Aircraft

Skai revises targets for its liquid-hydrogen, long-range eVTOL

Skai revises targets for its l...
Once certified and put into commercial operation, Skai says its "point to any point" air taxi service could cost about the same per mile as a standard Uber
Once certified and put into commercial operation, Skai says its "point to any point" air taxi service could cost about the same per mile as a standard Uber
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A ballistic parachute, deforming skids, seats and crumple zones will form the Skai's safety net in terms of catastrophic failure
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A ballistic parachute, deforming skids, seats and crumple zones will form the Skai's safety net in terms of catastrophic failure
The Skai air taxi uses liquid hydrogen to achieve its extraordinary range claims
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The Skai air taxi uses liquid hydrogen to achieve its extraordinary range claims
The Skai's 5-seat interior
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The Skai's 5-seat interior
Once certified and put into commercial operation, Skai says its "point to any point" air taxi service could cost about the same per mile as a standard Uber
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Once certified and put into commercial operation, Skai says its "point to any point" air taxi service could cost about the same per mile as a standard Uber
The Skai prototype looks terrific
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The Skai prototype looks terrific
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East Coast aviation company Alakai made big headlines last year when it became the first eVTOL air taxi company to put hydrogen on the table as a way to get around the battery bottleneck that's holding so many competitors down to short flight times with long charging waits in between.

Hydrogen, as beautifully explained by ZeroAvia's Val Miftakhov in our recent interview, could well become the fuel that finally gets aircraft running clean. With a much higher energy density than lithium batteries, it offers genuine long-range flight capabilities, as well as fast refuelling. Used in a fuel cell electric powertrain, its only local emissions are water, and there are a growing number of ways to produce it in a completely green and sustainable fashion, some of which are beginning to become cost-competitive with jet fuel.

So Alakai was able to draw up a plan for a comfortable, very schmick-looking air taxi design in partnerships with BMW's DesignWorks, and make a credible claim that it could blow all its competitors' range figures out of the water using liquid hydrogen as an energy source.

What's more, it could hit its 400 mile, 4 hour flight endurance targets using the simplest possible airframe: a 6-rotor multicopter. No wings, no tilting rotors, no transitioning between flight modes; Skai's hydrogen powertrain had so much energy to spare that no such complicated efficiency measures were necessary. This simplicity, CTO Brian Morrison told us, was key to another Skai advantage: he believed it would be far quicker to certify than the competition. "It'll fly next week, and it'll be certified next year," he told us last year.

The Skai air taxi uses liquid hydrogen to achieve its extraordinary range claims
The Skai air taxi uses liquid hydrogen to achieve its extraordinary range claims

Where Skai is at right now

"I think some of the statements made previously were perhaps overly ambitious, made in the excitement of the moment," says Skai's Chief Marketing and Sales Officer Bruce Gunter, almost exactly a year later as we catch up with Skai for an update on where things are at. "But that's what happens when you're pushing the envelope in that respect. We're working hard to get there."

"At that time," says Skai CEO Steve Hanvey, "we were running on the ground, putting all the systems in, but we were basically proving the science of a hydrogen powered vehicle of that size – between 3-4000 pounds. We're not aware of anyone in the world doing a liquid hydrogen powertrain at these kinds of weights right now."

So where is Skai actually at right now in the development cycle? Well, the original Skai airframe made it to the tethered flight stage, and now the company has iterated to a second generation design, which is approaching completion with mainly systems work remaining. The testing, up to this point, has been unmanned, and Hanvey has been flying the full-sized airframe with a drone-style controller. We've seen video footage of this, although it's not for public release as yet. The team is currently approaching 40 people, but has expanded as high as 70 with various contractors.

The target for manned flight has shifted to early 2021, but Hanvey won't be surprised if it takes longer: "Igor Sikorsky once said to me, when you're doing these kinds of technology projects, don't try to predict the future, because it tends to change," he says. "Right now, we're capable of flying it from the ground. In fact at this point I'm flying it about 50 feet away from the aircraft. Very quiet, very little air wash compared to a traditional helicopter. I'm flying it with my thumbs like you'd fly a drone. In the software, it's capable of taking off and hovering by itself, but we prefer to have physical control."

