The GSLV quest

Published : Mar 17, 2001 00:00 IST

The first developmental flight of the Geo-synchronous Satellite Launch Vehicle from Sriharikota will advance the country's quest for self-reliance in satellite launching capabilities.

WHEN the Geo-synchronous Satellite Launch Vehicle (GSLV) rises into the sky from its launchpad on the Sriharikota island later this month, it will be a landmark for the Indian Space Research Organisation (ISRO). While its predecessor, the Polar Satellite Launch Vehicle (PSLV), weighing 294 tonnes and 44 metres tall, injected a 1,050 kg Indian remote-sensing satellite into a polar orbit at an altitude of 820 km above the earth, the GSLV is designed to place a communication satellite into a geo-synchronou s transfer orbit (GTO). The 49-metre tall GSLV, which weighs 401 tonnes, will place the Geo-synchronous Satellite (GSAT) weighing 1,540 kg in a highly elliptical GTO with a perigee of 185 km and an apogee of 36,000 km. This orbit will be circularised by firing the satellite's propulsion systems into an orbit of 36,000 km by 36,000 km. The GSAT was built at the ISRO Satellite Centre, Bangalore.

Success in the mission will signal Indian capability to place in orbit its own communication satellites. INSAT communication satellites were all launched by Ariane vehicles from Kourou, French Guiana, or from the United States.

This is the first developmental flight of the GSLV, a three-stage vehicle. The first stage comprises a solid propellant motor, the core stage that is powered by 125 tonnes of solid propellants, and four liquid strap-on motors strung around the core. The liquid propellants in each of the strap-on motors weigh 40 tonnes. The second stage has 37.5 tonnes of liquid propellants. The third, upper stage is a cryogenic stage that uses 12.5 tonnes of liquid hydrogen and liquid oxygen. The GSLV thus uses five liq uid motors, which is a unique feature.

Dr. K. Kasturirangan, Chairman, ISRO, said the GSLV "is a major step at a higher level" because of the sheer "complexity of the vehicle." He told Frontline that the GSLV mission was a step after the PSLV in terms of "enhancing our capability in la unch vehicles and orbiting heavier class of satellites which are the communication satellites... The GSLV mission is primarily to realise this launch capability with INSAT class of satellites that weigh between 2,000 kg and 2,500 kg."

G. Madhavan Nair, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, called it " a major mission, a tapasya that we have been doing for ten years now."

According to D. Narayana Moorthi, Director, Launch Vehicles Programme, ISRO headquarters, Bangalore, "the entire planning of the mission has been a big development" because the past ten years had been "marked by visualisation; planning; design and develo pment of the systems, sub-systems and components; their testing; and simulation at the ground level." Narayana Moorthi said: "Enormous work has been done by teams of engineers, scientists and technicians at the VSSC, Sriharikota (at SHAR), the Liquid Pro pulsion Systems Centre at Mahendragiri, the Inertial Navigation Unit, the ISRO Satellite Centre and others."

At SHAR in Nellore district of Andhra Pradesh on March 2, the launch campaign was under way. The three assembled stages stood encased on the Mobile Service Tower (MST), a steel structure that is 79 metres tall and weighs 3,200 tonnes. It has platforms at various levels to stack up the three stages of the GSLV and the satellite on top of the upper cryogenic stage. A few hours before the launch, the MST will be moved away.

K. Narayana, Director, SHAR Centre, said that ISRO expected the launch to take place in the last week of March. "The three stages of the vehicle and the equipment bay have been assembled and checked out... We have done extensive checks on the performance of the electronic, control and guidance systems. We are in the phase of doing a total checkout of the entire vehicle and it will go on till March 15."

Since this is only a developmental flight, the GSAT is not crammed with payload. With five transponders, it will be used for demonstrating digital audio broadcast, Internet services, compressed digital television experiments and developmental communicati ons.

The GSLV is a complex vehicle. For the first time, an upper cryogenic stage and four massive liquid motors as strap-on boosters are to be used. The PSLV has six solid strap-on motors. In the GSLV, the liquid strap-on motors are ignited first and the core solid stage is fired after a gap of 4.6 seconds. The cryogenic stage, imported from Russia under an agreement signed between ISRO and Glavkosmos in 1991, provides the capability to inject heavier satellites into a GTO. Top scientists described the use o f upper cryogenic stage as "a major, new development." Cryogenics involved the use of liquid hydrogen at -265C and liquid oxygen at -240C. Kasturirangan said: "This is being done for the first time and the cryogenic systems are a class by themselves in terms of complexity and performance."

