Electrical Engines.

[KraytKing]

How could you have a jet or rocket engine that runs entirely on electricity? How efficient would they be compared to our usual fossil fuel burners? Just looking for plausible ideas, not possible ones.

[Terralthra]

Rocket, no, because you still need reaction mass to shove out the back end. It's theoretically possible to make a jet that operates off of electricity, though I can't think of any electrical mechanism that can heat air fast enough to push a plane into the air.

[Imperial528]

You could make a thermal jet that was purely electric, but you'd need either a combustion or nuclear power plant, so it'd be easier to just use them directly.

Ion engines run entirely off of electricity, but as far as "rockets" go they are very low thrust. Excellent exhaust velocities though, which is why they are used in spacecraft/satellites which can't carry much propellant.

I have toyed with the idea of ion engines which are essentially electron guns, but they would be even lower thrust, and likely not as energy efficient.

[Zixinus]

It depends on what you mean by "run on electricity".

Ion engines and similar engines need a great deal of electricity but still require something to throw out, just far less than you would with a chemical rocket or thermal rocket.

If you don't want reaction mass, then lasers with sails are the way to go. You will need ridiculously large amount of power for little thrust, but you will only be emitting photons. Yes, you can push stuff with laser (and light in general), it's just you need lots of it at high intensity to produce very little thrust.

[KraytKing]

It depends on what you mean by "run on electricity".

It doesn't need to be purely electricity, just something you could source renewably. For instance, a propeller plane with an electric motor instead of a gas turbine would work, except for not being a jet.

I want something that could fly in our atmosphere, and possibly something that could make launches from the Moon powered by a solar panel network.

[TimothyC]

Electric "Jets" are possible, and varius working on them. They don't heat the air, but simply use a motor to drive the main fan blades instead of the hot exhaust gasses driving a turbine.

[SpottedKitty]

Electric "Jets" are possible, and varius working on them. They don't heat the air, but simply use a motor to drive the main fan blades instead of the hot exhaust gasses driving a turbine.

I'm sure I remember reading about model planes with this kind of engine. Although there would probably be a bit of crossover with ducted fans, which are moderately common for "jet" models.

[KraytKing]

Somewhat related question: is it possible to launch unmanned cargoes from Earth using railguns, coilguns, or something else of that concept? I know the Gs would kill a human pilot, but how would a computer and your cargo hold up?

[LaCroix]

Somewhat related question: is it possible to launch unmanned cargoes from Earth using railguns, coilguns, or something else of that concept? I know the Gs would kill a human pilot, but how would a computer and your cargo hold up?

If long enough, and the endpoint is high enough to reduce athmospheric drag after exit, sure.
For example build a tube up the Mt. Everest or a similar mountain. With a barrel lenght of a couple km, you can use a pretty pedestrian acceleration and still come up with a substancial bit of speed at the end. At 8km altitude, you only face 30% of the air density after exiting the muzzle, which means a lot less drag. Put a small booster at the end of the payload for the last bit of ascension/orbit circularisation, and Bob's your uncle.

[Imperial528]

Such launch cannons are possible, but frankly on a atmospheric body such as Earth you're better off just using the materials to build more rockets to jumpstart asteroid mining.

For an airless body, they're rather practical if you don't mind the initial setup cost.

[LaCroix]

Such launch cannons are possible, but frankly on a atmospheric body such as Earth you're better off just using the materials to build more rockets to jumpstart asteroid mining.

For an airless body, they're rather practical if you don't mind the initial setup cost.

Asteroid mining is nice, but you still need the capability to lift up, as well. such a launch tube is certainly better than trying to lift small percentages of payload into orbit via convenional means.

You could probably try and curve the tube around the mountain a couple of times to get even longer acceleration time (or even have some more tube leading up to the mountain), so you can forgo most the booster (you still want some for steering).
After all, orbit is not a question of up - even (advanced) model rockets can reach 100km. You need the orbital velocity - so the muzzle should actally be pointed at 45° or thereabout, and have you exit with 8-10 km/s Vmo (depends on how much you will lose due to friction losses, I can't calculate that) in order to actually shoot you into orbit. Be sure to mount a heat shield nose cone, though.

It is most likely cheaper than a launch loop, especially since it will be much shorter. A loop would be more efficient, though.

Or you could go your way and start build an orbital ring from asteroid material to fill that need for the "up" capability. Especially since orbital rings provide a great "move anywhere on the planet within an hour" capability, too, especially with multiple ones at angle to each other.

[Imperial528]

I've thought about launch cannons before and my preference has been for a single cannon to bypass the atmosphere (mostly), and then using skyhooks to boost to orbital velocities.

