Former President Abdul Kalam used to say that FAIL is actually an abbreviation for ‘first attempt in learning’. ISRO has learnt a lot from its first attempt at a moon landing, the Chandrayaan-2. Accordingly, some crucial changes have been incorporated in Chandrayaan-3.
The basic reason why Chandrayaan-2 crashed was that the flow of fuel into the thrusters (engines on the lander, which provide an upward push against the descent, so that the vehicle slows down) did not come down.
As a result, the engines generated more thrust than they were supposed to, causing the vehicle to change its orientation. Because of this, the camera that was supposed to point at the chosen landing spot, pointed farther away. The software that was supposed to correct this apparently couldn’t carry out the correction. The vehicle somersaulted, but took time to come back to position, because the designed rate of rotation was small.
Having learnt from its previous experience, ISRO has incorporated seven correction measures in Chandrayaan-3. Here they are:
1. Engine thrust control throttle valve slewing rate reduced – can control the throttle valve better, better regulate fuel flow into the engines
2. Algorithms governing thrust modified
3. Instantaneous thrust regulation scheme implemented for the entire descent —for better control over thrust regulation.
4. Variability of individual thrust increased to 90N. There are four thrusters on Chandrayaan-3 (five on Chandrayaan-2). The tolerance of variation of thrust among the individual thrusters has been increased. Essentially, this means that if one thruster “misbehaves” a little, the vehicle can tolerate it.
5. ‘Laser doppler velocimeter’ or LDV has been brought in to regulate the flow of fuel into the engines. The LDV is a sophisticated instrument, which throws a laser beam onto a flowing liquid and determines the flow rate from the reflected light. This instrument, again, will help better regulate the engine thrust.
6. The rate of rotation of the lander has been enhanced to 25 degrees a second, from 10 degrees earlier – this will help the lander return to its position quicker
7. Multiple paths to handle contingencies — even if the lander is off-track, it can soft-land. In that case, it won’t be a perfect landing, but it won’t be a crash land either.
With all these, the hope is that India will achieve a soft-landing this evening. If it doesn’t, we can call it ‘SAIL’ -- ‘second attempt in learning’.