[btilly]

Elon Musk says that eventually we will move to VTOL supersonic electric jets, but he is too busy with other projects to implement it.

Have you thought about this idea, and in what time frame do you think someone might try to implement it?

Elon's basic idea is that air-breathing jets are limited to altitudes at which oxygen tops out, and then are mostly fighting drag from nitrogen. Electric ones can go to higher altitudes and experience less drag at much higher speeds. However wings designed to do that won't help you get off the ground, therefore you need vertical take off and landing. Which eliminates the need for a large airport, allowing you to locate more conveniently to major cities.

[schwarrrtz]

The specific energy of kerosene is ~40 MJ/kg [0] while the specific energy of lithium ion batteries is ~1 MJ/kg [1].

Unsure how much energy you'd save from the difference in drag, but I'd guess that significant improvements in electric energy storage would be required to make an electric plane doable.

[0] https://en.wikipedia.org/wiki/Kerosene#Properties [1] https://en.wikipedia.org/wiki/Lithium-ion_battery

[mikeyouse]

To be fair to our electric friends, electric motors are >90% efficient while turbofans top out at somewhere near 35% efficiency (as far as I can find).. So what appears to be a 40:1 advantage to kerosene is more like a 15:1 advantage -- substantial but much more accessible with ongoing battery improvements. Though as another commenter mentioned, fuel planes do definitely benefit from shedding weight the entire time they're moving.

[SeoxyS]

A jet engine (e.g. turbofan) relies on combustion in the engine in order to function. The ignition of the fuel from within the engine is key… it is not just there to just create energy to drive rotational force. You would you use an electric motor on a plane without going back to what is essentially a propeller design?

[gruturo]

Actually, a good 80% of a modern airliner's thrust comes from the huge fan in front of the engine.

A turbine after the combustion chamber steals some of the energy to move the compressor and the fan. Most of the air (>90%) just goes through the fan and bypasses the engine. You could argue that these engines are _already_ cleverly camouflaged propellers :)

https://en.wikipedia.org/wiki/Turbofan https://www.quora.com/How-much-of-a-jet-engines-thrust-is-de...

[lmm]

An engine is just a mechanism for converting stored energy into forward motion - the details are, well, details. There's nothing wrong with propellers (probably impellers will be more efficient), and those efficiency numbers are (presumably) including that.

[pjc50]

No, the details are completely critical to making it work!

I'm kind of surprised that Elon Musk, who clearly knows his stuff in rocketry, is handwaving the difficulties of an electric jet. Personally I see it as far more likely that we'll continue to use conventional aircraft but start synthesising fuel from renewable energy. The US Navy is already looking at doing this to make jet fuel from spare carrier nuclear power:

http://www.nrl.navy.mil/media/news-releases/2014/nrl-fuel-fr...

[Retric]

You can get supersonic by having air intake, expansion chamber, blade, high pressure chamber, exhaust port.

Think something like this: <=|=>

Importantly the air flows through the compression chamber at Subsonic speeds.

PS: While simplified this is the general approach for low mach speeds as burning stuff in a supersonic combustion chamber is much more difficult.

[eloff]

To be fair, you can manufacture kerosene from renewable electricity. If you use carbon from the atmosphere, it's even completely carbon neutral. Think of it as a less cost efficient, but more energy dense battery. I kind of doubt that batteries are the way to go when energy density matters.

[WatchDog]

Plus a conventional airplane sheds weight as it travels.

[flashman]

Nothing that recoverable drop-tank batteries can't solve. I'm mostly kidding.

[foota]

Refueling via amazon drone?

[schiffern]

And yet Musk claims transcontinental range (NY-LA) would be possible with 400 Wh/kg batteries and mid-70% battery mass fraction.

http://www.aviation.com/general-aviation/elon-musk-toying-de...

[schwarrrtz]

A 70% battery mass fraction seems like a huge engineering challenge for an aircraft. I did a quick calculation of fuel mass fraction for the various Boeing 777 models [0] which ranged from ~13% to ~36%.

Not saying it's impossible. Just hard.

[0] https://en.wikipedia.org/wiki/Boeing_777#Specifications

[lmm]

I don't see that the engineering is hard. You could just take an existing design and replace most of the cabin with batteries.

Building an economically viable mass-market aircraft with a 70% mass fraction might be another question, but if we're just talking about the engineering challenge of building it at all I don't think that's hard?

[xgbi]

And on top of that, "refueling" wouldn't cost the 250k figure that usually gets thrown in comments. I imagine it being more in the space of a 1-2k$ ; in which case the reduced passenger capacity might still be worth the premium in batteries.

[kuprel]

Specific energy of hydrogen is 142 MJ/kg. Why not a hydrogen fuel cell powered electric jet?

[taneq]

Because hydrogen is very hard to store in any reasonable size or shape, and that's even without focusing on the inefficiency of generating it.

[jessaustin]

Ammonia doesn't have this or several of the other drawbacks of hydrogen, but is otherwise equivalent.

[maaku]

Drag on a bigger airframe.

[bscholl]

exactly

[bscholl]

schwarrrtz and azernik nailed it.

Today's batteries don't store enough energy for long international flights. There are some battery technologies in the lab (Google Lithium-air batteries) that might make this feasible, but they're not ready yet. Yet alone proven safe to carry passengers.

As fans of speed, we want to bring supersonics to market as quickly as possible. So, we're only using core technologies that have flown on other airplanes and are accepted by regulators.

There's plenty of opportunity for radical innovation in V2 and V3 once V1 is working!

[rdl]

Batteries do make sense on aircraft in place of APUs for backup and start and such, of course, just not thrust.

[azernik]

Unfortunately, aerospace is a place where energy density really really matters. If your car's battery weighs twice as much as equivalent hydrocarbon fuel, that will decrease your range, but probably won't affect your performance or total energy usage all that much.

For planes, however, using a more environmentally-friendly but heavier energy storage medium is self-defeating, because you have to spend a lot more energy lifting the extra energy-storage weight. Unfortunate, but it is what it is.

[diskcat]

Yeah but you are getting 50:1 difference in energy density between hydrocarbon fuel and battery. Also, vertical takeoff is extremely inefficient because a wing can generate a lot of lift compared to drag. this in addition to the fact that a vertical lift system is only used during take off and landing while being dead weight the rest of the trip.

even elon can't override physics

[ungerik]

The big power requirement of VTOL is not the problem in the overall energy budget because the hovering time is very short compared to the total flight time. Main problem is the additional weight of the more powerful engines.

BUT: If you want to fly supersonic, you need very powerful engines anyways, so if you can re-use them for VTOL it's a win win.

[diskcat]

To fly supersonic you need an engine that is very powerful at supersonic speed.

VTOL requires an engine that is powerful at low speed.

The harrier jump jet, a most famous VTOL aircraft, didn't have supersonic capability when all its contemporaries did and can only take off vertically with reduced payload or it would burn all its fuel just taking off.

While I don't have the numbers I think VTOL supersonic aircraft would be VERY hard indeed and it would be something that has to be seen to be believed.

[Xixi]

The F35-B is both supersonic and VSTOL [1]: very hard indeed, but done. Just like the Harrier, the F35-B probably doesn't take off vertically with any significant payload though.

[1] https://en.wikipedia.org/wiki/Lockheed_Martin_F-35_Lightning...

[lmm]

The F-35-B has essentially two different types of engine (the main engines and the lift fan) to make that work.