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> Once you are no longer combusting fuel, all of that compression is simply wasted energy. Not true. Instead of burning fuel, you can use electricity (somethi

by etwigg 10y ago

> Once you are no longer combusting fuel, all of that compression is simply wasted energy. Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel. Thermal energy in chemical fuel and the potential energy in a battery or a raised weight have the same units, but there's actually a difference. You can turn energy in a battery or a raised weight into mechanical energy with 100% efficiency - not achieved in practice, but there's no theoretical limit. When you turn thermal energy into mechanical energy (via Otto cycle in a car or Brayton cycle in a jet), the peak efficiency you can achieve has a theoretical limit based on how much you can compress the working fluid (air) before injecting thermal energy (burning fuel or an electric arc). So if you've got electic energy that you can turn into mechanical energy in a fan with 100% efficiency, why would you take the thermodynamic losses of "burning" that electricity? Because we don't know how to go supersonic speeds with a fan or propeller - the only way we know how, for now, is with a jet running on the brayton cycle. And all the advances we've made in high compression fuel-burning jets would apply to electric jets just as well. If you're okay limiting your top speed to ~Mach 0.7, then you're correct spinning a large propeller using electricity is absolutely a better way to go, and you can make that propeller a lot bigger if you're able to tilt it. Karem Aircraft has done extremely interesting work on figuring out how to tilt rotors, and much of their research would apply to electric motors as well: http://karemaircraft.com/ http://karemaircraft.com/

