Where to Land...

louis · 2018-05-02 08:49:23

We've landed robot craft twice at Chryse Planitia so I presume that area's OK from that point of view.

If you are running a solar powered mission you also don't want to go much above 30 degrees north or south.

elderflower wrote:

Elevation is an issue as well. There has to be enough atmosphere to provide the braking on re-entry. The more fuel used to slow the vehicle, the less there is for final positioning of the spacecraft. There are seasonal effects as CO2 ice at the south pole sublimes into the atmosphere each southern summer and then reprecipitates making significant differences to atmospheric density. A position that is only accessible for part of the year wouldn't be a good one.

GW Johnson · 2018-05-02 10:12:41

Until some of the real landing issues get included in these discussions, like static and dynamic stability for rough ground, this is more akin to an angels-on-the-head-of-pin debate than it is to the realities facing Spacex or anybody else. Sorry, but that's the truth.

Elevation IS an issue for anything aerobraked. Having significant L/D >0.1-ish available during aerobraking is part of the counter to that, since the trajectory shape can be modified with uplift, at least once velocities have dropped below about 3.5 km/s on Mars. The BFS is a lifting body, none of JPL's (or anybody else's landers) have been. That's one huge difference right there.

The Justus and Braun JPL report on entry descent and landing is quite good as far as it goes, but it does NOT cover significantly-lifting descent. Nor does it cover retropropulsion. It does cover the 1-ton limit to landing with chutes the JPL has been doing.

To get landing accuracy out of BFS better than 5+ km error requires two things: (1) an equivalent to GPS to guide the direct-entry trajectory into exactly the right entry path, and (2) a radar transponder or beacon on the intended site to guide the last 5-10 seconds of the nail-biting retropropulsive touchdown. Do that and your error is meters. Do not, and your error is kilometers (plural).

Musk's original vision of a pathfinder on the site is still the correct one. Too bad Red Dragon is dead, he was going to use it for that purpose. Find the smooth spot, mark it with a beacon. That takes a small rover of some kind. Release a swarm of positioning satellites on the way in, or from another shot to Mars. BOTH of those still have to happen before BFS can go there: it is just too tall and narrow to make rough-ground landings.

Its 5-10 second touchdown timeline is too short for a human pilot to respond the way Armstrong did. And a BFS pilot will NOT have the visibility that Armstrong did. That's an inherent disadvantage of flying large vehicles.

GW

Last edited by GW Johnson (2018-05-02 10:18:22)

Oldfart1939 · 2018-05-02 10:57:21

I also regret that the Red Dragon will not fly. It was an excellent idea, and could still happen using the Falcon Heavy block 5 components. There needs be a sub-scale landing at least tried before expending a $500 million vehicle on an almost suicide mission. Musk could send a swarm of Red Dragons out for the cost of a single BFR/BFS.

louis · 2018-05-02 16:02:50

Getting the landing location right - particularly making sure you don't end up in a dud water resource area - is going to be more difficult I believe than a safe landing.

I don't think safe landing is going to be as problematic as you suppose thanks to (a) pre-surveying of landing area by satellites (satellites can certainly identify boulders of around 2 metres) (b) possibly landing in an area that has already been photographed by previous robot missions (c) the pre-landing of cargo BFSs which can have transponders on board and may even land mini rovers with transponders that can identify safe areas (d) laser guidance as to ground conditions and (e) laser communication with the cargo BFSs.

GW Johnson wrote:

Until some of the real landing issues get included in these discussions, like static and dynamic stability for rough ground, this is more akin to an angels-on-the-head-of-pin debate than it is to the realities facing Spacex or anybody else. Sorry, but that's the truth.
Elevation IS an issue for anything aerobraked. Having significant L/D >0.1-ish available during aerobraking is part of the counter to that, since the trajectory shape can be modified with uplift, at least once velocities have dropped below about 3.5 km/s on Mars. The BFS is a lifting body, none of JPL's (or anybody else's landers) have been. That's one huge difference right there.
The Justus and Braun JPL report on entry descent and landing is quite good as far as it goes, but it does NOT cover significantly-lifting descent. Nor does it cover retropropulsion. It does cover the 1-ton limit to landing with chutes the JPL has been doing.
To get landing accuracy out of BFS better than 5+ km error requires two things: (1) an equivalent to GPS to guide the direct-entry trajectory into exactly the right entry path, and (2) a radar transponder or beacon on the intended site to guide the last 5-10 seconds of the nail-biting retropropulsive touchdown. Do that and your error is meters. Do not, and your error is kilometers (plural).
Musk's original vision of a pathfinder on the site is still the correct one. Too bad Red Dragon is dead, he was going to use it for that purpose. Find the smooth spot, mark it with a beacon. That takes a small rover of some kind. Release a swarm of positioning satellites on the way in, or from another shot to Mars. BOTH of those still have to happen before BFS can go there: it is just too tall and narrow to make rough-ground landings.
Its 5-10 second touchdown timeline is too short for a human pilot to respond the way Armstrong did. And a BFS pilot will NOT have the visibility that Armstrong did. That's an inherent disadvantage of flying large vehicles.
GW

