Joby is using the technology developed by its subsidaries and partners, notably H2Fly which has a liquid hydrogen tank from Air Liquide:

https://www.ainonline.com/news-article/2023-09-13/h2flys-breakthrough-flight-demonstration-bolsters-case-liquid-hydrogen

' “Depending on the meteorological conditions and the flight altitude, the aircraft uses about three kilograms of LH2 per hour," he said. "So, for an eight-hour flight, we would need about 24 kilograms of liquid hydrogen." The HY4’s aluminum storage tank, designed and supplied by France’s Air Liquide, can hold up to 24 kilograms of LH2.'

This seems to be around the same rate of fuel usage as the Joby, and excitingly for future development down the road, whilst offering perfectly practical efficiencies for storage now, is nothing like what advanced composite tanks have the headroom for in future.

Here is more commentary on the system installed in the allied H2Fly prototype:

https://aviationweek.com/aerospace/advanced-air-mobility/h2fly-apply-lessons-learned-liquid-hydrogen-flights

'Air Liquide developed the double-walled, vacuum-insulated LH2 dewar tank installed in the cockpit area of the right-hand fuselage in the four-seat, twin-fuselage HY4. An internal heater pressurizes the LH2 in the tank, and a heat exchanger using waste heat from the fuel cell drives an evaporator that vaporizes the hydrogen for delivery to the fuel-cell cathode at 6.5 bar (94 psi) absolute pressure.

Evaporator control is identified as one of the technical achievements of the Heaven program. “The storage system needs an evaporator because the hydrogen in a liquid state has to go into the gaseous state. What we learned is this evaporator has to be controlled very exactly,” Kallo says.

“That is done by evaporating enough hydrogen, which then is directly used, and we control the pressure at the fuel cell. Including the energy that goes into the evaporator–which is mainly from the heat of the fuel cell–that pressure control has to be done extremely precisely so that we have a very stable pressure at the fuel cell,” he says. “The simplicity of the controller was astonishingly good.”

Control of the evaporator, pressure and fuel-cell power output was sufficiently fast and precise to eliminate the need to use a buffer battery to handle power transients. “We expected this, but we could show that during the 3-hr. cruise flight we did not need the battery,” Kallo says. '

I find it remarkable that the system does not need the battery to step in much, but handles the issue of warming the liquid hydrogen in a more or less energy neutral manner with the otherwise waste heat from the fuel cell in such a controlled manner.

Larger aircraft will also be needed, but obviously considerations of hydrogen handling, tankage etc remain constant, so these advances by Joby give good reasons for being optimistic there.

Here is an overview of hydrogen powered aircraft, including combustion as well as fuel cells:

https://www.sciencedirect.com/science/article/pii/S0376042123000386

And here an analysis for the regional Nordic market, likely to be a very early adopter:

https://www.sciencedirect.com/science/article/pii/S0360319924008243?via%3Dihub

What I have not a yet managed to get better information on is the fuel cell Joby are using, it is clearly PEM but I don't know if it is an HTPEM or not, which would give far better performance.

They have upped the energy density of the batteries compared to their batter only version, I don't know what trade offs they have had to make to enable this.

So we know that there is considerable room for improvement in the hydrogen storage by going to composites, and it seems likely that there is also a lot of room for improvement in the fuel cells, if they are indeed LTPEMS

We have then wholly acceptable boundaries right now for the application, with loads of room for improvement.

From the link already given:

https://www.sciencedirect.com/science/article/pii/S0376042123000386

' SAFs are more expensive than conventional jet fuel, which would increase the aircraft operating cost [22]. Furthermore, SAFs are unlikely to reach net-zero lifecycle emissions, optimistically reducing the climate impact of flying by 60% [23]. Farming the feedstock and chemically synthesizing fuel produce greenhouse gases that prevent SAFs from achieving net-zero lifecycle emissions [24]. Vast amounts of farmland are needed to grow the feedstock. To create new farmland, farmers can plow existing forests or grasslands. The plowed plants and soil release CO2 as they are decomposing or are burned [25]. Those plants were already absorbing CO2, which further hinders the emissions reductions [22]. These changes to the land are called land use emissions. Laborde [26] estimates that in Europe, they would cut the benefits of SAFs versus conventional fossil fuels in half. Lark et al. [27] find that the carbon intensity of corn ethanol biofuel is at least as high as gasoline, if not higher, largely because of land use change estimates. Another type of SAF is electrofuel (also known as e-fuel, or synfuel). To produce electrofuel, electrolysis and carbon capture powered by renewable electricity generate hydrogen and carbon dioxide, which are then used to create hydrocarbon fuels through chemical synthesis [28]. Electrofuel requires nearly three times the amount of energy to produce and distribute per unit of energy stored than hydrogen and is expected to be more costly [23].'

