Chandrayan-2 Mission

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sudarshan
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Re: Chandrayan-2 Mission

Postby sudarshan » 31 Jul 2019 03:15

disha wrote:Simple - orbital velocity formula is v = SQRT(G*M/R)., where R is radius of orbit, which will be avg. radius of moon + height above moon

Plugging in the numbers, for G, M and R for 10 Km at moon orbit, I think it will be around 70 Km/sec. Raising it to 100 km will be @22 Km/Sec. (I might have eaten a zero here or there :mrgreen: so please do not quote me or correct me on the numbers calculated :( ...)


Well the numbers are really off, though the formula is right. It's not just a question of eating zeros either.

The 70 km/s number you quote is greater than the escape velocity of Jupiter (yes, of Rahul Gandhi fame - for Jupiter, escape velocity is around 60 km/s). So if you have a body moving around or away from the moon at the speed of 70 km/s, it is never going to come back to the moon, much less orbit it. 22 km/s is greater than the escape velocity of the earth (11.2 km/s).

Please check - for a 10 km orbit around the moon, I get like 1.75 km/s. This is using the same formula as above. For 100 km, it is only slightly less, around 1.7 km/s.

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Re: Chandrayan-2 Mission

Postby SSSalvi » 31 Jul 2019 11:37

^^^
M=7.35*10^22
GM=49000
R=1738+10
V= 5.3 km/sec

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Re: Chandrayan-2 Mission

Postby sudarshan » 31 Jul 2019 13:19

SSSalvi wrote:^^^
M=7.35*10^22
GM=49000
R=1738+10
V= 5.3 km/sec


This is going OT I think, so this is my last post on this.

What's going on, madrasa math?

Here's the universal gravitational constant, G: https://en.wikipedia.org/wiki/Gravitational_constant

The value is: 6.6734e-11 in SI units.

So how in the world did GM, with M=7.35e22 kg, become 49000 :-? What units are these in? Certainly not SI.

Here's the escape velocity of the moon: https://en.wikipedia.org/wiki/Escape_velocity

Scroll down that wiki link, you will find escape velocity on the moon being 2.38 km/s. So your number of 5.3 km/s is more than twice this :-? Meaning if you go at this speed, you are going to fly off the moon and never come back. Certainly not orbit the moon at any height.

My radius was a little wrong, but even with the value which you have (which is right, thanks for the correction), I get 1.674 km/s at a 10 km height, and 1.633 km/s at 100 km height above the moon.

Again, my last post on this.

EDIT: (Ah I see - your value of G is off by a factor of 100 million, and you forgot to convert the radius from km to metres).

FURTHER EDIT (Don't wan't to prolong this OT discussion, so I'm editing my previous post instead of doing a new post):

SSSalvi wrote:^^^
^^^
My Apologies.. Sudarshan , for the post.


SSSalvi, didn't mean to be harsh, just that two of my pet peeves got touched off:

1. Reporting physical constants or properties of objects as numbers is meaningless, unless units are included - for ex., is the reported mass in grams, kg, pounds... is the reported radius in metres, km, miles... etc. Especially with some number like G (universal gravitational constant) units are critical. In your post, I strongly suspect that mismatched units are what messed up the final result.
2. When possible, always perform a sanity check after any calculation. In this case, one calculation reported an orbital velocity for the moon greater than the escape velocity of Jupiter(!), and another one reported 2.5 times the escape velocity of the moon! These escape velocity numbers can be readily looked up, and serve as sanity checks - they would have instantly shown that the reported orbital velocity numbers could not be true.

Sorry if I sounded like I was ranting.
Last edited by sudarshan on 01 Aug 2019 03:35, edited 4 times in total.

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Re: Chandrayan-2 Mission

Postby Mollick.R » 31 Jul 2019 13:50

A map of various previous Lunar Landing missions.

This screenshot is taken from a Youtube Video of TRT World (A Turkish Media Outlet)

https://imgur.com/WCQNBcY

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Re: Chandrayan-2 Mission

Postby SSSalvi » 31 Jul 2019 17:57

^^^
^^^
My Apologies.. Sudarshan , for the post.

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Re: Chandrayan-2 Mission

Postby disha » 01 Aug 2019 12:42

sudarshan wrote:2. When possible, always perform a sanity check after any calculation. In this case, one calculation reported an orbital velocity for the moon greater than the escape velocity of Jupiter(!), and another one reported 2.5 times the escape velocity of the moon! These escape velocity numbers can be readily looked up, and serve as sanity checks - they would have instantly shown that the reported orbital velocity numbers could not be true.

Sorry if I sounded like I was ranting.


Nah Sudarshan'ji it was not taken as ranting.

