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Bear in mind that Starship is Spacex’s new name for what was once the BFS second stage spacecraft article of its BFR/BFS system. To be useful at Mars or the moon, this spacecraft must be able to make rough-field landings. Its mass is heaviest when fueled for launch. Initially, it must be refueled for use at Mars, and eventually, also the moon. These static loads are larger than the landing weights, even factored for dynamical impact.
There are two parts to this:
(1) tip-over on rough or sloping surfaces, and (2) not exceeding the bearing load
capability of the natural surfaces.
There is also a new idea presented here for creating very large landing
pad surfaces that fold so as not obstruct airflow, in a very practical way.
The tip-over problem was well-explored in another article on
this site, as part of updates to the
basic performance evaluation article. That
article was “Reverse-Engineering the 2017 Version of the Spacex BFR” dated
April 17, 2018. The same article identified soil bearing strengths
as likely inadequate to support the spacecraft when refueled for launch.
The related article “Relevant Data for the 2018 BFS Second
Stage” dated September 24, 2018, included among other things a way to
reconfigure the round tip-mounted landing pads into oblong pads of increased
area. Those results were still
inadequate for the loose fine sand-like surfaces of much of Mars.
What is analyzed here is a different landing pad idea, depicted in Figure 1 (all figures at
end of article). Essentially, panels resembling landing gear bay doors are
built into each side of each fin tip,
with hinge lines at the aft trailing edge (which is the touchdown
surface otherwise). Unfolded hydraulically,
these panels become very large landing pad surfaces. Folded,
they do not protrude into the ascent or descent airstreams at all.
The same figure shows an 1100 metric ton fueled mass, which really could be as large as
1300-something tons. However, the BFS weight statement is still not known
publicly with any certainty. This figure
is in the same ballpark, given all the
other uncertainties.
Assuming 1100 metric tons of mass, the weights on Earth, moon,
and Mars are given in the figure.
The bearing pressures associated with those weights are also shown, assuming 2 m by 2 m parallelogram-shaped
folding panels are used. This assumes 2
panels per fin, and 3 fins. Just that initial assumption provides some 10
times the bearing area as the roughly 1 m diameter round tip pads shown in the
Spacex illustrations of this vehicle.
Figure 2 presents safe load-bearing strengths for civil
engineering purposes of various types of Earthly surfaces. These came from an older-vintage Marks’
Mechanical Engineer’s Handbook. No such
reference yet exists for lunar or Martian soils. However,
experiences from the Apollo missions verify that the lunar regolith is
similar to fine, loose Earthly
sand.
Experiences with the various Mars landers and rovers suggest
that much of Mars is similar to lunar regolith and to Earthly fine, loose sand.
Some of Mars seems to have a mix of sand, gravel and larger rocks, perhaps similar to Earthly surfaces such as
loose beds of medium and coarse sand, or
perhaps even as substantial as beds of coarse sand with gravel. These require picking, not a spade,
to remove. A hard clay requiring picking would also be
of similar bearing strength.
All of this is indicated in Figure 2.
Figure 3 compares applied bearing loads to soil strengths for
the moon, Mars, and Earth.
The moon is the least demanding problem because of its lowest
gravity. A design adequate for the moon
is adequate only for some of Mars: the
indicated folding-panel parallelogram dimensions would be 2.2 m by 2.2 m, for about 29 sq. m bearing area. On Earth,
sites must be strong, well-packed
sand/gravel or hard clay. No
beaches, no sand dunes, no soft desert.
If we increase that pad area to be capable all over
Mars, the folding panel parallelogram
dimensions become 2.67 m b 2.67 m, for
about 43 sq.m bearing area. This is part
of the comparative pad area summary and comparison given in Figure 4. There is a trade-off here: the bigger these folding panels can be, the more of Mars (that is otherwise fairly smooth and level) is a feasible landing and takeoff site.
The moon is not a problem, nor is
much of dry land Earth (most anything requiring picking or blasting would be
adequate).
This would simply not be the case with those 1 m diameter
round tip-mounted landing pads that Spacex illustrates.
That total pad area is about 2.4 sq.m,
give or take a small amount. What
is needed for rough-field capability on Mars (or the Earth) apparently falls in
the 30-45 sq.m range. You simply
cannot do that, in any practical
way, with smallish fixed-geometry
tip-mounted landing pads. Those will require
thick reinforced-concrete landing fields,
or else thick solid rock.
I know they have their hands full at Spacex trying to make
this vehicle a reality. But some of the
thinking I have explored here, also needs
to go into Spacex’s designs!
Figure 4 – Tradeoff of Increasing Pad Size vs Landing Site
Choices
Update 2-5-19:
As a follow-up, I put
some more-traceable masses for the BFS/”Starship” weight statements into a
spreadsheet, with gravity data, and empirical data for the safe bearing
capability of various Earthly surfaces.
This included the selections (and rationales) as for which surfaces
resemble the moon and Mars, and which
might be the widespread worst cases for emergency landings on Earth. Sources are indicated.
