Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by thunderbong on Wednesday August 21, 2019 @12:08AM
(#20754492)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by kvsankar_ on Wednesday August 21, 2019 @12:42AM
(#20754618)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by colechristensen on Tuesday August 20, 2019 @03:33PM
(#20751042)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by outworlder on Tuesday August 20, 2019 @04:32PM
(#20751760)
> 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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by aravindgp on Tuesday August 20, 2019 @12:38PM
(#20748943)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by spectramax on Tuesday August 20, 2019 @12:55PM
(#20749145)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by lmilcin on Tuesday August 20, 2019 @02:29PM
(#20750327)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by bdamm on Tuesday August 20, 2019 @01:29PM
(#20749565)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by ubertakter on Tuesday August 20, 2019 @12:30PM
(#20748833)
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."
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by olliej on Tuesday August 20, 2019 @12:55PM
(#20749149)
“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.”
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by ncmncm on Tuesday August 20, 2019 @04:17PM
(#20751605)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by ncmncm on Friday August 23, 2019 @12:06AM
(#20774442)
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.
Re:Chandrayaan-2 Update: Lunar Orbit Insertion
by saagarjha on Tuesday August 20, 2019 @01:05PM
(#20749279)
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.
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
Does anyone know what the teleportation glitch is right at Lunar capture?
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.
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 :)
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.
So, KSP's "Sphere of Influence" ?
https://en.wikipedia.org/wiki/Oberth_effect
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.
Are there plans to circularize this orbit, or will the lander be the only part that enters an orbit that isn’t highly elliptical?
"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."
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.
A 100 km X 30 km looks like elliptical orbit to me.
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.
3,574km X 3,504km
That helped me a bit.
100km x 30km orbit around moon, what was I thinking - facepalm (－‸ლ)
How many countries do you think are capable of doing it?