Where to Land...

louis · 2018-04-10 14:03:54

We don't often focus on where to land on Mars...

Does anyone have thoughts on:

(a) Where Space X might be planning to land...

(b) Where you think would be the best location to land.

If anyone has found any detailed consideration of potential landing sites recently, I'd be interested to see that. I don't think Musk has ever given any direct clues about the landing site.

Here's a helpful article:

https://www.popsci.com/if-humans-go-to- … and#page-2

It makes the point that sites where robot landers have landed previously are good places to land, as we know what to expect.

My own favourite would be Chryse Planitia either at the Viking Lander 1 landing site or, perhaps more to the North East from there at about latitude 20 N which is still flat plain but has a better water signature.

https://mars.jpl.nasa.gov/mgs/target/marsmap1b.jpg

I think Chryse Planitia would make a good base for exploration of key Mars features like Valles Marineris and Olympus Mons, the old shore of the Northern Ocean, and iron ore/other deposits.

Void · 2018-04-10 14:30:23

I am disobeying my current new rules, for you. I should not be reading this stuff:
Arcadia Planitia:
https://www.geekwire.com/2017/arcadia-p … pacex-ice/
Quote:

Possibly similar to Utopia Planetia in character.

That being ice 200-500 feet deep with significant inclusions of regolith 15%-50% with significant "VOIDS" in the ice which scares me for the landing event. Will it crumble?

But I will now go.

SpaceNut · 2018-04-10 20:23:00

Equally as tantilizing is https://marspedia.org/Hellas_Planitia

Radar images by the Mars Reconnaissance Orbiter (MRO) spacecraft's SHARAD radar sounder suggest that features called lobate debris aprons in at least some craters in the eastern region of Hellas Planitia are actually glaciers of water ice lying buried beneath thin layers of dirt and rock. Scientists believe that snow and ice accumulated on higher topography, flowed downhill, and is now protected from sublimation by a layer of rock debris and dust. Sublimation is when a solid changes directly to a gas without passing through a liquid phase. So on Mars, water ice will go directly into a vapor phase. This is common on Mars because of its thin atmosphere. Furrows and ridges on the surface were caused by deforming ice

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

The depth of the crater (7,152 m (23,465 ft)[1] ( 7,000 m (23,000 ft)) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 12.4 mbar (0.012 bar) during the northern summer .[8] This is 103% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or 0.09 psi) and above the triple point of water, suggesting that the liquid phase could be present under certain conditions of temperature, pressure, and dissolved salt content.

louis · 2018-04-11 03:14:24

Somewhere we want to and should go to asap. But we haven't sent any landers there before have we? It's southern hemisphere, a far less likely focus for initial colonisation I feel than the northern half. You could well be in the high 40s southern latitude if you are looking for water - certainly not ideal for solar.

SpaceNut wrote:

Equally as tantilizing is https://marspedia.org/Hellas_Planitia
Radar images by the Mars Reconnaissance Orbiter (MRO) spacecraft's SHARAD radar sounder suggest that features called lobate debris aprons in at least some craters in the eastern region of Hellas Planitia are actually glaciers of water ice lying buried beneath thin layers of dirt and rock. Scientists believe that snow and ice accumulated on higher topography, flowed downhill, and is now protected from sublimation by a layer of rock debris and dust. Sublimation is when a solid changes directly to a gas without passing through a liquid phase. So on Mars, water ice will go directly into a vapor phase. This is common on Mars because of its thin atmosphere. Furrows and ridges on the surface were caused by deforming ice
https://en.wikipedia.org/wiki/Hellas_Planitia
The depth of the crater (7,152 m (23,465 ft)[1] ( 7,000 m (23,000 ft)) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 12.4 mbar (0.012 bar) during the northern summer .[8] This is 103% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or 0.09 psi) and above the triple point of water, suggesting that the liquid phase could be present under certain conditions of temperature, pressure, and dissolved salt content.

elderflower · 2018-04-11 05:21:08

Wherever is selected in the end, it will need to be at low level if we are to use the atmosphere to brake a large object. Obviously Hellas, Chryse, Amazonis and other major basins would qualify on this account, however Hellas is a long way from the equator so photovoltaic power supplies will be adversely affected. This may not matter in the more distant, atomically powered future, but for early missions it will probably be either the main source or the backup source of power.

