Fifteen Minutes of Terror – Vikram Lander’s planned operations

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Tapan Misra

Rough Braking Phase

The terror part in title was coined by JPL team, to sum up the Curiosity landing on Mars. And this coinage got oft repeated in connection with landing of Vikram Lander on moon. I personally believe, scientists do great disservice to science by using such terms. Actually all so-called complex science, can be explained scientifically in “small small scientific parts” to make the resultant complex science simple. Science is never terror to me, but a quest of simplicity and simple logical explanations behind all the mysteries of nature, God’s beautuful creation.

There were so many queries from students, teachers, journalists, simple lay men about Lander (Vikram), as the hype in media was in crescendo. I found even experts, who clogged news channels, were under utter confusion.

I attempt to explain Lander operation in simple terms. Hope it benefits the common man.

a. Lander Trajectory: Lander was horizontal at 30 km altitude at a speed of 1.66 km/sec. When being brought to the lunar surface, it was supposed to be vertical. It’s maximum velocity was 2 m/sec. It followed a curved track during this operation.

b. The height of lander is reduced by firing thrusters in opposite direction. As the velocity is reduced, because of loss of energy, satellite loses height due to lunar gravitational attraction, roughly one sixth of gravity on earth.

c. There were two phases of lunar descent. Braking and hovering. Chandrayan 2 had 5 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.

d. Five big thrusters are positioned as follows: 4 at corners and one at centre. The resultant thrust of 4 corner ones, if fired equally, will combine in 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, i.e. by varying fuel injection rate, the resultant force vector is not aligned to vertical axis of lander, creating one horizontal and vertical component. And generally, operation of 4 corner thrusters and the central thrusters is made exclusive to make things simple. Thrust vector of central one will also pass through centre of gravity, aiding stability.

Let us assume operation of simultaneous operation of 4 corner thrusters. Now if one or more of them are not operating simultaneously or there is imbalance in thrust output among them, the resultant uncompensated horizontal force will spin the lander in horizontal plane. In that case the resultant vertical force vector will also not pass through centre of gravity and resultant couple will trigger spinning in vertical plane. In fact the controlled spinning by throttling is used to aid programmed tilting of the lander in the braking phase.

If spinning in two orthogonal plane goes out of control, it will essentially tumble down the lander. Tumbling of lander with thrusters on, will make things very complex, like firework burnt in Diwali, called spinning wheel or “Chakri”. The result will be simultaneous tumbling and zig-zag random motion of lander, beyond the control of on-board control system. So throttling of the four thrusters is a critical activity.

A very large component of lander is fuel tank. When 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 fuel tank, making life difficult. It may so happen that engine nozzle feed will be starved of fuel resulting in uncontrolled throttling.

e. The first phase of braking lasts from 30 km altitude to 400 m altitude where velocity is reduced from 1.66 km/sec (6000 km/hr) to 60 m/sec ( 200km/ hour). Orientation of lander is changed from horizontal to vertical. Throughout this period 4 corner thruster are operated to brake and central truster is switched of.

f. At 400 m height, the second phase of braking starts. The lander is vertical, two of four corner thrusters are switched off simultaneously and two diagonal thrusters are switched on. By the time lander descends to 100 m, these two thrusters brake lander to reduce vertical speed from 60 m/sec at 400 m height to less than 2 m/sec at 100 m height.

g. The braking control from 30 km height to 100 m is carried out by a series of time tagged commands, loaded in the lander a few hours before operation from ground. They are generated based on precise measurement of lander orbit, prior to deorbitting. This is a predictable operation.

g. When lander reaches 100 m height, the lander is three axis stabilised and it essentially floats. Moon’s gravity is compensated by upward thrust of two diagonal thrusters. Small thrusters are used to move lander sideways. The camera on lander takes photograph of lunar surface below. The resultant image is matched with stored images of landing site (captured by high resolution camera of orbiter earlier) and horizontal movement of lander is controlled. By slowly reducing vertical thrust by central thruster, lander is slowly descended. Radar altimeter keeps an eye on true altitude of the lander. This mode is called hovering mode. This is the most complex mode and fully autonomous. The software is loaded prior to launch and it cannot be changed afterwards as in the case of braking mode which can be changed even a few hours before operation.

h. Just 5 seconds before landing, the two diagonal thrusters are switched off and central thruster is switched on. It was apprehended that two corner thrusters, if active will blow the moon dust and it will create a centre jet upwards, covering the lander with dust. So central thruster will reduce this upward jet. All landers need to be prepared to operate under dusty condition at the last moment of landing.

Tapan Misra, the author of this blog is a Distinguished Scientist. He heads the Office of Innovations Management, ISRO, Bangalore. Earlier he was Director Space Applications Centre. For a brief period he also held additional charge of Director, Physical Research Laboratory, Ahmedabad.


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