decarbonisation. Planes are currently  only responsible for 2-3% of the world's  

carbon dioxide emissions, but that's  expected to rise to 25% by 2050. [1]

Most major polluters have clear technology  pathways to a cleaner future. The automotive  

industry has batteries and electric motors.  The shipping industry has a range of potential  

alternative clean fuels to choose from. Our  electrical grids are rapidly investing in solar  

and wind, and future nuclear energy projects  are being researched intensively. There is  

still plenty of work to do, but the path ahead  for these sectors has been surveyed and marked.

However, the aviation industry has no  clear way forward for replacing kerosene,  

and if the aviation sector can't find answers  to this problem, it's projected that with the  

continued growth of passenger numbers and the  expected decarbonisation across other industries,  

that it could represent as much as 25% of  total world wide emissions by 2050. [1]

To understand this problem, and the potential  technologies we could see in the future,  

we first need to understand the  current state of aviation fuel.

Today, nearly all jet engines use kerosene,  

but internal combustion turbine engines are not  actually that picky about the fuel they consume.  

Gas powered turbines power grids all over  the world [2] , and many of them are being  

converted to run on bioethanol [3].Early jet  engines were powered by mostly gasoline. If it  

burns hot and can be pumped into a combustion  chamber, chances are it can drive a turbine.

But, it's not quite so simple for a jet  engine that flies and carries humans.

There are two main types of jet  fuel used for commercial aviation.

Jet A and Jet A-1. Jet A is primarily  used in the United States and Jet A-1  

is used in the rest of the world. [4] So is this just another case of the United  

States insisting on being different because they  are too stubborn to admit the rest of the world  

may just have a better system? In this case, no.

The primary difference between  the two is their freezing point,  

with Jet A-1 having a lower freezing  point of -47 degrees versus Jet A at -40.

For domestic flights within the US, Jet  A's freezing point is just fine, but  

for colder climates, or colder international  routes like those that fly over the arctic,  

a lower freezing point is needed to  prevent the fuel from turning to wax. So,  

a lower freezing point is  desirable, but it comes at a price.

The United States uses Jet A because  it is cheaper. To understand why,  

we need to understand how crude oil is refined.

Crude oil is essentially just a  blend of many different hydrocarbons,  

all with different carbon chain lengths.  [5] We have short chain gas molecules like  

methane and butane, with 1-4 carbon atoms in each  chain. Then we have longer gasoline molecules,  

with chain lengths between 5 and 10. While,  kerosene molecules range from around 10 to 16.

We can separate each fuel type from crude oil  thanks to these chain lengths impacting the  

boiling point of each component, which allows us  to separate them with fractional distillation.

We simply heat the crude oil up and  pump it into a distillation tower.  

The longer chain hydrocarbons liquify  lower in the distillation tower,  

thanks to their lower boiling point, and  when they do so, they are tapped off.

The shorter chain molecules will remain  gaseous and continue rising through the tower,  

but the tower gets gradually colder as it rises.  Soon Kerosene will turn to liquid and be removed,  

then gasoline, and finally the lightest methane  and butane gases rise right to the very top.

So how does this explain Jet A-1's lower freezing  point? Freezing points and boiling points are  

generally linked, so Jet A-1 can lower its  freezing point by excluding hydrocarbons  

with longer chains, and therefore excludes  lower boiling point molecules from the mix.

Jet A, in comparison, is less  picky about the freezing point  

and can take a larger cut of this distillate.  Meaning, there is a broader percentage of  

the crude oil that can be included in  Jet A, making it cheaper than Jet A-1.

So, it makes perfect sense for a  country like the United States,  

that doesn't need to worry too  much about low temperatures,  

to manufacture a cheaper wider cut fuel  for their domestic airline industry.

So, these are our first two properties we need  to consider when choosing a future aviation fuel:  

freezing point and cost. The freezing point issue  rules out longer chain molecules like diesel.  

Diesel powered vehicles in Canada  and Alaska actually have to cut their  

fuel with kerosene to prevent the fuel  from freezing in the winter months. [6]

This is the same reason a different jet fuel, Jet  B, is used in parts of Canada and Alaska. It's  

also known as wide-cut fuel, which gets its name  because it takes a much larger cut of the crude  

oil distillate, with a mix of 30% kerosene and 70%  gasoline, giving it an even lower freezing point  

of -60. So if this wide-cut fuel can be used  in engines, why isn't it used in all engines?

