c5load
Posts: 344
Joined: Tue Sep 16, 2008 9:40 pm

Hybrid Jet Engines

Fri Jan 09, 2009 1:48 am

Okay, so here is a thought to ponder. I'm not very aircraft engine smart, but would it be possible to make a hybrid gas/electric jet engine. It would operate in the same sense as a hybrid car would, by using gas for power on takeoff, climb, and landing. But at cruise it would use the combo of airflow and electricity to keep speed. Is it possible?
"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
 
User avatar
Starlionblue
Posts: 17119
Joined: Fri Feb 27, 2004 9:54 pm

RE: Hybrid Jet Engines

Fri Jan 09, 2009 2:09 am

Possible? Yes. You'd have to put an electric engine in the core. I don't know how hard that would be.

The problem is that you run into the usual energy storage issue. Electric propulsion is very efficient but batteries have a relatively low energy density per weight. In other words, they are heavy compared to the equivalent energy stored in jet fuel. Other problems are battery aging and environmental issues.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Hybrid Jet Engines

Fri Jan 09, 2009 4:00 am



Quoting C5LOAD (Thread starter):
I'm not very aircraft engine smart, but would it be possible to make a hybrid gas/electric jet engine. It would operate in the same sense as a hybrid car would, by using gas for power on takeoff, climb, and landing. But at cruise it would use the combo of airflow and electricity to keep speed. Is it possible?

Starlionblue is right...it's technically possible but not economically viable.

Another major reason not to do it is that you don't get nearly the efficiency gains from a hybrid jet as you do from a hybrid car. The major efficiency problem in cars is that gasoline engines suffer a big efficiency hit when running at small fractions of their full power due to throttling. Car engines almost never run at full power, and they cycle up and down in speed a lot. Electric engines are much better for this so hybrids run electric when they can and then run the gas engine at a relatively constant speed at a high power (high efficiency) when they need to.

A jet engine doesn't really suffer throttling losses and it runs at nearly constant speed for most of it's whole life, so there's a lot less to be gained by going to hybrid.

Tom.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Hybrid Jet Engines

Fri Jan 09, 2009 5:58 am

Another big win for hybrid cars that most people forget about is regenerative braking. In fact it's probably the single biggest win for hybrids. The hybrid can recapture much of the kinetic energy of the car when it stops - and then use it again when the car starts moving again. It's the major reason why most hybrid get better city mileage than highway mileage (the reverse of conventional vehicles). That starting and stopping is what kills your city mileage in a conventional vehicle.

Of course that completely fails to apply to an aircraft.

That being said, the notion of a hybrid system for aircraft makes a certain superficial amount of sense, in that you want engines that are optimized for cruise for most of the flight, and then lots of extra power for takeoff and climb. The problem is that the engines are already a lot like that because of the effect of altitude. The engine that produces 10000lbs of thrust at full power cruising at 40kft, pretty much inherently can produce five times that at sea level. And for practical airliners, those numbers are both in the ballpark of what you need.
 
User avatar
SEPilot
Posts: 4972
Joined: Sat Dec 30, 2006 10:21 pm

RE: Hybrid Jet Engines

Fri Jan 09, 2009 3:07 pm

Rwessel's analysis covers most of the issues. As he says, the primary benefit of hybrids is regenerative braking, which in aircraft is called gliding. So the weight of batteries plus motor/generators would far outweigh any gains, and it really has no benefits at all.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
Scipio
Posts: 852
Joined: Tue Oct 16, 2007 4:38 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 12:24 am

If you can make jet engines with an electrical core, than you could theoretically power an aircraft with a small onboard nuclear reactor. Is that correct?
 
luv2cattlecall
Posts: 644
Joined: Fri Sep 28, 2007 6:25 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 2:34 am



Quoting C5LOAD (Thread starter):
But at cruise it would use the combo of airflow and electricity to keep speed

JW, where would you be getting the airflow from?
.
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Hybrid Jet Engines

Sat Jan 10, 2009 3:28 am



Quoting Scipio (Reply 5):
If you can make jet engines with an electrical core, than you could theoretically power an aircraft with a small onboard nuclear reactor. Is that correct?

Radiation.........shielding for the reactor.........weight of the reactor.........radiation contamination in the event of an accident, any one of those make it a bad idea.

Jet engines burn fuel to accelerate the air flowing through them, and extract that energy with the turbines. How much electrical energy would it take to create that much instantanious heat (3,000 + degrese F) as you have in the combustion chamber of a typical jet engine?

