Since most airliners currently in production are built to a specific configuration (turbine jets under the wings, ever bigger fan blades around smaller, more efficient cores=higher bypass), I was wondering if anyone can help with three questions. These are sort of expanding on each other:
1. I believe modern turbojets produce most of the thrust out of fan. Would an engine producing raw fan thrust (with fan only) work as well? I'm most curious in current fan/core thrust proportion in edges of flight envelope, like high and hot, or high altitude.
2. If so, natural replacement for core I can think of, is an electric engine. Would the power density be good enough to place an electric core inside of fan and be able to power it to speeds required?
3. if so, I can think of powercells in fuselage producing power (hydrogen cells). Again, would that be possible to put just enough of these to power up the engines as required?

I'm now just exploring "possible or not" question, not the cost or feasability (or a possible technical difficulties making it possible; I believe one of these would be providing proper amount of oxygen to powercells).

Cheers, Adam

Question is where your design target point is. As far as I understand, fan becomes beneficial at higher speeds, while prop may be more efficient at lower speeds - and for shorter range that is less important. So you loop back at the range of the airplane, which would bring you to fuel storage requirements - do you have enough juice to go long-haul to begin with?.
If you want to envision intercontinental range, you start with how to keep all the fuel on board, and how much such storage would weight. If you can magically store H2 with total energy density per weight and per volume better than that of kerosene, you may start talking drop-in replacements. Otherwise design would need to be optimized for storage.
If you will, this is true for Jet-A long haul as well where tank placement, volume, fuel weight impose significant limits on performance, and redesigning those tanks results in significantly different plane parameters (321 LR comes to mind)

kalvado wrote:

If you can magically store H2 with total energy density per weight and per volume better than that of kerosene, you may start talking drop-in replacements.

I'm well aware of lower energy density of H2 liq.



I'm not considering it at the moment, I'm not asking the question of range or economic feasibility. My question is as simple as "can it be done, or it fails to work at all?".

Cheers,
Adam

gloom wrote:

kalvado wrote:

If you can magically store H2 with total energy density per weight and per volume better than that of kerosene, you may start talking drop-in replacements.

I'm well aware of lower energy density of H2 liq.



I'm not considering it at the moment, I'm not asking the question of range or economic feasibility. My question is as simple as "can it be done, or it fails to work at all?".

Cheers,
Adam

Can I buy that $10M home and a cool Ferrari to drive to work? Sure, I can - but I have no money for that...
You basically want to disregard the main problem of entire system.

gloom wrote:

Since most airliners currently in production are built to a specific configuration (turbine jets under the wings, ever bigger fan blades around smaller, more efficient cores=higher bypass), I was wondering if anyone can help with three questions. These are sort of expanding on each other:
1. I believe modern turbojets produce most of the thrust out of fan. Would an engine producing raw fan thrust (with fan only) work as well? I'm most curious in current fan/core thrust proportion in edges of flight envelope, like high and hot, or high altitude.
2. If so, natural replacement for core I can think of, is an electric engine. Would the power density be good enough to place an electric core inside of fan and be able to power it to speeds required?
3. if so, I can think of powercells in fuselage producing power (hydrogen cells). Again, would that be possible to put just enough of these to power up the engines as required?

I'm now just exploring "possible or not" question, not the cost or feasability (or a possible technical difficulties making it possible; I believe one of these would be providing proper amount of oxygen to powercells).

Cheers, Adam

Energy is energy. To drive a fan or prop of size x, you need energy y. The fan or prop doesn't care whether that energy comes from dead dinosaurs or batteries.

AFAIK, an electric motor driving a large fan is perfectly feasible. That's not the problem. The issue is the fuel. The amount of energy in batteries by weight is more than an order of magnitude lower than in jet fuel. And it gets worse since the weight of the batteries stays the same as they are used, while the weight of the jet fuel goes away.

You could of course have a turbine that drives a generator that provides electricity for a motor that drives the fan, but now you're just adding a middleman.

Airplane design is extremely weight driven and how you drive the fan is, as mentioned, not the problem that you're trying to solve.



Hydrogen as fuel runs into volumetric issues.

The fan/core thrust proportion in modern turbofans is around 10:1.

gloom wrote:

kalvado wrote:

If you can magically store H2 with total energy density per weight and per volume better than that of kerosene, you may start talking drop-in replacements.

I'm well aware of lower energy density of H2 liq.



I'm not considering it at the moment, I'm not asking the question of range or economic feasibility. My question is as simple as "can it be done, or it fails to work at all?".

Cheers,
Adam

The chart really shows why fossil fuels are so necessary for flight—dense in energy without taking up space.

So basically there are no hard, technical problems there with such a concept, it's more an issue of fuel volume/range limitations as such, and fuel availability.