Alaka'i Technologies' Skai  can accommodate one pilot and four passengers, with a carrying capacity of 1,000 lb (454 kg)
Alaka'i Technologies' Skai  can accommodate one pilot and four passengers, with a carrying capacity of 1,000 lb (454 kg)

As for certification, Skai is about two years into what's typically a five-year process with the United States FAA. The company has an experimental ticket application ready to go for when its airframe is ready to fly off tethers and take a pilot, but hasn't asked the FAA to approve it yet. There's an approved FAA project plan, and the team is working with FAA regulators on how the certification process for these radically new eVTOL aircraft will be certified.

Hanvey stresses that he and the Skai team have a wealth of experience in this area. "I've been certifying aircraft since 1980," he says. "Military programs, helicopters, fixed wings, small and large, Our whole team brings experience from the aerospace business around what it takes to get an aircraft successfully certified, and what does it take to support it."

With battery-powered competitor Joby Aviation targeting 2024 for certification, and Germany's Lilium among several companies talking about 2025 as a launch date for air taxi services, it now seems unlikely that Skai will gazzump the field and be first to market. But Skai's liquid hydrogen special sauce could still confer it some huge advantages if it's ready around the same time.

The Skai prototype looks terrific
The Skai prototype looks terrific

Skai's liquid hydrogen system

The decision to use a liquid hydrogen powertrain is unusual, but based upon the simple fact that you can carry more energy for a given volume and weight of tank space if you use liquid rather than compressed gas hydrogen. The fuel weight itself becomes almost negligible with hydrogen, it's the weight of the tanks themselves and the accompanying apparatus that count.

Where ZeroAvia talks about its gaseous hydrogen systems offering a 10-11% mass fraction (the proportion of the total full tank weight that represents actual fuel), and Val Miftakhov speaks of mass fractions as high as 30% being realistically achievable with liquid hydrogen, the Skai team isn't ready to talk much about its tank technology just yet.

"The specific numbers are proprietary to what we're doing," says Hanvey. "We're working with our tank people as to how much we can get in a tank, what the weight of the tank is, and what percentage of the total aircraft weight that is. We're working with folk that are leading edge on this, but it's premature to be giving numbers now."

One challenge for anyone who wants to work with liquid hydrogen is that you need to keep it extremely cold to keep it in its liquid state. At atmospheric pressure levels, we're talking just 20.28 kelvins above absolute zero (−252.87 °C, or −423.17 °F).

That temperature can rise a little if you're willing to pressurize as well as cool (using a cryogenic system running between 250 and 700 bar of pressure), but Gunter says that's not part of Skai's plans, as "even a moderately pressurized system has significant weight penalties."

So, super-cooling it'll be, and while that entails extra energy losses in the liquefaction stage, the cooling equipment, the conversion back into gas for use in the fuel cell and in boil-off in the tank itself, the net result will still be a much longer range aircraft than anyone dealing with gaseous hydrogen – or certainly lithium batteries – will be able to deliver.

It'll be interesting to see how Skai gets the job done, as really you've got to look to NASA and other space programs to find liquid hydrogen being used in serious volumes.

"The good thing in all of this," says Gunter, "is the notable developments that occur in this space on an increasing basis. The efficiencies we’ve seen in fuel cells and the same the industry is seeing regarding H2 production all point to increasing effectiveness of any form of H2 as a future focused solution."

"There's a number of naysayers about what we're doing with hydrogen," says Hanvey, "but we believe we've gone from the question to the possible, and it's now the probable. We know we can fly with hydrogen, and the question is just how quickly we can get it to the market. And based on our experience, we think we can get there a lot quicker than perhaps the market will give us credit for."

We look forward to following Skai's progress, along with the rest of the growing hydrogen aviation industry. If Skai and its partners can get liquid hydrogen safely and effectively working in aircraft, then not only could the dream of air taxis become a more practical reality, but the door would then open up a crack to the possibility of long-range zero-emission airliners as well.

Alaka'i Skai - Hydrogen powered VTOL air taxi

Source: Skai

View gallery - 5 images
11 comments
owlpic
The Aeronautical physics is flawed. It is a copy of a toy, and does not scale up.
Brian M
Without a major break through in battery technology then we do need to look at alternative fuels.

