Where to Land...

louis · 2018-04-14 17:06:26

I don't think it would be impossible for Space X to have a robot rover exit one of the 2022 cargo BFRs, clear boulders and rocks, and then lay a landing pad. Not impossible, but would require a huge amount of R&D, so whether it is necessary needs to be established.

GW Johnson wrote:

When I said what I said about site investigation and prep for landing BFS's, it's not an absolute! Yes, you can take the risk of landing giant BFS vehicles on an unprepared pad, but you'd better be prepared to see some of them topple over! If crewed, that crew will die.
Wishful thinking cannot change that risk. Only appropriate actions can.
The experience to date landing Falcon stages supports no other outcome: every single successful landing was on a flat concrete pad or a flat steel deck. It would be interesting to see what landing on natural ground does to the landing success rate.
It's gaps like that, added to the foot-dragging schedules where NASA $ (or in the case of military satellites, USAF $) are involved, that have delayed actual Spacex accomplishments from the intended schedules Musk wanted. There's no mystery to that.
GW

GW Johnson · 2018-04-16 08:31:53

I checked Spacex's website for BFR/BFS information. What's posted there is from the 2017 presentation, a bit downsized from the 2016 presentation in Guadalajara. The BFR 1st stage and BFS 2nd stage/spaceship are 9 m basic body diameter. BFR is 58 m long, and BFS is 48 m long. The pressurized section and unpressurized cargo bay appear in the images to be half the overall length of the BFS.

By the way, if you dock two BFS's tail to tail (as proposed for orbital refilling), and then spin them at 4 rpm (the max that most people's inner ear mechanisms tolerate long term), you get 0.85 gee at the nose tips, and 0.42 gee at the inner faces of the unpressurized cargo bay. What that means is 0.5-0.7 gee spin gravity is easily available in any BFS mission to Mars where two ships are sent.

It ain't 1 full gee, but it's a lot higher than Mars gravity. What a way to keep passengers going to Mars fit upon arrival, even if you take the full Hohmann transfer time of 8.5 months one-way, to maximize thrown weight. And it comes a whole lot closer to keeping returning crews fit enough for an 11+ gee return at Earth.

No new technology for spin gravity here, since tail to tail docking with thruster microgravity is already proposed. Just ensure the ship structures are stout enough to take the side loads for spin-up and spin-down, and reconfigure the accommodations such that upside-down in spin looks right-side up. Reversible decks and fixtures, nothing more.

The posted images of BFS's on landing pads appear to show landing pads on a circle about 12-15 m in diameter, just eyeballing it off the images, knowing the hull is 48 m long. There are 4 landing legs shown. That's a height to span ratio of around 3 or 4, compared to just under 1 for the Apollo LM and all the JPL Mars landers.

The selected site needs to be rather smooth and flat, without much loose rock rubble, for this to be successful. That is another serious constraint to the landing sites that can be considered. The other constraint is easy availability of water ice. That's for making return propellants, per Musk himself.

Musk says cargo gets unloaded from BFS's with a crane sticking out of the cargo door. That's what he shows in the posted images. I see no reason why robotic or tele-operated equipment could not put more than one automated rover on the ground from each of the initial two uncrewed BFS's. Those rovers could grade-off and beacon-instrument landing pads for the crewed (and uncrewed) BFS's to come.

I still caution that of two unmanned BFS's to be sent in 2022 to unprepared landing sites, there is a high probability that at least one may tip over on landing, and be destroyed.

GW

Last edited by GW Johnson (2018-04-16 08:36:27)

louis · 2018-04-16 15:38:24

GW, well I am the eternal optimist here! I think there is no prospect of the Space X mission going ahead before they carry out multiple landing tests on Mars-analogue ground...maybe they will even build in tests on known unstable ground, to see how the landing goes, though I think that unlikely. I think the chance of one of the first two cargo BFSs tipping over will be close to zero.

A 1G or 0.5 G journey to Mars would be nice and your idea sounds like an elegant solution. I am sure Musk is thinking about that from time to time, but I just don't see it as the priority. Some people have been sent into space multiple times for several months at a time.

