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Comment by Animats | Hacker News Reader

The whole thing hinges on battery energy density more than anything else. The planning document says 20 seconds of hover at landing, with a 60 second reserve. The demo flight has 45 seconds of VTOL mode/hover at landing. That's cutting it close. You don't get a second chance at landing. No go-around.

Regard this as a bet on improved batteries. With current batteries, it's a nice demo.

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Yes, and it's a bet they will probably lose, if they really need 500 Wh/kg in three years time. Their battery graph is a fairytale at best.

The current state-of-the-art batteries that researchers build in laboratories that last 10-20 cycles can put out about 400 Wh/kg. You're simply not going to achieve first a 20% improvement over that, then a development of the tech to increase lifetime to something useable, and then scaling that to widespread commercial availability, within three years time.

If we speak about what's available in laboratories, Lithium–sulfur battery gives around 550 Wh/kg with up to 1500 charge-discharge cycles ([1]). Not available commercially, but there are a few companies that try to commercialize them ([2], [3], [4]).

I would not bet that these batteries arrive fast enough to save Lillium.

1. https://en.wikipedia.org/wiki/Lithium%E2%80%93sulfur_battery

2. https://lyten.com/

3. https://oxisenergy.com/

4. https://www.thebatt.com/news/lithium-sulfur-battery-startup-...

Yeah, I'm kind of assuming Li-S batteries won't work out. Sony announced in 2015 they were going to have commercial production in 2020. No news since then. Many others in that field have gone into bankruptcy, including one of those you link to.

Nice catch about one of the company being bankrupt.

I've taken a deeper look at Lyten and it does not seem to be a savior here either:

1. They need to produce graphene at scale and they don't yet have a solid plan to do that.

2. Lyten batteries are only 200 cycles.

Agree their chart is a fairytale based on current chemistries. It's basically a bet on commercially viable solid state batteries, which are targeting somewhere in that 500 Wh/kg range, but are probably a couple more years out than Lilium needs. I guess they could drop their viable range quite a bit at the start to compensate...

First Law of Aviation: Take-off is optional, but landing is compulsory.

I recall Vaclav Smil saying batteries are consistently about 2-3% per year over the long haul. It's not a Moore's law scenario, but a very slow crawl. "Miracles might happen," he says, "but would I bet on a miracle happening? No."

This link is to the point in the video where he says this, but the whole talk is quite interesting. https://youtu.be/gkj_91IJVBk?t=1705

Edit: here's a more recent video of Smil talking about energy density of batteries and the problems there, specifically mentioning the unlikelihood of flying on battery anytime soon. https://www.youtube.com/watch?v=91Cs-ZkAjCg

This was my take as well - and was wondering what is Fig. 11 based off of? We can predict, but we don't know how the market for batteries will form in the next decade or so. I.e. the COVID disturbed all sorts of markets bad enough that price of many commodities went up X-fold and haven't gotten down yet.

Sounds basically like the Apollo lander...only today we have better computers and control systems.

The Apollo Lunar Module had the ability to abort landing by dropping the descent stage and firing the ascent engine. So if they had run out of fuel while searching for a safe landing site they still had a safe alternative option.

On the other hand, Apollo had no option than to abort to orbit. Just ditching somewhere and hoping for the best wouldn't have been an option on the Moon, but it's commonly done in aviation on Earth.

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