How fast could an electric fan-jet fly?

[SomeGuy12]

Basically, I'm talking about a streamlined aircraft shape propelled by ducted fan engines like [URL="http://aviationweek.com/blog/econcept-eadss-hybrid-electric-airliner"]these[/URL]. The energy to run them would come from onboard [URL="https://en.wikipedia.org/wiki/Aneutronic_fusion"]aneutronic fusion[/URL] - yeah, yeah, a century away - that would produce electricity directly from the escaping helium ions. (by capturing the energy of the moving charged particles)

Anyways, basically, it's a weight in the tail (radiation shield is a shadow shield to protect from side reactions)

With this kind of 100% bypass, jet fan, can you even exceed the speed of sound? About how fast could you fly before the drag from the turbine blades limit your speed?

[Nuke]

ive always wondered how fast you can push an electric turbine. since i got my 3d printer ive been experimenting with a lot of ducted fan designs for various motors in my parts bin, with results ranging from 'thats cute' to 'omg what a monster' (usually depending on which motor i use). currently waiting on a new brushless motor for a centrifugal fan (stl file ready to go) for my hovercraft built out of trash. i saw [URL="http://hackaday.com/2015/11/16/ev-motor-not-powerful-enough-make-your-own/"]this build[/URL] the other day and was impressed. it has me wanting to re-wind some of my weaker motors with thick magnet wire to make them a little bit more powerful, and i certainly have the magnet wire for it.

i suppose if you use superconducting magnets cooled by liquid nitrogen or whatever, magnetic bearings, and a powerplant like a polywell or dpf reactor, or the thing that the skunkworks is building (does it have a name?). i figure those are going to be the only powerplants small enough to fit on an aircraft, tokamaks, stellerators, and laser machines are all out (too big to fit on an aircraft unless hovercarrier). you can probibly push a fan to the point where centrifugal force will shatter any known material. its an engineering problem, though not as hard as building a fusion reactor. Edited November 17, 2015 by Nuke

[wizzlebippi]

Most passenger aircraft use high bypass turbofan engines (bypass ratio of 5-10) with 80-90% of the thrust coming from the fan, so that's 500-550 KTAS (Knots True AirSpeed) guaranteed. Beyond that, since all jet engines eat air at < 0.3 Mach and many fighters use complex variable geometry inlets to slow intake air, it's probably possible to make an electric ducted fan that would produce thrust at up to Mach 2-2.5. How fast the aircraft would go depends on drag, but supersonic is probably possible.

[K^2]

I was thinking something along these lines as well. Though, you'll probably have a lot of trouble punching through transonic region. You might have to do what supercruise-capable jets usually do and have an afterburner for passing the sound barrier. Then you can kill the afterburners and continue on electric.

[Streetwind]

With battery tech constantly improving, we may not even have to rely on fusion reactors for all-electric planes in the years to come :) In fact, you can build battery powered planes today that can carry useful payload on intra-country flights of 1-2 hours tops. The main reason we're not doing that is because while the payload is useful compared to the cost of the flight, it's also not very large, meaning you will need to fly a large number of times to amortize the investment... a larger number of times than with conventional planes. Additionally, you need multiple electric planes to move the same volume as a conventional plane can - even if they are amortized and running very cost efficient, this creates additional ground crew and hangar overhead, as well as putting additional strain on airport capacities.

Since all of this makes airlines unlikely to be motivated to buy all-electric planes, no company has yet stepped up to try and build one for real (although multiple manufacturers are running research programs right now). However, as battery energy density grows at a steady 5% year over year (sometimes even exceeding that), the payload capacity of such a hypothetical plane continues to increase year over year as well, as less and less of its own weight is taken up by batteries. Right now I think I heard Elon Musk quoted that such a plane would require 70% of its own weight in batteries, but in 10 years, it may only be 30%. I'm fairly sure we'll start seeing such planes come to the market by then.

