Author Topic: LIVE: Chang'e-3 lunar probe and rover, CZ-3B - Xichang - December 1, 2013  (Read 309277 times)

Offline savuporo

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Actually i rechecked the video, you'll see that the ramp surface shown is "perforated" which presumably is designed so that wheels can engage with these like gears.

What i'm wondering is if they can open that ramp to two sides, or one only. Mechanically it would be just slightly more complicated to design it - probably dont even need an extra servo.

If it opens to one side only, they must trust their descent camera and autonomous hazard avoidance a lot - and if that works that would be a pretty big engineering achievement on its own.
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Offline Garrett

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...how do you assure that the rover drives only out onto the ramp and does not fall off the end?
Seriously? One wonders if they were that incompetent how'd they mangage to launch a rocket in the first place.

You seem to have assumed that I'm not asking questions because I want to know the answers and that I must be either stupid or assuming that the Chinese are dumb. I assure you that is not the case.

I note that you didn't provide any answers.

Note that getting a rover off a lander is a non-trivial piece of engineering, as JPL has demonstrated over two decades.
Yes, I presumed that you were assuming the Chinese are dumb. I found it difficult to see why you asked such a question (i.e. how to ensure rover does not drive off the ramp end).

Answers are numerous. Savuporo gave an example or two. Others include, but not limited to:
 -  having wheels that are controlled independantly (which is the case I believe), so if one wheel acts up, the others should prevent crazy movement
 - brakes on the wheels
 - locking pins on the rails
 - safety sensors that cut power to wheels
 - lots of pre-testing with single or multiple faults being present

I've never said that it was trivial, but rather my opinion was that your question was out of place. Based on this discussion, my question is:
- Assuming that all preventive measures fail, and the rover rolls over the edge of the ramp, I wonder if they've tested to see if it can survive such a fall? If yes, then that could be a back-up solution if ever the ramp lowering mechanism fails.
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Offline Garrett

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Actually i rechecked the video, you'll see that the ramp surface shown is "perforated" which presumably is designed so that wheels can engage with these like gears.

What i'm wondering is if they can open that ramp to two sides, or one only. Mechanically it would be just slightly more complicated to design it - probably dont even need an extra servo.

If it opens to one side only, they must trust their descent camera and autonomous hazard avoidance a lot - and if that works that would be a pretty big engineering achievement on its own.
To keep it simple, my guess is that it can only open to one side. The landing site is probably chosen, among other factors, so as to have a low probability of hazards for the off-ramp. That will require a bit of luck, but a very calculated luck.
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Offline Mike_1179

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The landing site is probably chosen, among other factors, so as to have a low probability of hazards for the off-ramp. That will require a bit of luck, but a very calculated luck.

But that would limit your landing sites to places where the landing ellipse does not include boulders over some given size at some given concentration.  This landing ellipse also has to take into account what range of lunar latitudes the trajectory would allow and what locations on the lunar surface allow for acceptable thermal and communication  requirements.

All of these impact how you meet your science goals.  If the rover is meant to sample rocks, it's not a very good mission if the only locations that meet these requirements are essentially rock-free.  What are the science goals of the mission (it's more than just "get something on the moon and then celebrate")

Therefore, it's not a dumb or insulting question to ask how the rover drives off the ramp and not off the sides or end if the lander is tilted or near another rock.  While it may seem like a flippant question, it's not - it's a question on how the design of the craft flows down from the mission goals and mission constraints (which themselves are driven by the design and goals).

Offline savuporo

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The mission doesnt have any specific rock related science goals, see the spaceflight101 link up in the thread for detailed overview of science instruments carried.
The overall mission goals seem much more engineering and tech development rather than science objectives.
Im pretty sure their mission scientists sstated that their autonomous landing hazard avoidance system is designed to steer clear of any potential problems like craters or rocks.
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Offline Danderman

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This all begs the question as to whether a digital surface map of the lunar surface exists. If not, the lander would use its radar to determine altitude and a "safe" landing stop.

If the digital map exists, then the lander radar would be used to compare real time data against a digital target landing area.

If the digital map does not exist, it should.

Offline savuporo

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The "digital map" exists but is not in high enough resolution to spot boulders of relevant size, thats why chang'e-3 is designed to enter a very low orbit and take up to date imagery of the landing site, plus all the way down the landing camera will look at the surface and run onboard hazard avoidance algo

Edit: Emily Lakdawalla just posted this recent update from LRO imaging the landing site. Thats your "digital map"
http://lroc.sese.asu.edu/news/index.php?/archives/828-A-Great-Place-to-Rove!.html
« Last Edit: 12/02/2013 06:41 PM by savuporo »
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Offline Blackstar

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Yes, I presumed that you were assuming the Chinese are dumb.

