Chandrayan-2 Mission

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Re: Chandrayan-2 Mission

Postby SaiK » 01 Sep 2019 04:55

How It’s Done:

On Monday, after the integrated spacecraft is stabilised in an orbit ideal for separation— which is estimated to be 121km X
125km— Isro will load the sequence of commands for the separation, which will then be autonomously executed by the
onboard systems.

The cylindrical structure on top of the orbiter— the extension of the fuel tank— is where the lander with the rover inside it is sitting. Both these modules are held together by clamps with two bolts. A scientist from the Isro Telemetry, Tracking and Command Network (Istrac) centre in Bengaluru, which is controlling Chandrayaan-2, said: “Two halves of a metal flat spring are used to hold orbiter and lander. The springs are pre-loaded using two bolts. These bolts will be sheared and once these two bolts are cut the lander gets released.”

Another scientist, while stating that this method is simple, reliable and has extensive flight history, said that the systems are released by operation of a pyrotechnic bolt cutter to cut the bolts that hold the clamp.

“This system has high strength and stiffness when clamped and releases quickly—typically in less than 50 milliseconds when command is given. Both of these operational characteristics are considered very important,” the scientist said.

Once the separation is successful, India will only be less than a week away from Moon landing, which is scheduled for 1.55am on September 7. And, a successful landing will make it only the fourth country after the US, Russia and China to have achieved such a feat.

https://timesofindia.indiatimes.com/ind ... 929834.cms


Is shearing the only way to do this?

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Re: Chandrayan-2 Mission

Postby RonyKJ » 01 Sep 2019 06:52

There are many other ways to separate two attached objects but none are as simple and reliable as cutting a bolt.
The bolt cutting method not only severs the connection but also provides a push in the same action due to the fact
that the holding clamps are springs. All that is needed is to turn a switch to pass a current to initiate the pryo bolts.
Other methods such as mechanical actuators have more complexity.

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Re: Chandrayan-2 Mission

Postby sanjaykumar » 01 Sep 2019 06:55

Any selfies of the orbitor/ lander? Or video of the separation?

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Re: Chandrayan-2 Mission

Postby RonyKJ » 01 Sep 2019 06:55

Forgot to mention that the bolt cutting method is also almost instantaneous, which is also a requirement considering the
precision required to achieve the pinpoint landing at a specific time.

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Re: Chandrayan-2 Mission

Postby SSridhar » 01 Sep 2019 07:38

This 1994 ISRO Magazine discusses in detail the various separation mechanisms employed. Satellites are separated using the same technique of lander separation in CY-2.

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Re: Chandrayan-2 Mission

Postby ramana » 01 Sep 2019 09:03

Amber G. wrote:
S^3 In such planetary travel/orbits 1km off is important no?

Perigee/apogee distance and minor variation here is not important. What's important is the orientation (inclination of the orbit) and phase (= timing -- where CY2 is at one particular point in the orbit relative to ground) so that final Vikarm orbit (30km x 100 km) is within the parameters of it heading towards the landing point. This is, actual timing issue as moon is spinning so your timing has to be exactly right so that the particular point on landing on the spinning moon is reached.

Actually final adjustment in the timing is done by adjusting (lowering or elevating) the orbit. If you are "behind" the spot you are supposed to, you actually "slow down" (less delta-V) ==> lower the orbit (thus increasing the angular velocity relative to moon) to "catch up". Pretty complicated and some times counter intuitive. When ISRO says "parameters normal" == much more than values of perigee/apogee.

One analogy I like to give: Hitting a wicket with pin-point accuracy is impressive but imagine a wicket where the cylinder shaped wicket is spinning fast and there is a small dot on the wicket. And one hits that fast spinning dot with pin-point accuracy. (And that too when dot is facing one particular direction -aka morning at a point on moon :!:)
-- Landing a rover on a preselected point is *many* times more difficult than putting a space-craft in a random orbit.


Tripurantaka moment.

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Re: Chandrayan-2 Mission

Postby ramana » 01 Sep 2019 09:05

RonyKJ wrote:Forgot to mention that the bolt cutting method is also almost instantaneous, which is also a requirement considering the
precision required to achieve the pinpoint landing at a specific time.


Is the cut bolt head retained in CY2 or else it will add a delta V?

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Re: Chandrayan-2 Mission

Postby SwamyG » 01 Sep 2019 10:26

RonyKJ wrote:Forgot to mention that the bolt cutting method is also almost instantaneous, which is also a requirement considering the
precision required to achieve the pinpoint landing at a specific time.