SpaceNut · 2018-05-02 19:13:27

Post 49 images from Chryse Planitia which are from orbital view flat but these are full of silt, sand drifting and good size rocks ect...

we should research some of these areas:
Amazonis Planitia Smooth water formed

Isidis Planitia

Around the Isidis basin magnesium carbonate was found by MRO. This mineral indicates that water was present and that it was not acidic, pH conditions more favorable for the evolution of life

Elysium Planitia

A 2005 photo of a locale within Elysium Planitia at 5° N, 150° E by the Mars Express spacecraft shows what may be ash-covered water ice. The volume of ice is estimated to be 800 km (500 mi) by 900 km (560 mi) in size and 45 m (148 ft) deep, similar in size and depth to the North Sea

This Map Shows Where We'll Live On Mars

Oldfart1939 · 2018-05-02 20:46:05

Louis-
I respectfully disagree with your assessment about landing difficulties. Smaller vehicles will find it less problematic than the huge BFR. Coming down directly atop a large boulder could destroy the entire engine complex, rendering a return impossible.

louis · 2018-05-03 01:49:17

Sorry, I didn't mean to disagree that safe landing was in principle easier for a small robot craft (with the legs below the unity ratio). But on the other hand the size of the BFS and the multiple landings (including pre-landings of cargo BFSs) make it possible to put in place a range of safety measures: laser guidance/identification of ground conditions, transponder guidance, terrain recognition systems, hydraulic legs.

Oldfart1939 wrote:

Louis-
I respectfully disagree with your assessment about landing difficulties. Smaller vehicles will find it less problematic than the huge BFR. Coming down directly atop a large boulder could destroy the entire engine complex, rendering a return impossible.

louis · 2018-05-03 01:52:04

Wouldn't we want to avoid an area with ice below ash as a landing zone, given the potential for instabilty from rocket exhaust (either melting ice or perhaps blowing away ash and allowing for sublimation) or for cracking of ice...?

SpaceNut wrote:

Post 49 images from Chryse Planitia which are from orbital view flat but these are full of silt, sand drifting and good size rocks ect...
we should research some of these areas:
Amazonis Planitia Smooth water formed
Isidis Planitia
Around the Isidis basin magnesium carbonate was found by MRO. This mineral indicates that water was present and that it was not acidic, pH conditions more favorable for the evolution of life

Elysium Planitia
A 2005 photo of a locale within Elysium Planitia at 5° N, 150° E by the Mars Express spacecraft shows what may be ash-covered water ice. The volume of ice is estimated to be 800 km (500 mi) by 900 km (560 mi) in size and 45 m (148 ft) deep, similar in size and depth to the North Sea
This Map Shows Where We'll Live On Mars

louis · 2018-05-03 15:05:54

https://www.youtube.com/watch?v=F48kjtsptfo

The above video (which I also put on a soil thread) also suggests where there are good places to land on Mars in terms of soil-like regolith and ice availability. My proposed landing area - Chryse Planitia is within striking distance of the richest "soils" on Mars.

SpaceNut · 2018-05-03 16:14:59

The real issue is sending a scout sized to selected for further investigation by units to land in each of the target zones to explore the area for the esenture items that we want on the sites wish list and then look to see if its a plausable site to land BFR in in the future. If so send the cargo ships to preload the area followed by the crews.

louis · 2018-05-03 16:42:03

For the BFR landing zone (Mission One) all that's required is water (= water ice either in a glacier or bound up in the regolith). The "scouting" for the water signature is being done by satellites essentially. I don't think Space X will be aiming for a glacier. It will be aiming for a water rich environment.