Yeah when I plugged in my numbers I myself knew that I am off somewhere. And I mentioned that as much. Always was the case. It took me decades after the exams why I used to fail even though I understood everything, teach my class mates as well, applied the right thing during exams etc and the result will be still way below passing marks :rotfl:

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Re: Chandrayan-2 Mission

Postby Ashokk » 02 Aug 2019 16:15

Chandrayaan2 update: Fourth earth bound maneuver
Fourth earth bound orbit raising maneuver for Chandryaan-2 spacecraft has been performed successfully today (August 2, 2019) at 1527 hrs (IST) as planned, using the onboard propulsion system for a firing duration of 646 seconds. The orbit achieved is 277 x 89472 km.

All spacecraft parameters are normal.

The next orbit raising maneuver is scheduled on August 6, 2019, between 1430 – 1530 hrs (IST).


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Re: Chandrayan-2 Mission

Postby juvva » 02 Aug 2019 16:51

Ashokk wrote:Chandrayaan2 update: Fourth earth bound maneuver
Fourth earth bound orbit raising maneuver for Chandryaan-2 spacecraft has been performed successfully today (August 2, 2019) at 1527 hrs (IST) as planned, using the onboard propulsion system for a firing duration of 646 seconds. The orbit achieved is 277 x 89472 km.

All spacecraft parameters are normal.

The next orbit raising maneuver is scheduled on August 6, 2019, between 1430 – 1530 hrs (IST).




Looks like a shortfall (0.84%) in the apogee : 277 x 89472 ( actual) vs 248 x 90229 ( planned ) - https://www.isro.gov.in/update/24-jul-2 ... spacecraft.

Anyway 2 more earth bound maneuvers (including TLI) to catch up , then we are on our way to the moon.

Don't think ISRO has published any plans / schedules for mid course correction maneuvers.

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Aug 2019 16:52

Only one more earth bound manoeuvres. The one after that is TLI

Image

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Aug 2019 17:46

juvva wrote:


Looks like a shortfall (0.84%) in the apogee : 277 x 89472 ( actual) vs 248 x 90229 ( planned ) - https://www.isro.gov.in/update/24-jul-2 ... spacecraft.

Anyway 2 more earth bound maneuvers (including TLI) to catch up , then we are on our way to the moon.

Don't think ISRO has published any plans / schedules for mid course correction maneuvers.


Nope it is bang on target based on the revised burn schedule which was 89743 Kms apogee which would have been further revised based on previous burn

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Re: Chandrayan-2 Mission

Postby Vivek K » 02 Aug 2019 18:41

Question for the gurus - Are the changes in actual achieved orbits caused by the additional fuel load in the satellite?

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Re: Chandrayan-2 Mission

Postby prasannasimha » 02 Aug 2019 20:05

Vivek K wrote:Question for the gurus - Are the changes in actual achieved orbits caused by the additional fuel load in the satellite?

Each orbit change is calculated based on the mass of fuel and craft before the burn.

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Re: Chandrayan-2 Mission

Postby Vivek K » 02 Aug 2019 22:58

prasannasimha wrote:Each orbit change is calculated based on the mass of fuel and craft before the burn.

Then did ISRO change the orbit details after the ones that were published before the launch of MKIII?

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Re: Chandrayan-2 Mission

Postby prasannasimha » 03 Aug 2019 00:30

Thry gave one additional set adter the launch.
The caveat also mentioned below it is that these are provisional subject to the orbit achieved in every burn. They changed the prohisional time table as the orbit of first burn was so cl9se to the actual injection so first burn time was considerably reduced sacing a lot of fuel. Station keeping consumes far less fuel than orbit change. Inclination change consumes the most

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Re: Chandrayan-2 Mission

Postby Indranil » 03 Aug 2019 00:37

These are not shortfalls. One should understand the objective here. ISRO is trying to increase the apogee of the spacecraft as efficient as possible (given hardware constraints). The goal is not accuracy.

The most efficient way to increase apogee is to give the largest possible kick instantaneously at perigee. But that is theory, an rocket engine has finite thrust. None-the-less, the larger the thrust (actually TWR), the better. ISRO's hardware here is 440 N LAM engine, which they currently have to burn for hundreds of seconds at the perigee to achieve higher orbit. It has smaller 22 N engines on board which will be required when accuracy matters, like orbiting the moon for an year in circular orbit (even with non uniform gravity field around the moon). But using those 22N engines in orbit raising is not efficient.

This is why I wanted to discuss the use of the new 800N engine that ISRO has developed for the lander as a future LAM engine. And electric propulsion for future interplanetary missions. In the latter case, the thrust and TWR is actually very low. EPS wins because its ISP is 10 times better than engines based on hypergolic engines. So, if you had a EPS with 440 N thrust, you would have seen a fuel saving of 10x. But, because the thrust is low, the efficiency of orbit raising is brought down, and the fuel saving is about 3-4 times.