Gasoline, thanks to it's  shorter carbon chain lengths,  

is too volatile for general use in aviation.  It's flash point is much lower than kerosene.  

Flash point is the lowest temperature vapors can  form from a liquid to create an ignitable mixture  

in air. So low flash points make unintended  explosions and fires much more likely,  

not something airports and planes are particularly  fond of. The lower temperature of vaporization can  

also cause problems with vapor locks in plumbing.  Where gas bubbles can form and cause blockages.  

This becomes an even larger issue for jet  engines, as boiling points lower as pressures  

decrease at altitude. So gasoline is not a  desirable jet fuel for general applications.

The US Navy and US Airforce even use two  different Kerosene grades for a similar  

reason. The U.S. Air Force uses JP-8 [1], which  is similar to Jet A-1, but with the addition of  

corrosion inhibitors and anti-icing additives  that are not required for the Jet A-1 standard.

While the US Navy uses JP-5. The primary  difference between the NAVY and Air Force  

fuels is that the navy fuel has a higher  flash point. 60 degrees versus 38 degrees.  

This makes it much safer to handle during  refueling operations on aircraft carriers,  

and makes explosions much less likely in the event  of an attack. This was a constant worry during WW2  

with the predominantly gasoline powered  piston engines. Fuel fires were not a  

rare occurrence during the war. [7] This is the  third property we need to consider: flash points.

But we aren't done yet. We haven't even  mentioned the most obvious. Energy content.

The primary function of aviation turbine fuel  is to power the aircraft. This is achieved by  

igniting the fuel, which releases  heat, which raises the pressure,  

which causes air flow. To fulfill this role  most effectively we want high energy content.

We can measure the energy content of  a fuel pretty easily. It's simply the  

heat released when a known quantity of the  fuel is burned under specific conditions.

There are two “quantity” measurements  however. Energy per unit mass,  

measured in megajoules per kilogram, and energy  per unit volume, measured in megajoules per liter.

In general a dense fuel with a high  volumetric energy content is desired,  

especially for military aircraft that always take  off with their fuel tanks filled to the brim,  

so volumetric energy density is a more important  metric. Commercial aircraft only fill their tanks  

with enough fuel to reach their destination,  with a little extra in case of emergency,  

but volumetric energy density is  still generally a better measurement.

Let's add this to our shopping list, and  start looking at potential alternative fuels.  

First, let's look at the numbers for our 4  main identified properties with a typical  

kerosene jet fuel. Cost, freezing point,  flash point and volumetric energy density.  

These will be our measuring  sticks for our alternative fuels.

The first stop on our proverbial  shopping trip is the biofuel aisle.  

We have a tonne of options to choose from here.

In terms of production volumes,  bioethanol and biodiesel  

are currently the most available biofuels.

Ethanol is a short chain alcohol. Similar to  the short chain hydrocarbons, it's freezing  

and flash point is quite low, minus 115 degrees  celsius and 13 degrees respectively. [8] The  

low freezing point is useful, but the low flash  point is a problem. This makes ethanol volatile,  

which makes it undesirable as a jet fuel.  It's volumetric energy density is about 61%  

of kerosene, meaning range would be reduced  if fuel tanks remained the same size. [9]

Biodiesel suffers from the opposite problem to  bioethanol because it's carbon chain lengths  

are much longer. As a result it's flash point is  very high, between 98 and 150 degrees depending on  

the feedstock used, and as expected comes with  a very high freezing point of about 1 degrees.  

This fuel would turn to wax in  the fuel tanks. It's unusable.

However, we can further process these biofuels to  create fuels that are so similar to kerosene that  

they can even be used in current generation  planes with very little modification. [10]

Airbus began testing a fuel composed entirely  of biofuel this year in an A350 powered by Rolls  

Royce XWB engines. [11] Testing the plane's  performance and emissions using the fuel,  

which was manufactured be Neste. A company  that manufactures biofuels from palm oil  

and waste oils, like cooking oil. Results  of this test have not yet been published,  

but NASA has already published data from their  own tests with a 50-50 fuel blend or traditional  

jet fuel and a similar plant oil derived biofuel.  [12] Their tests showed, with only a 50-50 blend,  

that particulate emissions in the contrail  were reduced by up to 70%. That's important,  

because those particulates have a much  larger impact on earth's atmosphere  

than the carbon emissions. This is  positive news, but these biofuels  

are a long way from being cost effective or  even environmentally friendly to manufacture.