Masses and gravity are given in Figure A below (all figures
at end of this update). The residual propellant remaining is nothing but a wild
guess, knowing that a dry-tanks landing
is truly risky. As it turns out, the presence or absence of residual propellant
mass at touchdown does not drive the sizing of landing pad area. Refilled launch weight drives this.
The safe surface bearing-pressure capability data for a variety
of Earthly surfaces is given in Figure B.
These are civil engineering data from an old-vintage Marks’ Mechanical Engineer’s
Handbook. These surfaces are rather
variable in properties, as is typical of
geology. They represent safe bearing
pressure loads so that your structure or object does not try to sink slowly
into the surface, even over long periods
of time. It is conservative, ethical practice to use the min pressure
values for design. In the
handbook, the table presented both
metric and US customary values, for
which it was obvious the metric were converted from the US customary source
values.
To this table I have added the notations about which
surfaces resemble the bulk of the moon and Mars, and the rationales for those selections. I have also indicated the most common
soft-surface emergency landing surfaces for Earth, excluding soft sand beaches and deserts (and
swamps). The rationale for that is
simple experience. These are my best estimates, if I had to do this.
I worked out local-weight weight statements for Earth, Mars,
and the moon, for two
configurations. One is arrival, with the larger payload sent from Earth, and only residual propellant left at
touchdown. The other is at
departure, with full propellant
load, but a reduced payload, for the return to Earth. This reflects exactly what Spacex says the
scenarios are for Mars.
Spacex says there needs to be no lunar refilling for return
to the Earth from the moon, but this
analysis anticipates that eventually lunar refilling might be attempted, for trips from the moon to destinations other
than Earth. It does not matter; as it turns out, the lunar launch case does not determine the
design requirement for landing pad area.
But I had to check, as a due
diligence item.
What I found was that local-weight launch weights exceeded
local-weight touchdown weights by roughly a factor of 5. Therefore,
it is only local launch weights that govern max bearing pressure exerted
upon the local surfaces. One of these is
the worst case that drives the design requirements.
Surfaces on the moon, and the great majority of Mars, resemble soft fine sand. There are places on Mars with somewhat-stronger
surfaces, but these are definitely not
the majority of possible landing sites. You
do not want a rough-field landing design restricted to rare site opportunities. That would be rather pointless. As for the range of properties, you have to select the min capability.
Figure C combines the local-weight weight statements with
the appropriate selected surface bearing capabilities, to produce min total landing pad areas for
Earth, Mars, and the moon.
Mars governs, and by a
significant margin. The total pad area result
obtained here is not at all far from the seat-of-the-pants 43 sq.m in the
original article just above. But this
updated result is more traceable, and
therefore the more reliable value. It is
just about 46.2 sq.m.
The same figure also gives a selection of parallelogram
dimensions for each fold-out landing pad panel.
This is a function of the number of landing-leg fins, the number of panels per fin, and the aspect ratio of those panels (the
height to base ratio for the parallelogram shape). Beyond scope here is the structural design of
such panels; it seems likely that the
lightest version would be nearer aspect ratio 1. That would be a panel 2.77 x 2.77 m
size, vs 2.67 x 2.67 in the original article. That’s pretty close!
Again, I must point
out that this is the sort of design, and
design analysis, that is needed by
Spacex to really provide a rough-field capability for its BFS/”Starship”
spacecraft on Mars (or anywhere else). The
round tip pads Spacex currently shows are roughly a meter diameter, for a total of about 2.4 sq.m total
area, roughly some factor 20 smaller
than what I determined here.
Implications
Without the large total landing pad area I found, the craft is restricted to very thick
reinforced concrete landing pads, or smooth, level stretches of thick, solid rock.
Otherwise, while you might
possibly land (and no guarantees about that!),
you’ll sink-in unevenly, and
tip-over “for sure”, upon refilling propellant
for launch.
At 2.4 sq.m and Mars launch weight, the applied bearing pressure is ~0.39
MPa, similar to the 0.38 MPa min capability
of Earthly coarse sand & gravel, and
similar to only a minority of Mars!
And even that still lacks the factor 2 margin you need for the dynamical
impact effects of the touchdown: the
fin tips will inevitably stab deeply into the surface, risking getting stuck like tent stakes. Very likely,
they will stab-in unevenly,
risking a tip-over, even if the
site is level. So the chances of
a successful landing with the depicted design are very poor, and there is no chance at all of a
successful refilled launch.
If the site actually resembles the soft sand that is the
bulk of Mars, there is no
chance at all of a successful landing,
and by far. The applied bearing
load is 0.39 MPa, even without the
factor 2 for the dynamics. The soft sand
capability is only 0.1 MPa. That’s about
factor 4 outside-of-the-ballpark, and
about factor 8 wrong with dynamics allowed-for!
That’s a crash, period. That’s exactly what will happen with
three 1-meter-diameter fixed-geometry fin-tip landing pads!
Conclusion
Something about the design Spacex currently shows, must change substantially, before even an unmanned cargo ship flies to
Mars. They need 47 sq.m of pad
area, not 2.4 sq.m. That’s what the best-available data actually says.
Figure C – Local-Weight Weight Statements and Bearing Loads Size Landing Pad Area
