogenic upper stage). If not for the issues with CUS we faced between 2010 and 2015, we would have completed C25 long ago," Sivan said.

But developing CE-20 engine was equally challenging, as it was a completely new technology than CE-7.5.CE-20 engine was designed to provide double the thrust and lift double the payload weight than the previous engine. "The beauty of this technology lies in the simplicity to correct design flaws faster," said S Somanath, director, Liquid Propulsion Systems Centre.

"The testing of the engine was also challenging. The systems had to be tested first in water medium, then in liquid nitrogen and in actual fluids progressively. Ignition and start sequence development were the key. It took us time to develop the engine due to the technology complexity , manufacturing and testing," Somanath said.

Meanwhile, GSLV Mk II tasted its first success in January 2004. With CE20, there were several tests that improved the design and analysis to make it flight ready. "We conducted around 200 tests on the system and its subsystems in the past few years. The advantage of the technology is that changes can be made as and when the sub-systems are tested," said PV Venkatakrishnan, director, Isro Propulsion Complex.

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