Every engineer works under constraints. For those designing the third generation geostationary launch vehicle ( GSLV ), the main constraints were the launch facilities at Sriharikota and the capabilities of Indian industry. They could design a big rocket, but the Indian Space Research Organisation ( Isro ) would have had to build an expensive place to launch them. They would have also had to find companies to make the vehicle components, as making big rocket components was beyond industry capabilities.When Isro started developing this rocket a decade and a half ago, it was far from being a thoroughbred organisation. It was not of great repute, its technical skills not cutting edge and its commercial potential, negligible. The first generation GSLV had just been test-flown, but its satellite had not reached its desired orbit. The GSLV Mark III was a complex vehicle and some of its critical technologies had to be developed from scratch. Isro’s cryogenic engine development had hit hurdles and got delayed beyond reasonable measure.However, by the turn of the century, Isro had shown glimpses of its current prowess. It had learned the art of making satellites. Its polar satellite launch vehicle (PSLV) had begun to put remote-sensing satellites into low earth orbit with precision, and this vehicle was slowly acquiring a reputation for reliability. The PSLV was relatively easier game.The GSLV was a different matter altogether, requiring powerful solid motors and liquid engines in the early stages and a cryogenic engine on top. GSLV Mark III was conceived as a heavy lifter, by Indian standards, capable of putting communication satellites into a geostationary orbit , 36,000 km above the earth. A powerful cryogenic engine needed to be developed quickly.Isro is now readying the vehicle for its first full flight at the end of this month, roughly three weeks after another flight of the current generation GSLV on May 5. Isro has used new ideas in its design, necessitating new methods in manufacturing. Some of these ideas will be tested for the first time in a flight from Sriharikota. It would be the first flight of GSLV III using India’s fully-indigenous cryogenic engine. If successful, it would also be India’s first launch vehicle qualified for human space flight. “This vehicle is going to be at the frontier for Isro,” says G Ayyappan , Mark III project director. “It can be used for human flight as well.”All of these combine to make it one of the most critical flights in Isro’s history. Although the space technology frontier has moved on, promising to keep Isro engineers busy for a long time, GSLV Mark III is the culmination of all that Isro initially set out to do when first set up in the 1960s. When fully ready, it would give Isro self-reliance and the ability to put any satellite into any orbit.The current f light of GSLV III is a developmental f light. Isro is planning another developmental f light a year later. It takes at least two f lawless developmental fights for the vehicle to be ready for commercial use. This year, India will put two communication satellites in orbit using the French Arianne launcher. One of these satellites will weigh 5.6 tonnes. It is beyond the capabilities of even the current GSLV III, which is now being developed to put four-tonne satellites into geostationary orbit. Later versions of the GSLV will be able to put satellites weighing up to seven to eight tonnes into geostationary orbit.Isro had gone through a difficult period a few years ago, when a launch of its GSLV Mark II failed. This failure had its impact on GSLV Mark III as well. “Because we had problems with Mark II,” says Isro chairman Kiran Kumar , “we had to rework some facilities of Mark III for Mark II. So Mark III got slightly delayed.” The successful flight of GSLV Mark II in 2014 was thus a major milestone for Isro. It also qualified India’s cryogenic engine, which was a reengineered version of the Russian cryogenic engines.The cryogenic engine in Mark III is entirely designed in India, and is twice as powerful as the Mark II cryogenic engines. Isro has used a different technology for this engine called the gas-generator cycle, primarily because it gave the engineers the freedom to test each component separately. The earlier engine used a method called staged combustion, where the entire engine had to be tested as one entity. “We have now made about 200 tests on the engine and its components separately,” says Kiran Kumar.For the cryogenic engine, Isro had to create new high altitude test facilities at Mahendragiri near Thiruvananthapuram. Isro tested the full engine in April 2015 for 635 seconds, and again in June 2015 for 800 seconds, well beyond the duration of its burning during a real flight. It had two more tests subsequently, one early in 2016 and another in December 2016. The performance of the new cryogenic engine would be the most crucial aspect of the flight later this month, as it is being tested for the first time in a flight. Although there are other new features in the vehicle, some of these have already been tested in a partial flight two years ago. “We had doubts about the configuration,” says K Radhakrishnan, former chairman of Isro. “So we decided to have an atmospheric test flight with a passive cryogenic engine.”This flight was on December 18, 2014, when Isro tested the recovery of a crew module. Isro had doubts about the configuration because it had two boosters on either side of the liquid core stage, with the core stage not firing with the boosters. GSLV Mark II had four strap-on motors surrounding the core stage. “The aerodynamics of Mark III special because the strap-ons are in one plane,” says S Somanath, former project director of GSLV Mark III and now the director of the Liquid Propulsion Systems Centre near Thiruvananthapuram. The absence of strap-ons in the other plane makes the pitching and yawing manoeuvres slightly different.At 3.2 meter in diameter, the strap-on motors are the third largest in the world. Apart from their size, the use of two strap-on motors provided another challenge for Isro. The two motors had to match their performance precisely. If not, one would tilt the vehicle to the other side during flight. To avoid this imbalance, Isro made the boosters from one casting, by splitting it into two. It was Isro’s first attempt at pair casting. It was to ensure uniformity of material and uniform degradation and it needed the development of new infrastructure.GSLV Mark III has a core liquid stage with twin engines, another – smaller – novelty in design. The liquid engines would switch on only a 100 seconds after lift-off, but well before the strap-on motors cease firing. The launch vehicle has redundant control electronics, a requirement for any vehicle used for human flight. Isro engineers have provided Mark III with other requirements for human flight vehicle, in terms of acceleration, noise and other safety margins. Isro has already designed and test-flown a crew module.In the end, the decision for a human flight rests with the politicians. Whether the country embarks on a human flight or not, it is necessary for Isro to design vehicles that can carry heavier and heavier payloads. Not just for communication satellites, but for future inter-planetary missions as well. The GSLV Mark III will fly this month with a 3.2-tonne satellite, the GSAT-19. The vehicle is designed to take a payload of four tonnes, but it would still not be enough for some of Isro’s future requirements.“Satellites are getting heavier and heavier,” says K Sivan, director of the Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram. “So we have to increase the capacity of the vehicle.” GSLV Mark III can be tweaked to later to carry more than six tonnes of payload into a geostationary orbit, by replacing the core liquid stage with a semi-cryogenic engine. This engine is under development, and might take three to four years. After its development, India would be able to put six to seven tonne-class of satellites into a geostationary orbit, and stop using expensive overseas facilities for launching its communication satellites.