INSUBCONTINENT EXCLUSIVE:
When Vikram, the lander in Isro's Chandrayaan-3 mission, safely touched down on the Moon at 6.03pm on August 23, a 500-strong team of
scientists looked back upon four years of work with pride
They were years when the team "breathed Chandrayaan-3," says Isro's associate project director K Kalpana
Hundreds of tests were done, thousands of simulations run.In the words of project director P Veeramuthuvel, "Failure was not an option." TOI
spoke to some scientists in Chandrayaan-3's 34-member core team to find out how they ensured everything worked this time
Isro chairmanS Somanath, and M Sankaran, director of UR Rao Satellite Centre - the lead centre for Chandrayaan-3 - give all the credit to
the relentless toil and unwavering commitment of their scientists and engineers
Chandrayaan-3 succeeded because nothing was left to chance, says Veeramuthuvel
"The lander was meticulously crafted to adapt to any descent path it encountered
No room for compromise was left." For this, the team developed multiple mission plans and designed tests to fix the shortcomings identified
Putting things to the testIt was crucial to anticipate every possible fault, and build systems capable of overcoming them
That's why, after hundreds of lab and field tests, the team was fully confident of success on landing day
They had done more than 80 integrated cold tests (no engines), integrated hot tests (with engines) and drop tests
The cold tests alone had involved 25 hours of flight time and 23 sorties in an IAF helicopter
As issues arose, they were addressed to guarantee reliability
The preparation differed from Chandrayaan-2 in one crucial way, says Veeramuthuvel: "In Chandrayaan-2, we used an aeroplane so we couldn't
hover or bring it to low altitudes, like 10 metres." As the equipment was tested on a flat bed at a 6km altitude, system-level sensors could
not be evaluated thoroughly for Chandrayaan-2
However, the use of a helicopter for Chandrayaan-3's testing allowed Isro to test sensors "at different levels of the power descent phase
We even did tests while it hovered at 800m and 150m, just as Vikram did while landing on the Moon," adds Veeramuthuvel
While the cold testing focused on sensors and navigation, the hot testing at Sriharikota in Andhra Pradesh checked for engine firing
It included dry runs, static tests, closed-loop evaluations, and truncated de-boost tests under conditions similar to those on the Moon
Sure-footed VikramThey also had to ensure that Vikram landed on its feet, so they built seven models of the lander, three of which were
scaled-down pieces, and did extensive lander leg drop tests at Chitradurga in Karnataka where craters and boulders served as a test bed for
These tests improved the team's know-how of powered descent and landing
"We closely assessed integrated sensors and navigation performance, ensuring every system was in sync," says Veeramuthuvel
The lander leg tests were done for conditions ranging from steep slopes to flat surfaces, hard and soft terrain, and various combinations of
horizontal and vertical velocity
Practice in virtual worldChandrayaan-3 had something Chandrayaan-2 did not - a dedicated simulation group
The 2019 mission's simulations were done by its control system teams and were less elaborate
Aditya Rallapalli, project manager (simulations), says, "We have 25TB simulation data from more than one lakh tests." Bharath Kumar GV,
deputy project director for navigation, guidance and control (NGC) simulations, says, "Testing for nominal conditions wasn't enough
We predicted various parameters that could go wrong and built models
Then corrections were carried out at each level." Four different simulation test beds - six degrees of freedom (6-DOF), onboard in-loop
simulation (OILS), software in-loop simulation (SILS), and hardware simulation - were used with one clear goal: making Vikram soft-land on
A team led by Prashant Kulshreshtha built and exhaustively tested the onboard software, a key component of the redesigned lander
Guiding it homeAnd finally, someone had to ensure that the systems behaved exactly like the simulations showed they would
Madhavraj, project manager (trajectory), Kuldeep Negi and their team were on the job
"Our role was to see that all the planning on the ground worked
It involved a lot of mathematics," says Negi
The five Earth-bound manoeuvres, the trans-lunar injection, lunar orbit insertion, five lunar-bound manoeuvres, and the two de-boosts to get
into the pre-landing orbit had to happen exactly as planned
"We had a Plan B for each of them, but our Plan A worked every single time," says Madhavraj
"If I've to sum everything up, I'd say, guidance couldn't fail," says Rijesh MP, also a deputy project director with NGC controls and
One of their key challenges was to ensure the guidance systems were in sync with the engines
If the engines were slow to respond to a command, the guidance system shouldn't have read it as an error
