pursue the exploration of the universe, to ignore this 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 sooty exhaust billow from its 5 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 liquid oxygen burned in the Saturn V first stage, it required 0.64

litres of kerosene, while its second stage needed 3.25 litres of liquid hydrogen for

every litre 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 Space X 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.

It 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 Clarks' 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 a 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 for it to burn completely in oxygen, as

they require more oxygen per gram of fuel to be completely oxidised 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 sooty carbon particulates that we saw in the Saturn

Vs launch.

This soot can easily clog up the intricate mechanism of a 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 in 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 oxidiser combinations could provide the same total impulse, but

we need to consider the weight of the fuel and oxidisers 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.

[4]

Going by this metric, a liquid hydrogen and liquid oxygen fuel mixture is by far the best.

(Table from [1]) Hydrogen has a much higher specific impulse

than kerosene [1] - around 390 seconds, against kerosenes'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 252.8 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 will the universe will always

win, and so it also requires boil 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 3 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 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 hasn't seen use yet.

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.[2].

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 liquify and use oxygen to be also used

for liquid methane.

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 synthesised 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.

Whilst 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 by-product of

combustion, instead of as a raw material, but it has the enormous potential to act as

a feedstock for the production of methane.

Over 100 years ago, a chemist called Paul Sabatier 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 it 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 Utilisation.

Getting carbon dioxide is a relatively easy task.

With an atmosphere made up from 95% CO2, 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' 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 gas.

This also naturally compresses the gas.

Then, when we need to use it in our sabatier reactor it can be simply warmed up to create

a high pressure stream of gaseous CO2. [3].

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 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 an efficient 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 the molar ratios would be important.

Here's why.

That 2:1 molar ratio of oxygen to methane, gives us a mass ratio of 4:1 of oxygen to

methane.

The propellant mixture employed by SpaceX's raptor engine uses a 3.4 : 1 ratio, so this

whole process gives us an excess of oxygen, which can be put towards the life support

systems in your Martian city [3].

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 1:1 mole

ratio of oxygen and methane, which isn't enough to burn our methane completely.

So, if we are going this route, we need a way to produce additional oxygen.

The best way is to use another process which would benefit from Mars' carbon dioxide

rich atmosphere - the Reverse Water Gas Shift reaction (RWGS).

This looks very similar to the Sabatier 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 electrolysed, and the hydrogen

recycled back into the reactor Again, when the equations of the processes

are combined, it gives us the 2 : 1 molar ratio between oxygen and methane that's

needed for the propellant mixture.