Chandrayan-2 Mission

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Amber G.
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Re: Chandrayan-2 Mission

Postby Amber G. » 19 Sep 2019 22:18

Dileep wrote:The "X cm" resolution of the camera is associated with the "distance to object" (or altitude for land imaging) na? So, 30cm was for what altitude?

Few comments about resolving power and resolution:

Yes, resolving power is actually measured in angle (radians). I can "resolve" 30 cm easily, without any aid, from a distance of 1 Km. My 20 cm home telescope can "resolve" 30 cm from a distance of 100 Km -- almost the same power as CY2's cameras..

- The "definition" or understanding of "resolving power of 30 cm" is this - if there are two LED's (point source of light) 30 cm (or more) apart one can see them as *two* separate light sources.. If they are closer than that they will look like one source.

- The resolving power depends on wavelength of the wave. It is about (wavelength) divided by (diameter_of_the_objective_lens/mirror).

- Though "resolving power of 30 cm" a good (very good) measure of what one can see clearly, BUT it is not everything. It certainly does not mean that any thing like 31 cm will be visible (seen/identified) and 29 cm will be not. MANY other factors (like lighting, surrounding, brightness/reflectivity of Vikram/ direction , shadows etc..) needs to be considered before one can identify Vikram.

- There are no "secrets" spilled if ISRO publishes high resolution photographs of the moon. The most likely reason it is not publishing it is NOT to protect it's technical secrets but because it is VERY difficult to identify Vikram from a few pixels.

Hope this helps in understanding and encourages people to learn more basics..:)

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Re: Chandrayan-2 Mission

Postby Amber G. » 19 Sep 2019 22:27

SSSalvi wrote:About current orbit data:
ISRO never publishes orbit data. ( State Vector or
TLE are the correct terms. )

Celestrak is an authentic source for latest TLE for Earthbound Satellites . It doesn't include CY2 after it entered Moon gravity.

For crafts in non-stellar orbits jpl/nasa horizons is the only known source. But it doesn't give in TLE format and more importantly it gives derived output. .. not the official values from actual satellite operators.

Other source for Earthbound satellites is n2yo

Thanks.. What I was looking for is *any* current authenticated ISRO data - for example exact time period of CY2 and/or perigee/apogee can help me make define the orbit shape correctly and then just few points in orbit can give all the things I want.

Thanks again. Nasa/JPL/Horizon system is quite nice.

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Re: Chandrayan-2 Mission

Postby Vayutuvan » 19 Sep 2019 22:39

FYI

before and after images from LRO of Beresheet at this Wikipedia page

https://en.m.wikipedia.org/wiki/Beresheet

we will have similar pictures had vikram crash landed. it gives hope that vikram did not crash land, may be.

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Re: Chandrayan-2 Mission

Postby SaiK » 19 Sep 2019 23:38

A report on what went wrong in the soft landing of Chandrayaan-2's Vikram lander on lunar surface will be released soon for the public, S Somanath, director of Isro's Vikram Sarabhai Space Centre (Thiruvananthapuram), said on Thursday.

Speaking about the moon mission, he said they were trying to establish contact with the lander. “We haven’t got any information yet. We have got help from other space agencies like Nasa. They have attempted in getting the images of the lander, we have not yet received them,” he said.

http://timesofindia.indiatimes.com/arti ... aign=cppst

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Re: Chandrayan-2 Mission

Postby ramana » 19 Sep 2019 23:49

Sudarshan thanks for applying mind and not just saying its stable.

From ISRO site: https://www.isro.gov.in/chandrayaan2-spacecraft


The Vikram lander weight is 1471 kg.
And from that graph of the Doppler returns the delta T was fraction of a second.
So can you back calculate?

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Re: Chandrayan-2 Mission

Postby Amber G. » 19 Sep 2019 23:50

Vayutuvan wrote:FYI

before and after images from LRO of Beresheet at this Wikipedia page

https://en.m.wikipedia.org/wiki/Beresheet

we will have similar pictures had vikram crash landed. it gives hope that vikram did not crash land, may be.

The analysis will take time... there will be more passes by LRO and CY2 ... Per NASA this close approach ..

"...long shadows in the area may be obscuring the silent lunar explorer "
“It was near dusk as the region prepares to transition from a two-week lunar day to an equally long lunar night, so shadows covered much of the region, and Vikram may not be in the LROC’s field of view "

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Re: Chandrayan-2 Mission

Postby ramana » 19 Sep 2019 23:52

Saik, From India Today:

https://www.indiatoday.in/science/story ... 2019-09-19

The Chandrayaan-2 orbiter is performing satisfactorily in its orbit around the Moon, the Indian Space Research Organisation said Wednesday as it dropped broad hint that chances of re-establishing contact with the Vikram lander are next to none now.