Thanks for your input.

Cheers,
Adam

Basically you are asking if an electric fan is possible.

They already have electric aircraft powered by electric fans. So there are no technical problems.

Good luck making an "electric core" that produces 65,000kw of power to run the fan of a GE90 in the 777. That's how much power is needed. The LM9000 generator is based off the GE90 so that gives you can idea how much power the core produces.

It is more efficient to move a large volume of air slowly than a small volume of air quickly when producing any given thrust level. Electric allows an effective infinite bypass ratio so an electric desgn will have much more fan area. Moving a huge volume of air at a slower speed.

Your idea of using a existing engine is a terrible idea. It probably halves the range with the same energy input as it is low volume high exhaust speed solution.

gloom wrote:

So basically there are no hard, technical problems there with such a concept, it's more an issue of fuel volume/range limitations as such, and fuel availability.

Thanks for your input.

Cheers,
Adam

There is a concept of "technology readiness level", TRL. It's a 1-10 scale, where 1 means basic principles were demonstrated in the lab, and 10 means you can place an order for established part number.
There were some great posts with insight on how project development involves risks of using lower TRL, and how difficult it may be to advance on the scale. I believe the post I remember was talking about Boeing regretting launching a project with 2 major TRL7 systems
Design you envision involves a lot of concepts which are low on TRL scale. There may be no fundamental physics issues other than energy storage. There will be problems - expensive and time consuming - possibly bringing project, or even entire concept into "unfeasible" category, or delaying it for decades.

What sort of compact light electric motors are there that could replace the gas turbine core on a commercial airliner?

We are talking motors in the 2,000 kw to 50,000 kw range. Is there anything like that at TRL 8?

What is the TRL of fuel cells capable of supplying that size motor?

I believe fuel cells are much more efficient than heat engines. Would a liquid hydrogen to fuel cell to electric motor VS liquid hydrogen to gas turbine be twice as fuel efficient?

Starlionblue wrote:

Hydrogen as fuel runs into volumetric issues.

Presumably we're discussing this in the context of environmental friendliness. Batteries obviously have energy density/volume issues. The Mass issue is even worse. As we know, as such an aircraft won't "lose weight" throughout a flight.

As for Hydrogen. Arch points out the volumetric issues with H2 as well. On top of this: substantially more energy goes into the necessary electrolysis, transport, containment, etc. etc. of H2 compared to refining Crude and pumping Jet-A into an aircraft.

rjsampson wrote:

Starlionblue wrote:

Hydrogen as fuel runs into volumetric issues.

Presumably we're discussing this in the context of environmental friendliness. Batteries obviously have energy density/volume issues. The Mass issue is even worse. As we know, as such an aircraft won't "lose weight" throughout a flight.

As for Hydrogen. Arch points out the volumetric issues with H2 as well. On top of this: substantially more energy goes into the necessary electrolysis, transport, containment, etc. etc. of H2 compared to refining Crude and pumping Jet-A into an aircraft.

Hydrogen is pretty complicated to deal with. Maybe I am overharassed by safety guys, but my perception of hydrogen as a fuel source for flight is not great.
Fire safety would be much more interesting than for Jet-A. If liquid H2 is involved... LH2 is a compressible fluid, so pumping it is a headache. Heat insulation - the smaller the vehicle, the more significant that weight becomes. Same for high pressure. Hydrogen can diffuse through metals, and embrittle them in process.
All those are somewhat solved in context of space flight, though - but remember, total resource of Shuttle engine was planned at <20 hours, and vehicle had to be defueled after 48? hours on the pad.
So even if hydrogen technology exists in principle, bringing it to the point of 10k+ service hours on the wing may be a totally different challenge.

eeightning wrote:

What sort of compact light electric motors are there that could replace the gas turbine core on a commercial airliner?

We are talking motors in the 2,000 kw to 50,000 kw range. Is there anything like that at TRL 8?

What is the TRL of fuel cells capable of supplying that size motor?

I believe fuel cells are much more efficient than heat engines. Would a liquid hydrogen to fuel cell to electric motor VS liquid hydrogen to gas turbine be twice as fuel efficient?

I doubt there’s anything near a TRL 8 65000 KW fuel cell and drive mechanism. Fuel cells emit water vapor, another GHG, so there’s that.

We have two warnings about airborne H2 powered vehicles—Hindenburg and the Challenger. It’s not a happy story.

GalaxyFlyer wrote:

eeightning wrote:

What sort of compact light electric motors are there that could replace the gas turbine core on a commercial airliner?

We are talking motors in the 2,000 kw to 50,000 kw range. Is there anything like that at TRL 8?

What is the TRL of fuel cells capable of supplying that size motor?

