‘Keep it simple’: Former Isro scientist explains what led to Chandrayaan-2’s Vikram crash
On September 7, Lander Vikram, with rover Pragyan, lost contact with the ground station during its final descent, just 2.1 kms above the lunar surface, minutes before the planned touch-down on the Moon.Updated: Sep 21, 2019 11:50 IST
As the Lunar night starts to fall early Saturday, hopes of re-establishing communication with India’s moon mission Chandrayaan-2’s Vikram lander seems to be all but over with its 14-day mission life about to end.
On September 7, Lander Vikram, with rover Pragyan, lost contact with the ground station during its final descent, just 2.1 kms above the lunar surface, minutes before the planned touch-down on the Moon.
A former official of the Indian Space Research Organisation (ISRO) suggested that keeping the technology simple; freezing the mission profile and not changing it till the last moment; carrying out the simulation tests to the maximum possible level -- are some of the learnings from the crash landing of India’s moon lander Vikram.
Speaking to IANS, the official said, “Space technology itself is complicated and one should not add more complications to it as it had happened with the lander Vikram. For instance, the four throttleable engines to work in unison is a technical challenge. Then there is the fifth engine at the centre.”
According to the ex-official, had it been known earlier that the GSLV-Mk III (Geosynchronous Satellite Launch Vehicle) rocket was available for the mission, the spacecraft could have been designed with a powerful single engine, and not with five engines.
“Ideally a moon lander of this kind should have a single 3,500 Newton (N) engine. This would have obviated the complexities of managing the four throttleable engines working in union. Other nations had used a powerful single engine while landing their crafts on the moon. Before the touchdown on the lunar surface, the single engine would be cut off and the lander would have made a soft landing,” he said.
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According to him, the mission profile underwent last minute changes and varied from the original plan.
“Originally the plan was to launch the Chandrayaan-2 spacecraft along with the lander using GSLV-Mk II that can carry a weight of two tonne,” he said.
After a while, the fifth engine was added at the centre of the lander in order to prevent the dust from the Moon surface from damaging the lander while it attempts to land, he added.
“This in turn increased the spacecraft’s weight and changed the other specifications that GSLV-Mk II was not capable of carrying. It was then decided to launch Chandrayaan-2 with higher capacity GSLV-Mk III,” he further added.
Originally, the idea was to switch off all the four throttleable engines first and allow the lander to slowly drop down. “The idea was to allow the lander to drop down from a height of 10 metres at 2 metres per second,” he added.
“After the fifth engine was introduced, there was a software change. How well it was tested is not known,” he said.
“The ISRO has to simulate various things and scenarios with the data on hand. It should also check and simulate what were the actions that were not done and the results that they had assumed,” another retired official, who preferred to remain anonymous, told IANS.
According to him, ISRO has to see whether any simulation prior to the launch was overlooked or waived any known deviation.
“ISRO should also check out the extent to which various failure modes were simulated. ISRO has to go into these details. It would take considerable time to arrive at a conclusion,” he added.
Space experts had told IANS that in case of data loss, it is assumed that the satellite suddenly changed its altitude. If the target/satellite experiences a large angular rotation, the communication link usually breaks.
According to them,the reason behind the lost communication link could be the wrong input being loaded on to the lander.
In other words, the loss of communication link with Vikram could be due to sudden disturbances -- in the form of engine propulsion or wrong data input, experts said.
The Vikram was designed to land on the lunar surface on its four legs at a speed of about 7 km per hour. But due to a few complications faced by the lander minutes before the touch-down, the Vikram might have hit the Moon’s surface at much higher speed than expected.
“The Vikram’s maximum tolerance landing level was 5 metres per second. But it seems during its final moments the landing speed was very high,” the retired official added.
“In the case of Vikram’s landing what was its point of no return -the point where its descent cannot be stopped or changed or what was the lowest or nearest point from the lunar surface that the lander could have been manipulated is not known,” he said.
Former ISRO officials said the Indian space agency would have got various engineering data from this mission which will be useful for the future course of action. Also adding, “There should be Plan B if something goes wrong with Vikram and whether it was activated is also not known.”
Originally scheduled for July 15, Chandrayaan 2’s launch was aborted due to a technical snag at the eleventh hour before take-off. After fixing the problem in about a week, India’s second lunar mission was launched from Sriharikota on board the GSLV Mk III on July 22 at 2.43 pm.
India’s second Moon mission has cost less than half the budget of Hollywood blockbuster ‘Avengers Endgame’. The total cost of the Chandrayaan 2 mission is estimated at 124 million US dollars, which includes 31 million US dollars for the launch and 93 million US dollars for the satellite.
(With inputs from agencies)