I kept wondering how are the clamps/bolts taken off. It is mind boggling that the bolts are sheared. 50 milliseconds it takes. Wow, is an understatement.
Last edited by SwamyG on 01 Sep 2019 10:33, edited 1 time in total.

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Re: Chandrayan-2 Mission

Postby SwamyG » 01 Sep 2019 10:27

sanjaykumar wrote:Any selfies of the orbitor/ lander? Or video of the separation?

Separation on Tuesday, no?

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Re: Chandrayan-2 Mission

Postby SSSalvi » 01 Sep 2019 11:45

^^
Of late, ISRO has started placing ( and also releasing the captured videos to public viewing ) cameras at inter-stage junctions so we surely can expect separation video ( mostly after sometime after the separation ..... they may not - and rightly so - want to publish certain intermediate steps )

====

Pyro separation method is used by ISRO in almost all object separation joints including inter-stage separation on rocket launcher itself.
It is used to unfurl the folded antennas also and also to open the heat-shield surrounding the payload after atmospheric resistance has reduced.

=====
Small deviations from desired orbit variations do not make any difference for overall system goal.

Afterall the impulse or next maneuver is computed just prior to execution based on real time situation.
So even if there is a small deviation it will be accommodated in next impulse direction/duration.

Especially in this mission it is of no big consequence because after separation there is a autonomous landing sequencer will takeover and reduce to speed and navigate in stages so that it softlands in a place where the land gradient is less than 12 deg to avoid toppling.
And these few minutes will be a nail-biting silence period for the operators.

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Re: Chandrayan-2 Mission

Postby SwamyG » 01 Sep 2019 16:28

^^ Dr. Sivan calls them "15mns of terror".

Do other countries use this type of shearing?

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Re: Chandrayan-2 Mission

Postby SwamyG » 01 Sep 2019 17:19

After separation why wait for Sep 7th? Is not the orbital position to commence the soft landing not achieved earlier than 7th? My layman's brain thinks the lander carrying the rover would complete more than on revolution in 4 earth days....

I know I am missing something very basic.

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Re: Chandrayan-2 Mission

Postby SriKumar » 01 Sep 2019 17:49

SwamyG wrote:^^ Dr. Sivan calls them "15mns of terror".

Do other countries use this type of shearing?
Routinely. Here is some old video released by NASA after shuttle program ended, showing bolts exploding to release the shuttle. All rockets are held in place before launch until the required minimum thrust is generated by the rockets.

https://youtu.be/iNmbRjS8-y4

15 minutes if terror sounds like a variation on the '8 minutes of terror' for the Mars landing by US craft, bandied about in the US press. A tad too dramatic.

Added later:
I said 'all rockets', but not sure about Russian rockets which have a different design approach, but physics is physics everywhere. A rocket will start to move upward if the thrust generated is even 1 Newton (or milliNewton) more than the weight of rocket. This is a miniscule thrust and can lead to launch issues if the rocket starts to move (and the thrust from each rocket nozzle can easily differ this amount, leading to alignment issues at liftoff). So, as I understand it, the bolts are blown up after a minimum pre-determined thrust well above the weight of the rocket, is achieved, and all nozzles are functional. Maybe Russian rockets are held by other means than explosive bolts, I dont know.
Last edited by SriKumar on 01 Sep 2019 20:46, edited 1 time in total.

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Re: Chandrayan-2 Mission

Postby juvva » 01 Sep 2019 19:13

https://www.isro.gov.in/chandrayaan2-latest-updates

September 01, 2019

Fifth Lunar Orbit Maneuver

The final and fifth Lunar bound orbit maneuver for Chandrayaan-2 spacecraft was performed successfully today (September 01, 2019) beginning at 1821 hrs IST as planned, using the onboard propulsion system. The duration of the maneuver was 52 seconds. The orbit achieved is 119 km x 127 km.

All spacecraft parameters are normal.

The next operation is the separation of Vikram Lander from Chandrayaan-2 Orbiter, which is scheduled on September 02, 2019, between 1245 – 1345 hrs (IST). Following this, there will be two deorbit maneuvers of Vikram Lander to prepare for its landing in the south polar region of the moon.

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Re: Chandrayan-2 Mission

Postby SSridhar » 01 Sep 2019 19:15

SwamyG wrote:After separation why wait for Sep 7th? Is not the orbital position to commence the soft landing not achieved earlier than 7th? My layman's brain thinks the lander carrying the rover would complete more than on revolution in 4 earth days....

I know I am missing something very basic.

We want one full lunar day and hence the delay.