SpaceNut wrote:

The real issue is sending a scout sized to selected for further investigation by units to land in each of the target zones to explore the area for the esenture items that we want on the sites wish list and then look to see if its a plausable site to land BFR in in the future. If so send the cargo ships to preload the area followed by the crews.

Last edited by louis (2018-05-04 05:16:46)

SpaceNut · 2018-05-03 17:24:55

The combined neutron gamma ray instrument is a detection of hydrogen and with the high oxygen content of the mars regolith the signature of water would appear. The maps detail the distribution of water-equivalent hydrogen ... By measuring neutrons, it is possible to calculate the abundance of hydrogen on Mars, thus inferring the presence of water.

https://www.nasa.gov/vision/universe/so … water.html 2003

ttps://www.nasa.gov/mission_pages/phoenix/multimedia/6433-20080513.html 2008

This map shows the estimated lower limit of the water content of the upper meter of Martian soil. The estimates are derived from the hydrogen abundance measured by the neutron spectrometer component of the gamma ray spectrometer suite on NASA's Mars Odyssey spacecraft.

The gamma ray spectrometer has measured the abundance and distribution of many elements of the periodic table, including hydrogen, silicon, iron, potassium, thorium, and chlorine.

GW Johnson · 2018-05-03 21:18:10

I think we've been through this landing thing a bunch of times, and the impediment to resolution is refusal to face facts. The analysis I posted over at "exrocketman" for the 2017 downsized BFR/BFS is there for all to see, and to replicate for themselves.

For landing statics, I showed a need for some rather large landing pad surfaces, totalling about 10 sq.m, for Martian soils as solid as a natural sand-and-gravel bank. Other than the solid rock surfaces or the frozen permafrost at the pole, I've seen nothing in any of the photos any different from that kind of dirt, and quite a lot that looks like loose sand. A refilled BFS will sink in and topple over in loose sand, even with 10 sq.m of pads. You CANNOT sense the difference in soil bearing strength from orbit. You might infer, but that requires assumptions, which get you into ASS-U-ME trouble. You MUST check this out in-situ on the surface to be sure.

Assuming hard enough ground and big enough pads, there is a static limit for a 4-leg vehicle of BFS characteristics of about 18 degrees surface inclination sloping directly toward one pad, or only 13 degrees sloping directly toward the middle between two pads. A lot of the photos I see show localized slopes equal or exceeding those magnitudes on a 10 meter scale. This kind of thing cannot really be determined from orbit. You MUST check it out on the ground.

From a dynamic aspect, I looked at setting one pad down on a 1-m boulder and having it suddenly slip off. The numbers are crude, but the assumptions I made to get them are rather realistic. I showed dynamics that quite literally threatened to topple the vehicle in such an event. If there is a 5 degree local slope, that threat of a topple becomes almost a certainty. And marginal soil strength makes that problem much worse. To the best of my knowledge, none of these satellite photos can spot 1 meter objects.

As for water resources, we've been through that, too. That notion of cooking moisture out of tens to hundreds of thousands of tons of regolith, to yield only thousands of tons of water, is a viable last resort, I suppose, but if you can find a buried glacier (and it looks like you can), that is the better option by orders of magnitude. Just land next to the thing and drive over with a mobile drill rig, sink a well, and get on with steam injection recovery. We've decades of history recovering oil that way.

These things may not be what you want to believe, but they really are as I have laid out. People making decisions based on preferences and fantasies will kill crews. Simple as that. I don't believe Musk and his people are stupid enough to ignore these huge mistakes to be made, but I do believe they have yet to think through all the nitty-gritty details. Focused on building the vehicle, I can understand that. But they WILL have to deal with all these issues, and more besides, before they attempt to land their vehicles on Mars.

GW

Last edited by GW Johnson (2018-05-03 21:24:25)

Oldfart1939 · 2018-05-03 22:09:01

In spite of the cheerleading going on by members of this forum, the camp divides into 2 quite naturally: dreamers and wishful thinkers versus pragmatists. To find out which group I associate with, check out my posts #24, # 53, and # 56.