Hydrogen is a good starting point, but not convinced that its the only solution, we really need a high energy density fuel similar to petroleum products, but made via renewable/green processes.
No reason why it can't be carbon based either, provided the carbon is used from carbon already circulating (air) or the carbon element is retained and recycled.

i.e. We might already might have the right chemical fuel, we just need to be able to produce it sustainably.
Laszlo KRUPPA
Question:

The statement "...simplest possible airframe: a 6-rotor multicopter" seems to be a contradiction. For 3D steering it is enough to have three (3) rotors without cyclic (swashplate) control.

If only 3 rotors are installed (keeping the footprint of the vehicle unchanged) diameters of each can be increased. Bigger diameter of a propeller means, as a rule, higher efficiency.

Please, clarify why 6 rotors are needed? I am ready to be corrected.

Thank you!
FB36
IMHO, using hydrogen as fuel for any kind of land/air/sea vehicle is an extremely bad idea because sooner or later there would be accidents/leaks/raptures & hydrogen fuel is explosive! It does NOT start fires (like gasoline etc); it explodes like a bomb!

IMHO, for any kind of vehicle, which batteries cannot provide enough power/energy, it would be best to use a biofuel, like biodiesel!
(Which is easy & cheap to produce (from countless kinds of plants/biomass) & energy dense & VERY unlikely to start any fires, after ruptures/leaks!)
freddotu
I applaud the use of Kelvin as a temperature reference without the "degrees Kelvin" reference that is so often incorrectly used. Unfortunately, "kelvins above absolute zero" is equally incorrect. Always Kelvin not kelvin or kelvins and it's always understood that 0 K is absolute zero. At least there are no descriptions of something being ten times smaller or ten times less than something else, as is all too common in these entries.
Username
Laszlo KRUPPA , I'm guessing it's for redundancy since it will carry humans. But I agree with you that the statement is flawed.
Eugarps71
To FB36:
While hydrogen may not be the best fuel from the standpoint of energy per unit of volume and the difficulty of storage, it exploding is not a huge issue. Pure hydrogen will only burn if it has oxygen and even then, it's a very controlled flame without the right mix of oxygen. It's only when a 1 to 1 H2 to O mix is reached that there is an explosion. It's not any different than gasoline, which is used safely in most current vehicles.
Towerman
Aaaah an update about my Favorite aircraft ! ! ! This is so refreshing.
@all the naysayers, what a bunch of shallow minded and uninformed comments !
@owlpic Your comment does not even to begin to make sense !, This is the best aerodynamics for a basic hexacopter design that is out there in this size ! The Beauty is in it's simplicity being a Hexacopter !
@Brian M. It might not be the only solution, however it is a solution that not only Skai is working on to perfect. Clearly the Skai Team is working hard on this and obviously knows where the limits can be pushed, it is what they do 24/7 ! Carbon based will just keep on producing the environmental headache we all face with fossil fuels these days.
@laszlo Kruppa O please...REALLY ???? It IS the simplest for as it has not tilting motors or mammoth wings sticking out the sides, The 6 rotors makes for brilliant Redundancy ! ! ! I would not want to see it done ANY other way !!!!

@FB36 Again another senseless comment from you as in previous topics ! ANY kind of propulsion is dangerous, maybe your blind but have you seen how many thousands of people have been scorched to a crisp in aircraft related accidents due to Aviation Fuel ! ! ! ! I have seen them and its real bad, YET they are certified. Skais Hydrogen system will be MUCH safer ! ! !
@Username, The only Flaw is the perception of what it is that they are doing that you don't understand, the Skai Craft is one of the very best concepts out there !

guzmanchinky
Towerman, maybe attacking everyone isn't the best way to convince people that this is a great idea? I agree these are amazing concepts, and I agree that Hydrogen is safer than avgas or jetfuel. I look forward to these machines becoming reality, but there will be many failed attempts along the way, as with any breakthrough startups...
Towerman
@guzmanchinky
I'm tired of deliberate trolling. Comments meant to confuse and derail reality. None of those posters understand what Exactly it is that Skai and other brilliant concepts are doing Exactly. They don't care for these concepts they only want their primitive fuel guzzlers to be the be all end all for eternity ! I have no respect for those preaching misinformation,uneducated hearsay and deceiving comments.