Why don't hybrid jet engines exist?
by 
Egao No Genki
ANSWERS: 6
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(Credit goes to "Jamaica Mistaica", who gave the copied-&-pasted answer here: http://forums.ebay.com/db1/thread.jspa?threadID=1000076918&tstart=0&mod=1126928380912 ) Pretty simple.. The performance characteristics of even the highest performance electrical motor cannot even come close to substituting for the performance of a typical modern high bypass turbofan engine. There would be no practical application.. On the other hand, many other vehicles have been using a combination gas/electrical propulsion system for years, such as locomotives and ships.
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One aircraft has flown since the early 1960s that is the definitive jet hybrid; the SR-71 Blackbird! This aircraft was the first practical use of a turbo-scramjet; combining the abilities of both a turbojet and ramjet into one. Pratt & Whitney J-58 engines are used to operate the aircraft for start-up, taxi, takeoff and obtainig supersonic speeds where the ramjet section then takes over and the J-58s are idled-back and the aircraft now flies using the supersonice ramjet sections of the egines to fly well beyond Mach-3. In the past the USAF had a project in the 1950s to make a hybrid jet-atomic powered engine. An aircraft that theoritically would have been able to fly for days on end but they could never solve the shielding probems and many others so this project seems to have been abadoned.
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I don't agree with Shultz, It's not the performance that's the problem with some kind of electric motor it's the weight. Electric motor can produce plenty of power but it comes with too much weight. A jet engine that powers a medium size jet (25000) is only 250 to 300 lbs. and produces 4000 lbs of thrust.
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because you can't, the combution of the fuel propels the plane and drives the engine, it would be more practical to have regular jet engines and electricaly powered propellers seperate from the jets.
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The reason this will not exist is mainly power to weight ration. In the aviation world this means everything. Another problem is the FAA, jet engines have changed little from the beginning of the jet age, the main differences are materials used to construct the engine and also fuel delivery. Other than that the engines have changed little. Another problem is finding a battery or source of power that is able to fly long distances at high speeds with quick turn around. It may take awhile to charge the batteries of a plane flying around the world, where it only takes a hour or hour(s) depending on the aircraft to fuel. I agree with the statement given about high bypass turbofans. There is no practical reason not to use these. The answer may be finding a alternate fuel. Airbus has a concept of hydrogen powered jet.
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Jet fuel is high octane and needs to burn extremely fast, Hybrid fuel are slow combusting and would not burn fast enough to create the revolutions needed to run a turbine engine effectively
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