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Laser-powered Jet Engine

Using the heat of a laser beam to replace combustion.
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Current jet engines run on the heat generated from chemical reactions in their combustion chambers. As the liquid fuel burns, it transforms into a heated gas that expands. This expanding gas is what provides thrust that moves the jet engine forward. There are some costs associated with this current configuration. For example, jet fuel is flammable, and accidents can result in dangerous fires. Also, the chemicals in the exhaust produced by current jet engines may be bad for the environment. Finally, the fact that jet engines rely on oxygen for combustion might be a limiting factor on how high they can fly (I'm unsure of how important that factor is, though).

What if instead of using combustion to run a jet engine, a laser or maser was used to heat the gas? The idea here is that inside of what would have normally been the combustion chamber of an ordinary engine, there is a ring of lasers or masers replacing the fuel injectors. Unlike conventional laser pointers which fire beams that travel through the air with little absorption, the devices in the jet engine would generate a wavelength of light that is readily absorbed by the nitrogen and/or oxygen gas present in the air. This might be in the infrared or microwave region of the electromagnetic spectrum, but I'm not sure exactly what wavelengths are absorbed by air.

When the lasers/masers pulse highly energized beams of light into the chamber, the air absorbs the light and heats up greatly. This heated, expanding gas produces thrust in the same way as a chemical jet engine does. However, since it doesn't rely on oxygen for combustion, the laser-powered engine could theoretically operate in oxygen-starved environments such as the very high atmosphere or in the smoke-filled air above forest fires. Also, the only product of the engine would be heated air and possibly some nitrogen oxides. There would be no issue with carbon dioxide or carbon monoxide pollution.

Depending on the construction of engine, we might call it a phototurbojet, a phototurbofan, a photoramjet, a phototurboprop, etc.

There is a major disadvantage associated with this design, however: it requires an external source of energy in order to operate. One option would be to use a large series of high-energy density batteries stored in the fuselage of the aircraft. Batteries, as I currently understand it, have a notably smaller energy-density than liquid fuels like gasoline. Hence, battery technology may need to advance before a laser-powered jet engine becomes competitive.

Another option would be to wait for the distant future when fusion reactors small enough to carry on airplanes can be developed (assuming they ever will). Electricity from such a reactor could be used to power the engine. It would provide an alternative to the classic direct-cycle and indirect-cycle methods of propulsion proposed for the old fission-based aircraft designs. The presence of hydrogen fusion fuel on such an aircraft would present a fire hazard, which eliminates one of the before-mentioned advantages of the laser-powered jet engine.

Kryptid, Mar 14 2008

Lightcraft Technologies http://www.lightcra...com/technology.html
A real device that uses a laser to heat air for propulsion. [Kryptid, Mar 14 2008]

Aircraft Nuclear Propulsion http://en.wikipedia..._Nuclear_Propulsion
Describes older methods for utilizing nuclear energy on aircraft. [Kryptid, Mar 14 2008]

Airborne Laser Weapon http://21stcenturys...20mount%20laser.jpg
How to get a powerful laser into a 747. It fires once, not for hours on end. [wagster, May 09 2008]

Aerospace Laser Jet Engine http://www.niiki.ru/eng/n-r-pr-lhrd.html
"Friggin laser beams!" [MikeD, May 10 2008]

[link]






       Because the major constituents of air are non-polar molecules and atoms, they aren't able to absorb much energy on a size-of-molecule scale. You have to hit them on the electron-orbital scale, which for nitrogen means about 100nm, or 1000 Angstroms, which is waaay up the ultraviolet.   

       You may want to familiarize yourself with some of the constituent problems you're gonna face. Example: what is the typical electricity-to-light conversion efficiency of a laser? (Hint - it's bad, at best. Usually worse than that.) Conversion of light-to-heat in your air-absorption step? (Hint - it's actually much the same mechanism as in the laser.) Parasitic losses in the conversion? (Hint - at that high frequency, most of the energy goes into pumping the atom's electrons into a higher energy level - and then they'll re-emit the photon, creating a very nice UV glowing exhaust wasting your energy.)   

       You'd get better results by powering an electric motor on the engine's shaft. At least by two, perhaps three orders of magnitude.
lurch, Mar 14 2008
  

       Or, alternately, use the batteries to power heating elements in the air stream. Not good, but simpler.   

