Chennai: The Indian space agency on Wednesday will be testing its ability to restart the fourth-stage engine of its rocket Polar Satellite Launch Vehicle (PSLV) on shutting it down after putting into orbit six Singaporean satellites.

Technically speaking, India will be testing its multiple burn fuel stage/rocket engine for the first time.

"The restart and shut off of the fourth stage engine is done as a first step towards launching multiple satellites but in different orbits," a senior official of Indian Space Research Organisation (ISRO), speaking on the condition of anonymity, told IANS.

Launching of multiple satellites with a single rocket is nothing new for ISRO and it has been doing that for several years. The challenge is however to launch several satellites at different orbits with one rocket and this is what ISRO will be testing out after PSLV ejects out six Singaporean satellites on Wednesday.

The PSLV rocket is a four stage/engine rocket powered by solid and liquid fuel alternatively.

"Restarting a rocket engine soon after it is shut off is a critical technology that has to be mastered. Once a rocket engine is activated, then the heat generated is very high. The trick is to cool it down in the space and to restart it at a short gap," an industry expert told IANS.

"This is entirely different from switching on and off the communication satellite's engines in the space. The interval between two restarts of a communication satellite engine will be in days. But in the case of restarting a rocket engine, the time gap will be in hours," the expert added.

"By that time the rocket's engine has to be cooled down. This part of the experiment is very critical," he explained.

On Wednesday, ISRO's PSLV rocket will blast off from the first launch pad at Sriharikota in Andhra Pradesh, around 80 km from here, with the six Singaporean satellites. All the satellites will be put into orbit around 21 minutes into the flight at an altitude of 550 km.

After that the rocket's fourth stage will be shut down.

"It will be coasting after that," an ISRO official told IANS.

Around 46 minutes after the launch of the sixth satellite, the fourth stage will be restarted when it is in a lower altitude of 523.9 km.

The engine will be in operational for four seconds and would go up to an altitude of 524 km before the stage is cut-off.

Meanwhile ISRO officials told the countdown for the December 16 evening launch of PSLV rocket is progressing smoothly.

On December 16, ISRO will be flying the 'core alone' variant of the PSLV rocket. The rocket will not have the strap on boosters, its standard feature

The successful launch of the six Singaporean satellites will take ISRO's total flights of foreign satellites to 57.

Out of the six satellites, the 400 kg earth observation satellite called TeLEOS-1 is the main passenger for the PSLV rocket and hence the mission is called TeLEOS mission by ISRO.

TeLEOS-1 is Singapore's first commercial earth observation satellite designed and developed by ST Electronics.

The other five co-passenger satellites are VELOX-C1 (123 kg), VELOX-II (13 kg), Kent Ridge-1 (78 kg), Galassia (3.4 kg) and Athenoxat-1.

The December 16 mission will be the last rocket launch mission for ISRO in 2015.

So far in 2015, ISRO has launched 14 satellites (three Indian and 11 foreign) from its rocket port in Sriharikota. Thirteen satellites were launched with PSLV rocket and one communication satellite - GSAT-6-with geosynchronous satellite launch vehicle (GSLV).

Last month India also launched its communication satellite GSAT-15 using the Ariane rocket of the European space agency which takes the total number of satellite launches in 2015 to 21 (17 foreign, four Indian).