I believe fuel cells are much more efficient than heat engines. Would a liquid hydrogen to fuel cell to electric motor VS liquid hydrogen to gas turbine be twice as fuel efficient?

I doubt there’s anything near a TRL 8 65000 KW fuel cell and drive mechanism. Fuel cells emit water vapor, another GHG, so there’s that.

We have two warnings about airborne H2 powered vehicles—Hindenburg and the Challenger. It’s not a happy story.

There are submarine motors, so there is something in high power domain. As far as I know, a lot of train engines run diesel-electric transmission, there are electric propulsion motors for ships. Weight and cooling would be the big challenges for an airborne version.
And I am pretty sure we can come up with a lot of oil fires. There are 2 burnt spots on highway right near my exit where trucks caught fire...
As for water - that is also a byproduct of Jet-A combustion. There is a reason those footsteps in the sky are called chemtrails! /s

We haven’t had any modern planes spontaneously start fire in flight using fossil fuel. I’m just saying that even fuel cells emit GHG, not denying fossil fuel combustion.

There is not only an massive difference in weight and size between locos and planes; planes also produce and require orders of magnitude more power than any loco. A GE90 produces 65000KW, a GE loco is about 4000KW. A freight might use four locos in distributed power; a B777 a pair of GE90.

Here is a scaleable Collins prototype that is 98% efficient at 9kw/kg.

A 2 megawatt replacement motor for an ATR would weigh 500 lbs. Half the weight of a PW127!

https://www.collinsaerospace.com/news/n ... rlander-10

I couldn’t find size weight specs for megawatt class fuel cells, but I gather their efficiency is around 60-70%.

So combined with a 98% efficient motor, they burn like 1/2 the fuel of a gas turbine!

Except that H2 has 2.8 times the energy density of JetA, so the fuel weight is 18%.

Engine weighs 1/2. Fuel weighs 1/5. That leaves a lot of room for fuel cells and tank insulation.

GalaxyFlyer wrote:

We haven’t had any modern planes spontaneously start fire in flight using fossil fuel.

TWA800. And I heard a rumor B-52 exploded at Sawer had a similar issue.

eeightning wrote:

Engine weighs 1/2. Fuel weighs 1/5. That leaves a lot of room for fuel cells and tank insulation.

I believe that pretty much sums the state of the art in hydrogen propulsion over past few decades.

kalvado wrote:

GalaxyFlyer wrote:

We haven’t had any modern planes spontaneously start fire in flight using fossil fuel.

TWA800. And I heard a rumor B-52 exploded at Sawer had a similar issue.

TW 800 was a wiring issue. Yes, and several other planes have had ground fires, usually lightning strikes or static discharge due to improper fueling procedural adherence.

eeightning wrote:

I couldn’t find size weight specs for megawatt class fuel cells, but I gather their efficiency is around 60-70%.

So combined with a 98% efficient motor, they burn like 1/2 the fuel of a gas turbine!

Except that H2 has 2.8 times the energy density of JetA, so the fuel weight is 18%.

Engine weighs 1/2. Fuel weighs 1/5. That leaves a lot of room for fuel cells and tank insulation.

It may weigh 1/5th, but the volume required is multiples of Jet. It requires a huge tank, think Space Shuttle and new levels of metallurgy to contain and transfer. If LH2 were so wonderful, surely we’d have found a use since the Apollo program.

GalaxyFlyer wrote:

kalvado wrote:

GalaxyFlyer wrote:

We haven’t had any modern planes spontaneously start fire in flight using fossil fuel.

TWA800. And I heard a rumor B-52 exploded at Sawer had a similar issue.

TW 800 was a wiring issue. Yes, and several other planes have had ground fires, usually lightning strikes or static discharge due to improper fueling procedural adherence.

Well, yet that was a release of energy stored in fuel.
Being similarly meticulous, Challenger wasn't a spontaneous fire but vehicle broken apart by aerodynamic forces, and Gindenburg - fire of termite paint.
Hydrogen is not pyrophoric. It has to be set on fire or detonated by something else - and that is not a simple task actually (if you want to do that intentionally. Getting into an accident is always easy).
In my book, H2 is a (much) bigger fire hazard than Jet-A or gasoline, though. But that doesn't mean gasoline is a joke.

Here's an article on the fuel safety piece: tl;dr H2 is not worse than Jet A: https://leehamnews.com/2020/08/14/bjorn ... en-safety/

The Challenger blew up because the SRB O-rings leaked and the launch team ignored Roger Boisjoly, nothing wrong with the H2 tanks. The Hindenburg had the paint issues and the static. Arguably Pan Am 214 could be counted as lightning/static igniting Jet A. There's precedent for both energy storage types releasing the energy quickly when something else goes wrong.