But even that is a structure that may well be a mile tall, and possibly even deeper underground than that, and you're subjecting the payload to a lot of forces along the way. And it needs to be precise.

I've never really liked the cannons that seek to impart more or less orbital velocities at the muzzle because they'd need to be taller than Everest to really escape the drag enough to be efficient. You also have to smack a heat shield onto your payload for launch.

And the entire accelerator tunnel has to be in at least partial vacuum unless you want to take drag losses along the entire thing. Which is a whole other problem.

Just seems like a lot more work going into the payload itself versus what goes into a rocket I guess. And a tremendous amount of effort into just maintaining the launcher.

[LaCroix]

And the entire accelerator tunnel has to be in at least partial vacuum unless you want to take drag losses along the entire thing. Which is a whole other problem.

Well, Elon got us covered with that. His whole hyperloop concept is pretty much just one step shy of a launch cannon, namely the higher acceleration, which he does not incorporate (yet) due to his concept being a passenger carrier, and humans being kinda squishy in nature.

But I would not be surprised if he isn't also looking into that as a future project.

You never know with Elon.

[Sky Captain]

During cold war there were experiments with nuclear jet engines that used heat from reactor to heat air in place of combustion. No reason why it couldn't work with electric heating elements if much better batteries are available.

[KraytKing]

You could probably try and curve the tube around the mountain a couple of times to get even longer acceleration time (or even have some more tube leading up to the mountain), so you can forgo most the booster (you still want some for steering).

Wouldn't the curve of the tube generate centrifugal force? That would slow it down immensely, both from friction and having to account for g-forces on a human. Same with leading up to the mountain and turning upwards. It seems to me like a straight line would be best.

[Elheru Aran]

A coilgun would be using magnets to suspend the payload in the middle of the tube, no? The only friction would be from the air? Is that what you mean? G-forces would only matter as far as the payload can deal with it, and if you're making most of the tube a vacuum, then air friction is negligible.

[KraytKing]

It seemed to me that LaCroix meant to wrap the tube around a mountain to make it longer. As in, the payload traveling through the tube in a long circle in order to slowly gain velocity. Certainly, an ordinary coilgun would not do this.

Also, out of curiosity, to coilguns spin their projectile if it's magnetic?

[Sea Skimmer]

Electric "Jets" are possible, and varius working on them. They don't heat the air, but simply use a motor to drive the main fan blades instead of the hot exhaust gasses driving a turbine.

That's not exactly jet engine then, it's just a ducted fan and would hit the same problem propeller driven aircraft do if you want airspeeds over ~450 knots. But nothing stops you from building an electrical jet engine with an electric heater element, it just wouldn't make much sense ever because the power to weight ratio would be very poor.

Wouldn't the curve of the tube generate centrifugal force? That would slow it down immensely, both from friction and having to account for g-forces on a human. Same with leading up to the mountain and turning upwards. It seems to me like a straight line would be best.

If you have a circular coilgun you don't have friction to worry about. Centrigual forces will absorb energy, but the point of a launch concept using EM drive is that electrical power is much cheaper then expending rockets, assuming your launch system is capable of several thousand launch cycles to account for its large initial fixed cost. Human launch via EM anything though hits serious problems with the human brain being very sensitive to such high levels of magnetic field.

A straight line system has the problem of enormous accelerations, which would greatly limit the types of cargoes that can be fired, and in any remotely practical configuration rule out anything living.

[Sea Skimmer]

Well, Elon got us covered with that. His whole hyperloop concept is pretty much just one step shy of a launch cannon, namely the higher acceleration, which he does not incorporate (yet) due to his concept being a passenger carrier, and humans being kinda squishy in nature.

But I would not be surprised if he isn't also looking into that as a future project.

You never know with Elon.

The hyperloop is nowhere near what a launch cannon needs to be and it won't scale up. Cold gas acceleration is not going to give you mach 25 muzzle velocity! Existing technology for other purposes is closer to launch cannon requirements then the requirements for hyperloop to work.

I also feel the need to point out that Musk did not invent this concept, nor has his company demonstrated anything very novel about it to date, except the ability to generate publicity while ignoring costs and low passenger throughput. RAND and the US government studied the crap out of it in the 1970s after the first oil crisis though the idea is in fact even older.

[Sky Captain]

Electric "Jets" are possible, and varius working on them. They don't heat the air, but simply use a motor to drive the main fan blades instead of the hot exhaust gasses driving a turbine.

That's not exactly jet engine then, it's just a ducted fan and would hit the same problem propeller driven aircraft do if you want airspeeds over ~450 knots. But nothing stops you from building an electrical jet engine with an electric heater element, it just wouldn't make much sense ever because the power to weight ratio would be very poor.