       Lasers are magic, in the Clarkean sense, in this idea. If the proposal was implemented with a gas laser, that would be obvious. The process would use electric power to heat up the gas in the chamber of a gas laser, precipitate that heat energy out as a laser beam, and then use the laser power to heat up the gas in the "combustion" chamber of a jet engine. Instead, it could use the power to heat up the gas in the combustion chamber, directly and by the same methods as would be done for the gas laser's chamber. The laser beam is a totally gratuitous step, with extreme losses, and no advantages in either focus, distance, coherence, speed or monochromaticism that are the usual reasons to employ a laser. But, "Ooh, it's a laser!"   

       And to title this as "Laser-powered" is wrong. "Laser-heated" would be the way to write it. [-]
baconbrain, Mar 14 2008
  

       The energy density of jet fuel is about 12 kWh/kg. Batteries are on the order of 0.1kWh/kg (even the good ones). So, given equal efficiencies, your aircraft is going to carry 120 tons of batteries to replace every ton of fuel.
MaxwellBuchanan, Mar 14 2008
  

       //The presence of hydrogen fusion fuel on such an aircraft would present a fire hazard//
haha!
ldischler, Mar 14 2008
  

       [+] This is an interesting concept to be implemented in upper stratosphere with the aid of solar powered orbiting satellites that can project a reflected beam to the particular craft. The idea is this: the satellites serve as a sort of solar energy collectors and pin-point beamers of electromagnetic radiation to the craft. The ionosphere would be the primary source of energy as beam intensifier and reflector. The potential energy in the ionosphere is readily tapped by the simulated radiation initiated by the beam. The beam is reflected to the craft. The craft focused the beam to heat the air chamber. Then, the heated chamber heats up the air to be directed to the nozzles propelling the craft forward. So, we can now have a heavier craft flying supersonic in the upper stratosphere almost indefinitely that could serve as an intermediary launching device for space missions. It will serve as catapult of deployed satellites to achieve orbital velocity.
rotary, May 09 2008
  

       reactionary thrust produced by heating gas gets really inefficient when the atmosphere is thin.
WcW, May 09 2008
  

       Is there huge weight and aerodynamic drag to contend with? Why do you care so much about inefficiency when energy is unlimited? (Imagine the untapped energy in the ionosphere!) Do you also mean to suggest additional blades to the windmills, replace jet engines with diesels, and scrap rockets for being too expensive and inefficient? So well, what's your alternative that is both more efficient and versatile? [WcW], you have none.
rotary, May 09 2008
  

       [rotary] <----- well if hot air is what's required...   

       anyways, give a try for an electric pulsejet in that case; toss some lightning through the chamber which heats up the contents, etc.
FlyingToaster, May 09 2008
  

       [rotary] - you might want to have a look at just what sort of energy density you have available in the ionosphere. A hint for you: a "plasma" is what you get when a gas is so highly excited that more than 5 percent of the atoms are ions. The ionosphere doesn't come anywhere close - and the density is very low. So "unlimited energy"? Not quite.
lurch, May 09 2008
  

       //simulated radiation //
Wouldn't it be better to use actual radiation?
coprocephalous, May 09 2008
  

       Oh, [lurch]. I am not an expert of plasmas, but I am too inclined to think that you underestimate your intelligence just to reason out your conviction. If you tap your knowledge bank carefully, you would fairly understand that radioactive particles play major roles of the properties of plasmas in the ionosphere. Think of this: a combustion plasma of your diesel engine is not radioactive enough...
rotary, May 10 2008
  

       maybe your lasers could be used to fire at little pellets of fusion-fuel, with the consequent reaction used to heat compressed air.   

       even though the "reactor" is not lined for energy collection in any way, the heat generated could power the compressor and give extra energy to run the lasers. kind of a direct-nuclear brayton-cycle engine.   

       this all assumes that preparing nuclear fuel is quick and easy, lasers are compact, powerful enough, and lightweight, and that elves are used in the assembly process.   

       [rotary]: i like what you said, just because it sounded so victorian, but i didn't get it. this is prolly coz i overestimate my intelligence by about half (i'm at the right place for that after all).
TIB, May 10 2008
  

       Didn't bother reading the roiling debate regarding this because this particular idea is ...   

       Baked.   

       I saw a documentary on this many moons ago. Here is a link.
MikeD, May 10 2008
  

       [MikeD], that's a different idea. This idea is for an engine with internal lasers, and an on-board power supply, thereby not restricting its operation to line-of-sight (in both position and orientation) from a ground station.   

       The author knew about, and linked to, the idea you linked.
lurch, May 10 2008
  

       *Nonchalantly backs out of room*
MikeD, May 11 2008
  
      
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