RobertDyck · 2018-05-04 05:04:49

GW Johnson wrote:

As for water resources, we've been through that, too. That notion of cooking moisture out of tens to hundreds of thousands of tons of regolith, to yield only thousands of tons of water, is a viable last resort, I suppose, but if you can find a buried glacier (and it looks like you can), that is the better option by orders of magnitude. Just land next to the thing and drive over with a mobile drill rig, sink a well, and get on with steam injection recovery. We've decades of history recovering oil that way.

Agreed. The manager for the Mars Homestead Project argued for that in 2005. I do have to point out this is not quite as easy as it sounds. According to Curiosity Rover as of May 1st (sol 2038), Mars surface pressure was 726 Pa, mean. That website has a detailed pressure graph for Sol 9.5 through 13. That's noon on the 9th Mars solar day after landing, through midnight of the 13th Mars solar day. Pressure in the graph varies from 690 to 780 Pa (6.90 mbar to 7.80 mbar).

You have to worry about what temperatures water will be liquid so it can be extracted. Water has a very narrow temperature range at that pressure. This is the phase diagram for water:

Water phase calculator
At 690 Pa, boiling temperature of water is 1.68°C. At 780 Pa, boiling is 3.40°C. Freezing temperature remains 0°C until very high pressure, or below the triple point pressure of 611.657 Pa. That's for clean ice, salt will drop melting temperature and raise boiling. Mars soil is salty, but when a large body of salt water freezes, it "squeezes out" brine, producing fresh water ice.

Can you ensure steam melted ice won't produce more steam, escaping your extraction chamber to condense/freeze on the surface? Or worse yet, escape to be blown away in wind?

Last edited by RobertDyck (2018-05-04 11:07:42)

louis · 2018-05-04 08:12:51

It sounds incredible, but apparently Mars survey satellites can see objects as small as 10 inches across and this has been further resolved to 2 inches across using image enhancement software:

https://gizmodo.com/new-image-enhance-l … 1773058237

So it sounds as though we will be able to identify one metre boulders with no trouble in the landing zone. You could even lay out a test range on Earth with boulders placed at exactly the same points and see how your landing guidance systems work. You could have a pre-planned exact landing spot.

No room for complacency though, for such an important aspect of the mission.

What do you think about 5 legs GW? I am thinking one unstable leg would not topple over a 5 legged craft, whereas with 4 the matter still be in doubt, depending on how the cargo is stored. It certainly could prevent unloading of cargo, which could be fatal to the mission.

I am of course more optimistic than you about determining the nature of the ground, so whether a a five legged craft is worth all the design trouble and extra mass, I don't know. With pre-landing of the cargo BFSs, even if there is no automated unloading of rover, they could still prospect the ground in the landing area, using lasers/radar and visually. Lasers could also be used to mark a friendly landing zone with a large X which could be identified by onboard camera software. [If Space X lose a cargo BFS, that might halt the mission, but it won't kill any human beings. If a cargo BFS topples over, it wouldn't in fact necessarily be the end of the mission either of the cargo could survive such an impact.]

While it would be nice to land close to a glacier, I just have a hunch that land around regolith could be exactly the sort of place to harbour nasty surprises, like underwater streams or old stream tunnels now devoid of water. And can we be sure there is no periodic flooding.

There might be an argument for landing at a distance from a glacier, away from any such dangers and then organise a robot rover operation to deliver water back to base. But that sounds a bit advanced for Mission One.

GW Johnson wrote:

I think we've been through this landing thing a bunch of times, and the impediment to resolution is refusal to face facts. The analysis I posted over at "exrocketman" for the 2017 downsized BFR/BFS is there for all to see, and to replicate for themselves.
For landing statics, I showed a need for some rather large landing pad surfaces, totalling about 10 sq.m, for Martian soils as solid as a natural sand-and-gravel bank. Other than the solid rock surfaces or the frozen permafrost at the pole, I've seen nothing in any of the photos any different from that kind of dirt, and quite a lot that looks like loose sand. A refilled BFS will sink in and topple over in loose sand, even with 10 sq.m of pads. You CANNOT sense the difference in soil bearing strength from orbit. You might infer, but that requires assumptions, which get you into ASS-U-ME trouble. You MUST check this out in-situ on the surface to be sure.
Assuming hard enough ground and big enough pads, there is a static limit for a 4-leg vehicle of BFS characteristics of about 18 degrees surface inclination sloping directly toward one pad, or only 13 degrees sloping directly toward the middle between two pads. A lot of the photos I see show localized slopes equal or exceeding those magnitudes on a 10 meter scale. This kind of thing cannot really be determined from orbit. You MUST check it out on the ground.
From a dynamic aspect, I looked at setting one pad down on a 1-m boulder and having it suddenly slip off. The numbers are crude, but the assumptions I made to get them are rather realistic. I showed dynamics that quite literally threatened to topple the vehicle in such an event. If there is a 5 degree local slope, that threat of a topple becomes almost a certainty. And marginal soil strength makes that problem much worse. To the best of my knowledge, none of these satellite photos can spot 1 meter objects.
As for water resources, we've been through that, too. That notion of cooking moisture out of tens to hundreds of thousands of tons of regolith, to yield only thousands of tons of water, is a viable last resort, I suppose, but if you can find a buried glacier (and it looks like you can), that is the better option by orders of magnitude. Just land next to the thing and drive over with a mobile drill rig, sink a well, and get on with steam injection recovery. We've decades of history recovering oil that way.
These things may not be what you want to believe, but they really are as I have laid out. People making decisions based on preferences and fantasies will kill crews. Simple as that. I don't believe Musk and his people are stupid enough to ignore these huge mistakes to be made, but I do believe they have yet to think through all the nitty-gritty details. Focused on building the vehicle, I can understand that. But they WILL have to deal with all these issues, and more besides, before they attempt to land their vehicles on Mars.
GW

Oldfart1939 · 2018-05-04 09:14:28

Louis-
Check my earlier post wherein I suggested 6 landing legs for distribution of ground pressure and added stability.

But no--the satellites cannot detect objects of 10", or at least not according to one of the satellite's designers--based on simply the pixel size in the sensor systems. We discussed this at a Mars Society meeting in Boulder, CO last year when the gentleman from Ball Research was present. He was the designer of one of the newer satellites. We had a speaker presenting his paper on the linnae on Mars, possibly indicative of water. The resolution is still greater than a meter.

GW Johnson · 2018-05-04 09:40:32

To answer RobertDyck's concern in post #65:

You have to case at least part of the well, so that you can operate the steam and the water recovery at pressure far above local Mars ambient. Here we do that with a pipe sleeve and cement. It's too cold there for Earthly cement, plus the water would boil out of it in the low pressure before it could cure. But there has to be a substitute we can use. This substitute can be tested aboard ISS, by the way. Any time.

To answer Louis's question in post #66:

Yep, 5 legs is more stable. See also Oldfart1939's 6-leg suggestion in post 67. However, these are heavier, and that mass must come directly out of payload capacity. That's just the tyranny of the rocket equation. Which is exactly what I used to estimate BFR/BFS performance characteristics from Spacex's own published data. Physics is physics, everywhere and for everyone.

Concerning resolution from satellites:

What Oldfart1939 said in post #67 is more in line with what I knew about spy satellites for many decades now. These use folded optical paths to package enormous focal lengths, and are quite large. The old KH-11's could resolve that a car had a license plate or not (as a difference of contrast within a blur), but no way could they read the license plate.

The old tales about reading plates and recognizing faces from 600 miles up were just tales. The KH-11's set the size of space shuttle's payload bay, by the way, at 15 ft diameter and 60 feet long, and about 15-20 tons delivered. That's how big optics like that must be. Physics has not changed since.

The resolution to the rather alarming issues I have been raising is exactly what Musk proposed about the time of his original BFR presentation in 2016 at Guadalajara. You send a small pathfinder to the site.

A camera on the scout can resolve the roughness of the terrain at a less-than-1-m scale, including localized crap like gullies and mounds. And boulders! If the scout is a rover, quite the large site can be covered in exquisite detail, we already know that from the other rovers we have sent.

Concerning soil bearing strength:

This rover needs a specialized arm equipped to test soil bearing strength. I have actually done that with nothing but a pickaxe and a jaundiced eye, laying engineered fire mains here on Earth. None of my sized thrust blocks ever moved. Fire main work is not like water main work even though it's the same slip-joint pipe: you test it unburied at 175 psi. Far more demanding. But for the rover, it's just a small flat pad mashed onto/into the soil at very high force. You are literally looking for the highest force per pad area just as the pad starts to dig in.