In a five-point update on the Chandrayaan-2 mission, Isro said that all the payloads that are present on board the orbiter are powered on and that initial trials to test the payloads, which will perform experiments on the Moon, were carried out successfully. "Performance of all orbiter Payloads is satisfactory," Isro also said. "Orbiter continues to perform scheduled science experiments to complete satisfaction."

Speaking about the Vikram lander, Isro only said that "national level committee" is examining why communication with the lander was lost. Interestingly, the space agency chose not to say that all efforts to make contact with the Vikram lander are on, as it has been saying so.

If the new Isro statement is a hint that it has given up on contacting Vikram, there's a good reason for it. Night is about to descend on the region of the Moon where Vikram attempted its soft landing on September 7. (It was in the last moments of this landing attempt that Vikram lost touch with ground control).....

During night, temperatures on the Moon plunge to less than minus 200 degrees Celsius. Vikram, which has no onboard heating equipment, cannot survive such cold. This means that once the south polar region of the Moon where Vikram attempted landing is full covered in darkness, the lander will be frozen out of operation.

Meanwhile, the Chandrayaan-2 orbiter, as Isro said, safely continues to orbit the Moon. The orbiter's mission life has been extended to seven years, during which it will study the lunar surface, the Moon's atmosphere and attempt to estimate the quantity of iced water on Moon.


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Re: Chandrayan-2 Mission

Postby SaiK » 19 Sep 2019 23:58

Chandrayaan 2: NASA fails to locate Vikram lander due to 'long shadows' over landing site
NASA's Lunar Reconnaissance Orbiter camera instrument has sent pictures of the intended Moon touchdown site but shadows in the area could not capture Vikram lander's exact position

https://www.businesstoday.in/current/ec ... 80024.html

I suspected NASA would fail due to poor dusk light

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Re: Chandrayan-2 Mission

Postby sudarshan » 20 Sep 2019 00:10

ramana wrote:Sudarshan thanks for applying mind and not just saying its stable.

From ISRO site: https://www.isro.gov.in/chandrayaan2-spacecraft


The Vikram lander weight is 1471 kg.
And from that graph of the Doppler returns the delta T was fraction of a second.
So can you back calculate?


I can recalculate with 1471 kg (though it will be close to what I already have with 2000 kg).

However, please go back and read the posts from near when that Doppler plot was posted. Delta T wasn't a fraction of a second, more like a minute (the x-scale is UTC as multiple posters pointed out, so the values on the scale are in hh:mm).

So I think there would have been ample time for the lander to tumble 180 deg., even with a 1% imbalance in thrust in one engine.

I'll repost with that 1471 kg calculation, but the other numbers are still just assumptions (the moment arm and the radius for moment of inertia).

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Re: Chandrayan-2 Mission

Postby g.sarkar » 20 Sep 2019 01:58

https://www.livemint.com/science/news/n ... 43490.html
NASA captures images of Chandrayaan-2's Moon landing site: Official
19 Sep 2019
HOUSTON : As the deadline to re-establish communication with Chandrayaan 2's Vikram lander nears, NASA's Moon orbiter has captured images of the lunar region where the Indian mission made an unsuccessful attempt to soft-land, a senior official with the US space agency confirmed on Thursday.
NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft has snapped a series of images during its flyby on September 17 of Vikram's attempted landing sight near the Moon's uncharted south pole.
Grey Hautaluoma, Senior Communications Team Lead at NASA told PTI in an email that "the LRO images are still being processed."
The probability of establishing contact with the lander has a deadline of September 21 because after that the Moon region will enter into a lunar night.
LRO deputy project scientist John Keller shared a statement confirming that the orbiter's camera captured the images, according to a report in cnet.com. "The LROC team will analyse these new images and compare them to previous images to see if the lander is visible (it may be in shadow or outside the imaged area)," Keller was quoted as saying in the statement. NASA is validating, analysing and reviewing the images. It was near lunar dusk when the orbiter passed over, meaning large parts of the area were in shadow, the report said.
......
Gautam

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Re: Chandrayan-2 Mission

Postby ArjunPandit » 20 Sep 2019 02:09

There was data received from chandrayan2 by NASA dsn..1kbps

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Re: Chandrayan-2 Mission

Postby Vayutuvan » 20 Sep 2019 02:18

sudarshan wrote:
ramana wrote:Sudarshan thanks for applying mind and not just saying its stable.

