Once again, a Geosynchronous Satellite Launch Vehicle (GSLV) is on the launch pad at Sriharikota. This launch will be crucial — after two successive failures of the rocket, the Indian Space Research Organisation can ill afford one more troubled flight. Moreover, the space agency needs to demonstrate that, after 20 years of effort, it has now mastered cryogenic technology.
The GSLV retains the first two stages of its predecessor, the Polar Satellite Launch Vehicle (PSLV). In order to carry heavier satellites than the latter, the third stage of the GSLV uses cryogenic propulsion. Running on liquid hydrogen and liquid oxygen, a cryogenic engine offers greater energy efficiency than those that use other propellants. The improved efficiency means that the upper stage can carry less propellant, with the weight saved translating directly into more payload.
ISRO tried to purchase cryogenic technology from what was then the Soviet Union, but the deal that was signed in 1991 ran into trouble after the U.S. imposed sanctions. Russia, which inherited the deal after the breakup of the Soviet Union, backed out of providing the technology but agreed to supply seven flight-worthy stages for the GSLV. (For more details see “The long road to cryogenic technology,” The Hindu , April 15, 2010).
Left with no option, ISRO began the Cryogenic Upper Stage Project in April 1994 for developing an indigenous version of the Russian cryogenic engine and stage. While this technology development was in progress, it could fly the GSLV with Russian-made stages.
The GSLV, equipped with a Russian cryogenic stage, first flew in 2001. However, unlike the PSLV, which shook off the failure of its first launch and went on to notch up 23 consecutive successes, the GSLV has been trouble prone. In its seven flights so far, three were outright failures and another two suffered serious problems.
In April 2010, the GSLV flew for the first time with an indigenous cryogenic stage. Close to five minutes after lift-off, the cryogenic engine came to life but only very briefly. With thrust from that engine failing to pick up, the rocket soon tumbled into the sea.
In December the same year, the GSLV was flown again, this time with a Russian cryogenic stage. But disaster struck yet again, with the vehicle going out of control less than a minute into the flight, breaking up into pieces and exploding into flames over Sriharikota.
ISRO has gone to great lengths to learn from those failures and adopt suitable changes, say senior officials of the space agency.
The Russian cryogenic engine and stage design is complicated. Booster turbopumps installed at the bottom of the liquid hydrogen and liquid oxygen tanks maintain a steady flow of propellants to the main turbopump.
Glitch rectified
Analysis of the data radioed down by the rocket during its April 2010 flight showed that the booster turbopump supplying liquid hydrogen had caused the problem. The turbopump had started up normally and attained a maximum speed of 34,800 revolutions per minute. But its rotation slowed after less than one second and stopped soon afterwards.
A detailed review concluded that one of the pump’s seals could have gripped the rotating shaft as a result of thermal deformation or some tiny contaminant becoming wedged somewhere. Alternatively, the casing of the turbine that drives the pump could have ruptured.
The review led to a tightening of manufacturing tolerances for the booster turbopump’s parts as well as more stringent procedures for its assembly. Extensive testing has also been introduced, including of the fully-assembled turbopumps.
The starting sequence for a cryogenic engine is a complex process, involving split-second timing. The cryogenic engine as well as the stage’s two small steering engines were tested briefly under simulated high-altitude conditions at ISRO’s Mahendragiri facility in Tamil Nadu to ensure that their ignition went smoothly.
In the GSLV’s December 2010 flight, a shroud, which protects the cryogenic engine during atmospheric flight, opened up as the rocket accelerated to supersonic speeds. In the process, it pulled apart connectors for electrical cables carrying control signals from onboard computers mounted near the top of the rocket to the rest of the vehicle. Out of control, the vehicle turned sharply and soon broke up. The shroud has now been strengthened and the connector mounting modified.
In its forthcoming mission, the GSLV is carrying GSAT-14, a communication satellite weighing close to two tonnes.
The rocket could launch seven more spacecraft over the next four years, according to ISRO Chairman K. Radhakrishnan. This could include four communication satellites, a meteorological satellite identical to the Insat-3D that was launched last month on Europe’s Ariane 5 rocket, the GISAT remote sensing satellite as well as Chandrayaan-2, the country’s next lunar exploration mission.
The cost of launching Insat-3D on Ariane 5, not including insurance, came to $82 million (Rs.490 crore), Dr. Radhakrishnan told this correspondent. The ‘marginal cost’ of each GSLV — that is, the additional expense the space agency incurs on the launch vehicle but which does not include all the organisational costs and investments for supporting the mission — came to about Rs.200 crore.
However, the current version of the GSLV will probably not be able to carry communication satellites weighing more than about 2.2 tonnes. ISRO has already launched several considerably heavier communication satellites aboard Ariane rockets. The Department of Space’s latest annual report shows eight more communication satellites being launched abroad over the next four years, including the GSAT-7 that will fly on the Ariane 5 later this month.
ISRO is in the process of developing a more powerful rocket, the GSLV Mark-III, that will be capable of carrying four-tonne-class communication satellites. The rocket’s giant solid propellant booster and its big liquid propellant stage have already been successfully tested on the ground. But an entirely new cryogenic engine and stage have also to be prepared.
Test firing of the GSLV Mark-III’s cryogenic engine would start soon and the intention was to have the entire vehicle ready for its first developmental flight by 2016-17, according to the ISRO chairman.
The graphic that accompanied this article has been removed as it had some factual errors.