I think I was more concerned about stability issues when I saw the crane illustration! There must be a limit on how many tonnes you can unload that way in one go. Yes, in theory a rover could be unloaded...but you have to get it to the crane...how? (presumably it's still packed)...you have to then somehow get it hooked to the crane...then you have to hope the crane mechanism works OK...in extreme temperatures...then you have to unhook the rover from the crane. It is possible, but I can't say I like it.

GW Johnson wrote:

I checked Spacex's website for BFR/BFS information. What's posted there is from the 2017 presentation, a bit downsized from the 2016 presentation in Guadalajara. The BFR 1st stage and BFS 2nd stage/spaceship are 9 m basic body diameter. BFR is 58 m long, and BFS is 48 m long. The pressurized section and unpressurized cargo bay appear in the images to be half the overall length of the BFS.
By the way, if you dock two BFS's tail to tail (as proposed for orbital refilling), and then spin them at 4 rpm (the max that most people's inner ear mechanisms tolerate long term), you get 0.85 gee at the nose tips, and 0.42 gee at the inner faces of the unpressurized cargo bay. What that means is 0.5-0.7 gee spin gravity is easily available in any BFS mission to Mars where two ships are sent.
It ain't 1 full gee, but it's a lot higher than Mars gravity. What a way to keep passengers going to Mars fit upon arrival, even if you take the full Hohmann transfer time of 8.5 months one-way, to maximize thrown weight. And it comes a whole lot closer to keeping returning crews fit enough for an 11+ gee return at Earth.
No new technology for spin gravity here, since tail to tail docking with thruster microgravity is already proposed. Just ensure the ship structures are stout enough to take the side loads for spin-up and spin-down, and reconfigure the accommodations such that upside-down in spin looks right-side up. Reversible decks and fixtures, nothing more.
The posted images of BFS's on landing pads appear to show landing pads on a circle about 12-15 m in diameter, just eyeballing it off the images, knowing the hull is 48 m long. There are 4 landing legs shown. That's a height to span ratio of around 3 or 4, compared to just under 1 for the Apollo LM and all the JPL Mars landers.
The selected site needs to be rather smooth and flat, without much loose rock rubble, for this to be successful. That is another serious constraint to the landing sites that can be considered. The other constraint is easy availability of water ice. That's for making return propellants, per Musk himself.
Musk says cargo gets unloaded from BFS's with a crane sticking out of the cargo door. That's what he shows in the posted images. I see no reason why robotic or tele-operated equipment could not put more than one automated rover on the ground from each of the initial two uncrewed BFS's. Those rovers could grade-off and beacon-instrument landing pads for the crewed (and uncrewed) BFS's to come.
I still caution that of two unmanned BFS's to be sent in 2022 to unprepared landing sites, there is a high probability that at least one may tip over on landing, and be destroyed.
GW

Last edited by louis (2018-04-17 03:51:27)

EdwardHeisler · 2018-04-16 21:14:07

I hope that Elon Musk decides to land the BFR with cargo and crews in the so-called "special regions" of Mars.

louis · 2018-04-17 03:50:27

What be they?

EdwardHeisler wrote:

I hope that Elon Musk decides to land the BFR with cargo and crews in the so-called "special regions" of Mars.

GW Johnson · 2018-04-17 23:39:15

I reworked my reverse-engineering analysis of the BFR in greater detail with more fidelity to the real world's complexity. This is based on the stuff posted at Spacex's website. My analysis is posted at "exrocketman" as "Reverse Engineering the 2017 Version of the Spacex BFR", dated 4-17-18. It supersedes the earlier, simplified analysis I did that was posted there in October 2017.

This takes on the booster, to include flyback, entry and landing burns. It takes on the spaceship journey to Mars, starting with an analysis of the Mars landing. That sets the propellant reserve required on board after the departure burn. It takes on the return trip in a similar manner, using the Earth landing to set the propellant reserve budget for the Mars departure. I have included estimates of gee levels wherever it is possible to estimate them.

