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?

  • 13
    $\begingroup$ Like an electric motor? $\endgroup$
    – Ron Beyer
    Apr 13 '16 at 21:30
  • 4
    $\begingroup$ Would a straight piston engine whose exhaust doesn't provide thrust count? $\endgroup$
    – Timpanus
    Apr 13 '16 at 21:56
  • $\begingroup$ Oh, thanks @RonBeyer, I didn't even know about such crazy thing like NASA Helios. This is really amazing! $\endgroup$
    – Stan Mots
    Apr 13 '16 at 22:00
  • $\begingroup$ @Timpanus, no, a piston engine is not what I needed. In simple words, I just wanted to know about engines that produce thrust without fuel combustion being involved. $\endgroup$
    – Stan Mots
    Apr 13 '16 at 22:15
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    $\begingroup$ Your title is confusing to me then. I consider "combustible fuel" and "a propellant" to be different things. $\endgroup$
    – BowlOfRed
    Apr 13 '16 at 22:39

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.

  • $\begingroup$ 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. $\endgroup$
    – Stan Mots
    Apr 13 '16 at 23:09
  • $\begingroup$ "Short of going nuclear" Not entirely unheard of though $\endgroup$
    – Ron Beyer
    Apr 14 '16 at 1:15
  • $\begingroup$ 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. $\endgroup$
    – slebetman
    Apr 14 '16 at 3:47
  • $\begingroup$ 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.. $\endgroup$
    – gusto2
    Apr 14 '16 at 15:44

Yes there are. Although they are not for commercial aviation. One example are solar powered engines:

Solar Impulse Source Wikipedia

Alternatively, there are other means of storing energy - rubber bands, but this one is just for model aitplanes.

Rubber band engine. Source: Pinterest

But in either case, you need store energy somewhere, and best way is to use oil-based fuels.

  • $\begingroup$ Big +1 for the rubber band airplane! $\endgroup$
    – abelenky
    Apr 19 '16 at 19:45
  • $\begingroup$ Wow, a radio controlled rubber band airplane? That's... wow! $\endgroup$
    – FreeMan
    May 4 '20 at 17:30
  • $\begingroup$ @FreeMan I see where your sarcasm is coming from. Corrected :) $\endgroup$
    – Alexus
    May 4 '20 at 20:11

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.

  • $\begingroup$ To be strictly accurate, those are not helicopters, they are autogyros. The terms have distinct meanings and all helicopters derive their power from engines. $\endgroup$
    – Simon
    Apr 17 '16 at 12:40
  • 1
    $\begingroup$ It is autogyro when descending. When going up vertically, it is probably not. $\endgroup$
    – h22
    Apr 17 '16 at 19:43
  • $\begingroup$ 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... $\endgroup$
    – xxavier
    May 1 '17 at 20:03

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