The jet engine has had a good run. It is one of the most successful machines humans ever built. It made distance feel cheap, it made oceans feel small, and it made airports feel like permanent construction sites with a side hustle in noise pollution.
Now the mood has changed. Aviation is getting squeezed from every angle. Regulation, fuel prices, local noise limits, public patience, and the simple fact that “we will offset it later” is starting to sound like a joke told by someone who has never opened a spreadsheet.
So propulsion is no longer a slow evolution story. It is a scramble. Everyone is trying to keep the benefits of flight while swapping out the part where we burn hydrocarbons at scale and pretend it is fine.
The current king is still the turbofan, and it is not going away tomorrow. The more realistic story is that the turbofan gets stretched, bent, cleaned up, and paired with new fuels while the next architectures fight to prove they are not just a slide deck.
Rolls Royce has been leaning into that with UltraFan, and it is not subtle about why. More efficiency, lower emissions, and a platform designed for the next few decades. It has also run the demonstrator on 100 percent SAF, which is basically the industry admitting the near term decarbonisation path is fuel swapping, not engine replacement.
Europe is pushing that fuel route hard. The EU’s ReFuelEU rules bake mandates into the supply chain, and the share ramps up over time. That is the regulatory ratchet. It does not care how pretty the engine is. It cares what comes out of the exhaust.
Electric flight sits on the other end of the hype scale. It sounds perfect. Quiet, clean at the point of use, fewer moving parts, the whole futuristic vibe. Then the battery shows up and ruins the party. NASA’s work on electric aviation lays out the problem in polite technical language, energy density, weight, packaging, safety margins. The maths punishes you the moment you want range and payload at the same time. Electric will carve out short routes and training missions and specialist roles. It is not a widebody replacement story unless battery chemistry does something heroic.
Hydrogen is the tempting middle ground because it promises long range without carbon at the tailpipe. Airbus has been pushing ZEROe for years, and it is openly working through the ugly bits like cryogenic storage and the fact that hydrogen needs to live at about minus 253 degrees C. Their own write up on the cold heart of the concept makes it clear this is not just “swap the tank and go”. It is a different aircraft problem. It is also a different airport problem. Airbus has even postponed parts of the timeline because the ecosystem is not moving fast enough, which is the real issue. Technology is one battle. Infrastructure is the war.
Then you have the option that looks like the past but keeps returning because the numbers are nasty and the fuel savings are real. Open rotor, or open fan, engines. No big duct. Big exposed blades. More efficient, more complicated to integrate, and always one noise compliance meeting away from a migraine.
CFM is betting hard on that direction with the RISE programme, explicitly aiming for a step change in efficiency and emissions, with the Open Fan architecture as the headline. That is the industry saying we will accept some aesthetic violence if it buys us fuel burn improvements that matter.
Adaptive cycle engines are the smarter cousin that came out of military programmes. Engines that can shift airflow paths to optimise for different flight conditions, more range when you want it, more thrust when you need it. GE’s XA100 is the poster child. It is not built for airliners, but the underlying lesson is that propulsion is moving toward flexibility and control, not just brute force.
And then there is the stuff that sounds like it should come with a warning label.
Rotating detonation is one of those ideas that reads like a dare. Instead of smooth combustion, you run continuous detonation waves in a chamber. Pressure gain combustion, better efficiency on paper, and a serious engineering challenge in reality. NASA is deep into RDRE development and it is not a hobby project. If detonation cycles mature, they change rocket engines first and then maybe feed into high speed air breathing concepts later. It is a long road, but it is the kind of road that ends with performance jumps that make old architectures look tired.
SABRE sits in its own category. The British engine concept that tries to blur air and space travel with a system that breathes air low down and then switches to rocket mode higher up. The signature trick is the precooler that can chill insanely hot incoming air fast enough to make the cycle possible. ESA has also covered the same Mach 5 performance milestone. If SABRE ever becomes a full vehicle system, it changes the boundary between aircraft and spacecraft. It is also a brutally hard engineering programme that lives and dies on materials, funding patience, and whether the real world behaves as well as the models.
So what wins.
Probably not one thing. More likely a messy blend. Better turbofans running more SAF because that is what regulation can force quickly. Open fan architectures if the noise and integration work gets solved. Hydrogen where infrastructure exists and policy backs it. Electric where the range is short and the economics make sense. Exotic cycles like RDRE and SABRE sitting in the background as the “if this clicks, everything moves” bets.
The sky is not the limit. The limit is the trade off you are willing to accept, and how much you are willing to pay to pretend you are not paying.
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