It does not take on the issue of very high gee levels during Earth free return entry. Nor does it take on the landing stability of a tall, narrow vehicle landing on rough, unprepared ground. And it does not analyze the tanker aspect of refilling in Earth orbit. These issues are mentioned, but not explored.

The "exrocketman" site is my blog site: http://exrocketman.blogspot.com.

GW

louis · 2018-04-18 05:27:40

Brilliant analysis GW, not that I am claiming to follow it all but I have taken in what I can. You seem to be giving the Space X architecture a thumbs up as possible but highlight the issue of very high G re-entry on the return to Earth. Couple of questions: can you not slow down the craft "naturally" by going into Earth orbit? And secondly, given Space X plan to refuel a craft on the back to back method for the outward journey, is it not possible to do the same when the craft returns to Earth? Couldn't a combination of deceleration (using the initial return propellant), orbital capture and refuelling in fact address the problem and allow for a low G descent?

GW Johnson wrote:

I reworked my reverse-engineering analysis of the BFR in greater detail with more fidelity to the real world's complexity. This is based on the stuff posted at Spacex's website. My analysis is posted at "exrocketman" as "Reverse Engineering the 2017 Version of the Spacex BFR", dated 4-17-18. It supersedes the earlier, simplified analysis I did that was posted there in October 2017.
This takes on the booster, to include flyback, entry and landing burns. It takes on the spaceship journey to Mars, starting with an analysis of the Mars landing. That sets the propellant reserve required on board after the departure burn. It takes on the return trip in a similar manner, using the Earth landing to set the propellant reserve budget for the Mars departure. I have included estimates of gee levels wherever it is possible to estimate them.
It does not take on the issue of very high gee levels during Earth free return entry. Nor does it take on the landing stability of a tall, narrow vehicle landing on rough, unprepared ground. And it does not analyze the tanker aspect of refilling in Earth orbit. These issues are mentioned, but not explored.
The "exrocketman" site is my blog site: http://exrocketman.blogspot.com.
GW

Last edited by louis (2018-04-18 05:28:36)

GW Johnson · 2018-04-18 07:46:31

"can you not slow down the craft "naturally" by going into Earth orbit? " -- sorry, that requires an enormous delta-vee burn of the same size as the one getting out of Earth orbit onto the interplanetary trip to Mars. That's most of the propellant that the vehicle can hold. Getting off Mars onto the interplanetary trajectory home is a similar burn of most of the propellant that the vehicle can hold. Refilling from Earth is possible by tankers in Earth orbit. Refilling for the return is done in situ on Mars.

"given Space X plan to refuel a craft on the back to back method for the outward journey, is it not possible to do the same when the craft returns to Earth?" -- no, there's no way to position a tanker on the return leg of the interplanetary trajectory home. That's beyond the capability of the vehicle design, even if you expend the vehicle.

"Couldn't a combination of deceleration (using the initial return propellant), orbital capture and refuelling in fact address the problem and allow for a low G descent?" -- if the vehicle could capture into Earth orbit, then entry from only Earth orbital speeds (around 8 km/s) is somewhere around 4 gees, as we have seen with capsules and the shuttle. Direct free return, entry speed is 17 km/s or thereabouts. To capture into orbit requires crudely 17 - 8 = 9 km/s worth of delta vee. After leaving Mars, the vehicle only has about 1-1.3 km/s delta vee capability left. And getting a fleet of tankers onto the return trajectory is a trip to Mars for each tanker, without landing, and then being lost in space. Musk is not to going to do this flying things expendably. I think we can bet on that.

The mission architecture Spacex has chosen precludes capture into Earth orbit. All the delta-vee for speed reduction is by aerobraking into Earth's atmosphere. That will be high gee because of the high speeds, regardless of whether it ends on the surface (the short process) or in some sort of parking orbit (a very much longer process).