GW Johnson · 2018-04-11 12:53:50

Weight (on Mars) of the vehicle at landing, divided by the total flat, touching area of the landing pads, is the average pressure exerted by the vehicle upon the local surface. Ignoring the effects of subsurface voids, the allowable bearing pressure of the soil must exceed "by considerable margin" the bearing pressure exerted by the vehicle. That covers dynamic effects touching down.

We'll need to do some real surface bearing tests, and those results are going to vary widely from place to place, but most of those Mars soils should be at least as strong as fine loose sand. That supports 1 to 2 tons per sq.ft (0.10 to 0.20 MPa). Those would be the 2000 lb US ton. An awful lot of Mars should be more like gravel and coarse sand in natural thick beds. That supports 4 to 5 tons/sq.ft (0.38 to 0.48 MPa). For locations more like a soft, easily-spaded clay, bearing strength is only 1 ton/sq.ft (0.10 MPa).

Those values (and much more) came from table 12.2.6 in my 1987 edition of Marks' Mechanical Engineer's Handbook.

As for the margin by which soil allowable bearing pressure must exceed the bearing pressure exerted by the vehicle, I'd recommend at least a 100% margin, or soil capability at least twice the applied pressure. Factor 4 or 5 would be even more reliable. Suspenders-and-belt, with armored codpiece, that's what's smart for those first missions.

GW

louis · 2018-04-11 16:51:13

One of the important and commendable aspects of Space X's Mars Mission plan is that the first part of the Mission involves landing two BFR cargo craft. If, by some mishappenstance, the landing area chosen proved to be treacherous, Space X will know that before any humans land. Yes, the mission might have to be abandoned at that location, should that occur, but at least no humans will have been sacrificed and the mission can start up again in two years' time.

Re what was learned from the Viking missions:

"The Viking landers dug trenches in the soil and revealed material unlike any found on Earth. By measuring the dimensions of the trenches and how much material collapsed, mission scientists estimated the soil's cohesion to be similar to that of wet sand."

https://www.space.com/33482-viking-mars … egacy.html

I think the description "wet sand" probably covers everything from very firm land to quicksand!

Intuitively the Viking 1 landing site looks v. firm to me, due to the stones and small boulders forming part of the terrain.

GW Johnson wrote:

Weight (on Mars) of the vehicle at landing, divided by the total flat, touching area of the landing pads, is the average pressure exerted by the vehicle upon the local surface. Ignoring the effects of subsurface voids, the allowable bearing pressure of the soil must exceed "by considerable margin" the bearing pressure exerted by the vehicle. That covers dynamic effects touching down.
We'll need to do some real surface bearing tests, and those results are going to vary widely from place to place, but most of those Mars soils should be at least as strong as fine loose sand. That supports 1 to 2 tons per sq.ft (0.10 to 0.20 MPa). Those would be the 2000 lb US ton. An awful lot of Mars should be more like gravel and coarse sand in natural thick beds. That supports 4 to 5 tons/sq.ft (0.38 to 0.48 MPa). For locations more like a soft, easily-spaded clay, bearing strength is only 1 ton/sq.ft (0.10 MPa).
Those values (and much more) came from table 12.2.6 in my 1987 edition of Marks' Mechanical Engineer's Handbook.
As for the margin by which soil allowable bearing pressure must exceed the bearing pressure exerted by the vehicle, I'd recommend at least a 100% margin, or soil capability at least twice the applied pressure. Factor 4 or 5 would be even more reliable. Suspenders-and-belt, with armored codpiece, that's what's smart for those first missions.
GW

Last edited by louis (2018-04-12 03:39:13)

SpaceNut · 2018-04-11 19:07:21

Well we would want to land at the lowest possible place near the equators out to maybe no more than 30' north or south

aHR0cDovL3d3dy5zcGFjZS5jb20vaW1hZ2VzL2kvMDAwLzAwMC85NDEvb3JpZ2luYWwvMDUwNDE4X21hcnNfZXF1YXRvcl8wMi5qcGc=

https://geology.com/articles/highest-po … mars.shtml

Or how about https://en.wikipedia.org/wiki/Schiapare … an_crater)
2.7°S 16.7°E

Its got lots of small crater for science and for building a dome cover as these would be within our capability once we are using insitu materials.

