The United States was the first country to develop cryogenic rocket engines. The Centaur upper stage, with RL-10 engines, registered its first successful flight in 1963 and is still used on the Atlas V rocket. America's early mastery of the technology paved the way for the J-2 engine, which powered the upper stages of the immensely powerful Saturn V rocket that sent humans to the Moon.
Other spacefaring nations followed. The Japanese LE-5 engine flew in 1977, the French HM-7 in 1979 and the Chinese YF-73 in 1984. The Soviet Union, first country to put a satellite and later a human in space, successfully launched a rocket with a cryogenic engine only in 1987.
ISRO recognised the importance of cryogenic technology fairly early. A rocket stage based on a cryogenic engine offered the simplest way of transforming the Polar Satellite Launch Vehicle (PSLV), intended to carry one-tonne earth-viewing satellites, into the far more powerful GSLV that could put communications satellites into the orbit.
In December 1982, six months after the PSLV project was cleared, a Cryogenic Study Team was set up. A year later, it submitted a report recommending the development of a cryogenic engine that could generate about 10 tonnes of thrust. The 15-volume report went into every aspect of developing the engine and rocket stage indigenously.
Then, strangely, ISRO went through a long period of indecision, dithering on whether to buy the technology or develop it on its own. Acquiring the technology from abroad would greatly reduce the time that would otherwise be needed, it argued.
But the U.S., Japan and France would either not provide the technology or do so only at an exorbitant price. Finally in January 1991, a deal was signed with the Soviet company Glavkosmos to buy two cryogenic flight stages as well as the technology to make them in India.
The 11D56 cryogenic engine had been developed for one of the upper stages of the mammoth N1 rocket, the Soviet equivalent of Saturn V. But after four successive launch failures, the N1 project was scrapped and its engines were mothballed. Under the Indo-Soviet deal, ISRO would get a stage built around the 11D56 cryogenic engine that could produce 7.5 tonnes of thrust. The stage would carry 12 tonnes of propellant.
But the deal violated the Missile Technology Control Regime, which was intended to prevent the spread of missile-related technology, and fell foul of the U.S. laws meant to enforce its provisions. Despite warnings from within the organisation, ISRO opted to go ahead with the import. In May 1992, the U.S. imposed sanctions on ISRO and Glavkosmos. A year later, Russia, which inherited the contract after the break-up of the Soviet Union, backed out of the deal.
ISRO then had no option but to develop the technology on its own. The Cryogenic Upper Stage project was launched in April 1994. Its aim was to develop a cryogenic engine and stage closely modelled on the Russian design.
At the time, ISRO gave the impression that much of the technology had already been acquired and further development would be quick. A GSLV with an indigenous cryogenic engine would be ready to fly in about four years, Chairman U.R. Rao told The Hindu in July 1993. The space agency's engineers were privately saying then that a flightworthy cryogenic stage was 10 years away. Instead, it has taken 16 years.
The Russian design involves a complicated ‘staged combustion cycle' to increase the engine efficiency. Hydrogen is partially burnt with a little oxygen in a gas generator. The hot gases drive a turbopump and are then injected at high pressure into the thrust chamber where the rest of oxygen is introduced and full combustion takes place. Before going to the gas generator, the incredibly chilly liquid hydrogen is used to cool the thrust chamber where temperatures rise to over 3,0000 Celsius when the engine is fired.
Reproducing the Russian design meant ISRO engineers also learning to deal with new materials and manufacturing methods. A process, known as vacuum brazing needed to make the engine's thrust chamber, for instance, took considerable time to master. Then there was the challenge posed by the powerful turbopump that rotates at a tremendous speed in order to send up to 18 kg of propellants every second into the thrust chamber. It must do so in the face of a sharp temperature gradient, with hot gases at over 5000 Celsius driving the turbine, which then spins the pumps for freezing-cold propellants.
Steps were also taken so that materials required for the engine and stage could be made within the country.
The Indian cryogenic engine is produced by Godrej and the Hyderabad-based MTAR Technologies working together as a consortium. Instead of ISRO first mastering the technology and transferring it to industry, the two companies were involved from the start and even the early prototypes were built by them. Failure on their part was not an option and the space agency had to make sure that these companies succeeded.
Finally, in February 2000, the first indigenous cryogenic engine began to be test-fired on the ground. According to one source, things went wrong in one test and an engine ended up badly damaged. However, by December 2003, three engines had been ground-tested for a cumulative duration of over an hour and half. One of those engines was fired continuously for more than 16 minutes, four minutes longer than it would operate in actual flight. More tests with the engine integrated into the full stage followed. The cryogenic engine that will fly in the forthcoming GSLV launch was tested on the ground for a little over three minutes in December 2008.
Meanwhile, the Russians had supplied ISRO with seven ready-to-fly stages. But their 11D56 cryogenic engine had not flown before and the Indians faced some unpleasant surprises.
The first was that the Russian-supplied stages turned out to be heavier than expected. In order to carry the extra load, it is learnt, the Russians increased the maximum thrust that the 11D56 engine was capable of — from 7.5 tonnes to a little over eight tonnes. The engine operates at the higher thrust for only part of the duration of its flight. The Indian engine too had to be tested and made to work at the higher thrust level. Moreover, the Indian stage is lighter than the Russian one.
When the GSLV was first launched in April 2001, the Russian cryogenic engine was found to be less efficient than predicted, based on a measure that rocket engineers call specific impulse. The increase in stage weight and decrease in efficiency together reduced the rocket's payload capacity significantly.
Where the GSLV with the cryogenic stage was intended to put 2.5 tonnes into the orbit, the rocket carried a satellite weighing just 1.5 tonnes in its first flight. With further optimisation of the Russian cryogenic stage and other parts of the rocket, the GSLV could successfully launch the 2,140-kg Insat-4CR in its fifth launch in 2007.
Sources told this correspondent that the last two stages supplied by the Russians carry an engine with a maximum thrust of over nine tonnes and are capable of accommodating an additional three tonnes of propellant. The GSLV with this stage would be capable of delivering a payload of 2.5 tonnes into the orbit. With further ground testing, the Indian engine too would be upgraded to a similar thrust level.
But the immediate challenge for ISRO and its engineers is to demonstrate in the GSLV launch that they have indeed mastered the intricacies of cryogenic technology.