As for the question of how fast it can fly - I'd like to ask: how fast does it [I]need[/I] to fly? Supercruising is ideal for getting passengers across the oceans, but cargo likely doesn't care if it is in flight for 5 hours or for 10. Additionally, intra-country flights are too short to really make it worth pushing for supersonic flight, and any supersonic overflight of any populated area at all is going to face massive regulatory issues because sonic booms are [I]loud[/I]. Even planes supercruising at 15km up and higher are very audible, while conventional subsonic planes at that altitude fall below the threshold of human hearing. The Concorde wasn't allowed to break the sound barrier until it left the continent behind, IIRC, because of the sheer noise level that produced. So I would wager that for the largest part of flights - 90% and up - our planes are going to stay subsonic for the foreseeable future, even if we switch to all-electric propulsion. Edited November 18, 2015 by Streetwind

[SargeRho]

Airbus have actually built an E-Fan plane. It's only a small prototype and amounts to little more than a glider with 2 E-Fans on it, but rome wasn't built in a day either. Last I checked their plan is actually to build a 100-passenger fully Electric plane.

[Streetwind]

Yeah, that's what I meant with "research programs" ;)

[KerbMav]

Something completely different but related:

Is there a point where it becomes more efficient (in terms of electrical energy/battery usage) to add two more propellors/jets than trying to rotate your first pair faster?

Also:

[quote name='Nuke']you can probibly push a fan to the point where centrifugal force will shatter any known material. its an engineering problem, though not as hard as building a fusion reactor.[/QUOTE]

This seems super relevant!

[url]https://www.youtube.com/watch?v=zs7x1Hu29Wc[/url]

[PB666]

Get rid of the battery replace it with an H2/O2 fuel cell.

BTW, there is one thing to remember about Fuel cell drive jets. They do not flame out over 15km alt. If you increase the wing surface area then you have less speed at lower altitudes, but use less power and can climb to FL600 or even 800. You have to provide a scrubber for passenger air, since you have O2 on board anyway. This technology could theoretically exceed the distance of the traditional jet. The engine is radically different since it cannot rely on airfuel expansion as in a traditional jet.

Breaking the sound barrier past FL550 is not that difficult, the problem is that to make it useful for SST you have to have a bigger and less efficient engine relative to low speed.

[fredinno]

[quote name='PB666']Get rid of the battery replace it with an H2/O2 fuel cell.

BTW, there is one thing to remember about Fuel cell drive jets. They do not flame out over 15km alt. If you increase the wing surface area then you have less speed at lower altitudes, but use less power and can climb to FL600 or even 800. You have to provide a scrubber for passenger air, since you have O2 on board anyway. This technology could theoretically exceed the distance of the traditional jet. The engine is radically different since it cannot rely on airfuel expansion as in a traditional jet.

Breaking the sound barrier past FL550 is not that difficult, the problem is that to make it useful for SST you have to have a bigger and less efficient engine relative to low speed.[/QUOTE]
Fuel cells are less efficient though.

[sashan]

EDF, or electric ducted fan are very popular among modelists who want an RC jet without the danger and expenses of mini jet engine. THe main downside is that the efficiency is 2x lower of the propeller of the same thrust.

[K^2]

[quote name='sashan']EDF, or electric ducted fan are very popular among modelists who want an RC jet without the danger and expenses of mini jet engine. THe main downside is that the efficiency is 2x lower of the propeller of the same thrust.[/QUOTE]
This is because of the scale and low speeds. On the scale and typical cruise speed of commercial airliners, ducted fan is actually more efficient than a prop. Hence the use of high-bypass turbofans in aviation.

[img]http://www.homebuiltairplanes.com/forums/attachments/aircraft-design-aerodynamics-new-technology/33926-ducted-fan-aircraft-propulsive-efficiency.jpg[/img]

As indicated, efficiency drops once you approach sonic speeds. It's hard to tell how much efficiency you can still squeeze out of an EDF in supersonic. It's entirely possible that if you're looking for an electric supercruise, you are going to be better off with some sort of a variation on thermal jet.

[SomeGuy12]

But if you also get tired of running out of fuel and you want some kind of vehicle that can stay airborne for weeks if not years (weeks is more realistic because of maintenance requirements), you could in principle do it with a nuclear energy source that is lightweight and provides electricity and electric fans.

