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#126 2019-05-15 17:35:02

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

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#127 2019-05-17 05:39:06

elderflower
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Registered: 2016-06-19
Posts: 1,115

Re: Mars InSight lander

I note the indent in the surface made by InSight's foot pad.
How much bigger will be the dent beneath the foot of a fuelled up BFR? Spacex must come up with a solution to this issue.

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#128 2019-05-17 15:47:22

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Mars InSight lander

Great catch as I was looking at the clouds...will need to do a repost

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#129 2019-05-17 18:33:28

kbd512
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Registered: 2015-01-02
Posts: 2,959

Re: Mars InSight lander

SpaceNut,

If a rather light lander like InSight makes that much of an impression in the surface, then Starship will require serious rough field landing capability or it will tip over when the ground shifts as it lands.  I think explosively driven anchors will be required to assure that the vehicle is secure when it touches down.  Anchors would also provide a more secure launch when the vehicle departs.  Some minor hardware replacement will be required, but that's still infinitely preferable to tipping over in loose regolith.

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#130 2019-05-18 16:16:29

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Mars InSight lander

Something else to make note of is with the solar panels that after dust was accumilating just happened to get cleaned off once more from the ability it has to move....

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#131 2019-06-01 12:03:11

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,643
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Re: Mars InSight lander

Landing pad sizing is actually fairly simple,  although not easy.  For landing only,  the weight (not mass,  local weight !!!) of your vehicle at touchdown gets divided by the total landing pad area to create a design bearing pressure figure.   

You factor that up by something in the 2 to 3 range to cover the dynamics of landing (factor 2) plus the uncertainties in your data such as uneven pad weight distribution on the transient (the other factor 1.5).  The estimated bearing strength (also a pressure) of the surface MUST EXCEED that factored design load pressure,  else you penetrate into the ground to an unacceptable amount. *** update *** below

If you are going to refill the propellant,  you use instead the launch weight divided by the total pad area,  not the landing weight.  For the Spacex Starship,  my best estimates say this is about 6 times the touchdown weight.  You need no factor of 2 for the dynamics,  and your weight distribution uncertainty factor is much closer to just 1.  That factored applied bearing pressure must not exceed the estimated bearing strength of the surface,  lest a pad sink in,  toppling the vehicle;  or pinning it to the ground like tent stake friction against takeoff thrust.

The biggest uncertainty here is really the max allowable bearing strength of the surface on Mars.  We're only just beginning to accumulate some real data on this.  What we do know is in accordance with the assumption that the great bulk of Mars's surface resembles either Earthly fine sand,  or a mixture of Earthly fine sand and loose rocks (same is true of the moon,  by the way).  You can get those figures from any Marks' Mechanical Engineer's Handbook (which is a big heavy volume,  not the usual notion of a small handbook).

Toppling over (a fatal outcome) is a risk from two sources:  (1) pads sinking in deeper on one side,  and (2) excessive slope leading to static instability.  With the Starship, these risks are enhanced over other craft we have landed on Mars (and the moon),  because the cg height of the Starship is about 2 or 3 times the span between its adjacent landing pads.  With all the other landers (Mars or moon) that number has been 1 or less. 

No explosively-fired anchor spear is going to be successful at preventing topple,  if the slope is too steep,  or the surface too soft,  or both.  The real trick is to design your lander so that you do not need some explosive-fired anchoring spear.  Keep your factored-up max applied load pressure under the min expected surface bearing strength,  and keep your landing pads spread very wide apart compared to vehicle length. Cg height to pad span ratio at or under 1 is the best practice (as demonstrated so far).

Simple as that.  And just as hard to do as it sounds.

GW

*** update *** a landing pad penetrating to an unacceptable depth means top of pad below surface level.  When that happens to a thin pad design,  dirt will pile onto the top of the pad,  raising the effective vehicle weight at takeoff.  This is not a problem for a one-way landing. 

Making the pad thicker acts toward preventing this,  but raises its weight,  again raising takeoff weight.  This is not a problem for a one-way landing,  but you only need enough pad thickness to carry the structural loads.  Weight is a design issue.

In the extreme,  if your pads come to resemble tent stakes more than flat feet,  they will stick deeper and deeper into the dirt,  sort of like piles driven in.  The first problem incurred is friction:  the extraction force for takeoff will be very high,  raising takeoff thrust required.  Not a problem for a one-way landing,  but a very,  very serious problem if you ever intend to take off again.

The second problem incurred is uneven penetration.  It is unlikely in the extreme that three or four landing "pads" that drive into the dirt like piles will ever penetrate evenly,  and for any of a variety of real-world reasons.  Not the least of which is the uneven distribution of subsurface rocks and boulders.  Uneven penetration leads inherently to large off-vertical attitude angle,  which very rapidly trends toward a fatal tip-over.  This is a problem even for a one-way landing.   The larger your cg height / pad span ratio,  the worse this is.

Last edited by GW Johnson (2019-06-02 11:03:29)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#132 2019-06-09 21:38:09

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Mars InSight lander

Still stuck at just 12 inches and needed to get lots deeper....
InSight's Team Tries New Strategy to Help the "Mole"

Working to move a structure to be able to see what is happening with the driving of the unit with each hammer blow....

