Vikram was in touch till 335m above lunar surface: ISRO
IT it is learnt that Chandrayaan-2’s Vikram lander was much closer than 2.1 km to the lunar surface when it went incommunicado. ISRO sources said the lander communication link was intact till it was 335 metres above the moon’s surface.
ISRO failed to soft-land the lander safely during its last 15 minutes of ‘terror’, denying India the credit of becoming the fourth nation to soft-land on the moon. Initially, ISRO chairman K Sivan said: “Vikram lander descent was as planned and normal performance was observed up to an altitude of 2.1 km. Subsequently, the communication from the lander to the ground centre was lost.” This suggested communication was lost at an altitude of 2.1 km. ISRO is yet to reveal what caused the glitch.
Sources said the first phase of braking of the lander — from a 30 km-altitude to 400 metres — was nearly successful. The lander’s orientation was changed from horizontal to vertical. Four-corner thrusters were operated to brake and the central thruster was switched off. At height of 400 metres, the second phase of braking started and communication was lost when the lander was 335 metres above the lunar surface.
ISRO senior adviser Tapan Mishra, who was instrumental in building India’s radar satellites, on Wednesday, further explained what may have happened.
Writing a detailed post on his Facebook page, Mishra said Chandrayan 2 had five big (800 Newton) thrusters and eight small thrusters. Thrusters are essentially small rockets, usually mono or bi-propellant based. Big thrusters are kept for braking/hovering and small thrusters are meant for orientation change and hovering. Without directly saying thrusters of the lander malfunctioned, Mishra said: “Five big thrusters (four at corners and one in the centre), if fired equally, will combine in the vertical direction, providing opposing force and the resultant vertical axis of vector will pass through centre of gravity, providing stability. If an imbalance is created by throttling four engines, ie by varying fuel injection rate, the resultant uncompensated horizontal force will spin the lander in the horizontal plane. If spinning in two orthogonal planes goes out of control, it will essentially tumble down the lander. Tumbling of lander with thrusters on, will make things very complex, like fireworks burnt in Diwali, called spinning wheel or ‘Charki’. The result will be simultaneous tumbling and zigzag random motion of lander, beyond the control of on-board control system. So throttling of the four thrusters is a critical activity,” Mishra said.
He also indicated depletion of fuel in the lander’s fuel tank. “A very large component of lander is its fuel tank. When the lander accelerates, decelerates, because of inertia, the liquid fuel gets into sloshing, akin to splashing of water in a tub. Sloshing becomes severe as more and more fuel depletes in the fuel tank, making life difficult. It may so happen that the engine nozzle feed will be starved of fuel, resulting in uncontrolled throttling.”