That being the case, the crew needs to be fully fit for the high-gee return. Musk is assuming that 0.384 gee on Mars will keep people essentially as healthy as Earth's 1.00 gee. That's an assumption. There is no proof or fact yet. Assume is spelled the way it is because it makes an ASS out of U and ME. The trip home is Hohmann transfer at about 8.5 month's exposure to zero gee. Even for 1-gee healthy folks, the microgravity weakening at 8.5 months is considerable. We have shown they can withstand 4 gees from orbit at 8 km/s entry. We have NEVER shown that folks in that state could survive Apollo's 11-gee ride home from the moon at 11 km/s entry. This entry at 17 km/s will be even more stressful.

I did suggest a way around that in another post not long ago. When a ship comes home, be sure it is 2 of them making the voyage home. Dock them tail-to-tail and spin the cluster up for spin gravity during the long transit home. At 4 rpm on a 48 m long vehicle, the gravity in the pressurized spaces will be around 0.6 to 0.7 gee. They should be much healthier for the harsh entry coming home, especially if 0.384 gee is as therapeutic as Musk is betting that it is.

GW

Last edited by GW Johnson (2018-04-18 07:59:09)

SpaceNut · 2018-04-18 14:10:57

The mars return from surface would need to settle into orbit to gain cargo from the on orbit safety ship that was used on going from earth to mars to do the spin or tumble gravity with the ship coupled end to end after the EDS burn and for the MDS burn for return each ship would then rendeveus with each other to do the tail coupling before spin up for gravity.

The ships would have time to make some fuel from waste recovery on the outbound leg while it waits in orbit befor the return trip with that fuel being used to do aerocapture plunging into the atmosphere to slow the ships before trying to do a retropropulsion landing after the on orbit burn. Your really do not care how long it takes on the slow down as you have the shield and can mate back up for gravity until you can land.

GW Johnson · 2018-04-18 14:32:55

Spacenut:

The Spacex ship designs, cargo or crewed, simply cannot stop in Mars orbit. That is quite far beyond their delta-vee capability. Way to hell-and-gone far beyond their capability.

What Spacex has done is figure out how to build a single stage vehicle with just enough delta-vee capability to get from Earth orbit to a Mars transit trajectory, with barely enough propellant to land, after aerobraking-away 97-98% the delta-vee required to capture at Mars, directly from that interplanetary trajectory. (The same applies to the Earth return leg, Louis.)

These ships hold 1100 tons of propellant. About 60-110 tons of propellant (depending upon payload mass) are required for the final retropropulsive touchdown burn, most of the rest (some 1040 to 990 tons) is required to get from Earth orbit onto the interplanetary trajectory. Those numbers include allowances for boiloff effects, midcourse correction budgets, hover/redirect at landing when obstacles crop up, and gravity and drag losses at both ends. There is no propellant to do anything else.

As for making more propellant in transit from wastes, the occupants will generate tons-only of waste, some smaller fraction of which could be made into methane. No oxygen at all will be generated from that waste, and you need 3 or 4 tons of it for every ton of methane. Somehow, I don't think that will be of any significant aid. Tons vs hundreds of tons at a very minimum. Outside the ballpark, that is.

Sorry, they've done a huge miracle figuring out how to do this with equipment not thrown away, and that miracle depends fundamentally upon making LCH4/LOX propellants on Mars (something not yet done). It's just not an unbounded miracle. There are very severe and very fundamental constraints on what this design can actually do, and there are still some very severe unaddressed risks, to doing it this way.

I honestly hope they succeed with this. But I am not going to hold my breath for it. The odds are against, until the unaddressed risks are properly addressed. That's just one skeptical old retired new-product-development aerospace engineer talking from common sense and long (and bitter) experience.

I ran the numbers on this design as best I could, given that Spacex has not revealed all the technical details yet. I posted the latest version of that analysis yesterday over at my "exrocketman" blog site. What I found is fairly convincing of the warnings in the paragraphs just above. That posting on "exrocketman" will get updated for a while yet, as I uncover or evaluate things regarding those still-unaddressed risks.