The largest electric propulsive unit that ISRO is currently building/procuring is 300 mN xenon-based unit. It will be used on its I-2 bus (for two-ton class satellites) and I-3 bus (for three-ton class satellites). You can see the details of its tender here. ISRO has also set up a facility to test EPSs up to 1N thrust. GSAT20 is supposed to be first all electric powered satellite. It was supposed to be using the I-3 bus. We will see if that materializes. What we know for sure is that GSAT9 uses a smaller EPS 75mN for station keeping, and it works! That is a validated proof of concept!

These are important related topics. It is a pity that we could not have had a civil discussion over it. I do want to say something here: discussing ISRO's limitations is not whining IMHO. At least, I know where ISRO is working, and what to expect! I can defend ISROs decisions MUCH MUCH better with that information. But that is my opinion!

ISRO is also studying a higher power Magnetoplasmadynamic Electric Propulsion Thruster, but this is more lab bound now. It requires a lot of electric power which solar panels + battery combined cannot provide. That's where you hear the discussion of nuclear powered propulsion. But that is quite far off in future.
Last edited by Indranil on 03 Aug 2019 03:56, edited 2 times in total.
Reason: Struck out unnecessary rant

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Re: Chandrayan-2 Mission

Postby SriKumar » 03 Aug 2019 09:39

A couple of general comments:
1 Agree that discussing ISRO's limitations is not the same as whining or criticising. THe discussion should go beyond (i) ISRO operate on a shoestring budget and have delivered greatly for a poor country, so what's your issue, or (ii) ISRO got no support from superpowers, plus fought sanctions, so its a home-grown product which is a stupendous achievement, let's appreciate what has been achieved, or (iii) this is an insult to the hardworking ISRO engineers..... khabardaar. If the discussion is technically focused and done respectfully, the some learnings accrue (I do recall vina mian dissing Vikas engines, and that went disrespectful after a point).
2. I frankly was not following the ion engine discussion, but I'll say that for Chandrayaan-2 thread, a little un-chandrayaan related discussion will happen as a matter of course, because chandrayaan-2 (much like mangalyaan did) will take weeks to get out of earth orbit. It does one orbit-raising maneuver every 3-4 days. That's a lot of waiting time in between orbital increments for a thread that relies on chandrayaan news for discussion. This waiting time can be filled with some extra discussion tangentially relevant to chandrayaan or space (By contrast PSLV launches are much faster, with satellites parked and settled in orbits, in a couple of days, so less waiting time for space gupshup).

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Re: Chandrayan-2 Mission

Postby juvva » 04 Aug 2019 16:04

Vikram clicked planet Earth:
https://www.isro.gov.in/first-set-of-be ... ram-lander

First set of beautiful images of the Earth captured by Chandrayaan-2 Vikram Lander

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Re: Chandrayan-2 Mission

Postby krishGo » 04 Aug 2019 17:37

Indranil wrote:
This is why I wanted to discuss the use of the new 800N engine that ISRO has developed for the lander as a future LAM engine. And electric propulsion for future interplanetary missions. In the latter case, the thrust and TWR is actually very low. EPS wins because its ISP is 10 times better than engines based on hypergolic engines. So, if you had a EPS with 440 N thrust, you would have seen a fuel saving of 10x. But, because the thrust is low, the efficiency of orbit raising is brought down, and the fuel saving is about 3-4 times.

The largest electric propulsive unit that ISRO is currently building/procuring is 300 mN xenon-based unit. It will be used on its I-2 bus (for two-ton class satellites) and I-3 bus (for three-ton class satellites). You can see the details of its tender here. ISRO has also set up a facility to test EPSs up to 1N thrust. GSAT20 is supposed to be first all electric powered satellite. It was supposed to be using the I-3 bus. We will see if that materializes. What we know for sure is that GSAT9 uses a smaller EPS 75mN for station keeping, and it works! That is a validated proof of concept!

These are important related topics. It is a pity that we could not have had a civil discussion over it. I do want to say something here: discussing ISRO's limitations is not whining IMHO. At least, I know where ISRO is working, and what to expect! I can defend ISROs decisions MUCH MUCH better with that information. But that is my opinion!

ISRO is also studying a higher power Magnetoplasmadynamic Electric Propulsion Thruster, but this is more lab bound now. It requires a lot of electric power which solar panels + battery combined cannot provide. That's where you hear the discussion of nuclear powered propulsion. But that is quite far off in future.


The most efficient way to get to the moon, is the 2 burn Hohmann transfer. This is what the Apollo, LRO, Change etc have done. Us using the mutli burn, orbit raising transfer, is due to combination of constraints like limited launcher capability, experience etc. (more of the former).

