Electric jet engines

[N_Molson]

Well it works with propellers, Solar Impulse proved it. Now the performance is... humble... It was never able to fly (prograde) against moderate winds.

 

[Urwumpe]

Well it works with propellers, Solar Impulse proved it. Now the performance is... humble... It was never able to fly (prograde) against moderate winds.

Yes, it was more like a motorized sail plane with poor glide number.

But still, a very successful demonstrator of what is possible. If not for getting drunken Germans to Spain and back again ASAP, then for example for having UAVs with literally unlimited endurance.

 

[Zatnikitelman]

Let's just get the ball rolling again with these: https://en.m.wikipedia.org/wiki/Nuclear-powered_aircraft

 

[Urwumpe]

Let's just get the ball rolling again with these: https://en.m.wikipedia.org/wiki/Nuclear-powered_aircraft

Why not nuclear ramjet cruise missiles for the fun of it? Think about mutation!

 

[perseus]

Yes, it was more like a motorized sail plane with poor glide number.

But still, a very successful demonstrator of what is possible. If not for getting drunken Germans to Spain and back again ASAP, then for example for having UAVs with literally unlimited endurance.

It's not necessary we can take the drink to the plane and get drunk on it
: lol

 

[boogabooga]

Back on point.

2) On a multi-spool turbofan, replace all but one spool with electric motors, and put a generator on the remaining spool (since motors and generators are heavy, you can save weight by connecting at least one fan/compressor stage directly to the turbine). If desired, you can also add solar panels as in the case above. Advantages: Mechanically simpler than a multi-spool turbofan. Disadvantages: Probably heavier.

I'm not sure why you have a problem with multiple spools, or why you think that they are very complicated. On the contrary, I think managing your power requirements right on a common shaft without having to convert energy any further is a simple and elegant solution. :tiphat:

With multiple spools, you can have stages of turbines (which are each broken down into more constant RPM stages). Each stage is taking a cut of the working fluid's energy. So you can efficiently match the turbine bucket (blade) design and the spool RPM around how much energy can/needs to be taken for each stage.

Having only one spool on a generator means that all power must be extracted from the working fluid using an inflexible and not necessarily the most efficient turbine design. Then you have to think about all of the energy losses associated with converting thermal energy to mechanical energy, to electrical energy, back to mechanical energy. Why not just keep the mechanical energy that you have?

Finally, think about all of the electrical equipment you would need to manage a several MW of electricity production. That will be a hefty generator, and electric motor. How is that simpler than just a turbine on a common shaft?

 

[Linguofreak]

While musing on electric jets, I came up with this non-electric concept, illustrated (to the limit of my drawing ability) below:

At low speed, air enters the engine and passes through the compressor in the central duct indicated by the "A"s (you could probably also have a fan, but I haven't illustrated one), and is burned in the combustion chamber at "B". Some of the exhaust passes straight out the tailpipe, some is diverted through bypass doors at "C" into ductwork going toward the front of the engine (as drawn, in an actual engine built like this all of the exhaust might be captured by the ductwork at low speed if the engineering details make it feasible and desirable). It passes through more bypass doors at "D" (ignore those for now), and is passed through turbines (Red lines, labeled with dark red "F"s, and muddled up with magenta arrows, I drew everything too small, so this part looks especially like crap).

Where a traditional jet would have turbine at the back of the engine, with the various turbine stages (if it isn't a one spool design) driving their corresponding compressor stages through concentric spools running from the back of the engine to the front, this design has each turbine stage as part of the same disk as the corresponding compressor stage (with the compressor blades terminating in a ring at the outer edge of the compressor duct, which is part of a seal between the compressor duct and the turbine ductwork, and the roots of the turbine blades being attached to the outside of that same ring). Bypass doors at "E" control the amount of exhaust admitted to each turbine stage in order to keep the corresponding compressor stage operating at its optimum RPM.

As airspeed increases and turbine inlet temperature limits are approached, the doors at "C" and "D" limit exhaust flow to the turbines, and the doors at "D" begin to admit inlet air to the turbine directly (so that the turbine is fed more and more by relatively cool ram air and less and less by hot exhaust, helping keep temperatures within limits), until eventually the whole flow through the turbine is inlet air. As inlet air begins to be used to feed the turbine, the afterburners at "G "are lit to accelerate the air coming out of the turbine to a comparable velocity to that coming out of the combustion chamber at "C".

