On July 22, 2019, India efficiently launched Chandrayaan-2, the second mission to the moon. Twenty-two days later (August 14), after a collection of orbit elevating manoeuvres, the spacecraft lastly escaped the earth’s gravity and adopted a path in direction of the moon. Six days later, Chandrayaan-2 was efficiently inserted into lunar orbit. Finally, on September 2, the Vikram lander separated from the Orbiter, carried out two de-orbit manoeuvres and on September 6, started its descent to the moon’s floor. The descent went as deliberate up to an altitude of two.1 km from the Moon’s floor earlier than communication from the lander to the floor stations was misplaced. The Vikram lander had apparently crash-landed on the Moon.
Soon a national-level failure evaluation committee headed by Dr. V. Narayanan, Director of the Liquid Propulsion Systems Centre (LPSC), was fashioned with consultants drawn from nationwide institutes and ISRO to research the reason behind failure and to suggest needed corrections. The committee pinpointed three essential errors that had occurred in succession leading to the crash.
There are 4 vital phases earlier than the landing on the moon — the tough braking section, the attitude-hold (orientation) section, the high-quality braking section, and the touchdown section.
Cause of crash
During the tough braking section, the velocity of the lander was efficiently decreased to a most. The drawback with the landing started when the Vikram lander entered the second section — the attitude-hold (orientation) section. During this section, the thrust had to be maintained at half the degree. “But when the engines were commanded to provide half the thrust, the achieved thrust was more than half. The dispersion level [variation in the thrust] was more than what we had assumed during the pre-flight simulations,” explains Dr. Ok. Sivan, former Chairman of ISRO underneath whose watch the Chandrayaan-2 was launched in 2019.
The steering system was supposed to have halved the thrust however it malfunctioned. “The guidance system was not designed to handle large dispersions, and this resulted in the guidance system malfunctioning leading to full thrust being given to the lander,” says Dr. Sivan. “At the end of the attitude-hold phase, the altitude and velocity were very different from the expected values. The guidance-system malfunction was the second crucial error.”
According to Dr. Sivan, throughout the high-quality braking section, the system was making an attempt to right the massive errors. The large-scale corrections to velocity and altitude demanded massive orientation manoeuvres. Changes to orientation are carried out by the management system, however sadly the management system was not designed to produce massive modifications. “Even though there was a large demand for orientation change, the control system restricted the rate. As a result, the control system was unable to provide the necessary large corrections to orientation,” Dr. Sivan says. “The control system was taking more time to correct the orientation due to rate restriction. But by that time the lander had reached the moon’s surface.” This resulted in the lander crashing on the moon.
The crash was brought on by three errors — the thrust (dispersion degree) was greater than assumed for design, the flaw in the steering system resulted in additional thrust being given, and the charge restriction in the management system failed to make large-scale modifications in the orientation. “The Chandrayaan-2 mission would have been a success if any of the three errors was not there. The three cumulative errors led to the failure,” he says.
Correcting the errors
Taking the learnings from Chandrayaan-2 failure, ISRO corrected for all the three main errors in the newest mission — the system was corrected to guarantee further dispersion doesn’t occur, the steering system was corrected, and the restriction in the management system was eliminated. “These were the major corrections in Chandrayaan-3 lander.
Redundancy was also built into Chandrayaan-3 lander in terms of extra sensors, and extra propellant to allow it to travel to an alternate landing site, if required. And the landing area was expanded from a narrow patch of 500 metres x 500 metres to a 4 km x 2.4 km region.
In addition, other improvements suggested by the failure analysis committee were also incorporated in the Chandrayaan-3 mission.
Extra fuel, sensors and stronger lander legs designed for higher landing velocity led to a net increase in the weight (about 250 kg) of the lander. Since Chandrayaan-3 was not carrying an orbiter like its predecessor, it became possible to accommodate the increased weight.
“Increased weight of the lander meant that a single engine was not sufficient. So the central engine was removed and two engines were used for landing,” Dr. Sivan says. “The central engine was added in the earlier mission as it was suspected that dust stirred by other engines would affect the electronic components. But our analysis found that dust wasn’t a problem.”