thom_nic 10y ago
This idea of an electrical-arc jet engine is new to me. Can you point to any published material on such a design or prototype? Very intriguing.
- nickff 10y ago
  Nobody who has taken a course in energy conversion (an second or third level thermodynamics course) would even bother to propose this. It is not coincidental that all mechanical engineers take this course, and mechanical engineers are also the people who design engines.
  - etwigg 10y ago
    If you take a course in energy conversion, and also an economics course, then you would consider this. https://en.wikipedia.org/wiki/Nuclear-powered_aircraft https://en.wikipedia.org/wiki/Nuclear-powered_aircraft Some joules are more expensive than others :)
    - nickff 10y ago
      The nuclear aircraft did not fail because nuclear power was too expensive, they failed because reactors were too heavy. If you read more about the American nuclear aircraft programs, you will find that they were most interested in using the heat from Nuclear reactions to warm up air; they did not propose to generate electricity with which to heat or push air (because it is too complex and inefficient).
      - vitus 10y ago
        Right. A nuclear reactor generates massive amounts of heat from fission, then uses that heat to boil water (sometimes heavy water for better radiation shielding) to make steam to cause turbines to spin, which generates the electricity. (in fact, fossil fuel burning plants also use this steam-electric model) If the end goal is to generate heat, then there's no good reason to add those additional steps.
  - rayiner 10y ago
    Using an RTG producing electricity as a heat source in a Brayton-cycle engine was on my Jet-Pro final exam in college.
nickff 10y ago
>"Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel." Turning chemical energy (low entropy energy) into electrical energy (low-medium entropy) into thermal energy (high entropy) to make mechanical energy (of intermediate) would be incredibly inefficient. Combustible fuels are much better than batteries at making things hot; I don't have the time to run the numbers right now, but what you are saying is incredibly wasteful. Please take a thermodynamics course. Fans are commonly used on supersonic aircraft. Most modern fighter aircraft use medium or low bypass turbofans, which are using fans to go supersonic. Turbofans are less efficient than turbojets over Mach ~1.6-1.8 (or in reheat), but a ducted fan could probably do better.
- etwigg 10y ago
  You are correct that it's wasteful, if one joule of electric energy and one joule of chemical energy cost the same. But if you are getting electric energy for free from solar panels, and you're getting chemical energy by pumping taxed dinosaur carbon into the air, then you might find that the electric energy could be more efficient. You're absolutely correct that it's wildly inefficient by any metric now. But if clean electric energy became wildly abundant, that would change. In computing, there are all kinds of wildly wasteful things (web browsers) that are the best way to do things because computation is extremely cheap. It's very interesting to think about what kinds of devices would make economic sense if electric energy had the same price collapse that computation has done. The fact that we "burn electricity" in electric water heaters, stoves, and toasters rather than only using high-entropy energy sources like gas water heaters and gas stoves is an example where present-day electric energy supply chains have beaten out the theoretical advantages of only using high-entropy energy sources when it is possible to do so.
  - nickff 10y ago
    This is not about financial cost, it is about weight and complexity. It would be less costly and more efficient to separate water into hydrogen and oxygen (using solar power if you like), then use them to power a rocket engine.
mrfusion 10y ago
Very interesting! I had no idea we could convert our existing jet engine technologies to electric. Could the Same conversion apply to scram jets?
- nickff 10y ago
  You can do this, but it is roughly equivalent to using a battery pack to heat water for a steam engine; it is a very bad idea.
  - etwigg 10y ago
    Which was seriously considered during the cold war: https://en.wikipedia.org/wiki/Nuclear-powered_aircraft https://en.wikipedia.org/wiki/Nuclear-powered_aircraft As bad of an idea as using javascript to do addition rather than assembly. You're assuming electric energy is the same order of magnitude cost as electric energy. It's possible that could change some day.
    - nickff 10y ago
      Even if cost is not a factor, it is still a bad idea to heat water with electricity in weight and time-critical applications. For instance, if you want to heat water using solar power, you should focus sunlight on the water in a black container rather than using photovoltaic (PV) solar panels.
hx87 10y ago
> Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. Why go through all the trouble of using electricity as the heat source in a Brayton cycle engine instead of the obvious solution, using electric motors? Your statement that "we don't know how to go supersonic speeds with a fan or propeller" is clearly wrong, since supersonic aircraft have been flying with turbofans and no afterburners for almost five decades now.
- etwigg 10y ago
  When a jet engine burns fuel, some energy goes into turning the fan, and some energy goes into creating thrust. At low-mach, the fuel burn is primarily producing torque which turns the fan. This fan could be turned by electricity. At high-mach, the fuel burn is primarily producing pure thrust, and the fan portion hurts the engine's performance. Supersonic turbofans are a compromise that have to produce thrust from takeoff through to high speed. I may be incorrect, but I don't believe we have ever built a ducted fan which can use pure torque to generate supersonic thrust, although I guess I don't see why it wouldn't be theoretically possible. We can put megawatts of energy into a fast-moving flow using thermal energy and a brayton cycle. In order to put megawatts of energy into fast-moving flow using rotors would require a totally new design - nothing like the supersonic turbofans currently in production.
  - nickff 10y ago
    You have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. No engine (of which I am aware) has air flowing past the fan blades at supersonic speeds, because the trans-sonic transitions tear engines apart. There is one prototype aircraft (a naval derivative of the F-84) which did have a supersonic propeller, but it was very inefficient and loud. All jet engines compress sub-Mach 1 flows, then expand them at the very back of the engine to produce supersonic exhaust.
    - etwigg 10y ago
      > You have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. Exactly. At high mach, almost all of the energy being put into the flow is from expanding combustion gases - not the compressor stage. If you wanted to go at supersonic speeds using only spinning fans, you will need a heck of a lot of fans. Maybe we could do it! But we haven't so far.
Florin_Andrei 10y ago
> Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. Egads, no. Please get out of the business of being limited by thermodynamics in your engines. The Carnot cycle is a hard stop whenever heat is being coaxed into doing mechanical work. You don't want that. Just convert electricity into mechanical work directly.
- etwigg 10y ago
  That's a great goal to have! But if you're putting energy into a flow using a propeller or fan, aerodynamic stall means that each propeller can only put a limited amount of energy in. This is why jet engines have so many compressor stages - each one adds as much energy as it can without stalling out: https://en.wikipedia.org/wiki/Gas_turbine#/media/File:J85_ge_17a_turbojet_engine.jpg https://en.wikipedia.org/wiki/Gas_turbine#/media/File:J85_ge... In the engine pictured, which has 8 compressors in a row, the energy which those fans are able to put into the flow is miniscule compared to the energy that you can dump in by burning fuel (or electricity) in the tiny combustion chamber. F-16 in afterburner is consuming 64,000 pounds of fuel per hour = 18 pounds per second A pound of Jet-A has 20 MJ per pound. So that's 360 MW of thermal power. Even if you're only turning 10% of that into usable thrust, that's 36MW of power. Here's what a 35MW power plant looks like: http://www.power-technology.com/projects/ctds_chp/ctds_chp5.html http://www.power-technology.com/projects/ctds_chp/ctds_chp5.... The people working on electric jets are well aware of the Carnot limit, and it's true that they don't want that. But for now, we'd need a lot of new tech to put 35MW of thrust power into a flow using electric motors and fans. Heck, we need a lot of new tech to put 350MW of power electronics onto an airplane!! My only point is that I don't think we should look down our noses at the people working on thermodynamic machinery. Until we can do better, they're the best we've got :)