The military experimented with a neuclear engine in the 50s, radiation was the big problem. The exhaust was radioactive, and the engine was also even after shutdown. They tested it, it worked, and then they scraped it after the tests. I think I saw the special on the Discovery chanel.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
dfwramper
Posts: 29
Joined: Fri Nov 28, 2008 2:06 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 3:33 am



Quoting Rwessel (Reply 3):
The hybrid can recapture much of the kinetic energy of the car when it stops



Quoting Rwessel (Reply 3):
Of course that completely fails to apply to an aircraft.

Regenerative braking doesn't apply....but altitude and speed sure have a bunch of POTENTIAL energy that's just ripe for the picking.....how to harness it and recapture it as a plane descends for landing?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 5:44 am



Quoting Scipio (Reply 5):
If you can make jet engines with an electrical core, than you could theoretically power an aircraft with a small onboard nuclear reactor. Is that correct?

As Ex52tech notes, it's been done. However, having a nuclear generator produce electricity to power an electric motor to power a fan would be a horrible way to do it. Much easier to just directly heat the air with the nuclear reactor and call it good.

Tom.
 
JA
Posts: 208
Joined: Sun Dec 12, 2004 9:25 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 6:47 am

Most hybrid vehicles are set up the wrong way.

Today's hybrid vehicles are set up so that the engine "kicks in when needed". Electric motors provide gobs and gobs of torque and power, but the issue of energy storage is an issue. The solution is to provide the electric motor with electricity using a generator powered by a fossil or alcohol fuel. The plane keeps its normal fuel tank and feeds a steady state generator (think small gas turbine) that sends electricity to the motor. The motor turns the fan and then helps create the compression to push the airplane. I am oversimplifying, but it might be able to work.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 7:39 am



Quoting JA (Reply 10):
The solution is to provide the electric motor with electricity using a generator powered by a fossil or alcohol fuel. The plane keeps its normal fuel tank and feeds a steady state generator (think small gas turbine) that sends electricity to the motor. The motor turns the fan and then helps create the compression to push the airplane. I am oversimplifying, but it might be able to work.

Except it violates conservation of energy. Current jet engines are very efficient at converting fuel energy to thrust. As long as you're using a given fan, the energy demand is basically fixed by the thrust. So your "small" gas turbine has to put out that much energy + enough to overcome the generator, transmission, and motor losses. Since that energy output is necessarily larger than the energy that your current jet engine puts out, the "small" gas turbine is, in fact, a large gas turbine...bigger than the jet engine you replaced.

Tom.
 
Alessandro
Posts: 4962
Joined: Wed Sep 12, 2001 3:13 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 9:47 am

I think it would only makes sense when taxing, if the weightpenalty isn´t too grand.
From New Yorqatar to Califarbia...
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Hybrid Jet Engines

Sat Jan 10, 2009 3:23 pm



Quoting JA (Reply 10):
The solution is to provide the electric motor with electricity using a generator powered by a fossil or alcohol fuel. The plane keeps its normal fuel tank and feeds a steady state generator (think small gas turbine) that sends electricity to the motor. The motor turns the fan and then helps create the compression to push the airplane. I am oversimplifying, but it might be able to work.

The K.I.S.S. method of engineering seems to me to apply here. How about we just burn fuel in regular jet engines like we do now, and call it good.

Re-inventing the wheel is not always the way to go.
The manufacturers and the airlines need to keep costs down, espically right now, not embark on something that will change everything from manufacturing to operations to maintenance. The bottom line here is to jam as much freight and passengers as you can into an airplane, not take up all that useable space with reactors, storage batteries, or some knetic energy recovery system.  duck 

We would be better off moving on to dylithium crystals and impulse or warp drive engines..............Scottie!  mischievous 
"Saddest thing I ever witnessed....an airplane being scrapped"
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Hybrid Jet Engines

Sat Jan 10, 2009 6:07 pm



Quoting Alessandro (Reply 12):
I think it would only makes sense when taxing, if the weightpenalty isn´t too grand.

Here's a possibly more feasible idea: Put some sort of electric system to drive the wheels themselves, and have the electricity come from the APU. This system would only be used when takeoff queues would be very long. I'm sure the fuel saved in those situations would justify the added weight.

Wasn't a company working on some sort of system like this with a motor to be attached to the NG?

