Chandrayaan-2 Update: Lunar Orbit Insertion | Hacker News Reader

Chandrayaan-2 Update: Lunar Orbit Insertion

isro.gov.in

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Here's a nice orbit simulation for Chandrayaan-2 mission: https://sankara.net/chandrayaan2.html

This is mind-blowing.

It's a really nice tool, I had several ah-hah moments!

Does anyone know what the teleportation glitch is right at Lunar capture?

>> Does anyone know what the teleportation glitch is right at Lunar capture?

Not sure! Author of this tool is here: https://forum.nasaspaceflight.com/index.php?topic=20324.msg1...

He might visit HN if notices traffic from here :)

n-body problems are hard, 2-body problems are easy (can by analytically solved)

One pretty accurate way of approximating orbits is the patched conic method where you break your n-body problem into multiple 2-body problems picking the most influential body for specific parts of a journey.

Interplanetary journeys will have three parts ex. earth-craft, sun-craft, and mars-craft.

The glitch might be some noise switching between two different patches.

> One pretty accurate way of approximating orbits is the patched conic method where you break your n-body problem into multiple 2-body problems picking the most influential body for specific parts of a journey.

So, KSP's "Sphere of Influence" ?

Yup, that's the term used in engineering as well.

They should use KSP:RSS to simulate their orbit lol. There won't be any glitch then.

Author of the animation here - glad you liked the tool.

I use data published by JPL/NASA HORIZONS interface which provides orbit data for all natural bodies and spacecraft. The "glitch" is because of the data I fetched and used - it has nothing to do with the animation per se.

As of now, the JPL site provides data only until 20th August - the time when I fetched the data a few days back, the data was available until 16th September. This was a "merge" of 3 orbit segments - the Earth bound orbits, the transfer orbit and lunar bound orbits. The former two fit the observed data. The latter was entirely based on predictions. This merged orbit has a discontinuity where the predictions start and that's why there seems to be a glitch.

Once revised data is available from JPL/HORIZONS, I presume the glitch will go away.

Why did they raise the orbit above the Earth so many times instead of just burning for the moon all at once?

Thrusters not strong enough to move the orbit in one go, they wait for it to along around to thrust again.

Presumably exploiting the Oberth effect as it goes round

https://en.wikipedia.org/wiki/Oberth_effect

The multiple loops are not for gravity assist (or gravitational slingshot [0]) as I had initially thought. It is for the Oberth Effect [1], which I only got to know of today.

From the Wikipedia article -

>> Oberth maneuver, is a maneuver in which a spacecraft falls into a gravitational well, and then accelerates when its fall reaches maximum speed. The resulting maneuver is a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well.

>> In some cases, it is even worth spending fuel on slowing the spacecraft into a gravity well to take advantage of the efficiencies of the Oberth effect.

Fascinating! To say the least!

[0]: https://en.wikipedia.org/wiki/Gravity_assist

[1]: https://en.wikipedia.org/wiki/Oberth_effect

The great thing is that it works in Kerbal Space Program, so you can plan out missions that use it in the game.

Author of the tool here - thank you for referring to the simulation here. Glad you liked it.

I love the fact the chandrayaan uses earths gravity for most of its journey. This is simply amazing idea of saving energy and realying alot on maths. That's incredible achivement over thousands of kms.

Not to be a thorn in your excitement but gravity assist is a standard technique for more than 40 years in space exploration. Literally every spacecraft uses it in some way or the other.

Right, didn't Beresheet just do basically exactly this orbital insertion pattern? And we have the Parker Probe which is shutting back and forth between the Sun and Venus to do gravity boosts in an even more dramatic way. Heck, both Voyagers did back in 1977 although those were very simple boosts compared to what we can do today.

Not to be a thorn in your excitement but as an expert in orbital mechanics (I have a certificate from University of Kerbin) there is absolutely no assist anywhere in this manouver. The pattern is just raising apogee of the orbit around Earth by performing prograde burn close to perigee.

The only reason this could have any mechanical advantage is due to Oberth effect if the engine was low thrust type. With low thrust it takes long time to perform the burn and so part of the burn would be performed away from perigee which makes it less efficient. Splitting it would allow them to overall perform it closer to perigee.

Other than that it has no advantage over straight burn (ie. directly from LEO to Moon using Hohmann transfer), mechanically, but it could have other advantages. Making last burn shorter makes it likely more precise than single long burn. It also probably allowed them to perform some burns to test the craft before the most important one. There might be other reasons like testing communications equipment which might be difficult in LEO but they might not want to perform it before committing to insertion.

I will get downvoted for this but man that website is pre 1980s

Many Indian government websites are old and some of them don't even have a proper HTTPS certificate.

> The orbit achieved is 114 km x 18072 km.

Are there plans to circularize this orbit, or will the lander be the only part that enters an orbit that isn’t highly elliptical?

From the linked article:

"Following this, a series of orbit maneuvers will be performed on Chandrayaan-2 spacecraft to enable it to enter its final orbit passing over the lunar poles at a distance of about 100 km from the Moon’s surface.

Subsequently, the lander will separate from the Orbiter and enters into a 100 km X 30 km orbit around the Moon. Then, it will perform a series of complex braking maneuvers to soft land in the South polar region of the Moon on September 7, 2019."

This doesn't answer my question of whether the orbiter will remain in its highly elliptical orbit or not.

It should have.

A 100 km X 30 km looks like elliptical orbit to me.

That's not really all that elliptical, especially compared to the eccentricity that the original orbit has. Also note that that figure was for the lander, not the orbiter.

EVERY orbit is elliptical.

100km X 30km are ELEVATIONS, meaning the real orbit is diameter of Moon + 100km X diameter of Moon + 30km.

If you had this scaled and drawn on a sheet of you would need very accurate instrument to detect any deviation from perfect circle.

> real orbit is diameter of Moon + 100km X diameter of Moon + 30km.

3,574km X 3,504km

That helped me a bit.

So maybe not so precise instrument, but still, if this was 15cm ellipse the difference would be about 1mm.

Yes, that makes sense.

100km x 30km orbit around moon, what was I thinking - facepalm (－‸ლ)

“On entering Moon's sphere of influence, on-board thrusters will slow down the spacecraft for Lunar Capture. Subsequently the orbit of Chandrayaan-2 around the moon will be circularised to a 100x100 km orbit through a series of orbital maneuvers.”

Thanks!

The series of 4 planned orbital maneuvers are outlined here https://www.isro.gov.in/update/14-aug-2019/chandrayaan-2-suc...

You have to imagine the eccentric orbit, 114 km perigee on the far side and a long wait going out to 18k km, toward and away from Earth, looping past the moon's poles.

Over time the eccentric orbit will swing away from pointing at Earth. They will do burns at perigee and the eccentricity will gradually contract until the orbit is about circular. After a week it has Earth in view full- time, a good time to do the final burns; after 2 weeks it has swung around to going behind the moon again, still going over the poles. They will land during the first period when the orbiter is in view full-time.

Congratulations to everyone from India. This has to be a proud moment.

Actually I was wrong about the initial and later direction of the orbit axis. It started with the orbiter swinging way to the left, as seen from earth (axis pointing at us), and is rotating now to be behind the moon, most of the time, come early next week, and in sight when it needs to do burns to circularize its orbit. By the time it lands, the orbit should have swung further and be coming up close to its axis pointing not too far from us, again. It should be in line of sight most of the time, by then.

Go go go go go!!!!!

To the moon and back! :-)

Any country that can build (or buy) rockets can do this now. So, why even bother?

So which country do you think can do it and still isn't doing it?

How many countries do you think are capable of doing it?

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