An efficient upper stage with restart capability and a good space thug, as a part of a medium-heavy/heavy launch vehicle should give us a lot more flexibility than a 800N LAM. When we have this (mind you we are not so far from that point), I believe ISRO's preferred lunar mission architecture will be the 2-burn Hohmann.

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Re: Chandrayan-2 Mission

Postby Anantha » 04 Aug 2019 21:43

How would the manned mission look like. Would the landing be done in 4 days? The current slingshot model will take 1.5 months to each the moon?

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Re: Chandrayan-2 Mission

Postby prasannasimha » 04 Aug 2019 23:26

No electric propulsion currently available will allow rapid transfer that allows man to get to the moon. EP will not allow a launch from earth - it can be used only for transfer after initial orbital injection and India will be doing one such GTO to GSO transfer in one of the coming flights.

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Re: Chandrayan-2 Mission

Postby disha » 05 Aug 2019 00:36

SriKumar wrote: A couple of general comments: ... (I do recall vina mian dissing Vikas engines, and that went disrespectful after a point).
...


Technical discussions and discussions of short fall is okay. However just like some persons would want to have grown trees in their backyard as of yesterday on constructing a new building and before move in., Sometimes posters fall in defensive depressive mode and start doing rona-dhona when they find that we do not have *all* types of rocket engine covered under our belt. That gets irksome.

Just rehashing some knowledge for general: GSLV was initiated in 1991. It was planned in 80s itself. It takes a while to completely master several technologies at play here. And nothing discussed here comes over night. Not even in a single or couple of years. The timelines are in several years.

Also creating sobriquets to diss one's own successful achievements and putting on public forums is plain stupid. Successes' small or big must be celebrated. And yes failures should also be discussed but with empathy and understanding. There is a thin line between objectively pointing mistakes and subjectively dissing the hardwork of the space scientists on their failures. Real or perceived.

Anantha wrote:How would the manned mission look like. Would the landing be done in 4 days? The current slingshot model will take 1.5 months to each the moon?


For humans to moon, one needs at the minimum the following:

1. A powerful restartable engine at the orbiting stage. Here powerful means power thrust in the order of several hundered Kilo-Newtons to Mega-Newtons.

2. An efficient engine. Preferably of ISPs >300 secs.

As one can see, very few fuel types can provide both. High thrust and High efficiency. Only LOH-LOX can provide very high thrust at very high efficiencies compared to others.

Electric propulsion can be 10x efficient than LOH-LOX, but their thrust capacity is abysmal. That is, 300 mN (current xenon ion thruster with ISRO) to 200 kN of CE-20. That is, one has to cluster some 700000 (or 700k) xenon ion thrusters to get the thrust equivalent of CE-20!

So coming back,

The first burn of the moon stage engine will be to park the moon bound vehicle in lower earth orbit. Here the vehicle has to be checked so that onward journey to moon is stable. If necessary, the mission can be aborted here and chandrayaatris can return back to Earth.

Once the decision to go to moon is taken a second burn to inject the chandrayaan with the yaatris into TLI has to be done. It will be like kicking the vehicle to some 10 Km/Sec to escape the gravity well in such a way that it will fall into Moon's gravity well. Too large a kick and it will escape Earth's orbit and will become an Helio centric object and lost. Too short a kick, the orbit will be raised but not out of Earth bound and exposing the crew to radiation till the vehicle is brought back safely to Earth.

Apollo did a 2-stage burn to each moon (earth orbit and TLI). It took them 3-4 days. It is okay to spend an extra day in space rather than risk losing the entire vehicle in the helio centric orbit. Hence the TLI has to be timed properly and with proper & right thrust.

So by itself a strong and efficient & restartable rocket engine will not help. Appropriate trajectory calculations will take time. That means sending more landers to moon. At least 3 successfully.

But is Apollo way the only way to send Chandrayatris? I do not think so. I have a separate tack on that.

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Re: Chandrayan-2 Mission

Postby ldev » 05 Aug 2019 01:52

krishGo wrote:
The most efficient way to get to the moon, is the 2 burn Hohmann transfer. This is what the Apollo, LRO, Change etc have done. Us using the mutli burn, orbit raising transfer, is due to combination of constraints like limited launcher capability, experience etc. (more of the former).

An efficient upper stage with restart capability and a good space thug, as a part of a medium-heavy/heavy launch vehicle should give us a lot more flexibility than a 800N LAM. When we have this (mind you we are not so far from that point), I believe ISRO's preferred lunar mission architecture will be the 2-burn Hohmann.