Quoting Ex52tech (Reply 13):

We would be better off moving on to dylithium crystals and impulse or warp drive engines..............Scottie!

What about conveyor belts running in the same direction as the plane for T/O? Big grin  duck 
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Hybrid Jet Engines

Sat Jan 10, 2009 10:05 pm



Quoting FLY2HMO (Reply 14):
Here's a possibly more feasible idea: Put some sort of electric system to drive the wheels themselves, and have the electricity come from the APU. This system would only be used when takeoff queues would be very long. I'm sure the fuel saved in those situations would justify the added weight.

Wasn't a company working on some sort of system like this with a motor to be attached to the NG?

They still are:
http://www.wheeltug.gi/

Tom.
 
OldAeroGuy
Posts: 3208
Joined: Sun Dec 05, 2004 6:50 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 2:20 am

Quoting Rwessel (Reply 3):
Another big win for hybrid cars that most people forget about is regenerative braking



Quoting DFWramper (Reply 8):
Regenerative braking doesn't apply....but altitude and speed sure have a bunch of POTENTIAL energy that's just ripe for the picking.....how to harness it and recapture it as a plane descends for landing?

Since descents from cruise altitude are made at flight idle, the kinetic and potential energy of cruising flight is already being used. Fuel mileage during descent is better than any other part of the flight profile.

Better ATC procedures and improved FMC capability to minimize holds and delay flap and gear extension can make further gains but they won't be dramatic.

[Edited 2009-01-10 18:35:44]
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Alessandro
Posts: 4962
Joined: Wed Sep 12, 2001 3:13 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 2:40 am



Quoting Ex52tech (Reply 13):


The K.I.S.S. method of engineering seems to me to apply here. How about we just burn fuel in regular jet engines like we do now, and call it good.

Re-inventing the wheel is not always the way to go.
The manufacturers and the airlines need to keep costs down, espically right now, not embark on something that will change everything from manufacturing to operations to maintenance. The bottom line here is to jam as much freight and passengers as you can into an airplane, not take up all that useable space with reactors, storage batteries, or some knetic energy recovery system.

We would be better off moving on to dylithium crystals and impulse or warp drive engines..............Scottie!

Nag, the bottom line is to fly airplanes as fuelefficent as possible, have a plane that flies for long periods without major technical problems and don´t cost too much.
My idea would be to replace ballast with batteries (I assume the ballast and batteries are about the same density) to recoup the energy during landing and use it during taxi.
From New Yorqatar to Califarbia...
 
OldAeroGuy
Posts: 3208
Joined: Sun Dec 05, 2004 6:50 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 3:06 am



Quoting Alessandro (Reply 17):
My idea would be to replace ballast with batteries

Very few commercial airplanes require ballast.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Hybrid Jet Engines

Sun Jan 11, 2009 3:10 am



Quoting FLY2HMO (Reply 14):
What about conveyor belts running in the same direction as the plane for T/O?

Trying to start that thread again I see.  cheeky 

Quoting Alessandro (Reply 17):
Nag, the bottom line is to fly airplanes as fuelefficent as possible, have a plane that flies for long periods without major technical problems and don´t cost too much.

Sounds like you are making reference to the K.I.S.S. method......... Not sure what the Nag means though.  scratchchin 
"Saddest thing I ever witnessed....an airplane being scrapped"
 
Alessandro
Posts: 4962
Joined: Wed Sep 12, 2001 3:13 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 4:20 am



Quoting OldAeroGuy (Reply 18):
Very few commercial airplanes require ballast.

So it died off with the use of DU as ballast?
From New Yorqatar to Califarbia...
 
Mir
Posts: 19108
Joined: Mon Jan 05, 2004 3:55 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 4:45 am



Quoting Rwessel (Reply 3):
Another big win for hybrid cars that most people forget about is regenerative braking. In fact it's probably the single biggest win for hybrids. The hybrid can recapture much of the kinetic energy of the car when it stops - and then use it again when the car starts moving again. It's the major reason why most hybrid get better city mileage than highway mileage (the reverse of conventional vehicles). That starting and stopping is what kills your city mileage in a conventional vehicle.

Of course that completely fails to apply to an aircraft.

Not completely - there is the kinetic energy on landing. Probably not enough to make a meaningful difference in flight, but it might be useful for the taxi-in.

Quoting FLY2HMO (Reply 14):
What about conveyor belts running in the same direction as the plane for T/O?