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Re: Chandrayan-2 Mission

Postby juvva » 01 Sep 2019 19:19

^
119 km x 127 km -achieved
vs
114 km x 128 km -planned

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Re: Chandrayan-2 Mission

Postby juvva » 01 Sep 2019 19:26

SSridhar wrote:
SwamyG wrote:After separation why wait for Sep 7th? Is not the orbital position to commence the soft landing not achieved earlier than 7th? My layman's brain thinks the lander carrying the rover would complete more than on revolution in 4 earth days....

I know I am missing something very basic.

We want one full lunar day and hence the delay.


Plus, they will have ample time for activating and checking out the various lander systems.

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Re: Chandrayan-2 Mission

Postby SSridhar » 01 Sep 2019 20:57

SSridhar wrote:This 1994 ISRO Magazine discusses in detail the various separation mechanisms employed. Satellites are separated using the same technique of lander separation in CY-2.

“It will all be very very quick, the process is similar to how a satellite separates from the launch vehicle,” Isro chairman K Sivan told TOI.

Link

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Re: Chandrayan-2 Mission

Postby SaiK » 01 Sep 2019 22:16

Image
The final and fifth Lunar bound orbit maneuver for Chandrayaan-2 spacecraft was performed successfully today (September 01, 2019) at 1821 hrs IST.

For details please visit https://bit.ly/2Uk5gjX

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Sep 2019 00:05

SwamyG wrote:After separation why wait for Sep 7th? Is not the orbital position to commence the soft landing not achieved earlier than 7th? My layman's brain thinks the lander carrying the rover would complete more than on revolution in 4 earth days....

I know I am missing something very basic.

This has been discussed extensively before. The important thing is that the Orbiter cannot just land on the moon at any place and at any time-It has a defined point of Landing and more importantly a defined date of landing as it must be on the first day of the" Lunar day" that lasts nearly 14 Earth days. This is required as the rover and lander are both solar powered and will not tolerate the intesne cold of Lunar night where it drops down to -173 Deg C or less. WE do not have an RTG heating system .
Incidentally as Dr Sivan mentioned they will try to wake up the Rover the enxt Lunar day(They have conducted some experiments on the hardware where there may be a chance but it is not a part of the actual mission profile) but we have lesss than one Lunar day to do all the science we can with the Rover and Lander.

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Re: Chandrayan-2 Mission

Postby Vivek K » 02 Sep 2019 01:42

One layman question -
Why didn’t CY2 carry the heater? Weight? Space? Or tech barrier?

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Re: Chandrayan-2 Mission

Postby Mort Walker » 02 Sep 2019 02:47

Vivek K wrote:One layman question -
Why didn’t CY2 carry the heater? Weight? Space? Or tech barrier?


The radioisotope thermoelectric generator was never designed for CY2. Typical source would be Pu238. Why wasn’t it used I don’t know, perhaps the implantation would change the design and make the GSLV-III payload heavier. Or cost would go up beyond what was sanctioned in the budget.

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Re: Chandrayan-2 Mission

Postby Amber G. » 02 Sep 2019 03:53

Mort Walker wrote:
Vivek K wrote:One layman question -
Why didn’t CY2 carry the heater? Weight? Space? Or tech barrier?


The radioisotope thermoelectric generator was never designed for CY2. Typical source would be Pu238. Why wasn’t it used I don’t know, perhaps the implantation would change the design and make the GSLV-III payload heavier. Or cost would go up beyond what was sanctioned in the budget.

From what I know, the main reason was existing restrictions/policy/GOI rules about RTG/RHU (Radioisotopic generator or heating units). ISRO did not decide to plan for RHU because they did not think that they will get clearance in the short time they were shooting for. (Weight etc was not the factor it is mostly that people don't like "nuclear" anything being sent into space. - politically it is quite difficult).. Better to design with what they can use. (
I hope there is new thinking there, and they use Pu238 as RHU unit for future space probes.

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Re: Chandrayan-2 Mission

Postby Mort Walker » 02 Sep 2019 04:31

A Pu238 thermal electric generator would need considerable shielding or an extended arm, but that would depend on the mass used. I don’t see why a small amount for 200-300 watts couldn’t have been used.

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Re: Chandrayan-2 Mission

Postby Vivek K » 02 Sep 2019 05:17

I mean isn’t it kind of a let down - if the rover lands as planned but is only effective for 14 earth days? Of course something is better than nothing but ISRO should have sent the rover better equipped.

