Chandrayan 3 Vikram lander`s path
Dr. Sivan eloquently captured the intricate nature of the mission’s final stage, which revealed the intricacies of Chandrayaan-2’s failure when the Vikram lander failed to transition correctly from a horizontal to vertical orientation during its critical “fine braking phase.” This misalignment led to its eventual crash on the lunar surface, occurring at a distance of 7.42 kilometers from the Moon’s surface.
Looking ahead to the Chandrayaan-3 mission, a pivotal technical maneuver is set to take place on August 23 as the lander approaches the last 15 minutes of its endeavor to gently touch down on the lunar terrain. This maneuver will entail shifting the high-velocity horizontal position of the spacecraft into a more gradual vertical orientation, facilitating a controlled and gradual descent onto the lunar surface.
As detailed by ISRO, the module will undergo internal checks and await sunrise at the designated landing site. The powered descent, crucial for ensuring a gentle touchdown, is anticipated to commence around 5:45 pm on Wednesday. The outcome of this phase will determine the mission’s success, marking India’s historic achievement as the first nation to achieve a soft landing near the moon’s south pole, with Chandrayaan-3’s significant descent slated for August 23, 2023, at approximately 6:04 pm IST. Several pivotal stages precede the actual touchdown, encompassing initial preparations, velocity reduction, orientation adjustments, attitude stabilization, fine braking, final descent, and landing.
The descent process
The descent process is divided into four phases. In the first phase, the lander must decelerate from its initial horizontal velocity of 1.68 km/s (with zero vertical velocity) to 358 m/s (about 1,290 km/h) and eventually 61 m/s (around 220 km/h) during the optimal “rough braking phase.” This phase spans 690 seconds, during which the lander descends from an altitude of 30 km to 7.42 km, traversing a distance of 713.5 kilometers across the lunar surface to reach the touchdown site.
Subsequently, the lander enters the “attitude hold phase” at an altitude of 7.42 km, lasting for nearly 10 seconds. Within this span, the lander adjusts from horizontal to vertical orientation while covering a distance of 3.48 km. The altitude is then reduced to 6.8 kilometers, with horizontal and vertical speeds set at 336 and 59 meters per second, respectively.
To ensure consistent thrust at the onset of the second landing phase, Chandrayaan-3’s thrust requirement is higher during the second attitude hold phase, at 740X4 N compared to Chandrayaan-2’s 400X4 N. These updated mechanisms streamline the transition from horizontal to vertical posture while maintaining thrust and angle stability.
During the third phase, named the “fine braking phase,” the lander completes its transition to a vertical orientation, lasting approximately 175 seconds. It covers the remaining 28.52 km to the landing zone, descending to an altitude of 800-1,000 m and achieving a nominal speed of 0 m/s.
Dr. Somanath explains that the phase from 30 km to 7.42 km involves rough braking, followed by an attitude hold phase at 7.42 km. At altitudes of 800 or 1,300 meters, sensor verification takes place, followed by hazard assessment at 150 meters, determining whether to vertically land or move laterally by a maximum of 150 meters to avoid obstacles.
Extensive simulations, revised guidance designs, and numerous algorithms have been employed to ensure accurate execution across all phases, even if nominal values vary. ISRO’s Director Dr. Somanath affirms that the lander will still strive for a vertical landing, even if conditions diverge from the expected figures.
The entire operation is autonomously conducted, encompassing timely engine operation, altitude-appropriate fuel utilization, and surface scanning for potential terrain obstacles before the final touch down.
At around 30 km altitude, the lander enters the powered braking phase, engaging its four thruster engines in retro firing to gradually reduce speed until it safely reaches the lunar surface. This controlled approach prevents a crash due to the Moon’s gravitational forces.
Before initiating descent, the lander’s sensors and cameras scrutinize the surface for obstructions. As it descends beyond an altitude of approximately 6.8 km, two engines are utilized while the other two are deactivated, providing the lander with the necessary reverse thrust for a soft landing.
