Leeham News and Analysis
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The state of alternative propulsion aircraft? Part 3.
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By Bjorn Fehrm
November 27, 2025, © Leeham News: In our series about alternative propulsion aircraft, last week we looked at the electric motor of an electric aircraft, which drives propellers or fans. It’s a component that is straightforward to develop, but the certification requirements for the aircraft and VTOL industry have slowed progress.
Now we look at the component that causes trouble for all alternative propulsion aircraft, except those that burn hydrogen in gas turbine engines: the batteries. Batteries work well in cars, where the requirement for energy capacity relative to weight is much lower and where energy recovery during braking reduces the energy required from the battery.
Figure 1. The battery-electric aircraft that flies operational trials as we speak, the Beta Technologies Alia CX300. Source: Leeham Co.
Aircraft batteries are a heavy component with very low energy storage capacity. Today’s aircraft battery system has about 60 times lower energy density per kg or lb than aircraft fuel.
The hope over the last decade has been that this relationship should improve. It has, it’s gone from about 70 times to 60 times in the last 10 years. This will improve further, the question is if it will continue at the slow pace of the last 10 years or not?
The state of alternative propulsion aircraft? Part 2.
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By Bjorn Fehrm
November 20, 2025, © Leeham News: In our series about alternative propulsion aircraft, we started last week by discussing what happened after the trigger by the Airbus E-fan flight in 2014 and the introduction of the Tesla Model S car two years before. The alternative propulsion aircraft projects that followed had a rocky path. They followed the Gartner Hype-curve, Figure 1.
There were hundreds of projects announced, more or less serious. A few of these came to producing hardware, and flew test flights, then stopped. Most stayed as PowerPoint presentations and fancy renderings, promising capabilities that were not possible to realize. The result was that we passed the Peak of Inflated Expectations and entered the Trough of Disillusionment. Investors fled to AI, and projects froze or stopped.
Figure 1. The Gartner Hype-curve. Source: Gartner Inc.
By 2025, we are in the Disillusionment phase. As there are signs we can now enter a phase of real, sustainable progress, it’s timely to take stock of where we are and what progress we can expect over the next decade.
To understand why progress has been so difficult, after explaining that learnings from Cars are not transferable last week, we start by focusing on two components that we find in every alternative propulsion concept, the Electric Motor and the Battery system. Of the two components, the motor is the most straightforward to develop and certify for an aircraft. Still, we have only limited progress so far, and we detail why in today’s article.
The Battery System is the most challenging component for alternative propulsion aircraft, both in development and in use. We will spend next week’s article detailing why and how it has slowed down progress so far, and how this can change going forward.
Bjorn’s Corner: Air Transport’s route to 2050. Part 32. Epilogue.
By Bjorn Fehrm
July 25, 2025, ©. Leeham News: In October last year, we began a series on how air transport is performing against the emission goals for the year 2050.
The ambition to reduce and eventually eliminate greenhouse gas emissions began in earnest 11 years ago, when Airbus flew the Airbus E-Fan at the 2014 Farnborough Air Show (Figure 1).
Figure 1. The Airbus E-fan flying at the 2014 Farnborough Air Show. Source: Wikipedia.
The result of this inspiring flight, which utilized technology that emitted no CO2 or other greenhouse gases (if the batteries were charged with green electricity), was an avalanche of projects from established players as well as upstarts. The optimistic view was that there was a solution to the emissions from airliners.
What’s the next new aircraft, Part 2
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By Bjorn Fehrm
July 21, 2025, © Leeham News: Our series about “What’s the next new aircraft” was introduced last week, where we look at what potential new aircraft could be introduced over the following decades, and what technologies these would use.
In Part 2 of the five-part series, we introduce some basics around aircraft efficiency and examine what areas these 13 new aircraft aim to improve to enhance their efficiency.
In the following Parts, we will look into these aircraft in more detail and write about how challenging it will be to develop and mature the needed technologies.
Figure 1. The 13 new aircraft concepts that we study. Source: Leeham Co.
Bjorn’s Corner: Air Transport’s route to 2050. Part 31, Conclusions.
By Bjorn Fehrm
July 18, 2025, ©. Leeham News: We have done a Corner series on the state of actions to mitigate the global warming impact from Air Transport. Now, we start to summarize what we’ve learned.
During the series, we compiled tables describing the warming effect of air transport in 2024 and a calculation of the effect during 2050. We made two tables, one with the most probable effects, Figure 1, and one where we downplayed the non-CO2 effects to the maximum given in the Lee et al. 2021 study, to a 5% probability, Figure 2.
Figure 1. The effects of Actions 1 to 4 on CO2 and NOx, represented as CO2e emissions during 2050. Source: Leeham Co. Click to enlarge.
Before we summarize by examining the tables, we will discuss the additive effects of CO2 and non-CO2 warming over a given time period, as the different components don’t have the same decay time of their warming effects.
Bjorn’s Corner: Air Transport’s route to 2050. Part 26.
By Bjorn Fehrm
June 13, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.
We have examined different ways to lower global warming over the course of the series. Over the last weeks, we have summarized what practical results we can expect from the different alternatives we have to reduce global warming from Air Transport. We looked at the following alternatives:
Bjorn’s Corner: Air Transport’s route to 2050. Part 21.
By Bjorn Fehrm
May 9, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.
Since we started in October last year, we have looked at:
- Alternative, lower emission propulsion technologies, ranging from electric aircraft with batteries as energy source, different propulsion hybrids, and new concepts for Jet-fuel and Hydrogen gas turbine engines.
- We have also reviewed recent research on the role of CO2, NOx emissions, and Contrails generated by airliners in global warming.
- Two weeks ago, we summarized the present situation around SAF, Sustainable Aviation Fuel.
Last week, we listed some base data about the present situation for Global Air Transport. We will now use this data to calculate the effect of air transport on global warming from the three alternatives.
Bjorn’s Corner: Air Transport’s route to 2050. Part 12.
By Bjorn Fehrm
March 7, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
Last week, we wrote about Pratt & Whitney’s announcement in January: their trials with critical components of their HySIITE engine, Figure 1, showed that they could increase the efficiency of a hydrogen burn engine by 35%!
It does this by intelligently using the water released when hydrogen oxidizes with the air’s oxygen. The water separated from the exhaust is reheated into steam and entered into the engine’s combustion, reducing NOx by 99.3% and increasing the engine efficiency by 35%.
Figure 1. A HySIITE engine with its backflow core part. Source: Pratt & Whitney.
Bjorn’s Corner: Air Transport’s route to 2050. Part 10.
By Bjorn Fehrm
February 21, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
Last week, we reviewed the present fallout of lower emission projects that have not reached their goals and where investors, therefore, have decided not to invest further.
There is a well-known project failing every month at the present pace. Some recent ones: Universal Hydrogen’s ATR conversions, Volocopter and Lilium’s bankruptcies, Airbus freezing the CityAirbus eVTOL (Figure 1) and pushing out the ZEROe hydrogen airliner, hibernation of the Alice battery aircraft, etc. There will probably be more in the coming months.
Bjorn’s Corner: Air Transport’s route to 2050. Part 9.
By Bjorn Fehrm
February 14, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
We have covered the progress of battery-based aircraft and hybrids. Last Corner started looking at hydrogen-fueled alternatives. A day after the Corner, the Airbus workers union Force Ovrier published information from an Airbus internal meeting, in which the airframer delayed the introduction of a hydrogen aircraft by 2035 to about 10 years later. As a consequence, it reduces the R&D spending on the development of hydrogen propulsion technologies.
Figure 1. The Airbus ZEROe concepts. Source: Airbus.