From ISRO site: https://www.isro.gov.in/chandrayaan2-spacecraft


The Vikram lander weight is 1471 kg.
And from that graph of the Doppler returns the delta T was fraction of a second.
So can you back calculate?


I can recalculate with 1471 kg (though it will be close to what I already have with 2000 kg).


1471 KG is the "wet" mass (fuel included). "Dry" mass is 627 KG, as per a post on Quora.

So the mass depends on how much fuel had been consumed at the time of the event.

Moment arm can be estimated from pictures with people in it. For example, the following.

Lander Vikram found in a single piece in a tilted position

Lander from another perspective below.

nasa-lro-orbiter-vikram-landing-site-photographed

Added later: No need to guess the dimensions of the lander. The following wikipedia page gives all the details. Moment arm can be estimated more accurately from the dimensions.

Chandrayaan-2 Vikram Lander Design

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Re: Chandrayan-2 Mission

Postby SaiK » 20 Sep 2019 04:52

Crashing at great speed, Vikram has long been dead

http://timesofindia.indiatimes.com/arti ... 209960.cms

The failure analysis team is inclined to believe that an error in the automatic landing programme (ALP) caused the accident Scientist who saw an image of Vikram after the crash-landing said the lander was either upturned or tilted, but not damaged beyond recognition The official wait for reviving Vikram gets over on Friday, when the lunar day (14 Earth days) ends.

CHENNAI: While the rest of India waited two weeks for Chandrayaan-2 lander Vikram to ‘wake up’, top Isro scientists knew that the lunar craft had died in the crash in the early hours of September 7. The failure analysis team is inclined to believe that an error in the automatic landing programme (ALP) caused the accident.

Isro insiders told TOI that the 1,471kg Vikram, with the 27kg rover Pragyan in its belly, had crashed on the lunar surface at more than 200kmph, and that the onboard systems are unlikely to have survived the impact. Scientist who saw an image of Vikram after the crash-landing said the lander was either upturned or tilted, but not damaged beyond recognition.

“What I saw appeared like a shadow of Vikram,” said a scientist who analysed the image. “It was definitely not on its legs. I could see at least two of its four legs protruding. It was either upturned or tilted.”

Another scientist privy to the failure analysis said Vikram must have spun out of control sometime during the final 10km descent, and lost contact with the mission control when it was about 330m above the lunar surface (not 2.1km as Isro had earlier said). “When it was upside down, the thrusters which were supposed to act as brakes would’ve worked as accelerators and hit the lunar surface at more than 200kmph, maybe faster,” he said.

Another Isro source said even at that altitude, Vikram could’ve escaped without much damage had it been upright and the thrusters brought down the speed to 10m/s (36kmph). “Had it crashed on its legs, the ‘shoes’ would’ve acted as a shock absorber,” he said. “But since there has been no signal from Vikram, the onboard computer and other systems must have been damaged.”

Two scientists said there could have been an error in the landing program written by a team at the UR Rao Satellite Centre, Bengaluru. “Teams are going through the program. We have to see if it was properly reviewed before execution,” said a scientist who did not want to be named. The software program was to guide the various systems for altitude and velocity control throughout Vikram’s 15-minute descent from Moon’s 30km orbit to the surface to ensure a soft landing.

The official wait for reviving Vikram gets over on Friday, when the lunar day (14 Earth days) ends. The lander and the rover were designed to work for these 14 Earth days, after which the systems would anyway have shut down during the long, frigid lunar night when temperature dips to minus 183 degrees Celsius.

Soft-landing has remained a challenge for even pioneers in lunar exploration, including the US, Russia and China, as landers, often asymmetric with shifting centres of gravity, had to be manoeuvred from great speeds to near-zero velocity to touch down on the lunar surface upright.

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Re: Chandrayan-2 Mission

Postby sudarshan » 20 Sep 2019 07:55

Vayutuvan wrote:1471 KG is the "wet" mass (fuel included). "Dry" mass is 627 KG, as per a post on Quora.

...


Well okay, with that info, seems like the moment arm might be 1 m or 1.3 m (since dimensions are 2.58 m X 2.0 m X 1.2 m). I'm assuming the 2.58 m X 2.0 m is the horizontal plane, and 1.2 m is the height. So the max. moment of inertia will be with a cylinder of 0.6 m radius (1.2 m dia).