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Re: Chandrayan-2 Mission

Postby Mort Walker » 02 Sep 2019 05:59

I think the first goal is to successfully land near the lunar South Pole. That in-itself is a huge goal. Less than half of the attempts to land on the moon have been successful.

I’ll be ready for a lungi dance on 7 September should all go well.

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 07:23

Thanks all for answering the question on shearing bolts, and why landing in 7th. I missed that a lunar day is longer than a. Earth day.

I see that after separation there are two more activities of deorbiting. How is deorbiting done and is the end result that the craft comes off the orbit?

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 07:26

Mort Walker wrote:I think the first goal is to successfully land near the lunar South Pole. That in-itself is a huge goal. Less than half of the attempts to land on the moon have been successful.

I’ll be ready for a lungi dance on 7 September should all go well.

I am doing a lungi dance after separation itself, when Vikram/Prayaag are on their own.

Soft landing at the polar site calls for group lungi dance.

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 07:37

Simha saar, thanks for the wonderful explanation. I know the part about precise landing site and time. That is why I asked about if that position is not achieved earlier. My mistake was that I thought the craft combo would have revolved around the Moon more than once in the 3-4 earth days. After all the Moon is so smaller :-))). My bad.

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Re: Chandrayan-2 Mission

Postby Amber G. » 02 Sep 2019 08:20

Mort Walker wrote:A Pu238 thermal electric generator would need considerable shielding or an extended arm, but that would depend on the mass used. I don’t see why a small amount for 200-300 watts couldn’t have been used.

It is less of technical problem, more is political/bureaucratic IMO.
As I said before, from what I know: (Disclaimer: This is NOT official/main newspaper sources but from what I know from people I know)
- There was supposed to test for Chandrayaan -2 and there were rumors (a few years ago) that they might use these for CY2, but AFAIK the idea was dropped for CY2.
- DAE has requisite facilities to prepare Pu-238 for (specifically for ISRO to use). ISRO had not official requested though.
- To produce Pu-238 is quite complicated. (India may have to depend some on other nations for some parts - I don't know). ESA for example went for Am-241 because of low supply of Pu. India may/could also use Th-228/Th-332.
Most of odd numbers (eg U235, Pu239 etc are used for bombs/reactors, even number isotopes are good for RTG type applications but are some what difficult to produce.

Bottom line, if India has political ok ISRO can get RTG etc .. and can integrate it with our power system.

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Re: Chandrayan-2 Mission

Postby Mort Walker » 02 Sep 2019 08:40

Amber G. wrote:
Mort Walker wrote:A Pu238 thermal electric generator would need considerable shielding or an extended arm, but that would depend on the mass used. I don’t see why a small amount for 200-300 watts couldn’t have been used.

It is less of technical problem, more is political/bureaucratic IMO.
As I said before, from what I know: (Disclaimer: This is NOT official/main newspaper sources but from what I know from people I know)
- There was supposed to test for Chandrayaan -2 and there were rumors (a few years ago) that they might use these for CY2, but AFAIK the idea was dropped for CY2.
- DAE has requisite facilities to prepare Pu-238 for (specifically for ISRO to use). ISRO had not official requested though.
- To produce Pu-238 is quite complicated. (India may have to depend some on other nations for some parts - I don't know). ESA for example went for Am-241 because of low supply of Pu. India may/could also use Th-228/Th-332.
Most of odd numbers (eg U235, Pu239 etc are used for bombs/reactors, even number isotopes are good for RTG type applications but are some what difficult to produce.

Bottom line, if India has political ok ISRO can get RTG etc .. and can integrate it with our power system.


IIRC, you're right about the ESA switching Americium-241. According to Wikipedia:
As 241Am has a roughly similar half-life to 238Pu (432.2 years vs. 87 years), it has been proposed as an active isotope of radioisotope thermoelectric generators, for use in spacecraft. Even though americium-241 produces less heat and electricity than plutonium-238 (the power yield is 114.7 mW/g for 241Am vs. 390 mW/g for 238Pu) and its radiation poses a greater threat to humans owing to gamma and neutron emission, it has advantages for long duration missions with its significantly longer half-life. The European Space Agency is working on RTGs based on americium-241 for its space probes as a result of the global shortage of plutonium-238 and easy access to americium-241 in Europe from nuclear waste reprocessing.

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Re: Chandrayan-2 Mission

Postby Amber G. » 02 Sep 2019 08:41

Mort Walker wrote:A Pu238 thermal electric generator would need considerable shielding or an extended arm, but that would depend on the mass used. I don’t see why a small amount for 200-300 watts couldn’t have been used.