The Apollo single burn direct to TLI makes sense for manned space flights because earth-moon transit is reduced to ~3 days. Typically those launchers have a LEO payload capacity of 130-150 tons, historically Saturn V and in the future, the in-development Space Launch System (SLS) launcher, SpaceX with it's BFI and the Chinese Long March 9, all have projected LEO payloads of 130-140 tons which would result in a TLI payload injection of 40-45 tons. Because they have been designed with the potential objective of setting up a space station orbiting the moon and/or establishing a moon base which will require large payloads to TLI.

For unmanned spacecraft where transit time is not of the essence, what ISRO is doing with Chandraayan 2 is perfectly acceptable with an ever increasing apogee in an elliptical orbit using it's LAM motors just past the perigee thereby maximizing TLI payload with available resources.

Added later:
For sending a manned mission to the moon ISRO will need a TLI payload injection capability of at least 18-20 tons i.e. the planned manned earth orbit mission space capsule weighs 7.8 tons. This weight will be at least doubled if not more with a lunar orbiter, a lander with engines to take off from the moon's surface for the astronaut's return. This 18-20 ton TLI payload will have to achieve a delta v of 3.2-3.4 km/s from LEO to achieve escape velocity. That will mean a rocket stage (re-startable in orbit) with fuel to achieve that delta v. So back of the envelope calculations means a gross LEO payload capacity of 50-60 tons (lunar lander + lunar orbiter + re-startable rocket stage in orbit + fuel for achieving escape velocity) That will mean a whole new generation of launchers for ISRO. GSLV Mk 3 is good enough for LEO manned missions and the Chandraayan 2 kind of missions to the moon but not more than that.

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Re: Chandrayan-2 Mission

Postby sanjaykumar » 05 Aug 2019 11:35

Why not use a modular approach and boost assets separately into orbit? Assemble by docking finally orbiting astronauts in a module that can orbit the moon and also return to earth.

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Re: Chandrayan-2 Mission

Postby juvva » 05 Aug 2019 13:28

If we do decide to go to the moon (HSF), the architecture should be sustainable over a long period ( not a flash in the pan, like the Apollo program.)

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Re: Chandrayan-2 Mission

Postby prasannasimha » 05 Aug 2019 14:08

sanjaykumar wrote:Why not use a modular approach and boost assets separately into orbit? Assemble by docking finally orbiting astronauts in a module that can orbit the moon and also return to earth.

That is Indias plan 2 rockets with earth rendezvous for docking TLI and lunar orviting and descent

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Re: Chandrayan-2 Mission

Postby ldev » 05 Aug 2019 19:09

sanjaykumar wrote:Why not use a modular approach and boost assets separately into orbit? Assemble by docking finally orbiting astronauts in a module that can orbit the moon and also return to earth.


Yes it is possible to do it. That is just what SpaceX is planning to do with it's Starship concept. To do that ISRO will have to master re-fueling in space, assembling/docking components launched separately and as a back-up astronauts being able to space walk to trouble shoot any potential docking malfunctions. But regardless the present LEO payload capacity of 10 tons for GSLV Mk 3 is still very low given that a realistic moon bound space craft including the re-startable engine and fuel will weigh 50-60 tons, so will need 5-6 launches plus most importantly, mastering re-fueling in orbit.

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Re: Chandrayan-2 Mission

Postby ldev » 05 Aug 2019 19:15

juvva wrote:If we do decide to go to the moon (HSF), the architecture should be sustainable over a long period ( not a flash in the pan, like the Apollo program.)


Correct. That is why IMO the US and China are designing their launchers with the capability for a 40-45 ton payload to TLI. That will allow sending components for setting up a lunar base as well as eventually commercial exploitation of resources such as helium. SpaceX with earth orbit re-fueling of it's Starship concept is planning on a 100 ton TLI payload.

It should not be that ISRO discovers water and helium deposits in certain locations on the moon and then other countries say, " Thank you ISRO for doing the scouting work for us. Now we will take over thanks to you pointing out the locations of these deposits."

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Re: Chandrayan-2 Mission

Postby prasannasimha » 05 Aug 2019 22:47

As far as EP propulsion goes let it be clear - you cannot beat physics and momentum = mass X velocity and current "regular" fuels have an ISP of max approx 450. However they have one great advantage they provide enough thrust in short period of time to allow us to escape earth. No ion engine will work in the lower atmosphere by its very nature and so that is out of question. Once launched if time is not a constraint we can use electric thrusters that ahve a high ISP but need to eb fired for thousands of hours.
There have been only very few probes that have used EP propulsion since the 1960's (See SERT1) and the latest has been Hayabusa
Why are we using the slingshot and a slower approach - simply because we do not have that powerful a rocket to inject our payload to the target and so we used earth's gravity to nudge our ship faster and faster. Its not something deprecated but used intelligently.Even the grand tour of planets by Voyager and many other interplanetary probes have used it
Unfortunately the longer we keep a human in a spacecraft we have to deal with their metabolic needs etc and will not have the luxury to keep them in the craft.Also repeated passage through the Van Allen radiation belts will also be an issue (You should see the recordings of the passage of Apollo aircraft through the belts)
So mission design is based on existing technology at hand with available constraints

As far as our engines go - let us just say that SC200 program is going forward and not static as some people claimed.