Seems like the four 500 N thrusters might be angled at 45 deg.? If so, the numbers will be different, I'm assuming that when they fire, they will swivel to point vertically down.

So here goes:

Image

Wet mass: 1471 kg
Dry mass: 637 kg
Median mass: 1054 kg

Now that makes it way worse, head-over-heels tumble within 30 seconds for any case.

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Re: Chandrayan-2 Mission

Postby ramana » 20 Sep 2019 20:23

Sudarshan Thanks. Shows that very little delta Thrust can tumble the lander.
We don't need exact numbers to confirm that. ISRO can and has done that.

With about 30 mins of total time you proved it. Kudos.



As to why it did not recover I think not enough distance to recover. 330m is not sufficient.

I suspect the requirements change in the middle of the schedule due to reviewers was a contributed to the software issue.

I still think it was the throttling fuel valve hydraulic instability at low flow.

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Re: Chandrayan-2 Mission

Postby Amber G. » 20 Sep 2019 21:30

From: LRO deputy project scientist John Keller's statment on cnet

“The LROC team will analyse these new images and compare them to previous images to see if the lander is visible (it may be in shadow or outside the imaged area)..
NASA is validating, analysing and reviewing the images. It was near lunar dusk when the orbiter passed over, meaning large parts of the area were in shadow..."

LRO has really been very busy..DSN and non-DSN stations were receiving lot of data...

I expect that it will take some time before we analyze all the pictures.

Meanwhile, From the recent LRO images: A closeup of the place where ISRO think Vikram has landed.
Image

(70.90278 S, 22.7811 E)..
From
Image

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Re: Chandrayan-2 Mission

Postby Haridas » 20 Sep 2019 21:32

That spreadsheet calculation gives out boundries for an open loop system. There is no evidence of IMU (inertial measurement unit) malfunction (it's any way very very unlikely). So engine thrust mismatching will be easily cancelled out by close loop control.
IMHO the close loop control will fail if :: software has error OR the thrusters have catastrophic failure. I have reason to believe thruster force was measured and not estimated from fuel flow rate.

Control software had many target value optimization loops, and proper time is needed to qualify it.
Also while it has main control software, there must also be a supervisory control software that guards mission safety albeit at less optimal accuracy on landing position & contact speed. The latter failing is indication of poor execution.
Last edited by Haridas on 20 Sep 2019 21:35, edited 1 time in total.

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Re: Chandrayan-2 Mission

Postby UlanBatori » 20 Sep 2019 21:34

Vikram could’ve escaped without much damage had it been upright and the thrusters brought down the speed to 10m/s (36kmph). “Had it crashed on its legs, the ‘shoes’ would’ve acted as a shock absorber,” he said.


Just posting 4 the benefit of the Expert who claimed I must be wacko because I said the landing speed just had to be somewhere less than 10 m/s.
(Accurate conclusion - that I am wacko - but based on wrong data, sorree onlee).
:P

BTW on the photos just posted. They are drawing circles in the wrong places. Look at the semi-cylindrical cavity just below the letters "nates" of coordinates. What would cause that? Also the very narrow conical shadow. What natural phenomenon could cause those? Another promising semi-cylinder under the word "Landing". But the one under "nates" has shadow also from its blocked end.

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Re: Chandrayan-2 Mission

Postby Haridas » 20 Sep 2019 21:44

UB saar, the composite image is by stitching images on multiple passes, Under similar lighting condition. The cylindrical artifact is due to stitching scaling errors from adjoining image patches.

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Re: Chandrayan-2 Mission

Postby ramana » 20 Sep 2019 21:46

Haridas if the control system failed its effectively open loop no?

The big question is why did the lander tumble?
Sudarshan showed it can tumble with very little over performance.
The four+ one thurster configuration has chance of tumble if there is thruster anomaly.

And if you see the location of the reverse thursters and the edge vernier thrusters there is no tumble recovery mechanism.

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Re: Chandrayan-2 Mission

Postby ramana » 20 Sep 2019 21:53

UB, Don't mind lekin this relying on NASA to pull a rabbit of the moon is height of hope.
All telemetry and calculations show there was a crash landing.
So why keep hopes up?
Need to move on.
The emotional moment is over.

Risks are known unknowns and can be planned for mitigated.
We can have a reliability engineer calculate the overall reverse thruster reliability from scratch starting from fluid supply, line pressure losses, throttle valve, thruster etc.
But these type of missions need to think like a Paki mind.
To deal with unknown unknowns or uncertainties.