by etwigg 10y ago

thom_nic 10y ago

This idea of an electrical-arc jet engine is new to me. Can you point to any published material on such a design or prototype? Very intriguing.

nickff 10y ago
Nobody who has taken a course in energy conversion (an second or third level thermodynamics course) would even bother to propose this. It is not coincidental that all mechanical engineers take this course, and mechanical engineers are also the people who design engines.
- etwigg 10y ago
  If you take a course in energy conversion, and also an economics course, then you would consider this. https://en.wikipedia.org/wiki/Nuclear-powered_aircraft https://en.wikipedia.org/wiki/Nuclear-powered_aircraft Some joules are more expensive than others :)
  - nickff 10y ago
    The nuclear aircraft did not fail because nuclear power was too expensive, they failed because reactors were too heavy. If you read more about the American nuclear aircraft programs, you will find that they were most interested in using the heat from Nuclear reactions to warm up air; they did not propose to generate electricity with which to heat or push air (because it is too complex and inefficient).
    - vitus 10y ago
      Right. A nuclear reactor generates massive amounts of heat from fission, then uses that heat to boil water (sometimes heavy water for better radiation shielding) to make steam to cause turbines to spin, which generates the electricity. (in fact, fossil fuel burning plants also use this steam-electric model) If the end goal is to generate heat, then there's no good reason to add those additional steps.
- rayiner 10y ago
  Using an RTG producing electricity as a heat source in a Brayton-cycle engine was on my Jet-Pro final exam in college.

nickff 10y ago

>"Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel." Turning chemical energy (low entropy energy) into electrical energy (low-medium entropy) into thermal energy (high entropy) to make mechanical energy (of intermediate) would be incredibly inefficient. Combustible fuels are much better than batteries at making things hot; I don't have the time to run the numbers right now, but what you are saying is incredibly wasteful. Please take a thermodynamics course. Fans are commonly used on supersonic aircraft. Most modern fighter aircraft use medium or low bypass turbofans, which are using fans to go supersonic. Turbofans are less efficient than turbojets over Mach ~1.6-1.8 (or in reheat), but a ducted fan could probably do better.

etwigg 10y ago
You are correct that it's wasteful, if one joule of electric energy and one joule of chemical energy cost the same. But if you are getting electric energy for free from solar panels, and you're getting chemical energy by pumping taxed dinosaur carbon into the air, then you might find that the electric energy could be more efficient. You're absolutely correct that it's wildly inefficient by any metric now. But if clean electric energy became wildly abundant, that would change. In computing, there are all kinds of wildly wasteful things (web browsers) that are the best way to do things because computation is extremely cheap. It's very interesting to think about what kinds of devices would make economic sense if electric energy had the same price collapse that computation has done. The fact that we "burn electricity" in electric water heaters, stoves, and toasters rather than only using high-entropy energy sources like gas water heaters and gas stoves is an example where present-day electric energy supply chains have beaten out the theoretical advantages of only using high-entropy energy sources when it is possible to do so.
- nickff 10y ago
  This is not about financial cost, it is about weight and complexity. It would be less costly and more efficient to separate water into hydrogen and oxygen (using solar power if you like), then use them to power a rocket engine.

mrfusion 10y ago

Very interesting! I had no idea we could convert our existing jet engine technologies to electric. Could the Same conversion apply to scram jets?