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Re: Chandrayan-2 Mission

Postby Sonugn » 06 Aug 2019 16:04

Chandrayaan2 update: Fifth earth bound maneuver

Fifth earth bound orbit raising maneuver for Chandryaan-2 spacecraft has been performed successfully today (August 6, 2019) at 1504 hrs (IST) as planned, using the onboard propulsion system for a firing duration of 1041 seconds. The orbit achieved is 276 x 142975 km.

All spacecraft parameters are normal.

The next maneuver is Trans Lunar Insertion (TLI), which is scheduled on August 14, 2019, between 0300 – 0400 hrs (IST).
https://www.isro.gov.in/update/06-aug-2019/chandrayaan2-update-fifth-earth-bound-maneuver

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Re: Chandrayan-2 Mission

Postby prasannasimha » 06 Aug 2019 18:54

Image

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Re: Chandrayan-2 Mission

Postby ramana » 07 Aug 2019 03:23

Sudharshan No need to rant on S^3.
We may a little bit more in one area but that does not mean we go off the handle.
S^3 has done a lot more than most of us for India.

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Re: Chandrayan-2 Mission

Postby sudarshan » 07 Aug 2019 07:27

ramana wrote:Sudharshan No need to rant on S^3.
We may a little bit more in one area but that does not mean we go off the handle.
S^3 has done a lot more than most of us for India.


I already said I was sorry, but unqualified apologies once again to SSSalvi.

I was genuinely puzzled about the units on G, and how, when the value of G seemed so far off from its SI units value, the answer could still be in the right ballpark. I think your value was off by just a factor of 10 (if GM=4900 instead of 49000, the whole thing would have worked out in km units instead of metres, and the answer would have been right!) That's why I was asking about the units (could have worded it more diplomatically though).

Anyways, I'll be more careful with my choice of words in future.

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Re: Chandrayan-2 Mission

Postby SSSalvi » 07 Aug 2019 12:42

^^^
Ramana ji .. I am not offended.
It was really a wrong post ( actually a careless computation to attempt at finding numbers without proper care to convert all the units to similar system of units) .. scientifically the numbers have no meaning unless accompanied by units.

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Re: Chandrayan-2 Mission

Postby Indranil » 07 Aug 2019 22:37

Salvi sahab, my salutations to you. It always heartens me to see learned people who can own up to a mistake. Inspirational. Good to have you here!

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Re: Chandrayan-2 Mission

Postby krishGo » 09 Aug 2019 02:59

ldev wrote:
krishGo wrote:
The most efficient way to get to the moon, is the 2 burn Hohmann transfer. This is what the Apollo, LRO, Change etc have done. Us using the mutli burn, orbit raising transfer, is due to combination of constraints like limited launcher capability, experience etc. (more of the former).

An efficient upper stage with restart capability and a good space thug, as a part of a medium-heavy/heavy launch vehicle should give us a lot more flexibility than a 800N LAM. When we have this (mind you we are not so far from that point), I believe ISRO's preferred lunar mission architecture will be the 2-burn Hohmann.

The Apollo single burn direct to TLI makes sense for manned space flights because earth-moon transit is reduced to ~3 days. Typically those launchers have a LEO payload capacity of 130-150 tons, historically Saturn V and in the future, the in-development Space Launch System (SLS) launcher, SpaceX with it's BFI and the Chinese Long March 9, all have projected LEO payloads of 130-140 tons which would result in a TLI payload injection of 40-45 tons. Because they have been designed with the potential objective of setting up a space station orbiting the moon and/or establishing a moon base which will require large payloads to TLI.

For unmanned spacecraft where transit time is not of the essence, what ISRO is doing with Chandraayan 2 is perfectly acceptable with an ever increasing apogee in an elliptical orbit using it's LAM motors just past the perigee thereby maximizing TLI payload with available resources.



Well, it is not just manned missions that use the 2 burn Hohmann. If we check even all the robotic missions to the moon, the 2 burn Hohmann should be the most prevelant architecture, followed by the mutli burn transfer & low energy transfers. The launchers that put these missions on lunar transfer weren't super heavy rockets like Saturn V rather rockets like Atlas V, H-2, Long March 3, all medium heavy / heavy launchers.