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Re: Chandrayan-2 Mission

Postby ramana » 20 Sep 2019 21:56

Nalla Baalu wrote:
https://www.indiatoday.in/amp/magazine/ ... ssion=true


Raj Chengappa (appears like he bides his time nowadays to write only on 'epochal' events) alludes to potential trouble during 'Absolute Navigation Phase' that was to last less than a minute to correct any residual errors of rough braking phase. I may have misread, but he says a small angle rotation was part of planned maneuvers to get line of sight for one of the sensors to aid some measurement. But here lander did a somersault and things started going haywire!





...
A little history about Vikram at this point can help one understand why a lunar lander is complicated business and why one out of two such missions ends in failure. India had not planned to make its maiden attempt at a soft landing on the lunar surface on its own. Even before the indigenously-built Chandrayaan 1 orbiter was launched, ISRO had decided that it could use the help and experience of Russia’s Federal Space Agency (Roscosmos) for the Chandrayaan 2 mission and signed an agreement with it in November 2007. For the joint Indo-Russian lunar mission, ISRO would have the prime responsibility for the orbiter; Russia for the lander and rover. The launch was planned for 2012. Though ISRO was ready with the orbiter on schedule, Roscosmos pulled out of the agreement in December 2011. This was after its Phobos-Grunt mission to put a lander and rover on a Martian moon in collaboration with the Chinese space agency failed. ISRO then decided it would build a lander and rover on its own and scheduled a launch for 2016. Meanwhile, the organisation repurposed its orbiter for the Mangalyaan, accomplishing it in record time.
....
Despite ISRO’s vast experience in building launchers and satellites, it soon found designing and developing a lander and rover a complex and uphill task. According to M. Annadurai, a former director of ISRO’s U.R. Rao Satellite Centre and till last year the in-charge of planetary missions, It is one thing to send an orbiter [to the Moon] as we did with Chandrayaan 1, fire an impactor probe to the Moon or send an orbiter to Mars. But to bring down an orbiting spacecraft and make it land softly on the lunar surface is vastly more complex and challenging. The key technologies ISRO needed to master were a flexible propulsion system that would regulate the lander’s descent, and a control system that would guide and navigate the spacecraft to a pre-designated spot on the lunar surface. Both these technologies have been developed in the past five years and are now the prime suspects in the premature termination of Vikram’s mission.

THE LANDING PLAN

After its separation from the Chandrayaan 2 orbiter on September 4, Vikram was orbiting the moon at a speed of 1,680 metres per second (or 6,048 km per hour, six times the speed of a commercial jet) and at a height of 30 km above the lunar surface. That velocity, along with the height, had to be brought down in a controlled manner to almost zero within 13 minutes of the descent phase. The lander would do so using the array of five rocket engines and eight tiny attitude control thrusters fitted on its base, which ISRO had developed for the mission. The engines were designed as throttle-able ones, their thrust varying with the regulating of the fuel flow, just like an accelerator in a car.

The control and guidance system was also developed to meet the complexities of a moon landing. With the distance between the earth and moon being 3.84 lakh km, there is a time lag of more than a second before commands sent from mission control reach the craft, and of another second when data about its implementation is relayed back. As decisions had to be taken in milliseconds during Vikram’s rapid descent to the lunar surface, ISRO developed a fully autonomous guidance and control system that would take care of all the exigencies and anomalies that may arise on the 15-minute flight. The craft was also equipped with highly precise measuring instruments to monitor its velocity, height, attitude, direction and position with relation to the moon’s surface, enabling Vikram’s onboard computer to take decisions in real time. The craft’s control and propulsion systems were also designed keeping in mind that the moon’s gravity is one-sixth of the earth’s. Both these systems were subjected to rigorous tests, simulating conditions corresponding to the moon’s erratic gravity profile.

THE BIG CHANGE

What was also under test was ISRO’s new plan for powered descent that was put in place just two years ago. When designing the lander, ISRO scientists had initially decided to work with only four engines instead of five. In this configuration, the engines and the guidance control system would gradually bring the speed and altitude down to around 10 metres above the moon’s surface. But then the concern arose that the engine thrusters, at this distance, would kick up a mini lunar dust storm that would envelop the craft and damage its vital equipment. ISRO then planned to shut all the four engines and instead strengthen the four legs of the craft to withstand the free fall from that height without damaging either the lander or the rover. A launch was scheduled for January 2018.