No mater where we go we will need to send down a scout mission to get a better lay of the land for the much bigger manned landings to follow.

elderflower · 2018-04-12 02:41:48

GW. Fine sand will have been blown away by the landing thruster exhausts. Only big rocks and consolidated sediments will resist this. What looks flat now may prove to be very uneven after this process. And maybe unstable!

Last edited by elderflower (2018-04-12 02:42:34)

GW Johnson · 2018-04-12 07:58:11

Hi Elderflower:

The effect you describe is another argument for a rather wide span between the landing leg pads relative to the size of the vehicle itself. The rocket jet streams will tend to create a crater of limited size right underneath the vehicle, with low-angle slanted profile radially outward.

To get beyond this effect (and the rocks it uncovers), the pads need to be radially far away from the rocket nozzles. Between that and the basic overturn static stability, you want a height / pad span ratio closer to 1 than the 3 or 4 Musk currently indicates for BFS.

You can actually accomodate quite a bit of surface roughness with sufficient stroke built into the landing legs. What worries me is two-fold: (1) cave-in of subsurface cavities (a complete unknown), and (2) a leg coming down on a big rock and then slipping off.

That second item puts a very fast stroke rate requirement on the landing leg, in order to recover before the vehicle topples over toward the effectively too-short leg. The taller the vehicle relative to the landing pad span, the worse this is.

The subsurface cavity problem is a real unknown. Perhaps a small rover probe with a ground-penetrating radar might help identify whether a potential landing area is safe. Perhaps not.

If the mission design didn't depend on direct entry from the interplanetary trajectory, then the vehicle could stop in Mars orbit. From there it could send multiple such small probes to multiple possible landing sites, and pick the "best" one based on actual "ground truth".

That's a higher delta-vee mission, but it offers more hazard mitigation potential. Maybe those old 1950's mission planners really did have it right.

GW

Last edited by GW Johnson (2018-04-12 08:00:28)

louis · 2018-04-12 18:53:14

Just an idea - what about inflatable (kevlar style) bags around the legs to provide additional stability? Or what about angled collapsible "spurs" beneath the pads as they land...designed to kick any major rocks or boulders out of the way or what about 5 metre probes that can detect large rocks at the last moment and result in a minor deviation to avoid them?

GW Johnson wrote:

Hi Elderflower:
The effect you describe is another argument for a rather wide span between the landing leg pads relative to the size of the vehicle itself. The rocket jet streams will tend to create a crater of limited size right underneath the vehicle, with low-angle slanted profile radially outward.
To get beyond this effect (and the rocks it uncovers), the pads need to be radially far away from the rocket nozzles. Between that and the basic overturn static stability, you want a height / pad span ratio closer to 1 than the 3 or 4 Musk currently indicates for BFS.
You can actually accomodate quite a bit of surface roughness with sufficient stroke built into the landing legs. What worries me is two-fold: (1) cave-in of subsurface cavities (a complete unknown), and (2) a leg coming down on a big rock and then slipping off.
That second item puts a very fast stroke rate requirement on the landing leg, in order to recover before the vehicle topples over toward the effectively too-short leg. The taller the vehicle relative to the landing pad span, the worse this is.
The subsurface cavity problem is a real unknown. Perhaps a small rover probe with a ground-penetrating radar might help identify whether a potential landing area is safe. Perhaps not.
If the mission design didn't depend on direct entry from the interplanetary trajectory, then the vehicle could stop in Mars orbit. From there it could send multiple such small probes to multiple possible landing sites, and pick the "best" one based on actual "ground truth".
That's a higher delta-vee mission, but it offers more hazard mitigation potential. Maybe those old 1950's mission planners really did have it right.
GW

elderflower · 2018-04-13 05:26:37

Yes we need a wide span and probably more than five legs, but you can't avoid the risk of toppling altogether due to surface collapses.
Land on an icy surface and blow away the dust covering and the surface will be unstable, but this one is likely to be fairly slow and blankets could be deployed to protect the ice after landing. Collapse of a cavity is much more dangerous and is likely to be fast.
Maybe the extra load spreading legs could be left behind when the BFR returns to earth to land on 3 or 4 remaining legs on the pad.
The next ESA lander is said to include ground penetrating radar and a drill so it could do some prior testing. we maybe need several of these to examine candidate landing sites.

