Chandrayaan-3: Embracing failure-based design for lunar success

The place where the lander can touch down has been made bigger. It used to be 500 meters by 500 meters, but now it's 2.5 kilometres by 4 kilometres. This change gives the software more room to control the lander's movements. -- Writes Girish Linganna, Aerospace & Defence Analyst.

Bengaluru (Karnataka): The Indian Space Research Organisation (ISRO) has repeatedly expressed confidence in the 'failure-based design' chosen for the Chandrayaan-3 mission, its third lunar mission. ISRO chairman S Somanath stated in a virtual address on August 15 that the entire design of the 'Vikram' lander ensures that it will be able to withstand malfunctions.

Chandrayaan-3's failure-based design focuses on what can fail and how to safeguard it and ensure a successful landing. Even if everything fails, if all the sensors fail, and nothing functions, Vikram will land. "That's how it was designed, assuming the propulsion system works well," said Somanath.

Somanath stated during a presentation that the Chandrayaan-3 lander should be able to manage a large number of failures if the algorithm works properly during vertical landing. He added that the ISRO's challenge is to land a horizontal "Vikram" in a vertical position on the lunar surface.

ISRO's chief stated that another challenge is ensuring that fuel consumption is reduced, that distance calculations are accurate, and that all algorithms function correctly. "We have ensured that a landing velocity of up to three metres per second will not cause damage to Chandrayaan-3. It ensures that the vertical velocity is constrained so that the structure does not fall'," Somanath explained.

Besides the above, due to mistakes in the software, a chain of problems occurred. The five retro-rockets on the lander, which were supposed to slow it down as it approached the moon, didn't work properly. Instead of slowing down, they actually made the lander move faster towards the surface. The software also caused the control systems to overcompensate, making the lander unstable in terms of its position, direction, and speed. Lastly, the lander's legs weren't strong and broke when it landed too forcefully.

The place where the lander can touch down has been made bigger. It used to be 500 meters by 500 meters, but now it's 2.5 kilometres by 4 kilometres. This change gives the software more room to control the lander's movements. They got rid of the fifth retro-rocket. They also added extra fuel to both the propulsion module (PM) and the lander. This helps to ease the time pressure during operations. They made the lander's legs stronger.

They used a big helicopter called Chinook and a crane to do a lot of tests on the software and the lander. The need for quick changes in direction just before landing has been reduced. This is because the Chandrayaan-2 Orbiter sent back lots of pictures, which helped create better landing instructions (including terrain having 28cms risks). The lander only needs to avoid rocks bigger than 30 cm now, instead of 28 cm.

The chairman of ISRO said the new guidance system is different. Before, it relied on things going well, but now it's ready for things to go wrong. It can fix mistakes, instead of assuming everything is perfect and making too many changes that might not work together.