Although the GSLV uses a Russian cryogenic stage, ISRO developed the electronic, control and guidance systems for it. Equally complex is the task of interfacing ISRO's electronics and control systems with the Russian propulsion system and ensuring that t hey met each other.

R.V. Perumal, Mission Director, said: "We designed and developed the control and electronics. We will carry out the operation of the stage in flight and take the satellite into orbit. ISRO has qualified these electronics up to the testing of the cryogeni c engine and the stage." He added: "We have gone through a very systematic process of testing and qualifying our electronics joined with their cryo. Based on this, we are confident that we will be able to operate the stage satisfactorily."

The complexity of the vehicle comes from the use of first and second stages as well, especially the liquid strap-on motors, all of which are derived from the PSLV. Perumal said that the PSLV is the real base on which the GSLV rests. He said: "When we ini tiated a project to build a launch vehicle to put a 2,000 kg communication satellite into a GTO, the guideline given to us was that we should make the maximum use of the PSLV systems." Accordingly, the GSLV was configured to make use of many PSLV systems .

There is an important difference between the PSLV and the GSLV in the firing of strap-ons. Perumal said: "In the PSLV, we first ignite the core motor and the vehicle lifts off. Then the strap-on motors are ignited. In the GSLV, we first ignite the four l iquid strap-on motors and check them for performance. After a gap of 4.6 seconds, the core solid stage is ignited. This gap is to confirm that all the four strap-on motors have developed the required thrust." He pointed out that an advantage in liquid mo tors was that if any one of them was not ignited, the flight can be aborted. The liquid motors could be shut down and started up again. This was not possible in solid motors that were used in the PSLV. Here, if a flight is aborted, the stages could be de fuelled and any defects analysed. The countdown could be resumed after refuelling.

Narayana Moorthi emphasised that the GSLV flight would be a test flight. While the individual systems could be tested on the ground, an integrated vehicle could be tested only in flight. "The first developmental flight is the real test-bed for the entire vehicle," he said. The first couple of flights had a lot of design margins. By studying the performance in the initial flights, the design margins were subsequently released and the vehicles would be able to launch heavier satellites, he added.

According to Kasturirangan, despite extensive ground tests there could be gaps when the integrated vehicle flew through the atmosphere and beyond. He explained: "It is these gaps which we try to understand in the first flight. These cover areas relating to separation of one stage from the other, parameters related to controls and the long-time burn-up of the cryogenic engine, which provides the final velocity to the satellite during injection into orbit."

Perumal pointed out that while the entire GSLV flight lasted 1,050 seconds to inject the satellite into the orbit, the cryogenic engine alone operated for 700 seconds. The cryogenic engine hurled the satellite at a velocity of 10 km a second into the orb it.

Narayana said that ISRO had built massive infrastructure in the past ten years to support the GSLV missions. ISRO was often innovative in this. The MST, which was originally built for the PSLV, was reconfigured by raising the height by 5 m so that the GS LV stages could be stacked. The launch pedestal, which was designed to withstand the weight of the PSLV (280 tonnes), was also reconfigured. Narayana said: "It is a very stiff structure. But we checked out and found that it can withstand 400 tonnes."

Russia having supplied the cryogenic stage, ISRO built massive ground facilities to store liquid hydrogen and liquid oxygen (at Mahendragiri) and to transfer them to the cryogenic stage with all the circuits (at SHAR). Narayana said: "The Russians suppli ed the equipment and ISRO installed, tested and commissioned them." Another major development related to augmenting facilities for earth storable liquids, that is, nitrogen tetroxide and unsymmetrical demethyl hydrazine, which combined to make the liquid propellants for the liquid stages.

Perumal pointed to the demands involved in handling the cryogenic liquids. While the earth storable liquid propellants could be loaded easily into the vehicle stages, the cryogenic propellants evaporated easily during storage and the temperature kept ris ing. So in the GSLV, the cryogenic propellants are loaded only three to four hours before the launch. The final level is achieved in the last ten minutes. This was because precise conditions were needed at the lift-off for the quantity of propellants, th eir temperature and pressure.

Narayana Moorthi said a lot of simulation had been done on the ground on the stages, their systems, sub-systems and components. Digital simulation was also done. Each item of hardware went through tests. The equipment bay, which forms the brain of the ve hicle with its electronics and gives commands to the vehicle, went through a number of tests, Narayana Moorthi said.

ISRO now looks forward to the launch. Madhavan Nair said: "Our ambition was to achieve self-reliance in launching remote-sensing satellites with our own vehicle. We achieved it with the PSLV. The next milestone will be to orbit our own geo-synchronous co mmunication satellite GSAT with our own vehicle, the GSLV. When two-tonne class of INSAT communication satellites and the GSLV are mated, national needs can be really met."

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