Comment by dreamcompiler | Hacker News Reader

Comment by dreamcompiler | Hacker News Reader

I'm skeptical.

They're claiming they'll be able to store 300kWh of energy on board. That's the equivalent of about 4 Tesla Model Y cars. Even if they could pack this much energy on board, flying that much weight 200km is almost certainly going to take more than 300kWh, especially with 7 passengers.

Lilium is very proud of their ducted fan designs and their power usage profiles -- and they should be! -- but they're counting on advancements in battery tech that are unlikely to happen in the timeframe they need. It's much more likely they will be limited to 100km range or 30-minute flights at a maximum when reserves are factored in.

This says nothing about how fast they can recharge those massive batteries. If they have the equivalent of four Tesla superchargers available they can probably recharge completely in 40 minutes or so, which is probably good enough. But that charging infrastructure needs to be in place at virtually every pad where the plane lands, so the "ordinary helipads" they brag about using are not going to work without roughly a $million of power infrastructure upgrades at each. If they try to go cheap on this, they will have to live with multi-hour recharge times.

1,000 times this, and this is why they are mired in an investor lawsuit.

Looks like I was overly optimistic. After doing some more research I found that the company has been saying "Our ducted fans are often assessed based on methodologies better suited to assessing open propellers, which are generally less efficient in flight than ducted fans" and "It is accepted in the aerospace expert community that ducted fans are aerodynamically more efficient than open propellers." [0]

This is just thinly-disguised lying. Ducted fans are more efficient than open propellers of the same diameter, which is a telling detail that they conveniently leave unstated. A ducted fan of 10 cm diameter cannot be compared with an open propeller of 4 m diameter, which is closer to what Lilium's competitors use. This doesn't by itself mean the ducted fan idea won't work, but it's ridiculous to imply that the ducted fan is somehow "more efficient" without talking about the propellers' respective diameters.

[0] https://investors.lilium.com/techfaq

> but it's ridiculous to imply that the ducted fan is somehow "more efficient" without talking about the propellers' respective diameters.

Why? Their point is that it's misleading to use data from open propellers and apply it to a ducted fan design. I would certainly hope that the critics tried to scale according to the diameter of the rotors. That is, they should at least assume something like a 10cm open rotor. But then it'd still be misleading to use data based on open propellers. Their point is that there are aspects of their design that increases efficiency that must be taken into account, and that's true as far as I can tell.

And why do you assume that scaling down the size of the rotor would make it less efficient, rather than more? I mean yes, if that's the only thing you do, it probably would be. But they're also changing the whole design of the aircraft specifically for distributed propulsion.

NASA is already demonstrating with the Maxwell X-57 that moving to many smaller rotors is more efficient than 1-2 big rotors, since you get additional lift distributed over the wing allowing for a smaller wing with less drag. I think NASA knows what they're doing.

The small distributed motors on the X-57 are only used on takeoff and landing to augment lift. The small propellers fold away in cruise, when the entirety of the aircraft’s thrust is provided by two large propellers at the wingtips.

> Even if they could pack this much energy on board, flying that much weight 200km is almost certainly going to take more than 300kWh, especially with 7 passengers.

The Pipistrel Alpha Electro can fly 2 people more than 100km today, with an aircraft that's not at all optimised for electric flight. The battery pack is 21kWh. Increasing that to 200km seems entirely reasonable with a more optimised design and newer battery technology. Let's say you could fly 2 people 200km with a 50kWh pack, then you can by definition fly 12 people 200km with a 300kWh pack. Trade 4 people for the trade-offs needed for VTOL, and you're down to 8.

> but they're counting on advancements in battery tech that are unlikely to happen in the timeframe they need

Are they? They're using Zenlabs batteries, which claims to have already been verified their energy/power densities with a third party laboratory. Question is as always if plans for mass manufacturing works out.

Seems like they're assuming numbers that are entirely reasonable for early mass manufacturing within the next 5 years. With the amount of money pouring into battery R&D, and the number of companies now in trial production phase with next generation cells, it seems more or less inevitable to me.

They can afford to pay more for the batteries than mass-market BEV companies. So it's also not unreasonable to assume they'll be able to buy batteries at the early stage of manufacturing.

> This says nothing about how fast they can recharge those massive batteries.

Charging these in 40minutes would take at most 500kW (I'm assuming 250kWh charged at 2C rate). Often it'd be much less (not always going to fly its full range). We have plenty of charging stations around me with way more total power than that. Chargers running at up to 350kW is already being built routinely. So you just need two of those, and they are usually built in pairs already. This is really not going to be a major issue at all in the initial phase. Maybe down the line when they've already built out the easiest locations, it could be a bigger challenge. But nothing bigger than the mass electrification of cars and trucks. The economics of the charging stations will be better than with BEV stations, since they'll be built on fixed routes, and have high utilisation during the daytime.

Doesn't feel like the skepticism passes the back of the napkin calculation tests. It's definitely within the realm of possibility. Hard? Yes, very, but if it was easy someone else would already have done it, as with all new development.

If you're going to be skeptical, at least look at the numbers of what's being done today, and see how far what they require is from that.

> Pipistrel Alpha Electro can fly 2 people more than 100km today, with an aircraft that's not at all optimised for electric flight.

Pipistrel Alpha Electro is basically a sailplane with an engine. It's designed to glide long distances without an engine. It's a beautiful design but it derives 0% of its lift from engine thrust, it has no VTOL capability, it's slow, and it's much lighter than the Lilium jet. And by the way it's marketed as a training aircraft rather than a passenger aircraft because Pipistrel is an honest company. There's no comparison.

> Are they? They're using Zenlabs batteries, which claims to have already been verified their energy/power densities with a third party laboratory.

Here's an article about the now-CEO of Zenlabs:

https://qz.com/158373/envia-the-mysterious-story-of-the-batt...

> Charging these in 40minutes would take at most 500kW (I'm assuming 250kWh charged at 2C rate).

You're assuming linear charging, and LiIon batteries don't work that way. You have to reduce the charge rate as the battery gets closer to full. This means if you want fast charging you have to break the battery into multiple pieces [electrically speaking] and charge each piece separately with its own high-current charger. And I seriously doubt they'd ever get FAA cert without a commitment to fully charge before takeoff.

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