The point is the compressor and turbine are mechanically independent. Along with that aerospike exhaust, the flight envelope should extend to the Karman line.
I get that the electric motor removes the gear/transmission issue to allow the compressing blades to run at any speed, but it does require conversion of the turbine engine to electric at sufficient quantities to also operate the compression. I can see how this is not a perpetual motion machine, because the turbine still uses fuel air mixture. I just wonder how much more efficiency gain from the compression cycle for the loss of the energy conversion.
Also, is he slowing down the compression blades at higher speeds such that they brake the air coming into the turbine?
One of the things about jet engines is the large fraction of the power generated by the turbine being fed back into driving the compressor. The practical efficiency of a jet engine has to do with having high mechanical efficiency of both the turbine and the compressor so that all of the power input into the turbine isn’t soaked up driving the compressor, leaving some power to propel the airplane.
An electric drive of the compressor (and propulsive fan in this application) has to be highly efficient for this to work, and it needs to transmit gobs of horsepower in relation to the horsepower doing propulsive work, too.
So it’s basically a big APU that uses an electric compressor.
It’ll work, sure, but is it compelling? Not so sure.
Might be interesting if one wants to locate the fan in a different location to the core.
This sounds like a variation on the Air Turbo Rocket, where the compressor is powered by a gas generator with no power fed back from turbine blades.
I’m unclear from the description if an optimized electric jet would have an optimized electrical power generating turbine separate from the jet or if the turbine is inside the jet.
Rocketlab has already flown a vehicle with batteries powering fuel pumps for a rocket motor. There didn’t seem to be any issues with delivering enough electric power to make the cycle work. Generating the electrical power in the vehicle for a mixed air breathing and pumped rocket cycle seems like the next step to try.
For the same mass flow rates and pressures air needs 1000x the power in the compressor. That’s why starting with LOX is a win. I just can’t see this powering a rocket any time soon.
In fact, you can make a very compelling case for a lox/fuel high bypass jet. Make a lox/fuel rocket that drives a turbine, which in turn drives fans. The materials science is hard for that, and the existing infrastructure would need augmentation. But it is a lot more efficient once you get past the heating issues of the turbine.
The article seems little garbled, the turbine produces power while the turbofan and the compressor consume it. Until the advent of the geared turbofan, all three were on the same shaft and operating at the same RPM. As the bypass ratios got bigger, the diameter of the much larger turbofan constrained the operating RPM of the core engine. Thus the eminently simple idea of the geared turbofan. Dealing with tens of thousands of horse power with the space, weight and reliability requirements was much less simple.
There is probably efficiency to be gained by decoupling the speeds of the three components. Will the increase compensate for what has to be a massive addition of weight, complexity and space? I note that large stationary gas turbines still operate on one shaft where the penalties of running the compressor separately would be much less. I’d bet that the much simpler geared turbofan gets most of that increase.
I wish him all the luck in the world, as long as it’s not my money.
“One of the things about jet engines is the large fraction of the power generated by the turbine being fed back into driving the compressor. The practical efficiency of a jet engine has to do with having high mechanical efficiency of both the turbine and the compressor so that all of the power input into the turbine isn’t soaked up driving the compressor, leaving some power to propel the airplane.”
Which is the reason why small efficiency increases in the compressor and turbine lead to large increases in useful power or thrust.
The mass of the alternator and motor will surely be quite high and the alternator will have an efficiency of 90% or so and the motor 95% so about 85% round trip at best. A geared turbofan would get much better than that. Gearboxes with fixed ratios seem quite efficient in the range 94 to 98%.
A geared turbofan is eminently simple is the way of the proverbial recipe for elephant stew: “First, cut an elephant into one-inch cubes.”
The first problem is sending what in my post above I described as a “gobnormous” amount of horsepower through a compact and lightweight gearbox. This, until recently, has limited geared turbofans to the smallest of aviation turbofan engines for use on cruise missiles, business jets and the smaller regional airliners.
The second problem is keeping the rotating shafts from flexing with respect to the gearbox, in so doing chewing up the gears. In turboprop aircraft, this is commonly done with a largish strut, which is impractical for geared turbofans powering something like an A320 or larger.
Pratt and Whitney’s patented solution is an arrangement to connect the shafts to the gearbox with a kind of flexing coupling in place of CV joints as used in automotive applications. One Neat Trick ™ but not obvious not simple to engineer.
General Electric’s solution going back to their Unducted Turbo Fan (UDF) from, what, forty years ago, is to greatly increase the diameter of the turbine wheel supplying direct drive to the turbofan. This limits the tip velocity of the turbine blades without the expense, weight, mechanical loss, and maintenance burden of a highly stressed gear train, the Pratt and Whitney solution, in trade for challenging “packaging” requirement of the engine within its nacelle.
This sounds akin to yet another attempt to make an electric turbocharger for a light aircraft, the idea being that it decouples exhaust turbine RPM from compressor turbine RPM. What it really does is add a heavy and complex motor-generator set to the engine.
Interesting.