With the brakes on, they would work.  biggrin 

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
OldAeroGuy
Posts: 3208
Joined: Sun Dec 05, 2004 6:50 am

RE: Hybrid Jet Engines

Sun Jan 11, 2009 5:09 am



Quoting Alessandro (Reply 20):
So it died off with the use of DU as ballast?

DU was replaced with tungsten, it's a lot less hassle.

However, DU and tungsten are used primarily as control surface balance weights to prevent flutter. In terms of volume, there is very liitle material of this type on airplanes today, certainly not enough to provide meaningful battery power. Besides, having batteries in the ailerons or elevators wouldn't be very convenient for running taxi motors.

Ballast is avoided because it's dead weight that doesn't add any structural strength and reduces airplane payload.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
baroque
Posts: 12302
Joined: Thu Apr 27, 2006 2:15 pm

RE: Hybrid Jet Engines

Sun Jan 11, 2009 6:32 am



Quoting OldAeroGuy (Reply 22):

DU was replaced with tungsten, it's a lot less hassle.

That is interesting indeed, and safer I dare say - back to WWII at last!

What would be the total weight of counterbalances in a plane such as a 747 or a 380 and are the amounts decreasing?

When the discussion about ballast started I thought they must be talking about the pax.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Hybrid Jet Engines

Tue Jan 13, 2009 7:18 am



Quoting Ex52tech (Reply 7):
Jet engines burn fuel to accelerate the air flowing through them, and extract that energy with the turbines. How much electrical energy would it take to create that much instantanious heat (3,000 + degrese F) as you have in the combustion chamber of a typical jet engine?

The energy density of Jet-A is about 43MJ/kg. Assuming equal efficiencies, that gives you a trivial conversion - remember that one Joule is one watt-second. So an airplane burning 1000lbs of Jet-A per hour is using about 5.5MW (continuous). Equivalently, that's about 7400hp. Scale those number up by a factor of about 25 for a 747...

OTOH, an electric motor driving the front fan will be rather more efficient that the turbine in raw terms, (probably about 80-90% of the energy would get sent to the front fan as opposed to 35-45% for the gas turbine). That however excludes storage losses. And any losses getting the electricity to the aircraft as well, but that's a different issue.

The problem is the lack of any storage device that can store electrical energy at the required densities - the very expensive Lithium Ion battery in your laptop stores less than *1* MJ/kg (typically around .5-.75MJ/kg). And we use those expensive Li-Ion batteries because they're particularly good t storing energy (the lead acid batteries in your car, while vastly cheaper, only store about .1MJ/kg).

If you could build a fuel cell, or similar device, that could convert Jet-A to electricity with much higher efficiency, it might be worth it to do that, and then use an electric motor to drive the fan. Unfortunately fuel cells are not available in the sizes required, nor are the ones that can run off non-hydrogen fuel sources nearly as efficient as the hydrogen/oxygen kind. And hydrogen has a huge volumetric energy density problem.

People keep forgetting just how good the medium chain hydrocarbons are as a fuel/energy source.
 
JA
Posts: 208
Joined: Sun Dec 12, 2004 9:25 am

RE: Hybrid Jet Engines

Tue Jan 13, 2009 4:35 pm



Quoting Rwessel (Reply 24):
If you could build a fuel cell, or similar device, that could convert Jet-A to electricity with much higher efficiency, it might be worth it to do that, and then use an electric motor to drive the fan. Unfortunately fuel cells are not available in the sizes required, nor are the ones that can run off non-hydrogen fuel sources nearly as efficient as the hydrogen/oxygen kind. And hydrogen has a huge volumetric energy density problem.

I would think, however, that an optimized generator powered by Jet A would still provide significant savings. For example, instead of 45% efficiency on two 50,000lb thrust engines engines, you may have one 80,000lb gas turbine and two electric engines on the wings. Yes, it's an extra engine, but electric engines are known for much lower maintenance because of fewer moving parts. The 20% thrust reduction saves fuel and the higher efficiency (you wire the output through a nice flywheel and to the engines) allows the same power to be generated.
 
