Pune | Updated: July 21, 2019 11:04:49 am
A sudden drop in pressure in one of the tanks containing helium gas on the GSLV Mk-III rocket was behind ISRO’s decision to abort the Chandrayaan-2 moon mission last week, The Sunday Express has learnt.
Less than an hour before lift-off on July 15, India’s first mission to land on the moon was scrubbed after what ISRO said was a “technical snag”. ISRO has said its expert committee had identified the problem and applied corrective measures, and that the system was now functioning normally. Chandrayaan-2 is now set to launch Monday.
But last week, scientists were unable to assess the exact cause of the sudden pressure drop. “The pressure in one of the helium tanks was observed to be falling. It happened while the cryogenic fuel, which is supercooled to temperatures well below -100 degrees Celsius, was being loaded. Because of the extremely low temperatures of the fuel, the environment surrounding the fuel tanks also get cooled,” said an ISRO scientist associated with the mission.
“This helium tank was in close proximity to the fuel tank. So, the pressure drop could be because of the loading of the fuel. A filled up balloon, if put on ice, would deflate. Something similar could have happened here as well. But at that time, scientists were not able to ascertain whether it was because of the fuel loading or because of a leak in the helium chamber.”
The scientist said that if it was not a leak, it was not a very serious problem. “The pressure recovers eventually. We have seen it happen in other missions. But at that time, the mission staff were not very sure. So it was decided to err on the side of caution, and postpone the launch,” said the scientist.
“The problem was not fatal. It was not something that could have prevented the mission from lifting off. The mission could have been successful as well. But this would have entailed taking a risk. And mission control, in its judgment, decided that it was a risk not worth taking. One has to make such judgments.”
According to the scientist, doubts arose since ISRO had little data from previous flights. “This was the first operational flight of GSLV Mk-III (India’s most advanced and powerful rocket). Before this, it has only undergone one experimental flight and two development flights. Their flight data cannot be relied upon for an operational flight. So scientists were faced with a situation in the very first flight,” said the scientist.
“With PSLV, for example, we have a wealth of data because it has done so many flights. We know exactly how it behaves, and what all to expect.”
The scientist did not reveal what the final conclusion of the expert committee, constituted to assess the problem in the rocket, was – whether the pressure drop was due to the super-cool cryogenic fuel, or because of a leak.
“The problem was noticed only during the time of fuel loading. The helium tanks were filled much before that. So why would the leak be detected only at the time of fuel loading?” said the scientist.
However, another ISRO scientist, now retired, while confirming the sequence of events, said leakage due to small problems in the joints could not be entirely ruled out.
“It could have flown, no doubt. But I think it was a much better decision to postpone it and check it all over again. This is a very prestigious mission, and it is absolutely essential to be correct. I think it was a wise decision to suspend the launch,” he said.
“Because it could ultimately prove to be fatal. The first experimental flight of SLV (Satellite Launch Vehicle, India’s first launcher, a precursor to the PSLV and GSLV, which was tested for the first time in 1979) had failed due to a similar problem. At that time we were using nitrogen, not helium, and there was a pressure drop in the nitrogen chamber,” he said.
Though the launch date has been delayed by seven days now, there is no change in the date of the moon lander and rover module missions, which will happen on September 6. Both scientists said the path that Chandrayaan-2 will take to reach the moon was flexible to accommodate this delay.
After its original launch, Chandrayaan-2 was supposed to spend 17 days in Earth orbit, and another 28 days in lunar orbit, before the lander and rover modules were supposed to separate from the main spacecraft and prepare for landing on September 6.
“I cannot tell you exactly what changes will be made in the journey. But making up for these seven days is totally possible. Adjustments in the flight path can be done. There is plenty of room in this mission,” said the serving ISRO scientist.
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