Why did ISRO detect fault in last minute?

Chandrayaan 2

Chennai: Now that the launch of Chandrayaan-2 has been called off, the next immediate question is when will the mission be launched.

It is said the launch might not happen any time soon as it might take another 10 days for the process of emptying out the fuel and taking the GSLV MK-3 for further investigation.

The question might arise as to why can’t the scientists solve the problem and get the vehicle ready for launch at the earliest. It is not that simple as the rocket should be launched in the optimal launch window for more efficiency and accuracy. It is said ISRO will now have a launch window of a minute every day till 31 July to attempt the launch of its second moon mission.

The launch window is the time frame on a given day that the rocket can be launched to reach its intended orbit. This can vary from as short as a second to an entire day. The perfect launch window requires the coincidence of orbits and their inclinations, precision, weather and lighting. Since we are planning to land on the south pole of the moon, it will be 14 days bright and 14 days dark. The launch should be timed in such a manner that we make optimal use of the brighter days.

Chandrayaan-2 will be first placed at the Earth Parking Orbit as it would be extremely challenging to schedule launches so that they happen at precisely the right time to launch a spacecraft directly from the pad into a trajectory to the Moon. So, instead, a spacecraft is launched into a stable orbit and the spacecraft then goes around the Earth. It stays in that orbit until the timing and geometry are right to fire its engine again, initiating a trajectory to its target. That temporary orbit is called an “Earth Parking Orbit.” From here, the space craft will be put on Lunar Transfer Orbit by performing a series of maneouvres.

On entering Moon’s sphere of influence, on-board thrusters will slow down the spacecraft for lunar capture. The orbit of Chandrayaan-2 around the moon will be circularised to 100×100 km orbit through a series of orbital maneouvres.

On the day of landing, the lander will separate from the orbiter and then perform a series of complex maneouvres comprising rough braking and fine braking.

Imaging of the landing site region prior to landing will be done for finding safe and hazard-free zones.

Why called off?
The propellant is usually filled three to four hours before the launch. There will usually be a tolerance limit for leakages. But if it exceeds the permissible limit, the launch might be called off, which is what happened in the case of Chandrayaan -2.
A former ISRO scientist said even at the last minute, our scientists have been able to identify a fault which is a good sign.
“There are instances where launches have been carried out despite such pressure drops. But in those cases, the launch vehicle just had to place the satellite in orbit; but here it has a more complex operation and aborting the mission was the best possible solution,” he said.
He further said the next launch will be scheduled based on the severity of the problem and the report submitted by the committee.


What happened?
It is said a technical snag developed in the launch vehicle system as the cryogenic fuel was being loaded.
“Scientists noticed 10 per cent pressure drop in one of the nine helium tanks used to maintain a pressure of around 300 psi to keep the temperature of liquid hydrogen at minus 253 degrees centigrade. To carry out the inspection, the fuel in the rocket has to be unloaded and the vehicle taken back for further investigation,” sources said.
It is said a team has been constituted to analyse the reason behind the fault.



Balasubramani Muniyandi