GW

Last edited by GW Johnson (2018-04-18 14:44:07)

Quaoar · 2018-04-18 16:31:10

GW Johnson wrote:

Why not just send a small robot bulldozer one way to your site to grade a flat spot, before the BFS gets sent? Fit it with a ground-penetrating radar and seismic shot equipment to look for subsurface cavities. That's the sort of thing getting left out of the mission planning.
GW

Hi GW,

How much precision can we achieve in a landing on Mars?

GW Johnson · 2018-04-19 11:31:32

Hi, Quaoar, long time no see. To answer your question, I don't really know. My gut hunch landing "blind" is within a small handful of km of the aim point.

There's not much time left after coming out of the radio blackout of hypersonics, but I would guess that a radar beacon at the site might reduce this error to around a km or maybe a tad less.

Once there is the analogue to GPS at Mars, this could be reduced to the few-meters accuracy we see here on Earth landing boosters.

All:

I did address the unresolved BFR issues a little bit in two updates to my recent post over at "exrocketman". Yesterday's update addresses providing artificial spin gravity in the BFR, for at least the trip home. You do it by docking 2 ships tail to tail (like for refilling) and spinning at 4 to 4.6 rpm.

Today's update addresses multiple issues regarding landing/takeoff statics and landing dynamics. I also looked at rocks flung by jet blast, and debris from a ship explosion.

That should pretty well cover the capabilities and vulnerabilities of the Spacex BFR system, until they publish more detailed data.

GW

GW Johnson · 2018-04-21 17:50:53

I added a third update today (Sat) speculating on the tanker issues: (1) what the tanker design could be, and (2) how many are needed for a complete crewed refill in LEO. That's my BFR reverse engineering article posted over at "exrocketman".

GW

louis · 2018-04-21 18:16:23

Looking forward to reading that additional analysis - will check it out soon...in the meantime can I just say that I disagree with you on landing accuracy. I think it's a lot to do with where we land. The following are relevant I think:

1. If we land near where we have previously landed rovers we can create some v. accurate topographical maps. Cruise missiles travel by topography and we could use that sort of landing technology.

2. Lots of Mars has already been surveyed by satellites.

3. Image enhancing is claimed to allow us to see 2 inch objects on Mars:

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

4. I think that without any pre-surveying of the landing sites, you could put a Mars BFR into Mars orbit and then eject small transponder landers, which in turn could be referenced to Mars satellites. These could then give you a v. accurate landing trajectory.

5. Space X's human lander BFS can reference the two cargo BFSs which will be there before the human mission gets the green light.

6. Potentially the cargo BFS could offload a couple of Rovers which will have transponders built in. They can then be positioned to direct the incoming human BFS to the exact right spot.

Put it all together and I would be surprised if Space X didn't achieve one metre either way accurate landing.

GW Johnson wrote:

I added a third update today (Sat) speculating on the tanker issues: (1) what the tanker design could be, and (2) how many are needed for a complete crewed refill in LEO. That's my BFR reverse engineering article posted over at "exrocketman".
GW

GW Johnson · 2018-04-21 21:04:00

Louis:

You may disagree with me about landing accuracy, but I'm pretty sure you would be wrong. The first determinant is knowing exactly where in orbit you are relative to the landing site. That takes some analog to GPS to get good accuracy here, same at Mars, and it ain't there yet. The Mars landers so far have demonstrated +/- 5 km-ish landing accuracy, in part because of that lack.

The second determinant is timing accuracy for firing the retros to get the descent trajectory you want, aimed at the site. That's just timing and a well characterized ignition transient response for your propulsion. That's in-hand with a properly-tested design.

There is nothing you can do to upgrade accuracy during the plasma-enveloped hypersonic entry, with radio blackout. You fly to your predicted path using inertial guidance. That is all you can do until the radio blackout is over.

Once the hypersonics and blackout are over, then a site transponder will help improve accuracy, if and only if (1) you have maneuver control, and (2) there is time to use that maneuver control. With the Mars lander probes coming out higher with chute deceleration, there was time, but little or no control. That's the biggest reason why the accuracy was +/- 5 km poor. That's at least 3 of those km.