GW Johnson · 2018-04-13 08:14:54

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

elderflower · 2018-04-13 08:44:14

Is it correct that Falcon Heavy can put 15Te or so on Mars' surface? If that is so we could send a hopping robot with fuel enough to allow investigation of more than one site. Then we might know where to land a really big mission safely.
Alternatively we might use several identical smallish robot missions, but I wouldn't know about multiple independent delivery of such items. I think it has only been done for warheads. Either way it would be stupid to gamble on a site, identified from orbit, being acceptable for the firsl BFR landing.

GW Johnson · 2018-04-13 08:58:14

Seems like the payload to Mars for Falcon-Heavy is now listed at something like 12-15 metric tons, although I am speaking from memory. It used to be nearer 4. At my age, memory is becoming less reliable.

But the Red Dragon thing showed that propulsive landing is indeed the way to go for things over 1 ton landed. It need only kill the last 0.7-1.0 km/s delta-vee, drag aerodynamics can kill the rest (7.5 km/s direct entry from an interplanetary trajectory).

It would be feasible to have around almost half the landed item to be the ultimate payload: some sort of rover to investigate and prepare the site. I get a mass ratio of 1.0 for 300 sec Isp (poorer than MMH-NTO) and 1 km/sec delta vee.

If the entry vehicle was 12 ton, that's maybe a 6 ton rover vehicle. Plenty of mass allowance there to investigate and minimally-prepare a site. Although, I have ignored the entry shielding. But that's not all that heavy, done as a 1-shot deal.

GW

Last edited by GW Johnson (2018-04-13 08:59:27)

louis · 2018-04-13 10:16:05

A Rover's not going to tell you if there's a cavity a few feet underneath or some hidden structural weakness. The first BFR landing will be a cargo craft. It seems a reasonable gamble to me. Are we really expecting there to be a problem with landing? I doubt it is a real threat, given the nature of the planet and the temperature range. As long as the landing site is chosen wisely (for me that means going to a site we already have a lot of knowledge, on preferably where a rover or lander has been sent before).

elderflower wrote:

Is it correct that Falcon Heavy can put 15Te or so on Mars' surface? If that is so we could send a hopping robot with fuel enough to allow investigation of more than one site. Then we might know where to land a really big mission safely.
Alternatively we might use several identical smallish robot missions, but I wouldn't know about multiple independent delivery of such items. I think it has only been done for warheads. Either way it would be stupid to gamble on a site, identified from orbit, being acceptable for the firsl BFR landing.

Void · 2018-04-13 11:18:02

I would add that in that landing, perhaps a radar would be able to scope out the surroundings under the soil. Fail or win, then you would have a map of what lies beneath. Then subsequent landings may have success, or a promoted chance of success.

GW Johnson · 2018-04-13 12:24:53

There are two good ways to detect subsurface cavities known to work on Earth: seismic waves from a small explosive shot, and ground-penetrating radar. The presumption here is that either or both techniques would work on Mars. Seems like a better presumption than just betting, even an unmanned cargo BFS landing. Certainly better than betting the lives on a crewed BFS.

Red Dragon is no more, and wouldn't be useful landing rovers anyway, excepting very tiny ones that would fit through its hatch. But it would seem to me that a Dragon heat shield and Super Draco thruster rig could be combined with a custom aeroshell and a fairly substantial rover. NASA couldn't be trusted to do this very quickly, but a somebody like Spacex could.

Small rover has grader blade and moves big rocks out of the way. Solar panels plus batteries power it. It emplaces a radar transponder to help guide the final touchdown of the BFS. It has either or both the ground penetrating radar and some seismic shock profiling gear. Maybe something about the size of the Curiosity rover. Wouldn't that fit inside a 3.7 m dia aeroshell shaped like a Dragon? On its end, maybe? Don't need legs, just land right on the heat shield, which is a 1-shot item.

Send it to the planned site a few weeks before the cargo BFS arrives. Let it pick out some 4-6 solid level spots, and grade off the bigger rocks on them. These would be landing zones about the size of a football stadium. Let it emplace the transponders on these zones. Could still serve as an extra robot construction machine after the BFS's start arriving.

At ~$120M per launch, plus around $1-200M to build the payload, this looks like good insurance to me. Just sayin'.

GW

Last edited by GW Johnson (2018-04-13 12:32:01)

louis · 2018-04-13 16:23:56

Yes, the scheduled 2022 BFR cargo missions are expendable really. If they could produce good sub-surface landing data that would be great and give reassurance for the human landing in 2024.