Concerning recoverable ice to mine:

This is more of a bet, because even the Canadian drill rover RobertDyck has described cannot drill deep enough. That design needs to be taken a further step to drill deeper, equipped with the soil tester and the site camera, and a landing beacon, and maybe a small grader blade. And send it to Mars.

You don't put this inside a Red Dragon, you replace the capsule pressure shell with it. It rides the heat shield and Super Dracos down the way Red Dragon would have, and lands on the heat shield itself. Then just drive off and get on with it.

The rover checks the site visually, tests the soil to confirm, plants the beacon, and pushes any boulders out of the way. Then it drives over to the suspected buried glacier, and sinks a test boring. If it strikes even dirty ice, we're good to go for steam recovery with bigger equipment that arrives on the BFS's.

Final comments:

You know somebody is serious about sending people to Mars in a big vehicle when they start doing what I just outlined. Not until then.

GW

Last edited by GW Johnson (2018-05-04 09:54:11)

Oldfart1939 · 2018-05-04 11:28:07

What GW stated here is exactly the point we need to consider. Send a pioneer mission-robotic is OK, but using retropropulsion to land.

The comments about what can or cannot be seen from space are essentially diffraction limited by the laws of physics. The resolution required is determined by the size of the objective lens diameter (or that of the mirror) in the telescope portion of the system, and the size of pixel in the digital camera onboard the spacecraft. just using the fantasy of increased magnification is fruitless. A magnified blur is still just a blur.

Terraformer · 2018-05-04 17:08:46

Could the BFR land on it's side, instead? I remember designs for Lunar missions (I think it was the ACES concept?) that would do something like that. I know it's not as God and Heinlein intended, but it would distribute the weight over much larger surface area. Obviously it would require retrorockets at the side, and stabilising legs to stop it from rolling.

louis · 2018-05-04 17:34:17

Regarding satellite imagery, I am reading that modern reconnaisance satellites can resolve down to 10 centimetres or better (clearly a lot of the info as to capability is under wraps). This satellite image gives an idea of what's possible but I doubt this is the highest resolution as they wouldn't have wished to make their best effort public:

https://upload.wikimedia.org/wikipedia/ … ide-15.jpg

The above photo is from 16 years ago, so I expect technology has moved on in any case.

I am confident that for a rock field on Mars we could identify all the boulders that would cause problems in a landing scenario. We may even be able to use the pattern of larger boulders as recognition markers to guide the BFS to the exact desired landing spot.

SpaceNut · 2018-05-04 19:22:01

Mars satellites are commercial vendor components and not military grade which is the ITAR of materials used in earth military use.

Sure commercial will catch up but it will take the military getting to the next level up of abilities before commercial will be allowed to even think about being able to use these devices.

Oldfart1939 · 2018-05-04 21:01:01

I had to go back and do some checking on the Recurring Slope Lineae (RSLs). The satellite used for their detection was the Mars Reconnaissance Orbiter, built by Ball Aerospace, with the most sophisticated optics yet placed in Mars orbit. The satellite optical designer was at our meeting, and his comment was the resolution was in the meter range, determined by the pixel size of the CCD and limits arising from the objective size. So--the resolution of the photos released is from several overlapping photos, digitally processed to enhance sharpness. No wishful thinking here; just solid scientific data. It was one of the most interesting talks in the past two years that I've attended.

louis · 2018-05-05 04:52:18

Sorry - that's a bit ambiguous...are you saying that 1 metre is the maximum obtainable or that with use of several overlapping photos and digital processing they can go under one metre?

Oldfart1939 wrote:

I had to go back and do some checking on the Recurring Slope Lineae (RSLs). The satellite used for their detection was the Mars Reconnaissance Orbiter, built by Ball Aerospace, with the most sophisticated optics yet placed in Mars orbit. The satellite optical designer was at our meeting, and his comment was the resolution was in the meter range, determined by the pixel size of the CCD and limits arising from the objective size. So--the resolution of the photos released is from several overlapping photos, digitally processed to enhance sharpness. No wishful thinking here; just solid scientific data. It was one of the most interesting talks in the past two years that I've attended.

Oldfart1939 · 2018-05-05 08:37:48

No, it's NOT under a meter. My statement was a bit ambiguous because his answer was such.

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