Yes, multi burn Hohmann gives you extra payload capacity, but this usually is not as big a concern if you have medium heavy/heavy launchers. With smaller/medium launchers, obviously there is a greater return of investment on payload capacity to be had by using the multi burn Hohmann.

ldev wrote:Added later:
For sending a manned mission to the moon ISRO will need a TLI payload injection capability of at least 18-20 tons i.e. the planned manned earth orbit mission space capsule weighs 7.8 tons. This weight will be at least doubled if not more with a lunar orbiter, a lander with engines to take off from the moon's surface for the astronaut's return. This 18-20 ton TLI payload will have to achieve a delta v of 3.2-3.4 km/s from LEO to achieve escape velocity. That will mean a rocket stage (re-startable in orbit) with fuel to achieve that delta v. So back of the envelope calculations means a gross LEO payload capacity of 50-60 tons (lunar lander + lunar orbiter + re-startable rocket stage in orbit + fuel for achieving escape velocity) That will mean a whole new generation of launchers for ISRO. GSLV Mk 3 is good enough for LEO manned missions and the Chandraayan 2 kind of missions to the moon but not more than that.



Image

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Re: Chandrayan-2 Mission

Postby kit » 09 Aug 2019 04:50

https://www.thehindu.com/sci-tech/technology/isro-too-has-designs-on-a-60-tonne-super-lift-rocket/article22724826.ece


ISRO, too, has envisioned developing super-heavy launchers that can one day lift loads as heavy as 50 to 60 tonnes. The technology is complex. Many supporting technologies leading to it, such as the semi-cryogenic fuel-based propulsion and reusable systems, are being explored under ongoing research, he told The Hindu recently.

“A [super] heavy lift vehicle of the future is on the drawing board as part of our R&D. We are doing a lot of preliminary research leading to it. Right now, we are developing a semi-cryogenic engine, which was approved some time back. Next, we must propose [for funding approval] a full semi-cryogenic stage. A lot of work is ahead of us, in this,” Dr. Sivan said.

There is no clear time frame or cost estimate for it, but if approved, it would be a single, innovative, two-stage vehicle, which would cater to all demands, he said. SpaceX’s FH, first heard about in 2005, is reported to have cost $ 500 million.

Such a heavy duty vehicle should be developed future-ready should the government green-flag a human space flight proposal, or it would cater to other large payloads in the coming years. A human space launch would have a roomy, safe, and habitable capsule for astronauts.

The present plan is to try out the semi-cryo stage at the core of MkIII in place of its liquid fuel engine, without changing the design. This would raise its muscle power 50% — from four to six tonnes. Gradually it would be enhanced to 10 tonnes and eventually 50 to 60 tonnes, by developing a modular super vehicle with many additions or strap-ons built around the main structure. FH used a similar tactic, according to Dr. Sivan, who oversaw preliminary semi-cryo related works while he was ISRO’s propulsion head until recently.

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Re: Chandrayan-2 Mission

Postby ldev » 09 Aug 2019 09:21

krishGo wrote:Well, it is not just manned missions that use the 2 burn Hohmann. If we check even all the robotic missions to the moon, the 2 burn Hohmann should be the most prevelant architecture, followed by the mutli burn transfer & low energy transfers. The launchers that put these missions on lunar transfer weren't super heavy rockets like Saturn V rather rockets like Atlas V, H-2, Long March 3, all medium heavy / heavy launchers.

Yes, multi burn Hohmann gives you extra payload capacity, but this usually is not as big a concern if you have medium heavy/heavy launchers. With smaller/medium launchers, obviously there is a greater return of investment on payload capacity to be had by using the multi burn Hohmann.


IMO different space agencies use the best possible tools that they have at their disposal e.g. China used the Long March 3b which can be optioned with a re-startable 4th stage. The combination of the re-startable 4th stage plus the fact that for the Chang'e 4 mission they are using a satellite launched earlier and parked at Lagrange point L2 for the lander-earth communication from the lunar surface which means that their actual spacecraft consisted of little more than the lander with a mass of 1200 kgs. I have no idea how they slowed down the spacecraft to enter lunar orbit. Did the 4th stage stay on till the moon insertion was achieved (and it was a circular orbit around the moon) or did the lander have a small LAM motor with just enough fuel to slow down the spacecraft to enter lunar orbit? In comparision ISRO did not have a light/small re-startable stage and hence more than 60% of the Chandraayan 2s launch weight was fuel for the LAM motors to increase the apogee in successive burns plus Chandraayan 2 is using an orbiter to relay communications to earth ground stations from the lander on the lunar surface.

