AFAIK the "jet" in classical jet engines is a jet of air. Unlike rocket engine, where hot combustion gases are your reaction mass, jet engines in aircraft use fuel primarily to spin turbines that compress and move air from the front of the engine to the back of it. You don't need combustible fuel to spin turbines - you can do that with electricity too.

Nitpick: a turbine extracts power from a moving fluid. A fan (or impeller) imparts power to a fluid. A traditional jet engine contains both components, which is why they are sometimes referred to as "gas turbines". In an electric jet, there are no turbines and the proper term is "ducted fan" or "electric ducted fan (EDF)".

You can also use any convenient source of heat - e.g. a nuclear reactor:

https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion

Wow, the 50s were so awesome! This is the kind of thing I wanted to do when I studied engineering. Would anybody else rather make nuclear-powered jets than web sites?

Oh hell yeah I would, but I sleep better knowing most of YOU ALL are making websites instead of nuke-drones. :P

Pretty sure everyone - including the people who worked on it - is happy that Project Pluto [0] / SLAM [1] never flew. Despite how awesome the projects were.

0: https://en.wikipedia.org/wiki/Project_Pluto

1: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...

Yes, thanks - I've just been looking that up. For some reason I'd never really thought about what was involved.

Me neither - I learned this on HN many years ago, so I'm just passing it forward :).

Isn't that also what a propeller does?

A propellor turns shaft torque into air momentum. You need some kind of motor to drive it.

A ducted fan also does this, though its aerodynamics are a bit different. Like a propellor, there is little compression.

A 'turbine' turns flow into torque.

A traditional jet engine turns fuel into torque with a compressor and a turbine. Confusingly it's sometimes called a 'gas turbine'. It also incidentally moves air, but not very efficiently.

A turboprop drives a propellor with a 'gas turbine'.

A modern 'high bypass' jet engine (or "turbofan") drives a ducted fan with a 'gas turbine'.

An 'electric jet' drives a ducted fan with an electric motor. No turbine is involved.

These definitions better be correct, becaus I will use your post as a reference from now :)

IANAAE (I Am Not An Aerospace Engineer), but I think the key difference here is the compression that happens with turbines as opposed to propellers which only move the air.

I've always wondered what exactly differs a jet from a ducted fan - this makes a lot of sense, thank you!

Modern "high bypass" jet engines are actually ducted fans, with the shaft powered by a "gas turbine" whose role is mostly to provide torque. All propulsion units compress the air to some extent, even propellers - if they didn't, they would be silent. However, in a gas turbine, the function of the "compressor stage" (which is NOT the "turbine" part) is to get a lot of oxygen in one place, where it can be combined with a lot of fuel, which makes it very hot, which causes it to adiabatically expand a lot, driving the turbine at the back (this is what a 'turbine' does - extract power). If the air weren't compressed, you wouldn't be able to burn much fuel with it, and it wouldn't get so hot and expand so much. (By the way, this is the same reason for superchargers and turbochargers in piston engines. More air = more fuel = more heat and expansion)

This process isn't actually very efficient at moving air though, which is why gas turbines are now mostly used to power ducted fans, propellors, and helicopter rotors.

In a propeller aircraft, basically you waste the energy left in your exhaust gas. But in a turbojet you make some use of it.

Also there is the distinction between just propeller, a turboprop, a turbojet and a turbofan. What we call jet airliners are typically high bypass turbofans. Turbojets (without bypass) have low efficiency at subsonic speeds, but they were used on Concorde and on fighter aircraft.

According to this article https://www.wired.com/story/lilium-electric-aircraft-prototy...

> The “Lilium Jet” uses 36 electric-powered ducted fans. Inside each, a small rotor ingests air from the front and pushes it out of the rear at higher speeds. They’re not technically jet engines (so the aircraft isn’t a “jet”). The lack of spinning blades improves efficiency, reduces noise, and eliminates the risk of turning passing birds into chop suey.

That article is not very accurate:

> The lack of spinning blades improves efficiency,...

A ducted fan is, in fact, a bunch of blades spinning in a duct. A well-designed ducted fan can be more efficient than a propeller of the same diameter. That efficiency is also more dependent on airspeed, as any given duct shape must be designed for a particular flow [1]. (it looks as though this design may have a variable nozzle, which could broaden the efficiency hump over a wider speed range.)

>...reduces noise...

To some extent, though the ducted fans on airliners still demonstrate a fair amount of noise from the fan itself. The video continues the practice of not giving a clear indication of how much noise is being generated.

>...eliminates the risk of turning passing birds into chop suey.

As there are rotating blades, this does not follow. It is not going to be healthy for either bird or fan when they collide. At least this airplane has a lot of fans.

[1] http://massflow.archivale.com/ductbook.htm

> eliminates the risk of turning passing birds into chop suey.

To be fair, it's a lot easier to put a debris (including birds) filter in front of a ducted fan than a prop.

There are efficiency issues, and it is not often done. It is hard to tell, but it does not seem to have been done here.

They say "20% as loud as a helicopter" and I found a Bell J2-A at 100 Ft is 100dB, so maybe 93ish dB at 100 feet? That's about as loud as a motorcycle at 25 feet, so significantly louder still than any road traffic.

In jet engines the thrust comes form shoving material out of the end - the jet itself.

In props the primary thrust comes form the force due to the pressure differential on the prop blades basically it’s like someone is pulling the aircraft.

High bypass turbofans and turboprops somewhat blur the line.

EDF - Electric Ducted Fan is probably the more accurate term for what they're using.

It's marketing... "electric jet" sounds cool.

One name for it is "motorjet" [1].

[1] https://en.wikipedia.org/wiki/Motorjet