The main challenges facing biofuels are scaling  the feedstocks in an environmentally friendly  

way and cost. Waste oil products as feedstocks are  fantastic and every country should be working on  

ways to collect waste products to feed this  growing industry, but sourcing oil from the  

palm oil industry is obviously problematic, as  the palm oil industry is driving the destruction  

of the Borneo rainforest. Sourcing enough  feedstocks to completely replace fossil fuels  

in the aviation industry is going to be a massive  problem to solve, and right now we have no answer.

Cost is also a huge issue. Norway announced a 0.5%  

biofuel mandate for the  aviation sector in 2019. [13]

This is a tiny fraction of the total fuel used,  but Scandavian Airlines has said that this 0.5%  

mandate will add an additional 3.3 million dollars  in fuel costs a year. Making it 100%, assuming  

prices wouldn't rise with the extra demand, would  cost 660 million dollars extra a year. That would  

Completely wipe out Scandinavian Airlines'  2019 profit of 84 million dollars. [14] So,  

these biofuels currently fail the cost metric,  despite being suitable alternatives to kerosene. 

Even if we ignore the questionable  environmental benefit of the feedstocks,  

the real issue here is the difficulty  in scaling up feedstocks to meet demand. 

So, are there any other alternatives? 

Hydrogen is also being explored  as a potential future fuel.

Airbus has published several concept  aircraft that could utilize hydrogen,  

because, unlike biofuels, hydrogen cannot be  used in existing planes. This would require a  

complete overhaul of airlines plane inventories  and would cost trillions over several years. 

Hydrogen's main advantage is that's feedstock  is just water, and we are surrounded by water. 

However, hydrogen currently needs very  pure fresh water to prevent corrosion  

to the electrodes that split the water  apart during electrolysis. Researchers are  

working on ways to extend the life of these  electrodes while preventing the salt ions,  

like chloride, that are found in seawater,  from breaking down the electrodes. [15]

The alternative is simply pairing the system  with desalination process, but this would draw  

even more electricity for what is  already a very expensive process.  

Hydrogen, right now, does not  satisfy our cost requirement.  

But let's move forward with the expectation  that we will have massive amounts of excess  

renewable energy looking for a home in the  future and assume these costs will come down. 

Hydrogen has insanely good  gravimetric energy density,  

at 120 MJ/kg. [16] Completely blowing kerosene  out of the water at around 44 MJ/kg. However,  

hydrogen's volumetric energy density, the quantity  we actually care about, is complete dog trash. 

The only way to get it to a reasonable number  is by pressurizing it or making it cold,  

but even then it's volumetric energy density  is terrible. At 700 bar, that's 700 times  

atmospheric pressure, hydrogen still  only has a volumetric energy density  

of 5.6 MJ/L, compared jet fuels 38.3 MJ/L. [17] Pressurizing a fuel tank to 700 bar comes with its  

dangers, as repeated pressure cycles can lead to  rapid failure due to fatigue. This is made worse  

by hydrogen's habit of attacking and embrittling  materials, a phenomenon that is also accelerated  

by higher pressures. [18] So, most designs for  hydrogen fuel tanks instead call for cryogenic  

storage. Where the hydrogen is cooled to  achieve a higher volumetric energy density  

with much lower pressures. [16] This also  results in higher energy densities of 8 MJ/L,  

but still much lower than the  38 MJ/L of traditional fuels. 

This low volumetric energy density, and need  to pressurize, makes hydrogen fuel tanks a  

nightmare to integrate to an aircrafts airframe. Planes these days place a large amount of fuel  

inside the wings. [19] This is ideal for several  reasons. It takes up no useful space inside the  

cabin of the plane. Aircraft wings need to be  hollow to increase the strength of the wings.  

The weight of the fuel being located so close to  the center of lift means the plane does not need  

to adjust it's control surfaces during flight  to compensate for changes in center of gravity  

as the fuel gets used up, which reduces drag. Finally, when flying, the wings deflect upwards  

due to the upwards lift force they create. This  creates stress in the supporting structures  

of the plane. So, by putting the fuel in the  wings it actually helps the wings deflect less  

as the weight of the fuel pushes them  down, and as the fuel is used up,  

the lift the wings need to generate reduces, and  the upwards lift forcing the wings up reduces. 

Storing the heavy fuel in the wings is an  incredibly elegant solution, and it's not possible  

with hydrogen. There simply is not enough space  in the narrow hollow structure of wings to fit  

the equipment needed. This space is also getting  even smaller as newer generation composite planes