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#133 2019-09-23 09:30:42

tahanson43206
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Registered: 2018-04-27
Posts: 1,055

Re: Mars InSight lander

In memory of Void #121 ...

Void mentions lava tubes in this post. 

The Space Show has scheduled an interview which will include discussion of lava tubes on Mars.

2. Tuesday, Sept. 24 , 2019: 7-8:30 PM PDT (9-10:30 pm CDT; 10-11:30 PM EDT): We DR. ANAHITA MODIRIASARI regarding lunar and Martian lava tubes.

Edit 2019/09/28 .... as a follow up

https://thespaceshow.com/show/24-sep-20 … odiriasari

Work done at Purdue University is reported in this link:
https://www.purdue.edu/newsroom/release … -moon.html



(th)

Last edited by tahanson43206 (2019-09-28 20:30:39)

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#134 2019-10-06 15:00:01

SpaceNut
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Re: Mars InSight lander

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#135 2019-10-07 20:39:53

kbd512
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Re: Mars InSight lander

SpaceNut,

We need another robotic mission to test a proper drill on Mars for core samples, ice mining, and seismic data gathering.  InSight's drill didn't work out.  This was only our first attempt, but landers with these tasks really should have proper drilling equipment.

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#136 2019-10-07 20:47:14

SpaceNut
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Posts: 16,150

Re: Mars InSight lander

Insight is a long metal probe that is hammer driven into the ground. It is believed that it hit a deep permafrost that is thick or a boulder.
There was a canadian drill but I am not sure whethers its going or not on the 2020 mission...its hard to remember everything...

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#137 2019-10-08 05:42:42

tahanson43206
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Registered: 2018-04-27
Posts: 1,055

Re: Mars InSight lander

For SpaceNut re #136

As you said in another topic recently, there is too much for one person to remember about activities on Mars.

In the case of post #136, there is an update if you have time for it.  The original theory (which you reported) has given way to a belief that the probe failed to advance because the soil does not have the characteristics needed to "grip" the probe after a hammer stroke.  As I understand the problem, the soil is like a spring which simply returns the probe to its starting point.

For an update:https://www.digitaltrends.com/cool-tech/insight-stuck-drill-pining/

At the start of this year, NASA’s InSight lander successfully placed its seismometer on the surface of Mars and covered it with a heat shield to protect it from the elements. But problems arose when InSight tried to deploy its second instrument, the Heat Flow and Physical Properties Package (HP3) or heat probe. The lander is equipped with a drill known as “the mole” designed to tunnel up to 16 feet beneath the surface so it can place the probe underground for more accurate readings. Unfortunately, after a few days of action and getting 14 inches down, the drill stopped moving in an issue the NASA team first thought was caused by hitting a rock, but later learned was due to the drill getting stuck in the soil.

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#138 2019-10-08 17:03:14

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Mars InSight lander

So the pounding of the rod into the ground heated the surrounding soil as it entered and formed ice as it went downward around the rod. That said slow hammer hits are not going to advance it as they need even more psi to force it to go. Sending many rapid hits means that the circuits must be able to respond with still the full force as a single hit for each leaving less time for recoil.

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#139 2019-10-09 07:48:18

elderflower
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Registered: 2016-06-19
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Re: Mars InSight lander

It was suggested that the stalling of the mole was due to a duricrust, which is quite widespread in earth deserts.

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#140 2019-10-10 08:31:50

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 3,643
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Re: Mars InSight lander

My own (unsupported) opinion is that they designed the thing using a "standard" surrogate for Martian dirt,  but ran into a big subsurface rock on Mars,  which would be something NOT in the Mars soil surrogate. 

Considering that Mars is heavily meteor-cratered,  there is a very unsurprising rich abundance of rocks and boulders scattered about the surface in many regions,  which clearly portends subsurface rocks and boulders scattered throughout the loose regolith between the surface and whatever bedrock there is deep below!  This is very little different from the moon,  but Mars has other regions buried in wind-driven shifting sand,  which the moon does not have.  And Mars has various rock-and-sand mixtures in between these extremes;  the rovers have seen this for some years now.

Which situation suggests the team that designed this "hammer mole" thing should have anticipated the easily-predictable presence of big subsurface boulders,  and put some of those into the soil surrogate they used in their development efforts.  Had they done so,  they would have ended up with a different design,  one with a better chance of working right.

This is more of a development team management error than an actual engineering error.  Their design is correct for dirt without big rocks mixed in.  But that was the wrong problem to solve,  given what we know already knew about Mars as the destination! 

A bunch of planetary scientists usually does not include engineers and technicians with real-world experience digging,  drilling,  and mining in real-world rocks and dirt.  But this team should have consulted with people like that,  and so very obviously didn't.

GW

Last edited by GW Johnson (2019-10-10 08:33:18)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#141 2019-10-10 16:02:33

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 16,150

Re: Mars InSight lander

We will soon know when the arm is pinched by side pessure as to where its a recoil bounce that is not allowing for downward progress or a boulder that we did not count on hopefully soon. If we are able when the pinch is tested to also swing the hammer at a quick as well as slow rate and listen to the mars quake sensor then we may be able to see it like an echo sonagram of the surrounding location.

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