And talking of the Long March 3b, with a launch pad mass of 425 tons it can put a payload of 11.5 tons in LEO. In comparison GSLV Mk 3 has a launch pad mass of 640 tons and has a payload capacity to LEO of 10 tons. Also, if you see the specific impulse of their various stages, they are in fact marginally less than ISRO's SC200 boosters and the L110 first stage. Is lighting up that L110 stage 100 + seconds after lift-off and then keeping the SC200 boosters on for some time, after they are burnt out and before they are separated the cause for this penalty? Will have to run some numbers on the equation to figure that out. I remember vaguely Vina used to talk about something related to this? By the way, where is he?

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Re: Chandrayan-2 Mission

Postby SSSalvi » 10 Aug 2019 18:58

Some Facts related to Chandrayaan 2 orbit/orbit manuevers
May be useful for any enthusiast. ( Compiled from various sources for my own personal use )



Table of Orbit Raising Manuevers (ORM ) while the Spacecraft is in Earth’s Gravitational sphere of Influence.

ORM No. Date Time ( IST ) Firing Duration ( Sec ) Achived Orbit (Perigee Kms x ApogeeKms )
1 July 24 1452 57 230 X 45163
2 July 26 0108 883 251 x 54829
3 July 29 1512 989 276 x 71792
4 August 2 1527 646 277 x 89472
5 August 6 1504 1041 276 x 142975



Some Facts related to Earth- Moon Relations.

Moon Orbit in July to Sept 2019 :
Closest Dist : 357176 Kms on 30th Aug
Farthest Dist : 406377 Kms on 13th Sept



Earth-Moon distance on 14th Aug 2019 : 402256Kms to 404101 Kms, (Say =403000 Kms for ease )



Mass of Moon : 7.35E+22 Kgs
Mass of Earth : 5.97E+24 Kgs
Ratio of Masses : Earth is about 81 times heavier than Moon ( 81.22 actual )

So the Neutral Point ( Where the Gravity of Earth and Moon becomes equal and so Zero pull in either direction ) is about ( Square Root of 81 = )9:1 ratio of Earth-Moon distance or about 362700 Kms from Earth on 14th Aug 2019 ( using 403000 Kms average distance )

Therefore if you release the spacecraft within that distance from Earth , then it will be pulled towards Moon on its own.

So you give a kick when the spacecraft is just before the Apogee so that after the Apogee it will continue further away from Earth (instead of returning towards Earth) and reach this barrier of Neutral point and gets into Lunar gravity.

But in Case of Chandarayaan2 it is not so simple because it has not just to be pulled into Moon’s gravity but to achieve the designated orbit maneuvers pertaining to:
1.Trans Lunar Injection
2.Lunar Transfer Trajectory and
3.Lunar Orbit Insertion
Last edited by SSSalvi on 11 Aug 2019 07:27, edited 1 time in total.

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Re: Chandrayan-2 Mission

Postby SSSalvi » 10 Aug 2019 20:26

Just enjoy!.. No scientific value.

We are used to see the 2 dimensional images of Chandrayaan 2 orbits .. e.g.

Image


But that is just half truth.
When you include the motions of various objects ( as shown below ) in a timeframe ( and project in 3 dimensions ) then we see breathtaking 'springs' as orbits.
In figure below we are showing two orbits - but also added the movement of Earth around Sun.( Yellow line )
Blue Dots represent two positions along the Earth's orbit and the spacecraft is orbiting around Earth and as the Earth moves forward the spacecraft also moves along the Earth around Sun and so both the movements combine to give a spring-like path.

The red dots are at fixed time interval. But they appear more spread around the perigee ( When moving nearest to Earth ) the speed is high so the dots are spread apart.
Image

Next we see Chandrayaan 2 from Launch to 10th August.

Notice the increasing apogee after each ORM.
( Note: In the image the orbit plane has been tilted to show the details so it appears to be in North-South direction . In reality actually the orbit is about 21 deg inclined to Equator - to match with the Ecliptic/Moon orbit plane.

On left is a overall view and on right is a detailed view. ( Drawn upto 10th Aug )

Image

After one more orbit the Chandrayaan 2 and Moon orbits will come closer ( and run almost parallel for some time ) that time on 14th Aug the TLI ( mentioned in earlier post ) will take place so that the CH2 will be pulled towards Moon.
Last edited by SSSalvi on 11 Aug 2019 18:23, edited 6 times in total.

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Re: Chandrayan-2 Mission

Postby ldev » 10 Aug 2019 21:36

SSSalvi wrote:Some Facts related to Chandrayaan 2 orbit/orbit manuevers
May be useful for any enthusiast. ( Compiled from various sources for my own personal use )
...............

1.Trans Lunar Injection
2.Lunar Transfer Trajectory and
3.Lunar Orbit Insertion


Thanks, much appreciated. You are a treasure trove of information!!


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