8 ms·

This is described as an 'electric jet' which doesn't make sense to me. Enclosed propellors perhaps? Edit: Apparently the rotors that compress the air are elec

This is described as an 'electric jet' which doesn't make sense to me. Enclosed propellors perhaps? Edit: Apparently the rotors that compress the air are electrically powered, so sort of.

TeMPOraL 7y ago
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.
- Wildgoose 7y ago
  Yes, thanks - I've just been looking that up. For some reason I'd never really thought about what was involved.
  - TeMPOraL 7y ago
    Me neither - I learned this on HN many years ago, so I'm just passing it forward :).
- arethuza 7y ago
  You can also use any convenient source of heat - e.g. a nuclear reactor: https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion
  - jl2718 7y ago
    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?
    - silversconfused 7y ago
      Oh hell yeah I would, but I sleep better knowing most of YOU ALL are making websites instead of nuke-drones. :P
      - DuskStar 7y ago
        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 https://en.wikipedia.org/wiki/Project_Pluto 1: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...
- ClassyJacket 7y ago
  Isn't that also what a propeller does?
  - moonrobin 7y ago
    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.
    - larkeith 7y ago
      I've always wondered what exactly differs a jet from a ducted fan - this makes a lot of sense, thank you!
      - dTal 7y ago
        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.
        deleted 7y ago
        [deleted]
    - semi-extrinsic 7y ago
      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.
- dTal 7y ago
  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)".
rjsw 7y ago
One name for it is "motorjet" [1]. [1] https://en.wikipedia.org/wiki/Motorjet https://en.wikipedia.org/wiki/Motorjet
dogma1138 7y ago
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.
- TheSpiceIsLife 7y ago
  High bypass turbofans and turboprops somewhat blur the line.
deleted 7y ago
[deleted]
ForHackernews 7y ago
According to this article https://www.wired.com/story/lilium-electric-aircraft-prototype-first-flight-jet/ 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.
- mannykannot 7y ago
  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 http://massflow.archivale.com/ductbook.htm
  - larkeith 7y ago
    > 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.
    - mannykannot 7y ago
      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.
NotPaidToPost 7y ago
It's marketing... "electric jet" sounds cool.
throwaway55554 7y ago
EDF - Electric Ducted Fan is probably the more accurate term for what they're using.

This is described as an 'electric jet' which doesn't make sense to me. Enclosed propellors perhaps? Edit: Apparently the rotors that compress the air are electrically powered, so sort of.

TeMPOraL 7y ago

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.

Wildgoose 7y ago
Yes, thanks - I've just been looking that up. For some reason I'd never really thought about what was involved.
- TeMPOraL 7y ago
  Me neither - I learned this on HN many years ago, so I'm just passing it forward :).
arethuza 7y ago
You can also use any convenient source of heat - e.g. a nuclear reactor: https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion https://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion
- jl2718 7y ago
  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?
  - silversconfused 7y ago
    Oh hell yeah I would, but I sleep better knowing most of YOU ALL are making websites instead of nuke-drones. :P
    - DuskStar 7y ago
      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 https://en.wikipedia.org/wiki/Project_Pluto 1: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil...
ClassyJacket 7y ago
Isn't that also what a propeller does?
- moonrobin 7y ago
  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.
  - larkeith 7y ago
    I've always wondered what exactly differs a jet from a ducted fan - this makes a lot of sense, thank you!
    - dTal 7y ago
      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.
      - deleted 7y ago
        [deleted]
  - semi-extrinsic 7y ago
    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.
dTal 7y ago
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)".

rjsw 7y ago

One name for it is "motorjet" [1]. [1] https://en.wikipedia.org/wiki/Motorjet https://en.wikipedia.org/wiki/Motorjet

dogma1138 7y ago

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.

TheSpiceIsLife 7y ago
High bypass turbofans and turboprops somewhat blur the line.

deleted 7y ago

[deleted]

ForHackernews 7y ago

According to this article https://www.wired.com/story/lilium-electric-aircraft-prototype-first-flight-jet/ 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.

mannykannot 7y ago
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 http://massflow.archivale.com/ductbook.htm
- larkeith 7y ago
  > 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.
  - mannykannot 7y ago
    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.

NotPaidToPost 7y ago

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

throwaway55554 7y ago

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