Meanwhile, a fierce debate had broken out among space scientists over the dangers of having a four engine-controlled descent for a moon lander. ISRO decided to circumvent the free fall by introducing a fifth engine at the centre of the lander’s base. This would have two advantages. The fifth engine would be fired only after all the other four engines were shut down at 10 metres and ensure a powered descent till touchdown. And since the engine was located at the centre of the craft, the plume of dust it would kick up would be pushed away from it. That decision would add more weight to the spacecraft: along with other changes in the configuration, the composite Chandrayaan 2 with the orbiter, lander and rover would now weigh 3.8 tonnes. This meant that Geosynchronous Satellite Launch Vehicle, or GSLV MarkII was no longer suitable as a launch vehicle, as it was capable of carrying a payload of only 2-3 tonnes. So the Chandrayaan 2 project team had to wait for GSLV MarkIII, ISRO’s heaviest rocket, then under development, to be validated. Rather than wait for the full range of trial flights, ISRO decided to take a risk by launching Chandrayaan 2 on GSLV MarkIII’s first operational flight. As it turned out, after an initial scare, the GSLV MarkIII fired beautifully on July 22, 2019, launching Chandrayaan 2 on its lunar journey.

THE WOBBLE

For Vikram’s descent to the moon, ISRO homed in on a parabolic powered descent trajectory divided into four distinct phases. The process would begin when the spacecraft was at a height of around 30 km above the lunar surface and 650 km away from the landing site. In the first phase, known as the Rough Braking Phase and lasting for 10 minutes 20 seconds, Vikram would use the brute force of its engines to brake its horizontal speed of 1,648 metres per second down to around 150 metres per second. In this phase, it would come down from 30 km to 7.4 km. While detaching from the orbit and independently revolving around the moon, Vikram was ejected with the exhaust funnels of its five engines facing the direction of its revolution instead of on the opposite side. At the beginning of the descent phase, its onboard computers ignited four of the five engines to steadily kill its velocity. To ensure that both the craft’s horizontal and vertical velocities were within parameters, all four engines had to fire with perfect synchronicity. If one of the engines deviated, the computer was pre-programmed to use the other engines to provide differential thrusts to correct the anomaly.

The live telecast by Doordarshan showed scientists clapping at the completion of the Rough Braking Phase, indicating it was successful. But some experts believe that there are indications that errors may have been building up in this phase. For while the horizontal velocity (the speed the craft was moving at) was to be around 150 metres per second at the end of the phase, the readings on the large console in the mission operations complex showed that it was around 200 metres per second, faster than what it should have been. On the other hand, the vertical velocity or the speed with which the lander was descending, hovered between 70 metres and 68 metres per second for several seconds.

It was at this point that the second phase, termed the Absolute Navigation Phase (ANP) and lasting around 40 seconds, kicked in. In this phase, Vikram should have corrected any errors in calculations of the key navigation parameters such as its height and velocity during the Rough Braking Phase. It did this by double-checking the readings of its on-board measuring instruments, including cameras photographing the lunar terrain, to measure Vikram’s velocity and height. Variations in the velocity, altitude or inclination of the spacecraft were to be corrected by the autonomous control systems, which arrive at their own logical decisions on the adjustments that need to be made. As a senior scientists put it, The number of exigencies and errors you can calculate and feed into the computer is only limited by your imagination. The best control systems are the ones where scientists let their imaginations run free and plan for as many contingencies as possible.
{Unknown unknowns or planning for uncertainities}

It was at the brief ANP phase that the anomalies in Vikram’s powered descent began to mount. In the control room, the large console simulating Vikram’s descent showed the lander deviating from its 45-degree inclination. It inexplicably executed a somersault, making the engines face upwards instead of downwards (see graphic: 15 Minutes to Despair). One explanation is that the onboard computer was correcting the spacecraft’s attitude to enable the cameras to position it properly for taking the pictures it needed to calibrate vital para­meters. But that manoeuvre went haywire and resulted in increasing the vertical descent velocity rather than decreasing it. The other explanation is that the control system noticed a drop in the velocity and corrected it even though it was still within the threshold. In doing so, it first erroneously rotated the craft by 140 degrees to boost the velocity, then reversed it to the original position. By then, the spacecraft had lost its orientation and control.


