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Comment by kybernetikos | Hacker News Reader

This is very cool. But it's also spending a lot of novelty points. I thought energy density was still a huge problem, so it seems weird to try to solve the problem of electric planes with acceptable range at the same time as tackling the enormously energy intensive problem of VTOL.

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Those are related. Part of what makes engine-powered VTOL difficult is the engines. The V-22 Osprey has two propellors and two engines, but in order to handle an engine failure, there's a complicated driveshaft and gear arrangement across the middle of the aircraft to send power from the working engine to the other propellor. I don't know that anyone has seriously tried more than 2 engines for a VTOL system.

Electric power allows you to have lots of small fans, a few of which can fail without disaster.

Also, throttling turbines up and down fast enough to stabilize an aircraft doesn't seem to work well.

There are a number of VTOL aircraft that have used multiple engines. The Yak-38 used two lift engines and one lift-cruise engine, the VJ101 used four lift-cruise engines and two lift engines, and the Do 31 used two lift-cruise engines and eight(!) lift engines. Out of these, only the Yak-38 was remotely successful, and not nearly as much as the single-engined Hawker Siddeley Harrier (and its derivatives).

There are plenty of ways to stabilize an aircraft without relying on pure engine thrust, and turbofan aircraft have some advantages here.

The distributed electric propulsion systems do have awesome redundancy, but they have significant losses in efficiency compared to fewer, larger props, which really isn't what you want in an aircraft with severe energy density limitations. I'm curious to see what the production Lillium's payload, range, and power margin end up being.

There have been a few flying VTOL prototype designs with more than two turbine engines. The extra engines were used for powered lift during takeoff and landing. But that approach turned out to be impractical due to safety, cost, and weight.

https://vtol.org/vstol/wheel.htm

XC-142 tiltwing, four turboprops

https://vought.org/products/assets/images/1758_005_o.jpg

It makes sense why they'd do this.

Given current energy density of batteries, electric planes are only useful on very short flights. If you can only operate out of airports (without vtol), the useful range of the airplane will be significantly reduced

Great point. Reading a little more https://lilium.com/newsroom-detail/technology-behind-the-lil... you're exactly right, and the same thinking drives the requirement of low noise and the ability to utilise helipads.

There aren't many helipads either. Outside of regular airports, only a few urban areas have any helipads available for commercial use. More can potentially be built but that requires large open areas free of obstructions. Adding helipads to existing building roofs is generally impractical due to weight and safety issues.

>large open areas free of obstructions.

https://www.beaconhealthsystem.org/beacon-medical-transport/...

You need roughly a football field of clear flat land for human-piloted helicopters.

In the US at least, that's nothing. It's comparable to one quarter of a clover-leaf offramp.

It's not nothing. In the affluent areas where passengers would potentially be willing to pay for air taxi service there aren't many large vacant lots left. So not impossible, but it's hard to see how the economics would work.

Medical transport helicopters have somewhat greater freedom to land in random places like roads and fields when necessary to pick up emergency patients. But that only works because police are on the ground to secure the area. And local residents don't complain about the noise from a single emergency flight. Those situations don't apply to air taxis.

>It's comparable to one quarter of a clover-leaf offramp.

Again, maybe you aren't in America, but outside of SF and NYC, literally every major city is brimming with already loud highway interchanges/off-ramps.

Just cap those and put the landing pad on top. Space is not the issue.

Building helipads on highway interchanges won't make air taxis viable. Passengers would still have to take a ground taxi to the final destination, thus eliminating any significant time savings on most routes. In order for air taxis to make economic sense they have to bring passengers directly to within a few blocks of the destination.

Again, maybe you're not in the US, but here we chose to build those interchanges right downtown,

Say what you will about America, but we built our environment to accommodate decentralized forms of transit, it's sort of our thing.

1-2 years ago the cto guy from lilium held a presentation at $itCompanyInGermany where I was at. The plan back then was to be same/twice the price of taxis, starting and landing on roofs.

Oh, and the cto was telling so much bullshit that it was hard to listen to. Blockchains was of course there, but also stuff I already forgot. I guess it's mostly to impress investors and to collect money, but bs is still bs.

"If you can only operate out of airports (without vtol), the useful range of the airplane will be significantly reduced"

Why do you say that? That's backward. VTOL consumes a huge amount of energy, far more than a traditional rolling takeoff.

"Acceptable" range is short - they're not really trying to compete with planes, but with other modes of transport.

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