Obviously, you could also use a basically open fission reactor core, where you suck air directly through pipes in the reactor core, and use variable geometry scramjet intake. The problem with this is you'd probably leak highly radioactive particles in your exhaust stream (with fusion, you can line the reactor with materials chosen to minimize neutron activation and use aneutronic fusion so that less than 1% of the reaction produces neutrons), and landing is hard because you have an engine that requires this constant flow of air or the reactor overheats, and you can't throttle down smoothly.

With the fusion method, you would land using batteries.

[SargeRho]

>Nuclear
>Lightweight
>Doesn't kill the crew

Choose 2.

[K^2]

[quote name='SargeRho']>Nuclear
>Lightweight
>Doesn't kill the crew

Choose 2.[/QUOTE]

That's why OP mentioned aneutronic fusion. It's one of possible ways to get all three. Nuclear isomers are another. Probably not something we're going to be doing in the nearest decades, of course, but no reason not to talk about possible applications for the future.

[SargeRho]

Is there an aneutronic fusion process that doesn't still produce neutrons from secondary reactions though? From what I know, proton-boron fusion still produces the occasional neutron, and D-³He fusion also ends up producing neutrons. Edited November 19, 2015 by SargeRho

[K^2]

What secondary reactions? The products of most of these are extremely stable, otherwise there would be neutrons flying about to begin with, and because recoil's all in gamma and/or beta, product nuclei don't end up buzzing about at several MeV. So if your fuel is just at the barrier for fusion, products are far, far from the barrier to continue fusion. E.g. proton-boron fusion, which yields slow alpha particles and gamma. The only way to get any of this to fuse further is to get some proton-proton reactions going.

And yes, proton-boron fusion is technically a fusion-fission reaction. H + [sup]11[/sup]B → [sup]12[/sup]C[sup]*[/sup] → [sup]4[/sup]He + [sup]8[/sup]Be[sup]*[/sup] → 3 [sup]4[/sup]He. The excited states decay via alpha emission rather than gamma emission. But it's still considered an aneutronic fusion reaction. Edited November 19, 2015 by K^2

[Aghanim]

[quote name='SargeRho']Is there an aneutronic fusion process that doesn't still produce neutrons from secondary reactions though? From what I know, proton-boron fusion still produces the occasional neutron, and D-³He fusion also ends up producing neutrons.[/QUOTE]

Helium-3-Helium-3 fusion doesn't produce any neutrons

[KerikBalm]

[quote name='K^2']What secondary reactions? The products of most of these are extremely stable, otherwise there would be neutrons flying about to begin with, and because recoil's all in gamma and/or beta, product nuclei don't end up buzzing about at several MeV. So if your fuel is just at the barrier for fusion, products are far, far from the barrier to continue fusion. E.g. proton-boron fusion, which yields slow alpha particles and gamma. The only way to get any of this to fuse further is to get some proton-proton reactions going.

And yes, proton-boron fusion is technically a fusion-fission reaction. H + [sup]11[/sup]B → [sup]12[/sup]C[sup]*[/sup] → [sup]4[/sup]He + [sup]8[/sup]Be[sup]*[/sup] → 3 [sup]4[/sup]He. The excited states decay via alpha emission rather than gamma emission. But it's still considered an aneutronic fusion reaction.[/QUOTE]


[url]http://forum.kerbalspaceprogram.com/threads/137590-Stellarator-class-fusion-reactor?p=2265768&viewfull=1#post2265768[/url]

[quote]If you fuse one proton with one boron, and create helium particles, there is no neutron produced, it is truly aneutronic.
However... if you have a mixture of Boron and protons at high temperature and pressure, and start fusing them....
You start to create alpha particles...
You now have a mixture of B, H, and He nuclei
You will then start to have two reactions taking place:
B^11+H^1 → 3 He^4 + 8.7 MeV (I think there is a C^12 intermediate... but C12 is stable, so I don't understand why it splits to 3x He^4)
and
B^11 + He^4 → N^14 + n + 157 KeV

Then if you don't use pure H1, and you've got a bit of deuterium, you'll get this:
B^11 + H^2 → C^12 + n + 13.7 MeV

Supposedly, a small fraction of the time, instead of:
B^11+H^1 → 3 He^4 + 8.7 MeV
You get:
B^11+H^1 → C^11 + n − 2.8 MeV
But these neutrons are less energetic and not as dangerous [/quote]