OT - But Pu238 does not really require any "shielding" (apart from protecting it's hot surface).. To touch it, it will feel warm/hot so one will need oven mitten but since there is no gamma radiation you don't need lead shielding or such. :). Human skin is good enough to block all harmful radiation. (Alpha rays can not pass through human skin or a thin paper :))
(After all they have pacemakers powered by Pu238).

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Re: Chandrayan-2 Mission

Postby Mort Walker » 02 Sep 2019 08:45

^^^Yes, I remember playing with some isotopes as an undergraduate that were primarily alpha emitters, But I didn't know that Pu238 had no gammas. I would still think any RTG would create noise in radio transmit-receive, as many digital transmissions are near thermal noise levels at a given bandwidth.

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 10:13

Amber G, ISRO is known for what it has achieved. So are the isotopes the only way to get power? I understand the power source has to be light in weight. So I am not wondering why we not have a giant battery pack.

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Re: Chandrayan-2 Mission

Postby navneeet » 02 Sep 2019 13:51

September 2, 2019

Chandrayaan-2 update: Vikram Lander successfully separates from Orbiter
The Vikram Lander successfully separated from Chandrayaan-2 Orbiter at 1315 Hrs IST today (September02, 2019). The Vikram Lander is currently located in an orbit of 119 km x 127 km. The Chandrayaan-2 Orbiter continues to orbit the Moon in its existing orbit. The health of the Orbiter and Lander is being monitored from the Mission Operations Complex (MOX) at ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru with support from Indian Deep Space Network (IDSN) antennas at Bylalu, near Bengaluru. All the systems of Chandrayaan-2 Orbiter and Lander are healthy. The next maneuver is scheduled tomorrow (September 03, 2019) between 0845-0945 hrs IST

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 14:31

Kudos....separation complete. Dr. Sivan says it went "perfect".

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Re: Chandrayan-2 Mission

Postby Ashokk » 02 Sep 2019 15:10

What happens next -
A separate entity now, the lander will de-boost with the firing of its four breaking engines.

This manoeuvre will bring it to a periapsis of around 18 km. (Periapsis is the point in the path of an orbiting body at which it is nearest to the body that it orbits.)

When the lander reaches this point, that is, at the height of 18 km, the onboard position-detection camera and hazard-avoidance sensor will study the landing site for accuracy.

Using the data obtained, the lander will autonomously determine the trajectory it will have to take to get to its pre-determined landing site and steer itself to a location 100 metres above the site.

Here, the lander will hover and allow the hazard-avoidance sensor to determine the safest landing point. The lander will then be guided to this point and hover at the height of 2 metres above the location.

At this point the thrust will be cut off and the lander will go into a free fall to the impact point with the landing legs attached to it absorbing the impact shock.

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Sep 2019 15:13

Amber G. wrote:
Mort Walker wrote:
The radioisotope thermoelectric generator was never designed for CY2. Typical source would be Pu238. Why wasn’t it used I don’t know, perhaps the implantation would change the design and make the GSLV-III payload heavier. Or cost would go up beyond what was sanctioned in the budget.

From what I know, the main reason was existing restrictions/policy/GOI rules about RTG/RHU (Radioisotopic generator or heating units). ISRO did not decide to plan for RHU because they did not think that they will get clearance in the short time they were shooting for. (Weight etc was not the factor it is mostly that people don't like "nuclear" anything being sent into space. - politically it is quite difficult).. Better to design with what they can use. (
I hope there is new thinking there, and they use Pu238 as RHU unit for future space probes.

Currently Pu238 is only being produced in Russia (In fact US had to get it from Russia for an RTG some time back) Strontium, Polonium and Americium have also been used. I think one iof the recent RTG's sent had Americium.
The original plan was to ahve a Rover from USSR but after Phobos Grunt mishap Russia backed out so we did not plan one with an RTG.

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Sep 2019 15:20

SwamyG wrote:Amber G, ISRO is known for what it has achieved. So are the isotopes the only way to get power? I understand the power source has to be light in weight. So I am not wondering why we not have a giant battery pack.


Battery has to be recharged and where and how will the recharging happen = from Solar panels and again this will prove to be a problem as the thermal amplitude is so wide that they aren not sure if the electronics will resist these fluctuations. They have done some tests and think it may work but can only say if Pragyaan rewakes.

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Re: Chandrayan-2 Mission

Postby SwamyG » 02 Sep 2019 15:32

So geo politics really weighed (pun intended) more than scientific considerations.

Anyways...next is the deorbiting test tomorrow where engines fired for 3 seconds? This is what an expert said on Times Now.


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