Eventually, as inlet heating becomes high enough that turbine / compressor inlet temperature limits are reached even with nothing but inlet air going to the turbine, bypass doors at "I" isolate the turbomachinery from the airstream, and doors at "H" open to allow inlet air directly into the duct to the afterburner at "G", converting the engine into a ramjet. (As illustrated, "I" seals off the main inlet completely, and "H" opens up an auxiliary inlet, but that's because it was convenient to draw that way, I figure a real engine built on this concept would be structured differently).

What do people think?

I'm not sure why you have a problem with multiple spools, or why you think that they are very complicated. On the contrary, I think managing your power requirements right on a common shaft without having to convert energy any further is a simple and elegant solution. :tiphat:

I got the impression that the concentric shafts add a fair degree of mechanical complexity, so my idea was to have the turbine spinning at its own best speed and powering a DC generator, and then have the motor for each compressor/fan stage operating at the best speed for that stage. It should be mechanically simpler, at the cost of more weight and less efficiency.

The impression that concentric shafts are mechanically complex is also why the concept outlined in this post has each compressor and the corresponding turbine as the inboard/outboard parts of the same disk. The general concept would still work with a shafted design (with ductwork to allow inlet air to bypass the compressor and combustion chamber exhaust to bypass the turbine).

 

[Urwumpe]

What do people think?

I think you underestimate how much energy you will loose by piping exhaust around in your engine to get to the turbine. Especially by touching more surface of the colder walls.

Also, if you put the turbine classically into the aft end of the shaft, you actually get a normal turbojet engine that way. Mixing bypassed air into the exhaust for cooling or performance optimization is not unknown, the J58 is famous for using this to the extreme by having many bleed air flaps and other systems for making the engine run at Mach 3.2.

And having a controllable flap/valve in the hot exhaust is not easy engineering.

Generally, you seem to attempt a tap-off cycle with this design, which is such a good idea, that it was already done. The J58 for example taps off a lot of compressed air from a middle compressor stage to prevent the remaining compressor stages from stalling and get more oxygen to the afterburner.

 

[RisingFury]

...

I have a few stupid questions:

1.) Given how complicated the piping for this engine is, why not just have to engines, a ramjet and a turbine that are fed from the same intake, where you can choose to divert the intake air into one or the other? Seems like a simpler design...

2.) Why would you think that the electric generator / fan combo would be simpler? Now you have multiple generators and motors that can fail and additional loss of energy.

3.) Why would you pipe hot, oxygen poor air back into the engine? Doing that would seriously decrease the engine's performance...

 

[Linguofreak]

3.) Why would you pipe hot, oxygen poor air back into the engine? Doing that would seriously decrease the engine's performance...

It's not being piped back into the compressor, it's being piped to turbine blades that are part of the same disk as the compressor, but sealed off from the compressor as far as gas flow. So moving outward, you have the hub of your disk, then you have your compressor blades, then you have a ring structure that fits into the wall between your compressor duct and the turbine ductwork and is part of a seal between the two spaces, and then you have the turbine blades. Exhaust gas only comes in contact with the turbine for each compressor stage, not with the compressor stage itself.

---------- Post added at 12:11 ---------- Previous post was at 11:52 ----------

Generally, you seem to attempt a tap-off cycle with this design, which is such a good idea, that it was already done. The J58 for example taps off a lot of compressed air from a middle compressor stage to prevent the remaining compressor stages from stalling and get more oxygen to the afterburner.

Pretty much, but with the details of what is tapped off and where it is sent changed.

 

[Urwumpe]

Pretty much, but with the details of what is tapped off and where it is sent changed.

Yes. Would I have more time right now, I would guesstimate a pressure/station diagram of this one for checking some obscure feeling I have: That the flow directions you painted are not working well with the total pressure inside your engine (eg, air/exhaust flowing from low pressure towards high pressure). I can't really use absolute numbers there, but this should work on a qualitative perspective: if a turbine stage needs to have zero pressure drop, it can't perform work. If your turbine would need to compress air, it can't work as well, it consumes work.

 

[Linguofreak]

Do you mean as drawn or in general? Keep in mind that this is a very rough drawing by someone with no artistic skills, not a rigorous blueprint.