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.
Bureau all ends in running.
It's estimated that about 650 million people watched the first
moon landing in 1969, nearly 1/4 of the world's population.
As the world watched those powerful Saturn 5 rocket engines
burst into life, they witnessed plumes of sunny exhaust billow
from the five shuddering nozzles as it pushed itself off
the ground, burning through a colossal volume of kerosene to
lift the rocket's massive 3 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 litre of oxygen burned in the Saturn 5
first stage, it required .64 litres of kerosene, while its
second stage needed 3.25 litres of hydrogen for every litre
of oxygen.
NASA engineers couldn't feasibly make the Saturn 5 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.
Starship's 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 honourable mentions in John D Clarke's Seminole 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 cooking the production of SODDY carbon particulates
that we saw in the Saturn 5's launch.
This soot 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 -252°C,
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 2 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 mega joules of energy
from each of these three fuels, we would need 11.9
litres of hydrogen, 2.2 litres of kerosene or 5.5 litres
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.
It's 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 Maris.
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
5% 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 1 mole of
methane and two of water.
When combined with our electrolysis process, this produces 1 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
5% and condense the carbon dioxide.
Currently, cryofreezing is the most viable option.
Carbon dioxide has the highest freezing point of the gases
present in Mars's 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 Sabadi,
a 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 2 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 one 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 Sabadier 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 Sabadi 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 Sabadier 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% in heat
generation, improving the efficiency of the whole process.
But despite how long the Sabadiae reaction has been around,
the research into optimizing the process is only a couple
of decades old.
For most of its life, the Sabadiae 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
realise 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 Sabadia 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 E gas 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 Maris 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
Sabadier 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.
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