Gedankenexperiment

[TheStu]

My friend and I were having an argument about whether or not it would be possible to design an effective jet engine without a combustible fuel source. It would run only on electricity.

Friend thinks it is possible because he has read about water jets which obviously don't require a combustion chamber. My counter-argument is that since water is non-compressible the impellers can effectively get the volume of water to jet out the back faster than it can was when it was brought in.

Is my logic wrong?

 

[MagnusTheBrewer]

Your 'thought experiment' is ersatz. How do you plan to get the water to the impellers fast enough to keep from running dry?

 

[TheStu]

The water jet would be in a body of water. As in instead of a traditional screw, the ship/boat would have the water jet, water gets brought in the front, gets squirted out the back.

The question is whether a jet engine, on an aircraft, can be run with just electricity.

 

[eLiu]

Uhh... I'm not sure exactly what you're asking.

Do you want to use electricity to turn the compressor and then expand the same air out the nozzle? (Don't need a turbine if you're driving the compressor...) Though this isn't really a jet engine per se.

Do you want to just spin a propeller like on a prop plane?

Do you want to heat the air with heating coils (instead of burning fuel in it)? (Not using combustion as a heat source has been done before--they built nuclear aircraft in the 50s/60s. They = US & USSR.)

All 3 of those would be possible ways to propel an airplane with only electricity. But the low energy density of batteries makes this plan generally pretty bad.


Jet engines run on a very simple principle--something called the "Brayton cycle." Basically, you draw air in through the intake. The air is compressed by the compressor (spinning fan blades); the power to drive the compressor typically comes from the turbine (coming up later). After compression, (atomized) fuel is injected & burned. Then the hot hot hot gasses are expanded through the turbine (which powers the compressor); the turbine only extracts enough energy to drive the compressor (on a big engine, the efficiency of this is really high... like 99&#37. It works exactly opposite from the compressor--expanding gasses cause the turbine blades to spin. Energy remaining in the air will be converted to thrust as it is expanded out through the nozzle. (i.e., expanding gasses push against the walls of the engine, producing thrust.)

So if you drive the compression w/electricity, you don't need a turbine. If you heat the air w/electricity, you don't need combustion. But accomplishing either of these tasks on a large scale (like an airliner engine) would be really difficult.

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edit: whoops, I see what you're saying now w/the water jet thing. Sorry, I'm an aero engy so I just defaulted to aero examples w/o reading thoroughly. Yeah I'm not sure you would really call that a "jet engine" since it's not following the aforementioned brayton cycle in any way.

But using an impeller to raise water pressure is pretty similar to the above idea of creating thrust by expending energy to compress a fluid and then expanding the fluid 'out the back' for thrust.

Also I don't understand your counter-example. Water is nearly incompressible yes, but that doesn't mean you cannot raise the pressure. And raising the pressure (a lot) via a pump is what makes water jets (for cutting or driving boats) useful.

 

[TheStu]

Uhh... I'm not sure exactly what you're asking.

Do you want to use electricity to turn the compressor and then expand the same air out the nozzle? (Don't need a turbine if you're driving the compressor...) Though this isn't really a jet engine per se.

Do you want to just spin a propeller like on a prop plane?

Do you want to heat the air with heating coils (instead of burning fuel in it)? (Not using combustion as a heat source has been done before--they built nuclear aircraft in the 50s/60s. They = US & USSR.)

All 3 of those would be possible ways to propel an airplane with only electricity. But the low energy density of batteries makes this plan generally pretty bad.


Jet engines run on a very simple principle--something called the "Brayton cycle." Basically, you draw air in through the intake. The air is compressed by the compressor (spinning fan blades); the power to drive the compressor typically comes from the turbine (coming up later). After compression, (atomized) fuel is injected & burned. Then the hot hot hot gasses are expanded through the turbine (which powers the compressor); the turbine only extracts enough energy to drive the compressor (on a big engine, the efficiency of this is really high... like 99&#37. It works exactly opposite from the compressor--expanding gasses cause the turbine blades to spin. Energy remaining in the air will be converted to thrust as it is expanded out through the nozzle. (i.e., expanding gasses push against the walls of the engine, producing thrust.)

