With less than a month to go for the launch of India's heaviest rocket, the GSLV Mark III, the Indian Space Research Organisation today rushed to clarify that the impending launch will be an experimental one, with only a dummy cryogenic engine.

The rocket launch that will take place from here sometime between December 15 and 20, is meant to test how the first two (lower) parts of the rocket behaves during its flight through the atmosphere, and the efficacy of the systems meant to enable the come-back of the chamber that will one day carry Indian astronauts.

At a press briefing held here today, ISRO officials said that the crucial cryogenic engine, critical for the objective of carrying heavy satellites, will be a dummy one. The cryo engine will not be fired; indeed it will not be capable of being fired, officials said.

The GSLV MK III rocket has been in the works for about 15 years and, as officials stressed, is not an extension of the 'regular' GSLV rocket. The MK III is an entirely new rocket and will be used in future satellites of 4 tonnes in weight to 36,000 KM above the earth.

The launch of next month will be but a small step for the rocket. It will fly to 126 km above the earth, and plunge back.

The idea is to test how the stage I and II behave during the atmospheric flight.

But on the way back, the ability of the 'crew module' to re-enter the atmosphere without getting burnt out in the heat caused by the friction while re-entry, will also be assessed.

The re-entry is crucial, to human flights. As an object approaches 80 km above the earth, or the 'sensitive atmosphere' the friction between the object and the atmosphere can heat the object to some 1600 degrees Celsius.

The crew module is designed to withstand such high temperatures and to prevent the heat from reaching inside.

By the time the rocket reaches 126 km the uses-up lower stages would have been jettisoned into the Bay if Bengal.

The upper, cryogenic stage, dummy in this rocket, gets cut off at 126 km altitude and begins to fall back along with the crew module, which is placed on the top, usually the home of a satellite.

Between 126 km and 80 km, engineers will practically steer the crew module's fall.

At 80 km, the atmosphere takes over. From then, the engineers can only check the speed of the falling object, which they do first by firing small rockets on the module in the reverse direction, and later by opening three parachutes one after the other.

Some 17 minutes after the rocket leaves the earth, the crew module will fall into the Bay of Bengal, 600 km of Port Blair in the Andamans, bringing to a close the Rs 155-crore mission.

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