Why Kerosene Is So Hard to Replace
Kerosene has powered commercial aviation for seventy years, and it is worth understanding why nothing has displaced it — because the reasons are also the reasons the alternatives are hard.
Energy density, twice over
Jet A carries about 43 MJ per kilogram at a density of 0.80 kg per litre, which works out at about 34.4 MJ per litre. It is near the top of what is achievable for a practical fuel on both measures at once, and aviation is the application that cares most about both.
The energy comparison tool makes the point by contrast. Liquid hydrogen beats it comfortably per kilogram — 120 against 43 — and loses badly per litre, at 8.5 against 34.4. A lithium-ion pack loses on both: 0.9 MJ/kg and 1.8 MJ/L.
The properties nobody talks about
Energy density gets the attention, and several less glamorous properties are equally load-bearing.
It stays liquid at altitude, where fuel in the wings can reach very low temperatures — freeze point is a specified property, and it is why Jet A-1 exists as a lower-freeze-point variant for international operations. It is not corrosive to conventional materials. It is stable enough to store for long periods without degrading. And it is safe enough to handle routinely: kerosene has a relatively high flash point, so it does not readily ignite from a spark at ambient temperature the way petrol does.
Any replacement has to satisfy all of these, not just the energy figure.
It doubles as a coolant and a lubricant
A detail that surprises people. Fuel is used as a heat sink on modern aircraft, cooling engine oil and hydraulic systems on its way to the combustor, and it lubricates parts of the fuel system itself.
Lubricity is a specified requirement for exactly that reason, and it is one of the properties that has to be matched by any alternative fuel — not simply the energy content.
Aromatics, and the 50% blend limit
Conventional kerosene contains aromatic compounds, which do a job nobody designed them for: they cause certain elastomer seals to swell slightly, which is how those seals seal.
Most sustainable aviation fuel pathways produce little or no aromatics, and this is a significant part of why SAF is currently certified only up to a 50% blend. Running fully synthetic fuel needs either seal changes or fuels engineered to include the missing compounds — solvable, but it is aircraft work rather than fuel work.
Drop-in is a demanding standard
“Drop-in” means a fuel can go into today's aircraft, through today's pipelines and trucks, with no modification anywhere. That is a very high bar: it requires matching kerosene's density, freeze point, flash point, thermal stability, lubricity and material compatibility, not just its energy.
It is also the reason SAF is the near-term option. A fuel that needs no new aircraft and no new infrastructure can be deployed at the rate it can be produced, which is a completely different proposition from one requiring a fleet and an airport network.
The bar to clear
Kerosene is not incumbent by accident or inertia. It is an unusually good match for the specific demands of flight, and the honest framing of the alternatives is that each gives something up — hydrogen gives up volume and infrastructure, batteries give up energy density, SAF gives up cost and supply.