And here is the link at ref [23] which is obviously critical:

Clean Sky 2 Joint Undertaking and Fuel Cells and Hydrogen 2 Joint Undertaking
Hydrogen-powered aviation: A fact-based study of hydrogen technology, economics, and climate impact by 2050: Report
Publications Office of the European Union (2020), 10.2843/471510

Of course there is no way at all for some time that hydrogen and fuel cells will be able to impact long range flight emissions, which are by far the greatest source of CO2, for many years.

What can be done, and should be done, it to levelise the costs of those emissions by repealling the ludicrous tax exemption on aviation fuel, and then progressively charging for emissions.

It is grossly irresponsible to simply carry on producing more and more long distance aircraft when the climate impact is clear.

Boeing has been very fortunate in the coincidental demise of their critics, who also seem to have destroyed their own evidence prior to suicide.

I can see no way at all that there is any coherent plan whatsoever to produce SAF economically in the vast quantities with consequent impact on land use, agriculture, water use and so on to make a substantial impact on emissions.

It seems to be a fake play, though those with complete confidence in Boeing and its management should raise their hands.....

My techological solution?
Don't fly long distance so much, and put up the costs to reflect the climate impact.
That would certainly help in creating the financial environment where real very low/zero emissions would be financed and developed at longer ranges as rapidly as possible.

I doubt that that will include SAF at any game-changing level.

Hi Roger.
You said:
' You will notice 200 miles out 750 does not seem to add up to a 76% reduction.'

It doesn't really work like that, as the rate of energy use and CO2 emissions vary across the flight profile, with landing and more particularly take off using disproportionate amounts of energy, and the energy needed to climb to cruising altitude, exactly how high that is, and the decent from cruising altitude all variables.

A well designed hybrid kicks in to provide assistance just where needed, although if your mission profile differs from the optimum it is designed for you can miss out!

The bottom line is that their claims are credible, although it is far from shown that their design is optimal.

Certainly hydrogen battery designs, if they can be pulled off, would reduce emissions much more.

Dave,

You wrote:

"So 76% is the absolute maximum, with who knows what assumptions put into the production of SAF etc.

For most trips it will be way less."

No. The shorter the trip the greater the emissions savings, since a greater proportion of the energy will come from the batteries. The 76% has nothing to do with SAF. It just how much CO2 is emitted by the aircraft compared to a traditional jet plane. If you use SAF then your net emissions of CO2 go down even further.

Also it appears that with the relatively modest extra energy expenditure required to get from 200 miles range to 750 miles range that a relatively modest increase in battery energy density would allow all electric flights of this range. All of this is based on the assumption that Odys technology can really walk the walk. Time will tell.

The fact that 24% of the emissions reappear by extending the range from 200 to 300 miles is not that impressive a result for the hybrid mode. So I am guessing that the distributed propellers are more about Odys method of producing vertical lift than about an attempt to improve overall flight efficiency.

Dave,

"I don't agree that only comparatively modest battery improvements would be needed to up the range to 750 miles range"

I don't agree either as my previous comment implies.

Thanks for the Lilium reference. I have no interest in handicapping evtol developers. I mentioned Odys because I knew they had a new method of producing vertical lift. I am not predicting that this method will revolutionize evtol. It's just something new. And it is new. Lilium rotates its engines (https://lilium.com/newsroom-detail/youve-never-seen-anything-like-this-an-introduction-to-the-lilium-jet):

"The canard wings each have two flaps and the main wings have four. We mount three of our pioneering all-electric jet engines on each flap, allowing them to pivot and change the direction of the thrust they create. The flaps can each move independently."

Maybe Lilium can do this with low cost and high reliability and run performance rings around the Odys scheme. I don't have a crystal ball. I just follow technology developments which I consider of potential interest.

By the way I don't like the fact that Odys is pushing hybrid technology. We need to be heading down to net zero ASAP and not promoting "more sustainable" technologies that are not going to get us there. However, that dislike does not imply that Odys vertical lift technology is worthless.

Another aspect where air travellers and airlines have been getting a free ride at everyone else's expense is air pollution from abrading their rubber tires in landing and take off.

Off course this is in common with car drivers, but one landing of a medium size airplane emits very large amounts of particulates, of course this is more at the expense of the health of the usually poorer people living in proximity to the airport rather than the important people who want to go skiing, so who cares?

There is a vague figure floating around on the web that each tire ( size of aircraft?) loses a pound of rubber on each landing.

