For vehicle designs that do not meet rough-field criteria
(Ref. 1), they should not be sent
to the lunar surface, until a
hard-surfaced, strong landing pad
surface has been constructed. The same
applies to Mars, most of its surface is
similarly weak.
Such involves a deep excavation, back-filled with size-graded rock of decreasing sizes toward the surface, with the rocks in direct touching contact, and the void spaces in each layer filled with appropriate hard-packed fines, to include sands and dusts. The last layer of fill is gravel with sand and dust fines, on top of which the final finished paved surface is laid. This is shown in Figure 1:
Figure 1 – How Roadbeds and Foundations Are Supported Upon
Softer Ground
There is a load-spreading effect of the backfill-rock
substrate, which is how you spread a
concentrated large force on the surface, onto a much larger area at the bottom of the
excavation. This behavior reduces the
applied bearing pressure at the surface,
to what the regolith or soil below the excavation, can actually withstand.
Estimates of the proper load spreading angle vary, but the usual civil engineering practice
assumes angles from about 30 degrees to as much as 45 degrees, with the usual default being 34 degrees. Angles as low as 26 degrees have been seen
experimentally, but are not common.
For
this rocket landing pad application, the
author recommends (in the absence of any real off-world experience) something
like 30 degrees, and an allowable soil
bearing pressure that is around factor 2 below measured failure pressures.
That sets the required excavation depth
for a given load force with a certain surface footprint, as Figure 2 shows:
Figure 2 – Excavation Depth Depends Upon Pressure Reduction
Ratio and Spread Angle
Bear in mind the virtually all the regolith on the moon
resembles nothing so much as “sand-dune sand” here on Earth. Fine dry loose sand here on Earth has a minimum allowable bearing
pressure of about 1 US ton per square foot,
equal to about 0.10 MPa. The
max allowable value is only twice that,
and the safety factor below failure is also only about 2 (meaning the
0.1 MPa allowable fails at about 0.2 MPa,
and the 0.2 MPa allowable fails at about 0.4 MPa). Most of Mars’s regolith is similar. The rocks embedded in these regoliths generally
do not touch, and so offer no
reinforcement, unlike a properly-built
roadbed.
These heavy load-bearing pad foundation construction techniques
described here, were developed and used
with great success by the Romans building roads, and are still used today for Earthly road-building. Unfortunately, doing it "right" is quite
expensive, so today our roads are built
with inadequate excavation and inadequate quantities and size-grading of the
rock fill underneath the paved finish surface.
So, most roads today
do not hold up nearly as well as the old Roman roads. Big heavy trucks do the most damage, by crushing the inadequate roadbed (and
pavement) down where the wheels track, one after the other, thus "rutting" the road. Rain fills these ruts, presenting a loss-of-control danger to
automobiles, if the water depth exceeds
the tread depth on the tires. This kind
of roadbed damage is illustrated in Figure 3, and is not properly repaired by simply
resurfacing the pavement:
Figure 3 – What a “Crush Rut” Looks Like
The photo was taken right near my driveway on the country
highway that runs by my house and farm.
It has so very clearly never been properly repaired! This highway is posted as “58,420 lb max
gross weight” (about like a 10-wheel dump truck), but I see 18-wheel big-rigs out here all the
time, and the big-rig grain trucks
during harvests appear to far exceed the statewide 80,000 lb gross weight limit! A rough guess puts them in the
100,000-120,000 lb range, pretty close
to the original factory gross weight limits for most of them.
This kind of back-filled excavation construction, whether here on Earth, or off-world on the moon or Mars, is all "dirtwork" to well-known
civil engineering standards, except
for the final pavement finish surface.
To land
rockets, that finish pavement must be
both heat-resistant and blast-erosion resistant! Concrete usually requires some repair after a
rocket landing because of heat-induced spalling, but concrete as we know it is simply unavailable
on the moon (or Mars).
Some
sort of tough, resistant “tiles” laid
like close-fitted flagstones might work on the moon or Mars. But they need to be thick, and as heavy as possible, in order not to get ripped away by jet blast
shear forces! It would help if there
were projections from their bottoms that penetrated into the top
gravel-and-fines layer of the bedding fill.
This is an approach option that needs development: it is not
experience-proven, and therefore not
yet ready to apply!
However, laying
such “tiles” directly upon the weak regolith will NOT work! They will simply be crushed down into it by the
large concentrated load forces, and also
tilted by those same forces acting off-center!
That kind of behavior is just unacceptable, especially since a rocket that topples over
is a rocket that fatally explodes!
References
(1 (1) “Criteria for Rough-Field Landings” by GW
Johnson, posted 12-01-2025
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