Well, you were certainly wrong there, weren't you?

But here's the kind of derivative questions that I would also ask:

-what is the maximum tilt angle that the lander can accept and still successfully deploy the rover? For instance, if the rover is tilted ten degrees down by one side, could that create a problem?

-is there any way to compensate for tilt? For instance, can the ramps be moved side to side to correct for tilt?

-what if there is a large rock blocking lowering of the ramp? Is there any way to compensate?

Note that JPL has struggled with the issue of rover deployment on Mars, and that was for landers that are much closer to the ground. Pathfinder and the MERs used dual ramps on either side of the lander, allowing the vehicles to drive off in either direction. Curiosity faced this problem in its early design studies and JPL considered things like a "crush lander" (with a type of crushable material underneath the lander). The Skycrane grew out of these and other considerations.

Offline Blackstar

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Some people may be interested in how the United States could respond to the Chang'e-3 mission. Here are some possibilities.

http://thespacereview.com/article/2413/1

Red Moon, Blue Moon
by Dwayne Day
Monday, December 2, 2013

Yesterday China launched Chang’e-3 on its way to the Moon, with landing scheduled for December 14. If it succeeds, it will be the first spacecraft to make a soft landing on the Moon in nearly four decades. Although the lander and rover have a modest scientific instrument suite, they are headed for a previously unexplored region of the Moon and will therefore return new and undoubtedly interesting data. Chang’e-3 will not be alone. NASA currently has two spacecraft—Lunar Reconnaissance Orbiter and LADEE—circling the Moon. But although NASA also has several other possible lunar lander missions that it could start building within the next decade, it is unlikely that a NASA spacecraft will join the Chinese on the lunar surface for many years to come.

NASA divides its planetary missions into three classes: flagship (large), New Frontiers (medium), and Discovery (small). The agency uses the planetary science decadal survey to select science goals for the first two categories. But NASA can also fly robotic missions in support of its human space exploration goals, and those missions are chosen by agency officials based upon their perceived requirements. In the past decade, the agency has flown lunar science missions to support the goals of President Bush’s Vision for Space Exploration, specifically the goal of sending humans to the Moon. Although the Vision for Space Exploration no longer guides NASA policy, some aspects of that previous policy continue to influence current NASA studies and plans.

Offline savuporo

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-is there any way to compensate for tilt? For instance, can the ramps be moved side to side to correct for tilt?
Pure guess but they probably drive the lowering mechanism with a regular servo, so i would think they can extend it to 100%, but also to 80% and 120% deployment positions

Quote
-what if there is a large rock blocking lowering of the ramp? Is there any way to compensate? Note that JPL has struggled with the issue of rover deployment on Mars,
I think they have placed the bet on the landing camera working well enough to avoid rocks. Consider that they can effectively have a human in the loop in the landing sequence, confirming or making the final selection between potential spots - that only requires a few more seconds of hovering time.
This is a luxury that Mars rovers do not have, nor did the last lunar landers four decades ago.

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Offline sdsds

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Consider that they can effectively have a human in the loop in the landing sequence, confirming or making the final selection between potential spots - that only requires a few more seconds of hovering time.

That's a fascinating suggestion! Have there been indications they might attempt this?
-- sdsds --

Offline savuporo

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Consider that they can effectively have a human in the loop in the landing sequence, confirming or making the final selection between potential spots - that only requires a few more seconds of hovering time.

That's a fascinating suggestion! Have there been indications they might attempt this?
No but if i was developing the landing sequence this seems like the logical way to structure it, and i dont see why they would do it any other way.

Your autonomous software identifies the primary and backup positions and orientations for landing, and there is a short time window where signal from ground can override the primary selection. If the corresponding command is not received for any reason, the software will proceed with the default sequence.

EDIT: for what its worth, Chinese seem to have learned a lot from Russian teleoperated lunar probes ( apparently even their RHU tech heritage goes back to Lunokhods ) , and if i recall correctly Russians had many human in the loop triggers for their "automated" sequences back then.

« Last Edit: 12/02/2013 08:23 PM by savuporo »
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Offline Garrett

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Consider that they can effectively have a human in the loop in the landing sequence, confirming or making the final selection between potential spots - that only requires a few more seconds of hovering time.