User avatar
SEPilot
Posts: 4972
Joined: Sat Dec 30, 2006 10:21 pm

RE: Hybrid Jet Engines

Tue Jan 13, 2009 5:02 pm



Quoting JA (Reply 25):

I would think, however, that an optimized generator powered by Jet A would still provide significant savings. For example, instead of 45% efficiency on two 50,000lb thrust engines engines, you may have one 80,000lb gas turbine and two electric engines on the wings. Yes, it's an extra engine, but electric engines are known for much lower maintenance because of fewer moving parts. The 20% thrust reduction saves fuel and the higher efficiency (you wire the output through a nice flywheel and to the engines) allows the same power to be generated.

You are still stuck with the inefficiency of the gas turbine driving the generator plus the inefficiency of the electric system, plus the added weight of the motor and generator. The only place a generator driving a motor makes sense is on a locomotive, due to the difficulty and expense of making a transmission capable of starting a train and continuing to provide constant power up to cruising speed. It has no place whatsoever on an airplane. Yes, the electric motor requires less maintenance, but it still requires some. And you'd never get away with one turbine driving the generator; you would now have a single-engine airliner. So all you would end up doing is adding two huge generators and motors and tying up most of your fuselage space as well as payload. For what?
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
User avatar
DocLightning
Posts: 19832
Joined: Wed Nov 16, 2005 8:51 am

RE: Hybrid Jet Engines

Tue Jan 13, 2009 10:34 pm



Quoting JA (Reply 25):
For example, instead of 45% efficiency on two 50,000lb thrust engines engines, you may have one 80,000lb gas turbine and two electric engines on the wings.

Until that single turbine fails midway between SYD and SFO.

You always need a minimum of two engines on a heavier-than-air craft that is going to be making long trips and carrying lots of people. It's not a dirigible; if the turbine stops, the passengers get wet. And passengers don't like getting wet.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Hybrid Jet Engines

Wed Jan 14, 2009 7:11 am



Quoting JA (Reply 25):
I would think, however, that an optimized generator powered by Jet A would still provide significant savings. For example, instead of 45% efficiency on two 50,000lb thrust engines engines, you may have one 80,000lb gas turbine and two electric engines on the wings. Yes, it's an extra engine, but electric engines are known for much lower maintenance because of fewer moving parts. The 20% thrust reduction saves fuel and the higher efficiency (you wire the output through a nice flywheel and to the engines) allows the same power to be generated.

To expand on what SEPilot said:

Unfortunately the best gas turbine generation systems (used for stationary power plants), run at closer to 55-60% efficiency. Knock off 15-20% for the electric motor, and you're right back to the mid 40% efficiency range.

And there's no way you'd translate that efficiency to something you can load onto an airplane. Straight gas turbine powered generators are in the same efficiency range as aircraft turbines (not surprising, they're rather closely related beasts). The big boost in efficiency comes from a "combined cycle" design, where the waste heat from the turbine is used to boil water then used to drive a steam turbine to generate more electricity. Typically the steam plant manages to generate about a third to half the power of the primary gas turbine.

Although I find the notion of a steam powered aircraft to be somewhat intriguing.  Wink
 
OldAeroGuy
Posts: 3208
Joined: Sun Dec 05, 2004 6:50 am

RE: Hybrid Jet Engines

Wed Jan 14, 2009 4:37 pm



Quoting Rwessel (Reply 28):
Although I find the notion of a steam powered aircraft to be somewhat intriguing.

Yeah, the idea of a water replenishment airborne tanker sounds pretty neat to me.

Perhaps steam propulsion would be better coupled with a nuclear power plant airplane though.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
User avatar
SEPilot
Posts: 4972
Joined: Sat Dec 30, 2006 10:21 pm

RE: Hybrid Jet Engines

Fri Jan 16, 2009 11:39 am

One thing that nobody has considered is the size and weight of electric motors/generators required to provide enough power for a transport category aircraft. We are talking about at least 10,000 hp; a motor or generator capable of that will be HUGE and HEAVY. That is not even considering batteries; there are no batteries in existence capable of that kind of power. If there were they would be far to heavy to be carried by an airliner. Also, either the voltage or the current would have to be astronomical; if the voltage is reasonable (500V) the current would be about 10,000 amps; this would require wires about the size of an elephant's leg, which would be very heavy. If the current was held to something reasonable, the voltage would be out of sight, which would require incredible insulation, and be very, very dangerous as well. All in all, just a totally unworkable idea.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Hybrid Jet Engines

Sat Jan 17, 2009 2:11 am



Quoting SEPilot (Reply 30):
One thing that nobody has considered is the size and weight of electric motors/generators required to provide enough power for a transport category aircraft. We are talking about at least 10,000 hp; a motor or generator capable of that will be HUGE and HEAVY.

