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Chandrayaan2 - Vikram can you Hear me!

So it’s all over the internet lately, and I have received couple of messages with Vikram can you hear me messages ;)

I thought I would write about it. You know a self-explanatory article of what went wrong with Vikram!

Chandrayaan-2 was indeed an ambitious mission from Indian Space Research Organization (ISRO). Months of hardwork from Indian scientists and engineers working for sleepless nights. I would not say it had gone in vain completely, the orbiter is booted up, good news is that orbiter has managed to save up its fuel we have enough fuel for continuing it forward for seven years.

More about the mission itself in this article:

The descent of Chandrayaan-2 lander module was supposed to last for 15 minutes, and initially everything seemed to be going according to the plan. Around 11 minutes after Vikram lander began its decent is when things started to get tricky.

At this stage, Vikram was supposed to make a slight rotation maneuver so that its on-board navigation cameras could map the lunar surface and pick up a suitable landing site.

During this crucial movement, Vikram unexpectedly performed a somersault.

What exactly made it to do this somersault?

Well this is when a bit of physics kicks in.

The lunar decent has two phases - Braking and Hovering. Vikram was equipped with 5 big thrusters and 8 small thrusters. These thrusters are actually small rockets, the bigger ones are kept for braking/hovering whereas the smaller ones are meant for tiny orientation changes and maneuvers.

So, these five big thrusters are positioned as - four at corners and one at center. The resultant thrust of four corner ones, if fired equally, will combine in vertical direction, providing opposing force and giving stability to the module.

Now the crucial part to be monitored is the center of gravity, if all the big thrusters are fired equally and simultaneously it will combine in vertical direction, providing opposing force and the resultant vertical axis of vector will pass through center of gravity which will keep the lander stable. If there is a slight imbalance created when throttling four engines (throttling is done by varying fuel injection rate) the resultant force vector will not be aligned to vertical axis of lander, creating one horizontal and vertical component.

Generally, the operation of all the four thrusters and the central thruster is kept exclusive to keep things simple. The thrust vector of the central one will also pass through the center of gravity as this will add in stability.

Things tend to go wrong, during the simultaneous operation of four corner thrusters, if one or more of them are not operating in synchronous with each other or there is an imbalance in thrust output among them, the resultant uncompensated horizontal force will spin the lander in horizontal plane. In this case the center of gravity which we discussed will start moving away from the stable position which will trigger spinning in vertical plane.

But this in fact, the controlled spinning by throttling is used to aid programmed tilting of the lander in the braking phase.

If spinning goes out of control, it will essentially tumble down the lander. Tumbling of lander with thrusters on, will make things very complex, 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.

Now who can be the culprits here which could have caused this unexpected tumbling?

  • Fuel tank, very large component of lander is its fuel tank. So, this adds up inertia to the lander whenever there is an acceleration or deceleration phase. The liquid fuel gets into sloshing like the splashing of water in our tub. This gets harder as more and more fuel deplete in fuel tank, making life difficult for Vikram. As there is this sloshing happening inside the fuel tank, it may so happen that engine nozzle feed could have starved of fuel resulting in uncontrolled throttling.

  • Dealing with Moon's Gravity. Space is hard, when Vikram reaches 100 m height, Vikram must be stabilized in three axis and it literally floats. Moon's gravity is compensated by upward thrust of two diagonal thrusters. Small thrusters are used to move Vikram side wise. The hovering mode takes over, radar altimeter keeps an eye on true altitude of the lander. This is when a bit of software computation happens, this most complex mode and fully autonomous - artistry and engineering at work! The camera on lander takes photograph of lunar surface below, the image is matched with stored images of landing site which was captured earlier by the orbiter and horizontal movement of lander is controlled. And by slowly reducing vertical thrust by central thruster, lander is slowly descended.

The result of these factors could have caused a hard landing of Vikram lander on the lunar surface and may have disabled the communication system.

So, What next? What is ISRO's Scientists are examining?

  • The final data of the last 20 minutes could hold the key as to what happened. Performance data of each subsystem, most crucially the liquid engines could tell the full story.

  • Signals or emissions from the lander that could establish an anomaly, software glitch or hardware malfunction before the crucial 2.1 km altitude position.

  • Last sets of sensor data downloads - Data gathered during descent by the Vikram lander's own sensors, in terms of its orientation, photographs of the Moon surface from its advanced cameras, and any other parameters that its sensors may have recorded.

  • Orbiter to be used to map area for clues - The orbiter has payloads capable of mapping the Moon's surface - its orbit takes it over the Moon's South Pole too as part of the mission, so there exists the possibility of some visual data of where the lander went down.

Well that said, ISRO is racing against time to bring Vikram back to life and salvage the lander-rover part of the Chandrayaan-2 mission.

More great links on Chandrayaan-2:

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