So if you drive the compression w/electricity, you don't need a turbine. If you heat the air w/electricity, you don't need combustion. But accomplishing either of these tasks on a large scale (like an airliner engine) would be really difficult.

I didn't realize that the nuclear powered planes were going to be jets, I thought that they were turbo-props. You say difficult to accomplish on a large scale. Difficult in which regard? Difficult in terms of power consumption? In terms of efficiency? If the only way to do it at all is with a nuclear power source, then I can see why they abandoned it.

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edit: whoops, I see what you're saying now w/the water jet thing. Sorry, I'm an aero engy so I just defaulted to aero examples w/o reading thoroughly. Yeah I'm not sure you would really call that a "jet engine" since it's not following the aforementioned brayton cycle in any way.

But using an impeller to raise water pressure is pretty similar to the above idea of creating thrust by expending energy to compress a fluid and then expanding the fluid 'out the back' for thrust.

Also I don't understand your counter-example. Water is nearly incompressible yes, but that doesn't mean you cannot raise the pressure. And raising the pressure (a lot) via a pump is what makes water jets (for cutting or driving boats) useful.

I was agreeing that my friend on the water jet thing, that those work, I apparently didn't spell it out very well, oops. My counter argument is that it is easier? to raise the pressure of the water to effectively use it as propulsion using just pumps/impellers/turbines than it would be to do the same with air.

And I apologize for my lack of correct terminology, I am not an engineer, I was just raised by them.

 

[Gibsons]

I didn't realize that the nuclear powered planes were going to be jets, I thought that they were turbo-props. You say difficult to accomplish on a large scale. Difficult in which regard? Difficult in terms of power consumption? In terms of efficiency? If the only way to do it at all is with a nuclear power source, then I can see why they abandoned it.

At least one functional nuclear powered ramjet was made.
Project Pluto
http://en.wikipedia.org/wiki/Project_Pluto

Talk about a doomsday weapon.

 

[eLiu]

I didn't realize that the nuclear powered planes were going to be jets, I thought that they were turbo-props. You say difficult to accomplish on a large scale. Difficult in which regard? Difficult in terms of power consumption? In terms of efficiency? If the only way to do it at all is with a nuclear power source, then I can see why they abandoned it.

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Difficult in terms of power consumption. I think GE's GEN-X (or is it GE-X) or say Rolls Royce's Trent-1000 require something like 30mega-watts to drive the compressor at speed. And the energy coming out the back end is something like 60MW? It's been a while since I've done the calculation, but those numbers should at least be the right order of magnitude. So you'd need something pretty powerful to just up and replace parts of a modern jet engine with electrically driven components.

From an efficiency point of view, usually "bigger is better" applies. Very small scale compressors/turbines are often fairly inefficient b/c the relative energy lost is larger.

As for the nuclear powered engine, the reactor *was not* used to generate electricity. The heat from the reaction heated the air directly; this was used in place of combustion. The US (i think GE?) built some working models but they never powered an airplane. The USSR strapped 2 such designs to their venerable "Bear" bomber and it flew. But the reactor was unshielded, so suffice it to say the crews didn't last very long & the idea was abandoned. The ultimate problem was weight--the weight of shielding to protect the crew was prohibitive. With modern tech we could probably build a practical model now, but the need for such things has long since vanished.

I was agreeing that my friend on the water jet thing, that those work, I apparently didn't spell it out very well, oops. My counter argument is that it is easier? to raise the pressure of the water to effectively use it as propulsion using just pumps/impellers/turbines than it would be to do the same with air.

And I apologize for my lack of correct terminology, I am not an engineer, I was just raised by them.

Easier? Uhh I'm not sure. This comparison is pretty difficult though, because air is 1000x less dense than water at standard atmospheric conditions. I do not recall what the energy per kilogram cost would be to raise the pressure of water vs air. (And with air it's even more complex, b/c the answer depends on how you raise the pressure.)