Void wrote:

I would add that in that landing, perhaps a radar would be able to scope out the surroundings under the soil. Fail or win, then you would have a map of what lies beneath. Then subsequent landings may have success, or a promoted chance of success.

louis · 2018-04-13 16:27:30

Well the 2022 BFR Cargo mission would be a good point to put a rover out to remove boulders and do ground radar. But I have seen contradictory info on whether cargo is going to be unloaded. Automated delivery of a rover to the surface might be possible. I'm thinking the equivalent of an airplane escape chute for the Rover to ski down.

GW Johnson wrote:

There are two good ways to detect subsurface cavities known to work on Earth: seismic waves from a small explosive shot, and ground-penetrating radar. The presumption here is that either or both techniques would work on Mars. Seems like a better presumption than just betting, even an unmanned cargo BFS landing. Certainly better than betting the lives on a crewed BFS.
Red Dragon is no more, and wouldn't be useful landing rovers anyway, excepting very tiny ones that would fit through its hatch. But it would seem to me that a Dragon heat shield and Super Draco thruster rig could be combined with a custom aeroshell and a fairly substantial rover. NASA couldn't be trusted to do this very quickly, but a somebody like Spacex could.
Small rover has grader blade and moves big rocks out of the way. Solar panels plus batteries power it. It emplaces a radar transponder to help guide the final touchdown of the BFS. It has either or both the ground penetrating radar and some seismic shock profiling gear. Maybe something about the size of the Curiosity rover. Wouldn't that fit inside a 3.7 m dia aeroshell shaped like a Dragon? On its end, maybe? Don't need legs, just land right on the heat shield, which is a 1-shot item.
Send it to the planned site a few weeks before the cargo BFS arrives. Let it pick out some 4-6 solid level spots, and grade off the bigger rocks on them. These would be landing zones about the size of a football stadium. Let it emplace the transponders on these zones. Could still serve as an extra robot construction machine after the BFS's start arriving.
At ~$120M per launch, plus around $1-200M to build the payload, this looks like good insurance to me. Just sayin'.
GW

elderflower · 2018-04-14 03:35:34

We want a well characterised landing site before we send a BFR. The risk of toppling one of the first three is too high. Especially as the manned flight will already be on the way when the third cargo flight lands. This is why I was considering a pre-BFR mission, using the Falcon Heavy, to scope out one or more landing sites. From what GW says this looks hopeful.
I discount any activity by NASA or the other national space agencies. They suffer from too much politics.

louis · 2018-04-14 04:48:15

But the human mission won't set off before the first two cargo BFRs have landed.

The landing area issue is a problem but not an insurmountable one. Through a combination of choosing an area where other craft have landed we substantially reduce the risk. The cargo BFRs can no doubt carry some further surveillance equipment, not least cameras to survey for big boulders. Add to that satellite surveying.

elderflower wrote:

We want a well characterised landing site before we send a BFR. The risk of toppling one of the first three is too high. Especially as the manned flight will already be on the way when the third cargo flight lands. This is why I was considering a pre-BFR mission, using the Falcon Heavy, to scope out one or more landing sites. From what GW says this looks hopeful.
I discount any activity by NASA or the other national space agencies. They suffer from too much politics.

GW Johnson · 2018-04-14 09:38:48

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

Last edited by GW Johnson (2018-04-14 09:39:44)

Oldfart1939 · 2018-04-14 09:54:25

It's my belief that the first uncrewed ships should have SIX landing legs in order to distribute weight more evenly, and that the actual "footprint" needs to be much larger as GW has already stated. The lunar landers paid a lot of attention to the possibility of a tip over, and so should SpaceX. These early "artist's conceptions" may not be at all what the actual engineered spacecraft will have as landing legs, though. Some of the early test "hop" flights in the McGregor test site may bring a sense of reality, if they try landing in the desert instead of on the concrete pad.

SpaceNut · 2018-04-14 16:58:23

https://en.wikipedia.org/wiki/Landing_Zones_1_and_2

The site consists of two main pads 282 feet (86 m) in diameter marked with the stylized X from the SpaceX company logo. An additional four 150 feet (46 m) diameter pads were initially planned to be built to support the simultaneous recovery of additional boosters used by the Falcon Heavy, although only two additional pads are planned for the near future.

barge landings

Falcon is just under 4 meter diameter and the BFR is going to be 12 to 15 meters making the landing area much larger as we need the legs to be farther appart.

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