Later this month, the Geosynchronous Satellite Launch Vehicle (GSLV) Mark III is expected to lift off for the first time from India’s spaceport at Sriharikota on an experimental flight that will assess the rocket’s performance as it hurtles through the atmosphere to reach speeds many times that of sound.

When operational, the GSLV Mark III will be the Indian Space Research Organisation's most powerful rocket, capable of putting four-tonne communication satellites into orbit, almost double the capacity of the current GSLV. The Mark III will weigh about 640 tonnes at launch, about 50 per cent heavier than the GSLV.

Should India decide to send astronauts into space, this will be the rocket that carries them. So it is perhaps appropriate that the forthcoming launch will also provide an early test of a crew module that is being developed.

During the 1990s, it became clear that a new launcher was needed to meet the country's requirements for communication satellites heavier than what the existing GSLV could carry, according to K. Kasturirangan, who was the ISRO chairman when the Rs. 2,498-crore project for developing the GSLV Mark III was approved by the Government in May 2002. Over four years of studies, simulations, debates on technical issues and several reviews went into finalising the Mark III’s configuration, he said.

Reducing the total number of propulsion modules that make up the GSLV Mark III was seen as crucial to increasing the rocket’s reliability and reducing launch costs, according to ISRO experts this correspondent spoke to. The GSLV Mark III has just four propulsion modules while its predecessor, the GSLV, has seven.

The GSLV Mark III has two huge solid propellants boosters, which are among the largest in the world, flanking a big liquid propellant core stage. Atop the core stage, sits a cryogenic upper stage that will provide half the velocity needed to put communication satellites into the proper orbit.

While the solid booster and the liquid propellant core stage completed ground tests and were qualified for flight about three years back, development of the cryogenic engine, running on liquid hydrogen and liquid oxygen, for the Mark III's upper stage is still in progress.

For the experimental launch, the Mark III will be equipped with a dummy cryogenic engine and stage that will simulate the weight and other characteristics of the flight version. Consequently, the rocket will not be able to put the crew module it carries into orbit.

The rocket will, however, give the crew module a velocity of 5.3. km/second when the latter separates at a height of about 125 km. The capsule will then descend and splashdown in the Bay of Bengal, about 600 km from Port Blair in the Andaman Islands.

The GSLV Mark III is “a totally new configuration,” observed K. Radhakrishnan, the current ISRO chairman, explaining the rationale for the experimental mission. “So if there are issues with respect to the configuration and we need to take care of that, it is better to take care of [them] early.” It was not necessary to wait till the cryogenic stage was qualified.

The GSLV Mark III is more sensitive than the Polar Satellite Launch Vehicle (PSLV) and the current GSLV to disturbances that might occur as it accelerates through the dense atmosphere, noted S. Ramakrishnan, who was the first project director for its development and retired earlier this year as director of the Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram. The ability of the rocket’s control systems to effectively handle such perturbations without violating the vehicle's structural capabilities will be tested during the experimental flight.

Building on experience with the big solid propellant first stage of the PSLV and GSLV, which carries 139 tonnes of propellant, the Mark III’s two giant ‘S200’ boosters each holds 207 tonnes of solid propellant. Only the solid boosters for America’s Space Shuttle and Titan IV-B as well as Europe's Ariane 5 have more propellant than the S200. Of these, only the Ariane 5 is still operational.

A separate facility has been established at Sriharikota to make the S200 boosters. The closely matched thrust levels required from the two boosters was achieved by carefully controlling both the quality of the raw materials used and their subsequent processing, said M.C. Dathan, who oversaw the development of the S200 and is currently director of VSSC.

Another major achievement is that the S200’s large nozzle has been equipped with a ‘flex seal.’ The nozzle can therefore be swivelled when the rocket’s orientation needs correction.

The GSLV Mark III’s two S200 boosters fire at lift off, together generating a thrust of over 800 tonnes. The sheer volume of sound produced at lift off could potentially damage the rocket and the spacecraft. A sound suppression system has therefore been installed on the launch pad that will be spray about 20 tonnes of water per second to reduce noise levels during lift off.

In flight, as the thrust from the S200 boosters begins to tail off, the decline in acceleration is sensed by the rocket’s onboard sensors and the twin Vikas engines on the ‘L110’ liquid propellant core stage are then ignited.

Before the S200s separate and fall away from the rocket, the solid boosters as well as the Vikas engines operate together for a short period of time. The thermal environment at the base of the L110 stage will be monitored during the experimental mission. The transition of control over the vehicle’s orientation from the S200s to the L110 will also be closely watched.

The L110 stage, with a diameter of four metres, carries 115 tonnes of liquid propellant. Although the Vikas engine has already flown on the PSLV and the GSLV, those on the Mark III have to operate for a longer duration.

The first developmental flight of the GSLV Mark-III, with a functional cryogenic engine and stage, could take place in two years’ time, according to Dr. Radhakrishnan.