Sriharikota: India on Wednesday launched six Singaporean satellites with its Polar Satellite Launch Vehicle's core alone (PSLV-CA) variant.
The PSLV rocket -- standing 44.4 metres tall and weighing around 227 tonnes -- took off from the rocket port in Sriharikota in Andhra Pradesh, around 80 km from Chennai. It rose into the evening skies with fierce orange flames at its tail.
Apart from launching the six foreign satellites, ISRO also tested the rocket's fourth stage/engine's ability to restart after it was cut off around 17 minutes into the flight.
Technically speaking, India was testing a 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," an ISRO official said.
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.
This is what ISRO plans to test when the PSLV ejects the six Singaporean satellites.
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 space and to restart it after a short gap," said an industry expert.
This is entirely different from switching on and off the communication satellite's engines in 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.
The PSLV's fourth stage/engine will be restarted just over 67 minutes into the flight or 50 minutes after the engine was cut off.
At the time of restart, the fourth stage will be in a lower altitude of 523.9 km while the satellites would have been ejected at 550 km altitude.
The engine will be operated for four seconds and is planned to go up to an altitude of 524 km before it is cut off again.
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).