But all internal combustion engines utilize the SSBB cycle
The point is the compressor and turbine are mechanically independent. Along with that aerospike exhaust, the flight envelope should extend to the Karman line.
I get that the electric motor removes the gear/transmission issue to allow the compressing blades to run at any speed, but it does require conversion of the turbine engine to electric at sufficient quantities to also operate the compression. I can see how this is not a perpetual motion machine, because the turbine still uses fuel air mixture. I just wonder how much more efficiency gain from the compression cycle for the loss of the energy conversion.
Also, is he slowing down the compression blades at higher speeds such that they brake the air coming into the turbine?
One of the things about jet engines is the large fraction of the power generated by the turbine being fed back into driving the compressor. The practical efficiency of a jet engine has to do with having high mechanical efficiency of both the turbine and the compressor so that all of the power input into the turbine isn’t soaked up driving the compressor, leaving some power to propel the airplane.
An electric drive of the compressor (and propulsive fan in this application) has to be highly efficient for this to work, and it needs to transmit gobs of horsepower in relation to the horsepower doing propulsive work, too.
So it’s basically a big APU that uses an electric compressor.
It’ll work, sure, but is it compelling? Not so sure.
Might be interesting if one wants to locate the fan in a different location to the core.
This sounds like a variation on the Air Turbo Rocket, where the compressor is powered by a gas generator with no power fed back from turbine blades.
I’m unclear from the description if an optimized electric jet would have an optimized electrical power generating turbine separate from the jet or if the turbine is inside the jet.
Rocketlab has already flown a vehicle with batteries powering fuel pumps for a rocket motor. There didn’t seem to be any issues with delivering enough electric power to make the cycle work. Generating the electrical power in the vehicle for a mixed air breathing and pumped rocket cycle seems like the next step to try.
For the same mass flow rates and pressures air needs 1000x the power in the compressor. That’s why starting with LOX is a win. I just can’t see this powering a rocket any time soon.
In fact, you can make a very compelling case for a lox/fuel high bypass jet. Make a lox/fuel rocket that drives a turbine, which in turn drives fans. The materials science is hard for that, and the existing infrastructure would need augmentation. But it is a lot more efficient once you get past the heating issues of the turbine.
The article seems little garbled, the turbine produces power while the turbofan and the compressor consume it. Until the advent of the geared turbofan, all three were on the same shaft and operating at the same RPM. As the bypass ratios got bigger, the diameter of the much larger turbofan constrained the operating RPM of the core engine. Thus the eminently simple idea of the geared turbofan. Dealing with tens of thousands of horse power with the space, weight and reliability requirements was much less simple.
There is probably efficiency to be gained by decoupling the speeds of the three components. Will the increase compensate for what has to be a massive addition of weight, complexity and space? I note that large stationary gas turbines still operate on one shaft where the penalties of running the compressor separately would be much less. I’d bet that the much simpler geared turbofan gets most of that increase.
I wish him all the luck in the world, as long as it’s not my money.
“One of the things about jet engines is the large fraction of the power generated by the turbine being fed back into driving the compressor. The practical efficiency of a jet engine has to do with having high mechanical efficiency of both the turbine and the compressor so that all of the power input into the turbine isn’t soaked up driving the compressor, leaving some power to propel the airplane.”
Which is the reason why small efficiency increases in the compressor and turbine lead to large increases in useful power or thrust.
The mass of the alternator and motor will surely be quite high and the alternator will have an efficiency of 90% or so and the motor 95% so about 85% round trip at best. A geared turbofan would get much better than that. Gearboxes with fixed ratios seem quite efficient in the range 94 to 98%.
A geared turbofan is eminently simple is the way of the proverbial recipe for elephant stew: “First, cut an elephant into one-inch cubes.”
The first problem is sending what in my post above I described as a “gobnormous” amount of horsepower through a compact and lightweight gearbox. This, until recently, has limited geared turbofans to the smallest of aviation turbofan engines for use on cruise missiles, business jets and the smaller regional airliners.
The second problem is keeping the rotating shafts from flexing with respect to the gearbox, in so doing chewing up the gears. In turboprop aircraft, this is commonly done with a largish strut, which is impractical for geared turbofans powering something like an A320 or larger.
Pratt and Whitney’s patented solution is an arrangement to connect the shafts to the gearbox with a kind of flexing coupling in place of CV joints as used in automotive applications. One Neat Trick ™ but not obvious not simple to engineer.
General Electric’s solution going back to their Unducted Turbo Fan (UDF) from, what, forty years ago, is to greatly increase the diameter of the turbine wheel supplying direct drive to the turbofan. This limits the tip velocity of the turbine blades without the expense, weight, mechanical loss, and maintenance burden of a highly stressed gear train, the Pratt and Whitney solution, in trade for challenging “packaging” requirement of the engine within its nacelle.
Handling giant amounts of electrical power will likewise be “interesting”.
This sounds akin to yet another attempt to make an electric turbocharger for a light aircraft, the idea being that it decouples exhaust turbine RPM from compressor turbine RPM. What it really does is add a heavy and complex motor-generator set to the engine.
You don’t solve a problem by creating a bigger problem.