The image in Figure 1 is some kind of artificially-generated image that I found on LinkedIn. It shows what could happen with a SpaceX “Starship” variant trying to make a rough-field landing on the moon, when not properly equipped to do so. It digs in unevenly and topples over, and a rocket falling over is a guaranteed fatal explosion!
This sort of thing is a definite risk, because the lunar regolith is a weak
“soil”, quite unable to support large
loads concentrated upon small areas. It actually
rather-closely resembles Earthly sand-dune sand. The rocks in it that do not touch each
other, cannot reinforce its strength. (The same is true of most Martian regolith.)
Figure 1 – Concept Image of Vehicle Not Configured for
Rough-Field Landings
The "how to design for rough field landing" image in Figure 2 shows what I estimated for a lunar "Starship" variant properly equipped for a rough-field landing on the moon. A rough-field design for Mars would be similar, but the numbers would be different, and it would be very difficult to protect such externally-mounted legs during Mars atmospheric entry.
Figure 2 – Options for Adding Rough-Field Capability to
Lunar “Starship” Variants
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There
are three critical design criteria a rough-field lander must meet:
(1) The
transient pressure underneath the pads (or other contact surfaces) during the
touchdown event cannot be allowed to be any greater than the bearing strength
of the lunar regolith, which is rather
similar to Earthly sand-dune sand.
(2) The
minimum span across the polygon created by the landing leg outer contact points
must exceed the height of the vehicle center of gravity above the surface.
(3) The
pads need to “tip toward the center”, so
that the lead pad cannot “dig in” if there is horizontal velocity at touchdown.
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These three critical design criteria were amply demonstrated
to be appropriate, by the Apollo LM
lander, and by the Surveyor probes before
it. There are too many today who
ignore, or never learned, these well-established criteria. It shows in the recent overturned commercial
lunar landers.
There is an image of the "Blue Ghost lander" in Figure
3. It is the Firefly Aerospace
commercial lander design that was actually quite successful landing on the
moon. Note the squat low form
relative to the landing leg pad span,
and the large size of the landing pads,
that do indeed “tip” toward the lander body in the center. It meets the same criteria by which the
Apollo LM and the Surveyor probes were designed. So, its
success at a rough-field landing should not be much of a surprise.
Figure 3 – Image of the Firefly Aerospace “Blue Ghost” Lunar
Lander
The take-home lesson here is simple: if making rough-field landings upon the moon (or
Mars), the lander vehicle design must
meet the three critical design criteria listed and highlighted above.
Vehicle designs that do not meet these criteria should not
be sent to the moon (or Mars) until a properly-constructed, hard-surfaced, and very strong landing pad has been built
there to receive them! How to build such
landing pads in such hostile places is the topic for a future article.
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ReplyDeletethere weren't an "edit" option so I tried deleting its previous version!
DeleteGreat insight! The same idea also applies to asteroids either. The gravity is low and there haven't been any "chemical-mechanical processes/forces" to make them "bound" and form a rigid rock-and-soil-like material — Like climate did on the earth for its terrain, soils and rocky hills, etc. Celestial objects without an active climate environment + weak gravity just can't have those somewhat 'bonded' materials on their crust. They are just left "crumbs of rock just settled together and sat there, didn't particularly join or fuse together" but "NOT a bounded/bonded rock".
ReplyDeleteI actually thought about this around 7 years ago while pondering asteroid defense methods, and I was quite pleased when the DART mission confirmed this idea — the impact ejected far more material than expected, suggesting Dimorphos was essentially a loosely-bound rubble pile.
I actually doubt that Martian soil would be as 'crumbly' as you described, Mars historically had an active climate, for billions of years, and still has an atmosphere (albeit thin) and wind blows there. Those should have made the rocks and soils are 'bounded' particularly (or Claude told me, this is called the 'cementation') — unlike the pristine and unbonded dust of the Moon or asteroids.
Very early on, the moon may have had an atmosphere for a brief amount of geologic time, but the impact pounding since has buried any surface rocks (igneous) with smashed rubble to a depth of several meters. Mars is a bit different: the weathering is there along with the impacts. But the bulk of that weathering is windblown, and has been, for around 2 billion years, they say. That creates a lot of loose sand and dusts (as in Earthly deserts), and that is exactly what we see there. Rocks are there at the surface, too, but not really touching each other, so they don't reinforce the sands and dusts against mechanical penetrations. There is rock below the sand, and the depth to it varies a lot from place to place. Where the rock is exposed, it is quite craggy. The asteroids and comets are different, and largely differ from each other only in the amount of ices they contain. Some were formed with a lot of ices, others not so very much, but all have lost ices to sublimation over geologic time. None of those ices are long-term stable if exposed directly to vacuum. -- GW
Delete