LOSS OF CONTROL

It was at this point that the third phase, the Fine Braking Speed Phase lasting 90 seconds, began. To bring down Vikram’s horizontal and vertical speeds to near-zero and the craft to an altitude of 400 metres, two of the four engines were to shut down. There is evidence to show that the spacecraft was desperately trying to regain its orientation and was pitching from side to side. The console showed that the vertical speed had increased; it was also at this juncture that all communication with the control room snapped. There was no evidence to show that the two engines had shut down as per plan. All the console showed was that the horizontal velocity was still a high 48 metres per second and the vertical velocity 59 metres per second. Both these key parameters should have been considerably lower for the lander to go into its terminal descent phase. Its speed at this point should have been near-zero and it should have been hovering over the lunar surface at a height of 400 metres. Its onboard cameras were then to take pictures for its control system to check whether the landing site was suitable.

ISRO had decided that Vikram would land near the colder South Pole where water molecules were expected to be found in greater abundance. This was the first time a lander was doing so for good reason, as there are more craters on the lunar poles than its equatorial belt. Vikram’s control system, using its instruments including the cameras, was to ensure that the craft would land on a flat surface. If it landed on any surface that had an incline beyond 12 degrees, it would topple over. Vikram was to then descend to 10 metres before its on-board control system would switch off two engines. The fifth engine located at its base would then be switched on for a controlled descent. All this was to happen if everything had gone well in the earlier phases. But, since communication snapped at a height of 2.1 km, there is no evidence so far to show whether the terminal descent phase was activated or not.

According to experts, Vikram’s abrupt end can be attributed to three major reasons, but they do not quite agree which one was the main culprit. 1) Some believe that the propulsion systems malfunctioned during the transition from the Rough Brake Phase to the Absolute Navigation Phase, when the engines were to fire synchronously to reduce the lander’s speed. Since the throttle-able engines were based on a new technology, there is suspicion that one of them could have misfired, causing unstable conditions beyond the system’s tolerance, and confused the command and control system. Others 2) believe the error lay in the control system itself, with an improper logic built in, that made the lander do a complete turn during the transition between the absolute navigation and fine braking phases. Yet 3) another section of opinion argues that it was a combination of errors in both propulsion and control systems that led to the setback. Meanwhile, ISRO scientists are gathering every bit of data the lander transmitted before its signal was lost. They are using such data to simulate all possible scenarios and explain Vikram’s aborted landing.

........

Haridas
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Re: Chandrayan-2 Mission

Postby Haridas » 20 Sep 2019 22:46

ramana wrote:Haridas if the control system failed its effectively open loop no?.
when multi actuator, multi variate, control loop fail it is much worse than open loop control.

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Re: Chandrayan-2 Mission

Postby Vayutuvan » 21 Sep 2019 01:24

UlanBatori wrote: What would cause that?


An almost spherical meteorite hitting almost tangentially?

Free eBook from Lunar and Planetary Institute

This book has a lot of nice black and white pictures as well.

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Re: Chandrayan-2 Mission

Postby Amber G. » 21 Sep 2019 03:03

There is huge amount of data, including publicly available NASA datasets, APIs, visualizations, methods etc.. for moon. For those who are scientifically curious, one place to start may be https://data.nasa.gov/ and https://svs.gsfc.nasa.gov/index.html..

Let me share a visualization which was generated by using NASA’s Moon Trek portal (https://trek.nasa.gov/moon) and data from the Lunar Reconnaissance Orbiter’s (LRO) WAC) and LOLA.
Vikram's location (approx landing site is a relatively smooth plane between the craters Manzinus C and Simpelius N.)

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Re: Chandrayan-2 Mission

Postby Vayutuvan » 21 Sep 2019 04:23

AmberG. ji, nice list of websites there.

I think what is called Ndarts would be useful to simulate the dynamics of the Vikram lander. You need to register and request the opensource program. NASA will review your request and grant you access or deny the same.

Here is the link to NDarts

https://software.nasa.gov/software/NPO-47703-1
Last edited by Vayutuvan on 21 Sep 2019 08:10, edited 1 time in total.

Gagan
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Re: Chandrayan-2 Mission

Postby Gagan » 21 Sep 2019 05:27

Probably OT for this thread, but this is what the NASA Mars missions did for balloon landing on Mars.
Multiple variations of this are possible, and there is no limit to what a space agency is comfortable with.



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Re: Chandrayan-2 Mission

Postby SaiK » 21 Sep 2019 06:20

Amber ji, is that video a simulation of the actual path Vikram took?

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Re: Chandrayan-2 Mission

Postby sudarshan » 21 Sep 2019 06:30

ramana wrote:Sudarshan Thanks. Shows that very little delta Thrust can tumble the lander.
We don't need exact numbers to confirm that. ISRO can and has done that.

