Elon Musk has said that he has a design for a vertical take off and landing super sonic electric jet. I was under the impression that jet engines work by burning fuel, expanding volume and thus creating force. How can this be done electrically?
- a rapid stream of liquid or gas forced out of a small opening
A Jet Engine is an Engine which uses a Jet for propulsion. Typically, a "rapid stream of gas", in which the gas is air. Really, we could count pretty much any aircraft engine as a jet, but it depends on how "rapid" you require the air to be before it's a jet.
In a typical jet engine as we define it now, the jet is created by burning fuel and air... but there's no requirement for that to be the case. An electric jet would simply use an electric motor to create a rapid stream of air.
Would it really be a "jet" engine? That depends how fast the air is - but most people see "jet" as being a fast, powerful, reliable type of aircraft, so it's as much about marketing as anything else. An Electric Prop or Electric Plane doesn't sound as cool as Electric Jet... (Although I'd argue that ElectroJet/ElectroProp/ElectroPlane are awesome names and Elon Musk has let himself down by not using one of those names)
As far as I understand, the use of the word jet here has no relation to the engine type, except to exclude non-faired propellers.
It is being used as a synonym for aircraft, while keeping the appeal of modern jetliners as opposed to propeller planes, which are often considered "outdated" by the general public. This same public cannot usually tell a piston engine apart from a turboprop or even a propfan, so it is understandable.
My personal understanding or Mr. Musk's remarks is that he wants to develop a "more electric" or "all-electric" aircraft capable of both VTOL and supercruise at very high altitudes, more likely the later.
An all-electric aircraft uses batteries to store its energy, which is fed into electric propellers, fans, flapping wings or any other devices that convert it into thrust. On the other hand, the more electric approach usually keeps a fuel tank, a combustion engine of some sort and an alternator to transform the chemical energy in the fuel into electrical energy for an electric propulsion method. This is because the energy density in any type of aviation fuel is orders of magnitude higher than that of even the best modern batteries, so it's more efficient to carry fuel than batteries.
PERSONAL OPINION AHEAD
Mr. Musk is a businessman, not an engineer (he hires pretty good ones to do the engineering work, since he can afford it). So when he gives interviews, consider he's doing business, not arguing the details of his ideas.
Furthermore, while I think there is no aviation enthusiast who wouldn't love to see an operational, all-electric, supersonic, VTOL-capable airliner, the chances of it happening in the next couple of decades without some major breakthrough in fuel cell or battery technology are close to nill. Until such a thing happens, energy density will continue to keep such designs firmly in the realm of interviews and 3D renders
There are instances of interviews where Musk describes in detail what the intention is.
The aircraft's main advantage would be in his opinion, would be the ability to conduct high altitude flight in envelopes where engines requiring atmospheric oxygen to combust could not operate, thus able to move faster with less resistance.
He also talks about gimballing engines for control surfaces, and using it to have VTOL capacity.
The biggest issue right now is power density for it to even exist, but from a design stand point, I'm not sure that it would be wisest to leave your control surfaces to JUST the engines. The biggest benefits to having tails, canards etc, is that in the event of engine failure, you don't turn into a brick.
And also in higher altitudes, around (At least higher than 20km) 40-60 km, I imagine you might need RCS like control systems as backup.
But getting to the 'jet' part, I believe that it's precisely what you think of it as, a 'turbofan' with the core powered by a motor and gimballed.
Perhaps he means an electrostatic fluid accelerator. Mr Musk or his engineers are the appropriate parties to clarify what he has in mind. It could be a turbofan without a turbine (= just a fan).
I know he has proven the naysayers wrong about electrically propelled cars, but in the matter of electrically driven, supersonic VTOL aircraft, I remain a naysayer. I would be very excited to see a more detailed and viable design.
Near as I know, this was a hypothetical proposal for an aircraft making use of gimballed electric motors, allowing it to redirect the thrust for VTOL flight operations. As for the supersonic part, that's going to be a tall order for Musk. So far there has only been one marginally successful supersonic VTOL aircraft put into production and that's the F-35B. It was nearly stillborn due to weight and complexity issues, can only hover with less than 40% total internal fuel, and whether the complicated lift fan system can operate in the demands of real combat remains to be seen. I can imagine the technical design challenges for doing all this and still making an ALL ELECTRIC aircraft as well makes me think Musks eyes are far bigger than his stomach.
UPDATE - Another idea which occurred to me about designing a supersonic VTOL vehicle like this would be a hybrid propulsion system using a gimballed electric drive ducted fan/cold diffuser for powered lift flight and acceleration combined with a chemical ramjet for high speed flight. It would be more advantageous than an all electric system and might be possible in the near future. It would require quite a powerful electric drive system to propel the craft up to the 400 - 500 KIAS required for the ramjet engines to become effective.
After having worked in the aerospace world as a design engineer for 12 years, I'm going to give you all a little wisdom about the business.
There are a whole lot of fancy pieces of concept art and CGI in social media but little actual flight hardware which ever sees the light of day.
Airplanes are EXTREMELY complex vehicles in their current state to design and manufacturer, requiring billions of dollars and thousands of man years to bring into fruition. Increase the complexity of that design, even in trivial ways, and you drive the costs through the roof and imperil the enterprise to be stillborn, ensnared in its own complexity.
Musk, as stated above is an entrepreneur and software whiz, not an engineer. His PayPal system was brilliant but I can't say the same for SpaceX or his current Hyperloop boondoggle where design work seems to be pushed off to student design teams without even the slightest efforts made the real issued of designing a safe, evacuated tunnel for this thing to operate in and all with a laughably unrealistic price tag.
Anytime someone proposes such a design, ask yourself this one question: If it was that simple, why aren't Boeing/Lockheed/Northrop Grumman/Bombardier/Sukhoi/Chengu/Textron/Embraeer/Mitsubishi designing similar vehicles for lesss as they have the economies of scale on their side for these types of projects? This usually ferrets out the reasons why such a design won't work very quickly.