As far as I know, all engines used in aircraft produce thrust by means of some propellant: kerosene, hydrogen peroxide etc. Maybe I've missed something, but could you tell me, are there any engines that can produce thrust without using exhaust gases expelled from the nozzles? Is it even possible?
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15$\begingroup$ Like an electric motor? $\endgroup$– Ron BeyerApr 13, 2016 at 21:30
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4$\begingroup$ Would a straight piston engine whose exhaust doesn't provide thrust count? $\endgroup$– TimpanusApr 13, 2016 at 21:56
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$\begingroup$ Oh, thanks @RonBeyer, I didn't even know about such crazy thing like NASA Helios. This is really amazing! $\endgroup$– Stan MotsApr 13, 2016 at 22:00
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14$\begingroup$ Your title is confusing to me then. I consider "combustible fuel" and "a propellant" to be different things. $\endgroup$– BowlOfRedApr 13, 2016 at 22:39
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1$\begingroup$ @Storix: A water bottle rocket uses water as propellant but doesn't combust fuel. A piston driven propeller uses no propellant (it propels the air instead) but burns fuel. An ion engine for spaceships uses xenon gas as propellant but burns uranium as nuclear fuel. $\endgroup$– slebetmanApr 14, 2016 at 3:42
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4 Answers
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Yes there are. Although they are not for commercial aviation. One example are solar powered engines:
Source Wikipedia
Alternatively, there are other means of storing energy - rubber bands, but this one is just for model aitplanes.
Source: Pinterest
But in either case, you need store energy somewhere, and best way is to use oil-based fuels.
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No matter which kind of engine, you need some way to supply the engine with energy and you need some way to store that energy until you need it in flight.
Short of going nuclear (and there are some nontrivial engineering problems involved in designing a halfway safe nuclear reactor that you can put on a plane and keep it lightweight enough that flying it isn't pointless), chemical fuels for burning have the best available ratios of weight to energy stored. This is particularly important for aviation because more weight means more induced drag which means more thrust needed.
Joule for joule, the efficiency of fuel-burning engines is not great, but the superior energy density of hydrocarbon fuel more than makes up for that and allows them to supply more useful thrust per kilogram of energy storage lifted than either batteries (low losses, but very heavy) or fuel cells (medium losses, but require small-molecule fuel such as hydrogen or methane that still have markedly worse energy densities than kerosene).
Since you speak about "exhaust gases" in the question, note that current aircraft derive at most a small fraction of their thrust from ejecting exhaust gases. Propeller planes don't generally make use of the exhaust at all, and "jet" engines are practically always high-bypass turbofan engines where most of the energy from the combustion goes to drive the fan that blows intake air backwards without burning anything in it.
answered Apr 13, 2016 at 22:47
hmakholm left over Monicahmakholm left over Monica
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$\begingroup$ Thank you, especially for the part about the exhaust gases! I see I had totally wrong idea about the creation of thrust. Things are more clear now. $\endgroup$ Apr 13, 2016 at 23:09
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$\begingroup$ "Short of going nuclear" Not entirely unheard of though $\endgroup$ Apr 14, 2016 at 1:15
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$\begingroup$ The Soviets did build a nuclear aircraft engine: the Kuznetsov NK-14 which was meant to be used on the nuclear powered TU-119. The plane was never completed due to cost constraints but the engine was built and ground tested. $\endgroup$ Apr 14, 2016 at 3:47
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$\begingroup$ Even flying a glider you need to get it up somehow.. well - lets see the paraglider :) Though the paraglider has no engine. Talking about the engines, today we always 'burn' something to get the energy out of it.. $\endgroup$– gusto2Apr 14, 2016 at 15:44
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$\begingroup$ Even in a pure turbojet most of the air is just heated, but does not take part in any chemical reaction. The nitrogen, which makes up almost ⅘ of the air, is inert, and even of the oxygen just a small fraction is actually used to burn the fuel. $\endgroup$ Jan 1 at 10:48
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Some simple helicopter models like bamboo copter use rotary inertia as the power source. Either rotor or, in some cases, the whole simple model is spun up on the ground, providing enough rotating energy for the short flight with the help of the blades, same as for the real helicopter. However I am not aware of the full scale machines relying on such a power source.
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$\begingroup$ To be strictly accurate, those are not helicopters, they are autogyros. The terms have distinct meanings and all helicopters derive their power from engines. $\endgroup$– SimonApr 17, 2016 at 12:40
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1$\begingroup$ It is autogyro when descending. When going up vertically, it is probably not. $\endgroup$– h22Apr 17, 2016 at 19:43
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1$\begingroup$ The 'bamboo copter' uses 'rotary inertia' as power source only when climbing. When descending, it gets its energy from the column of air that traverses the rotor disk while losing altitude... $\endgroup$– xxavierMay 1, 2017 at 20:03
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If you mean rocket engines and jet engines, excluding prop aircraft, there is of course the ion engine. It doesn't use "propellant" in the sense of heated gas thrown out the back, but it does exploit a stream of what we call "protons" for thrust.
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$\begingroup$ Wrong. "Ionized xenon" is plasma, not gaseous. Plasma phenomena cannot be modeled or understood in terms of the behavior of gases. $\endgroup$ Jan 1 at 8:04
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$\begingroup$ Ah, I accidentally misread "protons" as "photons". $\endgroup$ Jan 1 at 8:14
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$\begingroup$ Might be worth linking to en.wikipedia.org/wiki/Ion-propelled_aircraft, and being clear you are not talking en.wikipedia.org/wiki/Ion_thruster which are notable useless in atmospheres. $\endgroup$ Jan 1 at 8:18
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$\begingroup$ @fertilizerspike You seem to be mixing up the two kinds of ion engines that work quite a bit differently. $\endgroup$ Jan 1 at 11:04
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