That's a fascinating suggestion! Have there been indications they might attempt this?
The few articles I've read suggested that the entire sequence is fully automated. Of course, it is probable that those articles do not have all the detailed info.
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Offline Garrett

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Yes, I presumed that you were assuming the Chinese are dumb.
Well, you were certainly wrong there, weren't you?
As Prober pointed out, your use of the term "dubious" as an adjective for the rover ramp was quite patronizing. I can only go on the words you actually type, not on what you truly deep down were thinking while typing.
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Offline savuporo

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The few articles I've read suggested that the entire sequence is fully automated..
It would have to be fully automated, as you cant rely on radio signal working at the critical moment, however having a fully automated sequence never prevents human overrides - for high level decisions that are not made within milliseconds.
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Offline savuporo

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« Last Edit: 12/02/2013 09:42 PM by savuporo »
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Offline Blackstar

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-is there any way to compensate for tilt? For instance, can the ramps be moved side to side to correct for tilt?
Pure guess but they probably drive the lowering mechanism with a regular servo, so i would think they can extend it to 100%, but also to 80% and 120% deployment positions

Not what I meant. Think of it this way: it lands on an incline. From the rover's view straight ahead, the horizon is tinted 15 degrees sloping up to the right, meaning that the right hand ramp is higher than the left hand ramp. Can it compensate? Or is there a risk that when they uncage the rover from the hold-downs on the top of the lander that it could tumble to one side? Related question, what is the maximum tilt that it can accommodate and safely drive off?

I'm not necessarily expecting anybody to have an answer to this (and I may have more technical documentation on this mission than is available to many people, and I haven't found the answer yet). But I am curious about the question.

Looking at Chang'e-3 compared to the Lunokhods, the rover is much higher up and has smaller wheels and a smaller wheel base. The Lunokhods did not have a high center of gravity to worry about.

Then again, lookit this:
« Last Edit: 12/02/2013 09:45 PM by Blackstar »

Offline lcs

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An interesting and previously-unknown (at least by me) fact: study of the lunar exosphere's disturbance by Chang'e 3's landing is expected to be performed by LADEE. Unforeseen space cooperation between China and the US! :)

You mean in the sense that we will be figuratively, if not literally, eating their dust?
« Last Edit: 12/02/2013 09:52 PM by lcs »

Offline Rocket Science

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Some people may be interested in how the United States could respond to the Chang'e-3 mission. Here are some possibilities.

http://thespacereview.com/article/2413/1

Red Moon, Blue Moon
by Dwayne Day
Monday, December 2, 2013

Yesterday China launched Chang’e-3 on its way to the Moon, with landing scheduled for December 14. If it succeeds, it will be the first spacecraft to make a soft landing on the Moon in nearly four decades. Although the lander and rover have a modest scientific instrument suite, they are headed for a previously unexplored region of the Moon and will therefore return new and undoubtedly interesting data. Chang’e-3 will not be alone. NASA currently has two spacecraft—Lunar Reconnaissance Orbiter and LADEE—circling the Moon. But although NASA also has several other possible lunar lander missions that it could start building within the next decade, it is unlikely that a NASA spacecraft will join the Chinese on the lunar surface for many years to come.

NASA divides its planetary missions into three classes: flagship (large), New Frontiers (medium), and Discovery (small). The agency uses the planetary science decadal survey to select science goals for the first two categories. But NASA can also fly robotic missions in support of its human space exploration goals, and those missions are chosen by agency officials based upon their perceived requirements. In the past decade, the agency has flown lunar science missions to support the goals of President Bush’s Vision for Space Exploration, specifically the goal of sending humans to the Moon. Although the Vision for Space Exploration no longer guides NASA policy, some aspects of that previous policy continue to influence current NASA studies and plans.
Nice article Dwayne... While other nations have a systematic progression in abilities, the U.S. seems to have giant surges and then pulls back resting on its laurels...
"The laws of physics are unforgiving"
~Rob, Physics instructor, aviator, vintage auto racer

Offline edkyle99

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Looking at Chang'e-3 compared to the Lunokhods, the rover is much higher up and has smaller wheels and a smaller wheel base. The Lunokhods did not have a high center of gravity to worry about.

Then again, lookit this:
Thanks for that photo.  I think that it provides an important clue about what China may be planning for future uses of its lander stage.  The Yutu rover seems too small for its lander stage, necessitating the moving ramp arrangement.  Yutu seems to me like a precursor test article - like (a) Pathfinder.  A pathfinder for bigger things.

 - Ed Kyle
« Last Edit: 12/02/2013 11:58 PM by edkyle99 »

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