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

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 15,587

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

elderflower
Member
Registered: 2016-06-19
Posts: 1,087

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: 15,587

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: 2,834

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
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 15,587

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: 3,523
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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
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 15,587

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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