Why Chandrayaan-3 lander had four engines, one fewer than 2019 mission

“The original design of the propulsion system for the lander is what was used in Chandrayaan-3. The original design…was modified and an additional engine was brought in for the Chandrayaan-2 mission,” Dr Mylswamy Annadurai, who was project director for the Chandrayaan-1 mission (2008) and the initial portion of the Chandrayaan 2 mission, said.

“From 2009 to 2016-17 it was meant to be four thrusters with two being good enough, and two acting as backup engines,” Dr Annadurai, who was director of ISRO’s U R Rao Satellite Centre in Bengaluru from 2015-18, said.

The Chandrayaan-3 lander did not have to use the safe landing instruments on the spacecraft — such as the laser doppler velocimeter — during its descent on the lunar surface on August 23, ISRO chairman Dr S Somanath has said. The lander was able to make a soft landing by using the navigation, guidance and control system, sensors, and the four thruster engines on board.

From 4 engines to 5; back to 4

The Chandrayaan 2 lander was supposed to have four engines to provide the braking thrust during the landing process. However, in the final stage of the mission, a fifth central engine was brought on to the lander. The presence of this central engine created “confusion” in the spacecraft in the final stages of the landing, Dr Annadurai said.

The four-engine configuration was planned because India did not have a big engine at the time of the Chandrayaan-1 mission, Dr Annadurai said. The fly-by mission landed a Moon Impact Probe (MIP) on the lunar surface, the first Indian object on the Moon.

“We needed a 3000 Newton throttleable engine, so that would have been located in the middle of the lander. We did not have an engine of that capacity — and even if we did, it could not have been carried on the GSLV rocket that was available at the time (2008), Dr Annadurai said.

For Chandrayaan-2, the design was changed — and an additional engine was added, so there were five engines on the lander. “This is what created the confusion. In the last year before the launch in [2019], a lot of changes were made to the lander design. The configuration that was planned after the Chandrayaan-1 mission, and what was done for Chandrayaan-2 in the end were totally different,” Dr Annadurai said.

In Chandrayaan-3, the original four-engine configuration was restored and the software was fine-tuned so that it would resolve any velocity problems immediately, he said.

“Although the Chandrayaan-3 landing was divided into the broad segments of rough braking, fine braking, and vertical descent, the standard deviations were corrected in small segments. Five-engine configuration is no longer a big issue because of the success of this landing,” Dr Annadurai said.

Chandrayaan-2: 4 engines plus 1

In a paper presented to the 51st Lunar and Planetary Science Conference in 2020, ISRO scientists involved with the Chandrayaan-2 project including M Vanitha, project director, P Veeramuthuvel, who was project director of the Chandrayaan-3 mission, and associate director K Kalpana, reported that the Chandrayaan-2 spacecraft had four 800 N throttleable engines and a fixed-thrust 800 N central engine (apart from eight small engines of 58 N for directional control).

In a statement to Parliament regarding the failure of the Rs 970-crore Chandrayaan-2 mission (Rs 603 crore for the spacecraft and Rs 367 crore for the GSLV Mk III rocket) the union government said that mission performed nominally during the first phase of descent from an altitude of 30 km to 7.4 km above the Moon’s surface, during which its velocity was reduced from 1,683 m/s to 146 m/s.

“During the second phase of descent, the reduction in velocity was more than the designed value. Due to this deviation, the initial conditions at the start of the fine braking phase were beyond the designed parameters. As a result, Vikram hard-landed within 500 m of the designated landing site,” the government informed Lok Sabha in November 2019.

Smooth landing for Chandrayaan-3

Ahead of the launch of the Chandrayaan-3 mission, Dr Somanath had indicated that ISRO had reverted to the four-engine configuration on account of the lander being nearly 200 kg heavier than the previous mission. He also said the propulsion system on the lander could by itself ensure a safe landing.

“If all sensors fail, if everything fails, it will still make a landing provided the propulsion system works well. This is how it has been designed. Even if two of the engines do not work, the lander will be able to land. There are four engines. Earlier there were five engines. We are going to land with two engines. Earlier the plan was to land with a single (central) engine,” the chairman said ahead of the landing on August 23.

“The lander’s four engines are used to reduce the velocity and there are also eight small engines to control the direction of the descent. The engines are throttleable and the thrust can be varied from 800 Newton to almost a lower value. It can keep the lander hovering on the Moon’s gravity,” Dr Somanath said.

After the successful landing, the chairman praised the landing manoeuvre, which did not require the use of the redundancy measures installed on the spacecraft.

“We had a very soft landing and it actually achieved most of the nominal conditions for landing including a velocity of touchdown. The velocity was far less than the 2 m/sec that was considered as the nominal velocity and this gives us lot of confidence that the health of the craft will be very good and we will be able to continue with experiments including the Pragyaan rover coming out and doing experiments,” he said.