Why no tri or quad jet fighters?

botsing · by **botsing** » 18 Jun 2018, 16:40

Agreed with sprstdlyscottsmn indeed.

sferrin · by **sferrin** » 18 Jun 2018, 17:12

Sorry guys. I've blocked him so that should be that.

madrat · by **madrat** » 19 Jun 2018, 02:04

Wow, the guy that went over dramatic has now saved the day by blocking his victim. Yay. /sardonic tone

count_to_10 · by **count_to_10** » 21 Jun 2018, 13:00

sferrin wrote:Why go through all the waste though? What does this get you vs a standard turbofan in increased efficiency, thrust to weight, etc.? I've yet to hear an advantages.

As I wrote earlier, there are some interesting things you could do with the ability to independently set the RPMs of each stage or physically disconnect a compressor from any turbine. It would take considerable study to work out if the electrical losses would be worth it. Maybe TEG could give us an idea if it’s feasible, but I don’t think the rest of us can dismiss the idea out of hand with any credibility.

sferrin · by **sferrin** » 21 Jun 2018, 15:03

count_to_10 wrote:
sferrin wrote:Why go through all the waste though? What does this get you vs a standard turbofan in increased efficiency, thrust to weight, etc.? I've yet to hear an advantages.

As I wrote earlier, there are some interesting things you could do with the ability to independently set the RPMs of each stage or physically disconnect a compressor from any turbine. It would take considerable study to work out if the electrical losses would be worth it. Maybe TEG could give us an idea if it’s feasible, but I don’t think the rest of us can dismiss the idea out of hand with any credibility.

There are two different issues. One is would an electric turbofan have advantages over a JP-4 powered engine. (Setting aside the energy source for the moment.) Sure, there are a lot. The problem is an electric turbofan doesn't exist without a source of electricity and that's where it all falls apart. There is no kind of battery, real or theoretical, that has the energy density of jet fuel. It's not even close.

sprstdlyscottsmn · by **sprstdlyscottsmn** » 21 Jun 2018, 17:38

sferrin wrote:
count_to_10 wrote:
sferrin wrote:Why go through all the waste though? What does this get you vs a standard turbofan in increased efficiency, thrust to weight, etc.? I've yet to hear an advantages.

As I wrote earlier, there are some interesting things you could do with the ability to independently set the RPMs of each stage or physically disconnect a compressor from any turbine. It would take considerable study to work out if the electrical losses would be worth it. Maybe TEG could give us an idea if it’s feasible, but I don’t think the rest of us can dismiss the idea out of hand with any credibility.

There are two different issues. One is would an electric turbofan have advantages over a JP-4 powered engine. (Setting aside the energy source for the moment.) Sure, there are a lot. The problem is an electric turbofan doesn't exist without a source of electricity and that's where it all falls apart. There is no kind of battery, real or theoretical, that has the energy density of jet fuel. It's not even close.

I think you may be missing something, or I am. As I understand it Count to 10 isn't saying to get rid of the jet engine or jet fuel, just the mechanical linkage between the turbine and the Compressor. There would be losses in converting turbine power to electrical and back to compressor, but Ct10 is wondering if it might be worth it to have "customized" RPMs in the compressor state. Much like how a super charger may take 100hp to tun but it is providing 300hp net gain. It's not a FREE lunch, but it is a bigger lunch.

botsing · by **botsing** » 21 Jun 2018, 18:01

count_to_10 wrote: Maybe TEG could give us an idea if it’s feasible, but I don’t think the rest of us can dismiss the idea out of hand with any credibility.

I agree, we should be prudent with basing any conclusions on the limited dataset we have.

sferrin wrote:The problem is an electric turbofan doesn't exist without a source of electricity and that's where it all falls apart. There is no kind of battery, real or theoretical, that has the energy density of jet fuel. It's not even close.

Lithium–air batteries already have a theoretical specific energy very close to that of jet fuel:

* Specific energy of Lithium–air battery: https://en.wikipedia.org/wiki/Lithium%E ... ir_battery (43.2 MJ/kg)
* Typical energy content of Aviation Fuel: https://www.cgabusinessdesk.com/documen ... review.pdf (~43.54 MJ/kg)

However, significant advances in science are needed to be able to reach those theoretical limits (for both batteries and liquid fuels).

Another advantage of electric engines is that directed energy can give them a (limited) wireless fuel line.

sferrin · by **sferrin** » 21 Jun 2018, 19:03

botsing wrote:
count_to_10 wrote: Maybe TEG could give us an idea if it’s feasible, but I don’t think the rest of us can dismiss the idea out of hand with any credibility.

I agree, we should be prudent with basing any conclusions on the limited dataset we have.

sferrin wrote:The problem is an electric turbofan doesn't exist without a source of electricity and that's where it all falls apart. There is no kind of battery, real or theoretical, that has the energy density of jet fuel. It's not even close.

Lithium–air batteries already have a theoretical specific energy very close to that of jet fuel:

* Specific energy of Lithium–air battery: https://en.wikipedia.org/wiki/Lithium%E ... ir_battery (43.2 MJ/kg)
* Typical energy content of Aviation Fuel: https://www.cgabusinessdesk.com/documen ... review.pdf (~43.54 MJ/kg)

However, significant advances in science are needed to be able to reach those theoretical limits (for both batteries and liquid fuels).

Another advantage of electric engines is that directed energy can give them a (limited) wireless fuel line.

I'll see if I can dig up the AvWeek article discussing this. It was within the last couple months if anybody else wants to look. Airbus said it would take something like 340 tons of batteries for an A320 or somesuch. You also have the disadvantage that the batteries don't get lighter as the flight progresses.

sferrin · by **sferrin** » 21 Jun 2018, 19:27

sprstdlyscottsmn wrote:I think you may be missing something, or I am. As I understand it Count to 10 isn't saying to get rid of the jet engine or jet fuel, just the mechanical linkage between the turbine and the Compressor. There would be losses in converting turbine power to electrical and back to compressor, but Ct10 is wondering if it might be worth it to have "customized" RPMs in the compressor state. Much like how a super charger may take 100hp to tun but it is providing 300hp net gain. It's not a FREE lunch, but it is a bigger lunch.

I did miss that. 3-shaft engines were an attempt to accomplish some of that but they don't seem to have caught on. Most are still 2-shaft. Now if there were such a thing as room-temperature super conductors. . .

count_to_10 · by **count_to_10** » 21 Jun 2018, 22:21

sprstdlyscottsmn wrote:I think you may be missing something, or I am. As I understand it Count to 10 isn't saying to get rid of the jet engine or jet fuel, just the mechanical linkage between the turbine and the Compressor. There would be losses in converting turbine power to electrical and back to compressor, but Ct10 is wondering if it might be worth it to have "customized" RPMs in the compressor state. Much like how a super charger may take 100hp to tun but it is providing 300hp net gain. It's not a FREE lunch, but it is a bigger lunch.

That’s it exactly. You might have some battery storage to raven things out, but otherwise the electrical energy should be generated by a turbine as fast as it is being used up in the fan/compressor. It all comes down to how efficient the electrical system can be made, and yes, room temperature superconductors would help that a lot.
Also, you might only disconnect the “outer” stages, leaving a core compressor and turbine system that share a shaft.