we have been given the scientific
knowledge
the technical ability
and the materials to pursue the
exploration of the universe
to ignore these great resources would be
a corruption of a god-given ability
it's estimated that about 650 million
people watched the first moon landing in
1969
nearly a quarter of the world's
population as the world watched those
powerful saturn v rocket engines burst
into life they witnessed plumes of soddy
exhaust below from the five shuttering
nozzles as it pushed itself off the
ground burning through a colossal volume
of kerosene to lift the rocket's massive
three million kilogram weight off the
ground
soon the first stage shut off and
separated allowing the second stage to
roar into life this time powered by a
different fuel liquid hydrogen a fuel
capable of providing better performance
but it occupied too much space
liquid hydrogen is considerably less
dense than kerosene so needs to be
combined in a much higher ratio with
liquid oxygen
for every liter of oxygen burned in the
saturn 5 for a stage it required 0.64
liters of kerosene
while its second stage needed 3.25
liters of hydrogen for every liter of
oxygen
nasa engineers couldn't feasibly make
the saturn v first stage fuel tanks any
larger so liquid hydrogen was not an
option
the science of rocket fuel is a
fascinating and complicated field
combining not just physics chemistry and
engineering but also logistics
that's the challenge facing spacex as it
develops the next generation of heavy
lift rockets designed to take us not
just to the moon but further to mars
starships raptor engines will not use
kerosene or liquid hydrogen they will
use methane a fuel that was considered
frequently over the past century of
rocket fuel research with a few
honorable mentions in john d clark's
seminal book ignition
but it has never seen widespread use
so why is spacex using it now
putting people on the moon in the 1960s
was one of the greatest technological
challenges we'd ever faced but getting
humans to mars is a considerably more
difficult task especially when you
factor in the enormous challenge of
keeping humans there of creating and
sustaining a human settlement on the red
planet
how do you reduce the cost of launches
how do you make the oxygen needed to
stay alive
how do you provide water for growing
food and for drinking and how do you
make the fuel to power the return trip
to earth
kerosene and hydrogen are not perfect
kerosene is extracted from crude oil via
fractional distillation and is made up
of a mixture of long-chain hydrocarbons
reaching up to around 20 carbons in
length the longer the hydrocarbon the
harder it is to burn completely in
oxygen as they require more oxygen per
gram of fuel to be completely oxidized
into carbon dioxide and water and so
even in its refined form kerosene often
burns incompletely decomposing instead
into smaller reactive radicals
the result is coking the production of
sodium carbon particulates that we saw
in the saturn 5's launch
this such can easily clog up the
intricate mechanisms of the rocket
engine which is a problem for spacex and
its goal to make its engines reusable
with minimal maintenance especially on
mars where the facilities to fix these
issues will not be available
liquid hydrogen of course doesn't have
this problem and it has the advantage of
burning more efficiently than kerosene
we can quantify this efficiency with
specific impulse
specific impulse describes how
efficiently a fuel can convert its mass
into thrust
to understand this let's first look at
total impulse which describes the thrust
force generated over the entire burn
period of the engine
we can graph this rather easily by
plotting the thrust the engine is
providing for each second of its flight
that may look something like this the
total impulse is found by finding the
area under this graph which gives us the
total energy the rocket released
this is a useful metric in itself but
specific impulse is better because not
all propellants are born equal
two different fuel and oxidizer
combinations could provide the same
total impulse but we need to consider
the weight of the fuel and oxidizers
themselves after all the initial weight
of rockets is always dominated by the
weight of their own fuel
to find the average specific impulse we
divide the total impulse by total
propellant weight the rocket expelled
going by this metric a liquid hydrogen
and liquid oxygen fuel mixture is by far
the best
hydrogen has a much higher specific
impulse than kerosene around 390 seconds
against kerosene's 285 seconds however
as mentioned earlier hydrogen is much
less dense than kerosene requiring much
larger fuel tanks
hydrogen also has an exceptionally low
boiling point at minus 252 degrees
celsius and so the tanks need to be
heavily insulated to avoid expansion of
the liquid hydrogen but thermodynamic
equilibrium is a war of attrition that
the universe will always win and so they
also require oil off valves to release
gaseous hydrogen to prevent an explosion
this all adds mass and complexity to the
rocket hydrogen also degrades and
weakens metals in a process known as
hydrogen embrittlement
this is a massive issue for spacex's
reusability design ethos
combining two parameters the density and
the specific heat of combustion we can
get an idea of the difference between
these three fuels
if we want to release 100 megajoules of
energy from each of these three fuels we
would need 11.9 liters of hydrogen 2.2
liters of kerosene or 5.5 liters of
methane
methane is much closer to kerosene than
hydrogen allowing methane fuel tanks to
be smaller than liquid hydrogen fuel
tanks but not small enough to offer much
performance benefits over kerosene
when the necessary design changes are
made to switch from kerosene to liquid
methane like increasing the fuel tank
volume the increase in specific impulse
is all but negated this is why methane
has never seen widespread use
methane simply sits in an awkward middle
ground between the two most popular
fuels
it provides better performance than
kerosene but not as good as hydrogen and
it's easier to store than hydrogen but
not as easy as kerosene
its benefits are only now becoming
useful as spacex works to unlock the
magic of reusable rockets
methane is a single carbon hydrocarbon
so unlike the long chain molecules found
in kerosene it produces significantly
less soot when burnt leading to less
damage to the engine over time
its boiling point is actually higher
than liquid oxygens allowing much of the
necessary infrastructure to liquefy and
use oxygen to be used for liquid methane
very important when working with limited
infrastructure on mars
but most importantly the real reason
methane has suddenly become very
attractive to spacex is that it can be
synthesized from the carbon dioxide rich
atmosphere of mars
mars atmosphere is almost entirely