nickff 10y ago
You can do this, but it is roughly equivalent to using a battery pack to heat water for a steam engine; it is a very bad idea.
- etwigg 10y ago
  Which was seriously considered during the cold war: https://en.wikipedia.org/wiki/Nuclear-powered_aircraft https://en.wikipedia.org/wiki/Nuclear-powered_aircraft As bad of an idea as using javascript to do addition rather than assembly. You're assuming electric energy is the same order of magnitude cost as electric energy. It's possible that could change some day.
  - nickff 10y ago
    Even if cost is not a factor, it is still a bad idea to heat water with electricity in weight and time-critical applications. For instance, if you want to heat water using solar power, you should focus sunlight on the water in a black container rather than using photovoltaic (PV) solar panels.

hx87 10y ago

> Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. Why go through all the trouble of using electricity as the heat source in a Brayton cycle engine instead of the obvious solution, using electric motors? Your statement that "we don't know how to go supersonic speeds with a fan or propeller" is clearly wrong, since supersonic aircraft have been flying with turbofans and no afterburners for almost five decades now.

etwigg 10y ago
When a jet engine burns fuel, some energy goes into turning the fan, and some energy goes into creating thrust. At low-mach, the fuel burn is primarily producing torque which turns the fan. This fan could be turned by electricity. At high-mach, the fuel burn is primarily producing pure thrust, and the fan portion hurts the engine's performance. Supersonic turbofans are a compromise that have to produce thrust from takeoff through to high speed. I may be incorrect, but I don't believe we have ever built a ducted fan which can use pure torque to generate supersonic thrust, although I guess I don't see why it wouldn't be theoretically possible. We can put megawatts of energy into a fast-moving flow using thermal energy and a brayton cycle. In order to put megawatts of energy into fast-moving flow using rotors would require a totally new design - nothing like the supersonic turbofans currently in production.
- nickff 10y ago
  You have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. No engine (of which I am aware) has air flowing past the fan blades at supersonic speeds, because the trans-sonic transitions tear engines apart. There is one prototype aircraft (a naval derivative of the F-84) which did have a supersonic propeller, but it was very inefficient and loud. All jet engines compress sub-Mach 1 flows, then expand them at the very back of the engine to produce supersonic exhaust.
  - etwigg 10y ago
    > You have to distinguish the conventional (high efficiency) cycle of a jet engine at low Mach, from the reheat (low efficiency) operation usually used for high Mach. Exactly. At high mach, almost all of the energy being put into the flow is from expanding combustion gases - not the compressor stage. If you wanted to go at supersonic speeds using only spinning fans, you will need a heck of a lot of fans. Maybe we could do it! But we haven't so far.

Florin_Andrei 10y ago

> Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. Egads, no. Please get out of the business of being limited by thermodynamics in your engines. The Carnot cycle is a hard stop whenever heat is being coaxed into doing mechanical work. You don't want that. Just convert electricity into mechanical work directly.

etwigg 10y ago
That's a great goal to have! But if you're putting energy into a flow using a propeller or fan, aerodynamic stall means that each propeller can only put a limited amount of energy in. This is why jet engines have so many compressor stages - each one adds as much energy as it can without stalling out: https://en.wikipedia.org/wiki/Gas_turbine#/media/File:J85_ge_17a_turbojet_engine.jpg https://en.wikipedia.org/wiki/Gas_turbine#/media/File:J85_ge... In the engine pictured, which has 8 compressors in a row, the energy which those fans are able to put into the flow is miniscule compared to the energy that you can dump in by burning fuel (or electricity) in the tiny combustion chamber. F-16 in afterburner is consuming 64,000 pounds of fuel per hour = 18 pounds per second A pound of Jet-A has 20 MJ per pound. So that's 360 MW of thermal power. Even if you're only turning 10% of that into usable thrust, that's 36MW of power. Here's what a 35MW power plant looks like: http://www.power-technology.com/projects/ctds_chp/ctds_chp5.html http://www.power-technology.com/projects/ctds_chp/ctds_chp5.... The people working on electric jets are well aware of the Carnot limit, and it's true that they don't want that. But for now, we'd need a lot of new tech to put 35MW of thrust power into a flow using electric motors and fans. Heck, we need a lot of new tech to put 350MW of power electronics onto an airplane!! My only point is that I don't think we should look down our noses at the people working on thermodynamic machinery. Until we can do better, they're the best we've got :)