Much of it is later scraped off the runway, but obviously some substantial proportion will end up as atmospheric particulates.

VTOL pretty much solves this issue.

Roger said:

' LH2 will be stored in feather-weight polyurethane tanks'

I believe that is the insulation of the tanks, not its pressure containing components, which need to be a bit beefier:

' A lightweight polyurethane foam insulation for liquid hydrogen tanks of space vehi- cles was developed that (1) could be foamed in place, (2) did not crack when chilled to liquid hydrogen temperature, and (3) had a thermal conductivity of 0.0137 W/(m)(K) (0.0079 Btu/(hr)(ft)('R)) at a mean temperature of 136 K (243' R'

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://ntrs.nasa.gov/api/citations/19690011428/downloads/19690011428.pdf&ved=2ahUKEwjZwLC73KiHAxWlYEEAHSvbAMYQFnoECBMQAw&usg=AOvVaw39tYbWwbAMgFxAQuqxc67l

For the containment vessel:

' NLR has developed additional facilities for testing composite materials at 20 Kelvin. Several semi-crystalline thermoplastic composites (Toray) have been screened regarding their properties at this very low temperature. Screening of some thermoset composite materials from Toray will follow shortly. The materials are also characterised regarding their permeability properties and resistance against thermal cycling down to 4 Kelvin'

https://www.nlr.org/case/case-liquid-hydrogen-composite-tanks-for-civil-aviation/

The illustration at that link shows the layers of the tank

The basic point remains that if we make composite tanks for LH2 work, and progress is good, then the weight/energy ratio of liquid hydrogen becomes better than kerosene, although volume hassles remain, so it is far from optimal to stick hydrogen tanks in an existing kerosene airframe.

Here are details of the remaining steps the FAA are imposing on Joby ( and others ) to meet for certification of VTOLs:

https://www.ainonline.com/news-article/2024-04-15/faa-finalizes-airworthiness-criteria-jobys-evtol-aircraft

Standards have at least partially been harmonised with Europe.

Sometime in 2025 seems to be the hope, I am a bit unclear on when.

And here is a pretty meaty critique of the whole VTOL notion, and Joby in particular:

https://www.kerrisdalecap.com/wp-content/uploads/2023/10/Joby-Aviation-JOBY.pdf

' Underlying Joby’s project is the premise that battery-powered electric flight can be cheaper and safer than current alternatives. But is battery-powered flight even possible? Well, barely. Even the most advanced lithium-ion (Li) technology can’t simultaneously optimize on the 3 axes of energy consumption: power, capacity, and rechargeability. Joby’s eVTOL requires all 3: immense power for takeoff, landing, and climbing; capacity to enable range; and rapid recharge for efficient refueling.
Joby claims its eVTOL will have 100-mile range and a 10,000-cycle life. But we estimate that, constrained by both Li limits and regulatory reserve requirements, maximum range will be 35 miles and the battery will last a few thousand cycles at best. That’s not a jet; it’s a science project.

Joby’s plan to manufacture hundreds, or even thousands, of eVTOLs annually at a unit cost of just $1.3 million is only slightly less naive. The production forecast ignores the experience of seasoned airplane manufacturers, which – using the same materials from the same vendors – took years to scale their production lines, and even then barely got to 100 units/year. And that’s at a lower degree of complexity and less rigorous demands for airframe robustness. The cost projection ignores, well,
everything: there’s no aircraft the size of Joby’s in the world that can be manufactured at that cost, and Joby’s competitors – who are by no means pessimistic – are projecting a number 3 times greater. Is Joby immune from the laws of manufacturing? We think not.

Nor will it be immune from the laws of economics. Joby claims that fuel and maintenance savings will enable eVTOL flights at a fraction of the cost of comparable helicopter flights. But we broke down the cost of flying and found that the savings are negligible and don’t account for the cost of the battery and the aircraft, which, when considered, make the eVTOL flight more expensive than a comparable helicopter. Just another instance of Joby’s selective math and wishful thinking.

Speaking of which, Joby is guiding to type-certification by 2025, boasting of having completed 3 of the 5 certification stages. But those were mostly comprised of paperwork. Little real-life testing, analysis, and verification (Stages 4 and 5) have been achieved for the purpose of certification, and those make up the lion’s share of time, cost and effort expended in the certification process. It’s clear that it’s still early days in that respect, particularly given that major safety concerns – such as battery
fires and rotor-related accident scenarios – have yet to be appropriately addressed.

The logistical hurdles of pilot training and air traffic control also remain, both of which may take years to clear given recent FAA proclamations.

Yikes!
And there is more.