For a big heavy item like BFR (or even a Red Dragon, the limit for not being "big" is 1 ton landed), you come out of hypersonics rather low with very little time to do anything. Chutes are infeasible, because there is not the time to get any effective deceleration from them, even if you get it deployed. That's why Musk chose retropropulsion; it can be made to work on a short timeline, although you pull gees, and you will bite your nails.

With retropropulsion, there is maneuverability, so if there's a transponder on the site, you have a few seconds to adjust closer to it as you set down. Without the transponder, my guess is accuracy near +/- 1 km-ish. With it, nearer 0.2 km, maybe better.

I dunno about any +/- 1 m stuff, but they have been demonstrating +/- ~5 m landing Falcon stages. I'd guess about that same value given a site transponder and a GPS analog at Mars. Larger if either is missing, probably quite a bit larger.

GW

Last edited by GW Johnson (2018-04-21 21:06:24)

Oldfart1939 · 2018-04-21 21:52:23

Would it not make sense for SpaceX to use one of the Falcon Heavy launches to place a GPS constellation around Mars? That would seem very prudent if precision/accurate landings are desired? That's something they could accomplish in the 2020 Hohmann transfer launch window.

louis · 2018-04-22 06:16:21

This is how cruise missiles find their way - not by GPS.

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

I don't see why that wouldn't work on Mars, especially if used in conjunction with ground transponders. In the case of the human BFS, it could pick up transponder signals from the two cargo BFSs.

GW Johnson wrote:

Louis:
You may disagree with me about landing accuracy, but I'm pretty sure you would be wrong. The first determinant is knowing exactly where in orbit you are relative to the landing site. That takes some analog to GPS to get good accuracy here, same at Mars, and it ain't there yet. The Mars landers so far have demonstrated +/- 5 km-ish landing accuracy, in part because of that lack.
The second determinant is timing accuracy for firing the retros to get the descent trajectory you want, aimed at the site. That's just timing and a well characterized ignition transient response for your propulsion. That's in-hand with a properly-tested design.
There is nothing you can do to upgrade accuracy during the plasma-enveloped hypersonic entry, with radio blackout. You fly to your predicted path using inertial guidance. That is all you can do until the radio blackout is over.
Once the hypersonics and blackout are over, then a site transponder will help improve accuracy, if and only if (1) you have maneuver control, and (2) there is time to use that maneuver control. With the Mars lander probes coming out higher with chute deceleration, there was time, but little or no control. That's the biggest reason why the accuracy was +/- 5 km poor. That's at least 3 of those km.
For a big heavy item like BFR (or even a Red Dragon, the limit for not being "big" is 1 ton landed), you come out of hypersonics rather low with very little time to do anything. Chutes are infeasible, because there is not the time to get any effective deceleration from them, even if you get it deployed. That's why Musk chose retropropulsion; it can be made to work on a short timeline, although you pull gees, and you will bite your nails.
With retropropulsion, there is maneuverability, so if there's a transponder on the site, you have a few seconds to adjust closer to it as you set down. Without the transponder, my guess is accuracy near +/- 1 km-ish. With it, nearer 0.2 km, maybe better.
I dunno about any +/- 1 m stuff, but they have been demonstrating +/- ~5 m landing Falcon stages. I'd guess about that same value given a site transponder and a GPS analog at Mars. Larger if either is missing, probably quite a bit larger.
GW

Oldfart1939 · 2018-04-22 08:59:01

Louis-

There's more than an order of magnitude difference in velocities, comparing a hypersonic planetary reentry and a cruise missile. Comparing cherries to watermelons.

SpaceNut · 2018-04-29 19:30:47

Bump: Where to Land on Mars? It's not as Easy as It Looks

It took the viewing of the movie to generate interest to think about Mars...

In The Martian, a stranded astronaut has to survive a hostile Red Planet.

If Humans Go To Mars, Where's The Best Place To Land?NASA is taking suggestions this week By Charles Q. Choi October 27, 2015

2P192767805EFFAO55P2271R1M1.JPG?itok=DIAhMhje
Gusev Crater sure seems uninviting with all the slopes and rocks...