With about 30 mins of total time you proved it. Kudos.



As to why it did not recover I think not enough distance to recover. 330m is not sufficient.

I suspect the requirements change in the middle of the schedule due to reviewers was a contributed to the software issue.

I still think it was the throttling fuel valve hydraulic instability at low flow.


Thanks! These things must be proved bindaaz ishtyle, or not at all :).

Now that I think about it, I think that max. moment of inertia calc. isn't quite right for the rotational axis we have in mind, but the ballpark figure should hold. It shouldn't take more than a minute or two for even a 1% excess thrust on one engine to flip the lander over. Time goes as square-root of moment of inertia, so a four-fold increase in I will only increase the time by a factor of 2.

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Re: Chandrayan-2 Mission

Postby UlanBatori » 21 Sep 2019 06:49

Haridas wrote:UB saar, the composite image is by stitching images on multiple passes, Under similar lighting condition. The cylindrical artifact is due to stitching scaling errors from adjoining image patches.

Hmm... that explains the sharp conical sectors with different colors. But the cylindrical cavities (there are 2 of them) are very curious, I have not seen anything like that before.
Why don't the craters look elongated I wonder, if there is stitching error. My take is that it is one of the ramps down to Godzillaji's Kave Kamplex. Probably chomping on the Pragyan's wheels as snacks.

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Re: Chandrayan-2 Mission

Postby Vayutuvan » 21 Sep 2019 08:14

ubji, you are watching too many Japanese movies from MST3K. :twisted:
good night sans Godzilla nightmares.

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Re: Chandrayan-2 Mission

Postby SSSalvi » 21 Sep 2019 10:07

UB
It is a crater of which left and right portions have been cut during stitching

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Re: Chandrayan-2 Mission

Postby sudarshan » 21 Sep 2019 20:41

Now we have lunar night. Goodbye Vikram, nice knowing you :(.

kit
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Re: Chandrayan-2 Mission

Postby kit » 21 Sep 2019 22:14

Wonder whether there will be a C 3 mission !! .. hopefully with a GSLV IV powering it all upto lunar orbit

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Re: Chandrayan-2 Mission

Postby arvin » 21 Sep 2019 22:55

There is no gslv 4. L110 on mk3 will be replaced by semi cryo sce 200.
IMO if the lander goes in for a redesign, it should be optimized for pslv xl fairing, so that moon access can be done at much cheaper and regular intervals.

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Re: Chandrayan-2 Mission

Postby somdev » 22 Sep 2019 01:48

Two scientists said there could have been an error in the landing program written by a team at the UR Rao Satellite Centre, Bengaluru. “Teams are going through the program. We have to see if it was properly reviewed before execution,” said a scientist who did not want to be named.


It is hard to believe this was due to software error. AFAIK, ISRO uses Ada/Leon3 and the Ravenscar profile in Ada providing a subset of the tasking facilities is specifically meant for high integrity mission critical systems.

ISRO's T Mookiah Sir (Director, ISRO Propulsion Complex) is a legend in this area!

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Re: Chandrayan-2 Mission

Postby ramana » 22 Sep 2019 02:43

Thanks for that input.
Means this is not a simple fault.

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Re: Chandrayan-2 Mission

Postby UlanBatori » 22 Sep 2019 04:30

Skip the Moon. It is about as interesting as Wichita Kansas minus the thieves, or Gobi desert minus the LeEducation Centels. Go directly for Venus. Low-ballistic lander feasible. The sulfuric acid clouds will make Venus-Skram feel right at home, like Bengaluru. :mrgreen:

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Re: Chandrayan-2 Mission

Postby UlanBatori » 22 Sep 2019 07:00

BTW, one ISROwallah claims to have seen "at least 2 legs" sticking out of the V-Kram as it reposed on the Moon. Why is ISRO not releasing any such images? Any comments so far? Maybe that Pagal guy and the other one (in The Hundi) were not so far off the mark, demanding answers?

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Re: Chandrayan-2 Mission

Postby juvva » 22 Sep 2019 07:15

UlanBatori wrote:BTW, one ISROwallah claims to have seen "at least 2 legs" sticking out of the V-Kram as it reposed on the Moon. Why is ISRO not releasing any such images? Any comments so far? Maybe that Pagal guy and the other one (in The Hundi) were not so far off the mark, demanding answers?


Maybe this will help:
https://www.isro.gov.in/right-to-information


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