carbon dioxide 95 of the martian
atmosphere is co2 with the remaining
five percent being made up from gases
like nitrogen argon and a trace amount
of oxygen
while this carbon dioxide rich
atmosphere may be a disadvantage when it
comes to establishing a city on mars it
provides huge advantages when it comes
to creating rocket fuel we have come to
see carbon dioxide as a waste gas
something produced as a byproduct of
combustion instead of as a raw material
but it has an enormous potential to act
as a feedstock for the production of
methane
over 100 years ago a chemist called paul
sabadier came up with a process of
converting carbon dioxide into methane
by passing it through a catalyst usually
nickel with hydrogen gas at an elevated
temperature and pressure the reaction
takes one mole of carbon dioxide and
reacts with four moles of hydrogen to
produce one mole of methane and two of
water when combined with our
electrolysis process this produces one
mole of methane to two moles of oxygen
the ratio of moles the stoichiometry is
going to be important soon but the first
question is how do you get your chemical
reagents on mars well for this we need
in-situ resource utilization
getting carbon dioxide is a relatively
easy task with an atmosphere made up of
95 carbon dioxide extracting a pure
sample of the compound is
straightforward enough but we do need to
get rid of that other five percent and
condense the carbon dioxide
currently cryo-freezing is the most
viable option carbon dioxide has the
highest freezing point of the gases
present in mars atmosphere so in a
process that is essentially the opposite
of distillation we can cool the air to
separate the carbon dioxide which will
freeze into a solid while the other
gases remain as a gas
this also naturally compresses the
carbon dioxide
then when we need to use it in our
sebatia reactor it can be simply warmed
up to create a high pressure stream of
gaseous co2
however getting the necessary hydrogen
is much more difficult the first option
is to import it directly from earth but
given how much is required and the
difficulty storing it for long periods
this isn't a great option so long term
we will want to extract it from the
resources available on mars
water is contained within martian soil
but most significantly it's found in the
form of ice in the polar regions of the
planet
if we can find a way of mining the water
we can then convert it into oxygen and
hydrogen using an electric current and
then the hydrogen can be combined with
carbon dioxide to produce methane
remember when we said that the molar
ratios would be important here's why
that two to one molar ratio of oxygen to
methane gives us a mass ratio of four to
one of oxygen to methane the propellant
mixture employed by spacex's raptor
engine uses a 3.4 to 1 ratio so this
whole process gives us an excess of
oxygen which can be pushed towards the
life support systems in our martian city
a win-win but extracting large
quantities of water from mars either
from ice in the polar regions or from
the small quantities of liquid water in
the soil would not be easy and the
technology to do this on a large scale
doesn't exist yet
over the short term if we brought our
own hydrogen to mars we end up with a
one to one mole ratio of oxygen and
methane which isn't enough to burn our
methane completely
so if we're going this route we need a
way to produce additional oxygen the
best way is to use another process which
would benefit from mars's carbon dioxide
rich atmosphere the reverse water gas
shift reaction
this looks very similar to the sabati a
process and involves the reaction of
carbon dioxide with hydrogen but instead
of producing methane and water it
produces carbon monoxide and water
the water that's formed in this reaction
can then be electrolyzed and the
hydrogen can be recycled back into the
reactor
again when the equations of the
processes are combined it gives us the
two to one molar ratio between oxygen
and methane that's needed for the
propellant mixture
this gives us options if mining water on
mars proves difficult but the additional
advantage of this approach is that these
two reactions the sabbatier and reverse
water gas shift can be done in the same
reactor as demonstrated by pioneer
astronautics in 2005
this combination of these two reactions
has an enormous thermal advantage
because the sobatiya reaction is
exothermic it releases heat and the
reverse water gas shift reaction is
endothermic it absorbs heat this leads
to an overall reduction of 35 percent in
heat generation improving the efficiency
of the whole process
but despite how long the savani a
reaction has been around the research
into optimizing the process is only a
couple of decades old for most of its
life the sabbati-a reaction has been
used to remove carbon dioxide from
hydrogen in the production of ammonia as
it damages the catalysts used in the
process of making ammonia but in the
last 20 years scientists have begun to
realize its potential in helping
mitigate the effects of climate change
instead of letting the carbon dioxide
formed from burning fossil fuels into
the atmosphere what if we could collect
it and turn it back into fuel using the
sabatiae process
this would require a method of not only
efficiently capturing carbon dioxide
from the air but a method of creating
hydrogen too
these processes are currently expensive
but work is being done to reduce the
cost in both energy and money this could
create a realistic economy for carbon
capture to help mitigate climate change
but we are a long way off from it being
cheap enough
if the technology advances it could
offer a method of storing renewable
energy over extremely long periods of
time as methane
there are already a handful of power to
gas plants and projects in operation
like the audi egas plant in germany but
it has not yet reached wide scale use
in december of last year elon musk
tweeted that spacex is funding programs
into carbon capture and storage adding
that the research would be important for
spacex's mars missions
it's not yet clear exactly what they are
developing but it will likely be very
similar to the power to gas projects
already in development based on the
century-old sabbatier reaction
people often criticize nasa and other
space initiatives as a waste of time why
work on scientific research to help us
live in space or on other planets when
we have so many issues that need
research here on earth but it's clear
for anyone paying attention working on
difficult problems to make mars
habitable will directly lead to helping
solve the greatest problem facing earth
today
this problem needs all hands on deck we
need as many talented engineers and
scientists working on climate change as
possible that's part of the mission of
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