Well, you would probably be looking at a superconducting design of some sort. And an issue is that no one has really worked the size/weight issues on that sort of thing (compared to conventional designs they're rather more compact already), so any off the shelf design will likely be considerably heavier than what you’d actually build to put on an aircraft.

For example, there's a prototype 39MW (49kHP - about 93% efficient) motor intended to drive a ship, that's only 75 tons. It's not quite directed at aviation, as it's got a 120rpm output, but it uses 6600V (at about 6000A).

http://www.amsc.com/products/motorsgenerators/shipPropulsion.html

But yes, that's pretty heavy (it's actually not physically that big - just dense – by eyeball it looks similar in size to a GE-90). For comparison, a GE-90-115 generates about 110,000HP, and weighs about 9 tons, vs. 170t for a scaled up version of the above.

If you scaled that down to the cruise power requirements, you're a lot closer - about 30t for the GE-90-115 equivalent, but that leaves you with the need for some other source of thrust for takeoff and climb. Then again, this thread is about Hybrid Jet Engines...  Wink

Quoting SEPilot (Reply 30):
That is not even considering batteries; there are no batteries in existence capable of that kind of power. If there were they would be far to heavy to be carried by an airliner.

That's by far the killer problem. As I pointed out upthread, batteries are on the order of 100 times less dense (power-content-wise) than Jet-A. Forget the big electric motors, replacing the 180t of Jet-A on a 777-200ER would require on the order of 5000t of Li-Ion batteries - assuming you doubled the energy efficiency of the system by going electric.

Quoting SEPilot (Reply 30):
Also, either the voltage or the current would have to be astronomical; if the voltage is reasonable (500V) the current would be about 10,000 amps; this would require wires about the size of an elephant's leg, which would be very heavy. If the current was held to something reasonable, the voltage would be out of sight, which would require incredible insulation, and be very, very dangerous as well. All in all, just a totally unworkable idea.

Nah - rather higher voltages are no real problem to handle. And you'd be shooting yourself in the foot trying to run an electric motor at 500 volts an 10K amps - if nothing else the conductors will be huge if you run at that low a voltage (and the magnetics in the motor will be much bigger - and thus heavier - than necessary too). BTW, for a 10kHP motor, you'd be looking at something like 9MW input (not 5MW), which you'd want to target closer to 3000V and 3000A. And since weight is such an issue, I'd be tempted to go for a higher voltage.

Frankly, so long as you keep it under about 20KV, you're not looking at any particularly difficult requirements. Spark plug cables in cars typically carry over 20KV. Used as insulation, Teflon, for example, has a dielectric breakdown limit of nearly 60kV/mm (although, of course, you want to be nowhere near that limit - at least a factor of ten is typically required). For 20KV, and 450A (9MW), you'd be looking at a conductor about 15mm (.6 inches) in diameter, with 6-7mm of insulator on top of that. Just keep your fingers out of the works.  Wink
 
wingscrubber
Posts: 806
Joined: Fri Sep 07, 2001 1:38 am

RE: Hybrid Jet Engines

Sat Jan 17, 2009 9:48 pm

Remember that turbofans produce the majority of their thrust from the fan and not just simply the hot gasses escaping the core causing reaction thrust although they do contribute some - the design intention is for the fan turbine to be driven by those hot gasses.
Simply heating the core gasses with electricity or nuclear energy is hair-drier technology and would be extremely wasteful.
Enough energy to spin the fan with a large electric motor could come from a small nuclear reactor, the Russians already did it right? Feasibly, a boost of electric assist power during cruise could offset the fuel burn rate which could then be very economical, but as already mentioned, the weight and safety issues would be the major limiting factors.
The areas which would benefit most from hybrid technology are on the ground - taxiing obviously, i.e. taxiing with engines off, electric motor turning the wheels with power from the APU; this is already in use. The other obvious application IMO is during take off runs and landing roll out; a boost of electric drive from the wheels during the take-off roll would be a big fuel saver, the energy for which could be re-claimed later during landing as regenerative braking, but of course this brings us back to battery size and weight as the limitation again!

I still like to imagine airports with a powered rail system to negate the need for any engine-run while taxiing, and then either a cable catapult system, electric pantographs for ground powered run up or maybe even maglev runways!
Resident TechOps Troll

Who is online

Users browsing this forum: rjsampson and 15 guests