With more mars mission by probes, landers and rovers we might look to NASA Eyes Potential Landing Sites for 2020 Mars Rover Mission

louis · 2018-04-30 06:30:22

Is landing in a crater a good idea?

SpaceNut wrote:

Bump: Where to Land on Mars? It's not as Easy as It Looks
It took the viewing of the movie to generate interest to think about Mars...
In The Martian, a stranded astronaut has to survive a hostile Red Planet.
If Humans Go To Mars, Where's The Best Place To Land?NASA is taking suggestions this week By Charles Q. Choi October 27, 2015
https://www.popsci.com/sites/popsci.com … k=DIAhMhje
Gusev Crater sure seems uninviting with all the slopes and rocks...
https://mars.jpl.nasa.gov/spotlight/ima … gGusev.jpg
With more mars mission by probes, landers and rovers we might look to NASA Eyes Potential Landing Sites for 2020 Mars Rover Mission

GW Johnson · 2018-04-30 10:31:10

See the size of the Gusev crater landing ellipse in post #44? That's what happens from a direct-entry trajectory with a positioning satellite constellation of adequate capability. Plus the drift and the inability to steer, from using chutes.

As for the sloping, rough, rocky ground, that's what big landing pads, hydraulically-stroked landing legs, and a height-to-pad-span ratio under unity are for. There is no way BFS could ever land on terrain looking like that in the photo of post #44.

GW

Last edited by GW Johnson (2018-04-30 10:33:46)

elderflower · 2018-04-30 11:11:48

There are very few contours in the landing ellipse so slopes should be quite gentle.. Contour interval is 200metres, I think, so the area should near flat considering the size of the ellipse.
The photo doesn't look to me like it shows a part of the landing zone.

Last edited by elderflower (2018-04-30 11:17:29)

louis · 2018-04-30 12:43:59

Agreed. And craters, however large, must naturally collect dust and debris which could make them unstable for large craft. I think we are better off on a plain like Chryse Planitia.

GW Johnson wrote:

See the size of the Gusev crater landing ellipse in post #44? That's what happens from a direct-entry trajectory with a positioning satellite constellation of adequate capability. Plus the drift and the inability to steer, from using chutes.
As for the sloping, rough, rocky ground, that's what big landing pads, hydraulically-stroked landing legs, and a height-to-pad-span ratio under unity are for. There is no way BFS could ever land on terrain looking like that in the photo of post #44.
GW

SpaceNut · 2018-04-30 19:55:33

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

https://mars.jpl.nasa.gov/mgs/sci/fifthconf99/6176.pdf

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

Name Coordinates Size (km)
Acidalia Planitia 50°N 21°W 3400
Amazonis Planitia 26°N 163°W 2800
Arcadia Planitia 47°N 176°W 1900
Argyre Planitia 50°S 43°W 900
Chryse Planitia 29°N 40°W 1500
Elysium Planitia 3°N 155°E 3000
Eridania Planitia 38°S 122°E 1100
Hellas Planitia 42.5°S 70.5°E 2300
Isidis Planitia 14°N 88°E 1200
Utopia Planitia 47°N 118°E 3600

elderflower · 2018-05-02 04:25:17

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.

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#26 2018-04-14 17:06:26

Re: Where to Land...

#27 2018-04-16 08:31:53

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#37 2018-04-19 11:31:32

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#38 2018-04-21 17:50:53

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#40 2018-04-21 21:04:00

Re: Where to Land...

#41 2018-04-21 21:52:23

Re: Where to Land...

#42 2018-04-22 06:16:21

Re: Where to Land...

#43 2018-04-22 08:59:01

Re: Where to Land...

#44 2018-04-29 19:30:47

Re: Where to Land...

#45 2018-04-30 06:30:22

Re: Where to Land...

#46 2018-04-30 10:31:10

Re: Where to Land...

#47 2018-04-30 11:11:48

Re: Where to Land...

#48 2018-04-30 12:43:59

Re: Where to Land...

#49 2018-04-30 19:55:33

Re: Where to Land...

#50 2018-05-02 04:25:17

Re: Where to Land...

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