But like I said, water jets aren't really jet engines.

 

[TheStu]

That is a ridiculous amount of power consumption. Alright, so for a smaller vehicle, nigh impossible given current technology. For something the scale of the B36, not impossible, but still difficult. Los Angeles class submarine has a 26MW reactor according to Wikipedia, and I would imagine that is one of the smaller available nuclear reactors out there.

Alright, so unless I had a ludicrously powerful source, some sort of combustible is required for a jet engine.

 

[narzy]

I know there was some experiments with nuclear powered aircraft, the US experimented with them in the 1950's (1958) but the aircraft only carried a nuclear reactor and never used it to power the plane.

The soviets had a similar project, the aircraft was the Tupolev TU-119. The soviets used it to power the aircraft "They chose to focus on the direct cycle system from the start, testing ramjets, jet engines and even turboprops."

It is possible to run a "jet engine" using electricity, I use the words loosely in this context as it is more of a ramjet then a true jet engine. But for the sake of your argument I would call it a Jet .

eLiu stole my thunder . Though to add there is no reason the reactor couldn't be used to generate electricity to power the aircraft.

Here is another interesting article and a nice picture.

As you can see, the reactors were massive, although with today's nuclear technology I imagine that they could be developed to be much smaller. Probably one of the big sticking points was the size and cost of the reactors. Today the stigma of nuclear safety (and possibility of a HUGE accident) would prevent any such project although I think it would be highly interesting to see.

 

[narzy]

I didn't realize that the nuclear powered planes were going to be jets, I thought that they were turbo-props. You say difficult to accomplish on a large scale. Difficult in which regard? Difficult in terms of power consumption? In terms of efficiency? If the only way to do it at all is with a nuclear power source, then I can see why they abandoned it.

I believe it was because of cost and complexity, it was also very hard to shield the aircraft operators from the radiation that was emitted. The shielding added enormous weight to the aircraft but the nuclear design allowed for virtually unlimited 'fuel'

 

[eLiu]

That is a ridiculous amount of power consumption. Alright, so for a smaller vehicle, nigh impossible given current technology. For something the scale of the B36, not impossible, but still difficult. Los Angeles class submarine has a 26MW reactor according to Wikipedia, and I would imagine that is one of the smaller available nuclear reactors out there.

Alright, so unless I had a ludicrously powerful source, some sort of combustible is required for a jet engine.

Well so the thing is, if you wanted to run a jet engine electrically, I'm sure the setup would not be exactly the same as like the GENX. (And by electric jet, I'm thinking like... compression + heat = thrust)

I'm guessing that something like this
http://en.wikipedia.org/wiki/Ducted_fan
would be the most efficient route.

But back to electrical requirements. Part of the reason large engines require so much power is that these things do not scale linearly. So to get a high compression ratio, you need a powerful turbine. But to make is to that you don't spend like 90% of the energy just to keep the engine running (i.e. powering the compressor), you make the thing even bigger so there will be plenty of excess energy to use as thrust. But that costs you in terms of thermodynamic efficiency, so there's a trade-off in the classic Brayton Cycle.

To run the whole thing electrically raises a different set of questions. Now "all" the energy you put into the flow (through compression & heating) can be turned into thrust. So there'd be some kind of balance to strike btwn energy used to compress the air & energy used to heat it. The heating would be a complex issue by itself... I am not sure how the best heat-exchangers (like on the back of your refridgerator, but more powerful) compare to just lighting a fire (combustion). If I weren't lazy, I might work this out in more detail. Maybe I could get it posed as a design problem to an undergraduate thermodynamics course.


narzy: haha, yeah I discovered nuclear powered aircraft through some documentatries on youtube a while ago. To be honest the idea had never crossed my mind before, as I guess going through engineering school trained me to think about jet engines in one specific way. But it was pretty cool.

When the B36 was flying with the reactor on board, they had a chase plane full of paratroopers following it. If an accident occured, the paratroopers were supposed to jump out and lock down the affected area. Other units called those guys the "glow in the dark regiment" or something like that