CHANDRAYAAN-3 - Technological challenges for a soft-landing
Pranav Patil, TOGAF,CSM,PGDIPR,
PLM Architect and Director - Iristechsys Software Services
Chandrayaan-3's smooth landing has been the feather in the cap for us all Indians. India made history as the 4th country to successfully soft-land a craft on the moon's surface. As far as the entire sequence goes, the most complex maneuver was the soft landing.
Why is a soft landing on the moon so challenging?
I would like to bring about the numerous technical challenges that ISRO overcame to achieve this extraordinary feat.
1. Absence of Atmosphere on the moon:
On Earth or any other planet with an atmosphere, the spacecraft can use aerodynamic drag (a.k.a parachute) to slow down during descent. However, the Moon lacks a substantial atmosphere, making a parachute useless. Therefore the only way to achieve soft landing on the moon is to use thrusters against the moon's gravity.
Chandrayaan-3 used four 800N throttleable liquid engines (variable thrust engine) and eight engines of 58 N. Why a variable thrust engine is required? I will come to that later.
2. Navigation and Guidance:
We have the luxury of using GPS satellites to accurately determine the position of any craft/airplane on the Earth's surface.
We don't have anything similar on the moon. Therefore the only way the position could be determined was using a slew of sensors to predict the position accurately.
Chandrayaan-3 used a variety of sensors and cameras like a Laser Altimeter, laser Doppler velocimeter, and Lander Hazard Detection and Avoidance Camera (LHDAC)
The Lander Hazard Detection and Avoidance Camera (LHDAC) system, took pictures in real-time while it was landing and correlated them with stored images to find out if the landing was taking at the right place and adjust the track of the lander accordingly so that it lands on the decided landing site.
3. Vertical Descent Control:
This is the crux of the soft landing and is a very interesting dynamics and control problem.
An analogy could be seen when you attempt to control a drone in the air. The thrust of all the fans has to be precisely controlled - or the drone will either drift or capsize.
Imagine holding a 2-tonne object steady and bringing it down to the surface of the moon in a near-vertical position.
That is where the variable thrust engines came into the picture (refer to point#1) that adjusted the thrust as per the need.
The impressive dynamic and control algorithms of Chandrayaan-3 fetched real-time data from the sensors, processed it, and accordingly controlled the variable thrust engines for a smooth landing.
4. Transition from Horizontal to Vertical:
The animations show the lander being vertical while landing on the moon. But while it is orbiting the moon, it is inclined. So even before the descent on the surface of the moon, the lander's orientation has to be managed and changed precisely, or else it will crash.
Chandrayaan-3 used advanced inertial guidance systems to manage its bearing on the surface of the moon.
5. Limited Real-time Control from Earth:
We cannot have real-time control over the lander because of its large distance from the Earth.
Real-time control of the landing process from Earth is not feasible due to these communication lags. Therefore Chandrayaan-3 had to be made completely autonomous in its operations during the soft landing
6. Low-Gravity of the Moon:
The Moon's low gravity significantly alters the dynamics of the descent- there are limited ways to simulate this descent on the surface of the earth.
Thrusters must be carefully calibrated to counteract this lower gravitational pull. Chandrayaan-3 engineers carefully worked on the thrusters to ensure that there was the right amount of thrust in response to any deviation in the altitude or position.
7. Software and Hardware Redundancy:
The moon is hostile due to a variety of factors such as extreme temperatures (+120 C to -150C). Another aspect: spacecraft is exposed to high levels of radiation from the sun and cosmic rays. This can cause malfunctioning of the sensors which could be catastrophic.
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Chandrayaan-3 had substantial redundancy built in - for eg, a standby camera in place in case the LHDAC malfunctioned.
8. Limited Margin for Error:
Another important point is that unlike Earth, where an aircraft or spacecraft can circle around for another landing attempt, the Moon offers limited options for a safe go-around. A single landing attempt must be well-executed to avoid disaster.
Therefore Chandrayaan-3 team had to carry out numerous simulations and as the Chairman of ISRO stated, had to go for a "failure-based design"
All these efforts by the ISRO engineers have made the Chandrayaan-3 a mega success and made all the Indians proud.
This post would not be complete without a poem that is an ode to the moon, written by poet Raja Badhe:
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