What would be best way to go supersonic using electrical engines? Asuming batteries or supercapacitors with energy density close to jet fuel exist. If jet engine with heating elements would be too heavy what other options would be possible to go supersonic using electricity. Some kind of motor driven ducted fan with afterburner that uses fuel only when extra performance is needed?

[LaCroix]

Don't think the afterburner wil work the way you think. You already need high airflow for this to work, or it will cause problems due to backfires, afik.

For an electical jet, you would need a way to ionize the air and then magnetically push it out the back. Like an ion drive, just with using the existing air as fuel. The thrust will be miniscule, though, I fear.

[Elheru Aran]

Would probably work well for long haul, endurance type flights though. If ion engines have anywhere near enough thrust to be effective at high atmosphere and the whole combination is cheap enough, I could see sending small payloads up to orbit by slowly flying them up into the jet-stream to give them a little boost, then carrying on till they're high enough to kick on an ion engine and push into orbit.

Just carrying chemical rockets would probably be faster and cheaper, though.

[Sea Skimmer]

What would be best way to go supersonic using electrical engines? Asuming batteries or supercapacitors with energy density close to jet fuel exist. If jet engine with heating elements would be too heavy what other options would be possible to go supersonic using electricity. Some kind of motor driven ducted fan with afterburner that uses fuel only when extra performance is needed?
[/quote]

The problem with a supersonic electric engine is that even if your battery could equal the power density of jet fuel, which is notionally possible but presently unbuildable, the battery weigh is fixed mass. Aircraft performance is rather predicated on the fact that as you fly along you loose weight to fuel burn. In the case of the Concord for example the dry aircraft weight was 203,000lb, payload 29,500 pounds, and fuel weight no less then 213,000lb.

Military aircraft can be much more skewed towards fuel, the SR-71 was closer to 2/3rds fuel by weight, but actually couldn't takeoff with more then a fractional fuel load, and always had to inflight refuel to get its incredible sustained flight performance. By only taking off light it could have a much smaller wing and thus much less supersonic drag.

An electric aircraft has to takeoff heavy and stay heavy, that's always going to make it suck on range and altitude performance. That the engine itself is somewhat heavier isn't the real problem for a plane, at least a big one. It certainly would be for a fighter. The lower and slower you go the more acceptable fixed weight would become.

If you want a rocket for launch meanwhile then an electrical engine will suffer much more from not only not shedding fuel mass, but also its higher weight, and the fact that since it needs an extremely high thrust to weight ratio to be useful at all you'll also need some really potent electrical switch gear. And if you can't use atmospheric air for cooling, or fuel, the way jet engines and high performance rockets cool themselves, you may have another weight guzzling problem to deal with too.

Best way to use electrical power is employ it to power a catalytic cracking facility to make hydrogen; though better still would be to build some 4th generation nuclear plant that's just outright hot enough to power the cracking facility directly, or possibly a solar thermal oven for the same purpose; in space that should be a lot easier then on earth.

For an electical jet, you would need a way to ionize the air and then magnetically push it out the back. Like an ion drive, just with using the existing air as fuel. The thrust will be miniscule, though, I fear.

Yeah the thrust will be minuscule and such an idea while plausible, ionic wind thruster, will only work at low subsonic speeds due to lack of compression. This might have applications some day for a low altitude silent recon drone though or similar project.

More typical space type ion thrusters have such bad thrust performance, and bad concept in general for anywhere with drag, that even if your electrical power was free via magic they still wouldn't be able to make an aircraft fly worth a damn. The engine can't even lift itself.

[Sky Captain]

Thanks, that's very interesting. So essentially a ducted fan electric aircraft would be useful in short to mid range passenger and cargo flights where performance penalty of having no weight loss during flight may be acceptable, but high performance military and long range comercial planes would best remain using conventional jet engines.

Seems that if development of nuclear aircraft would continued during Cold War engineers would faced similar problems of heavy engines, reactors, shielding and performance penalty of having no weight loss in flight ruling out high performance aircraft.

[SpottedKitty]

Seems that if development of nuclear aircraft would continued during Cold War engineers would faced similar problems of heavy engines, reactors, shielding and performance penalty of having no weight loss in flight ruling out high performance aircraft.

In this one case I'm not sure it would matter; remember that Project Pluto was intended to develop a cruise missile powered by a nuclear ramjet. The proposed mission profile included a low-level supersonic dash towards the target, which I think qualifies it for "high performance". The reactor would have produced so much power — potentially, for months — that any sort of conventional performance penalty calculation was sort of irrelevant.