# Why does reverse thrust work? **URL:** \ **Category:** Factual Questions\ **Created:** [January 10, 2013, 4:38pm UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758 "2013-01-10T16:38:48Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:05am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/81 "2013-01-17T02:05:25Z")
> [@Richard\_Pearse](#): > > If the exhaust is not allowed out then nothing gets used either. I’m not sure how many times I have to say it. The exhaust is necessary for the thrust. If the airmass is not accelerated backward, there is no thrust and nothing moves. How can the airmass then be called “wasted”. It’s entire purpose is to be ejected forcefully from the rear of the engine, that is the design principle of the engine. The thrust of a jet engine is DEFINED by the acceleration of the airmass rearward. It seems you are thinking of the plane itself as the force. the expansion happens in suspension in the combustion chamber. when the expanding gasses hit the plane, (the metal) that is what moves/pushes it, what comes out the back is the expansion from the center of the combustion that didn’t go in the direction of the metal of the plane, and afterwards, what bounces off the metal of the plane which was not absorbed to move the plane forward because of the resistance the mass of the plane of the combustion going in that direction you could do away with the fan and pipe air in and it will be the same. like the pebble dropping in the middle of the water, wouldn’t you agree that the combustion in the cans has a central point suspended in the cans where each force vector is emanating from? vyVY ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 17, 2013, 2:07am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/82 "2013-01-17T02:07:17Z")
> [@virtually\_yours](#): > > yes but SOME of my energy north is used to move him north if he is on wheels (remember the wheels of the plane). if his brakes are on and mine, nothing happens we are counteracted… > vyVY Some of your energy is used to move him north and some of the energy is used to move you south, none of it is wasted. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:10am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/83 "2013-01-17T02:10:42Z")
> [@Richard\_Pearse](#): > > Some of your energy is used to move him north and some of the energy is used to move you south, none of it is wasted. I don’t wanna go south I wanna go north vyVy ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:15am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/84 "2013-01-17T02:15:43Z")
> [@Richard\_Pearse](#): > > I wonder if he considers it “wasted” because it is not being used to drive the turbine? no, whats used to drive the turbine is absorbed converted, none of that energy comes out the back, the energy was converted, so you find more power before the turbine than after it. otherwise it would be kind of perpetual energy thing going on.something for nothing. vyVY ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 17, 2013, 2:17am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/85 "2013-01-17T02:17:50Z")
> [@virtually\_yours](#): > > It seems you are thinking of the plane itself as the force. the expansion happens in suspension in the combustion chamber. when the expanding gasses hit the plane, (the metal) that is what moves/pushes it, what comes out the back is the expansion from the center of the combustion that didn’t go in the direction of the metal of the plane, and afterwards, what bounces off the metal of the plane which was not absorbed to move the plane forward because of the resistance the mass of the plane of the combustion going in that direction you could do away with the fan and pipe air in and it will be the same. like the pebble dropping in the middle of the water, > wouldn’t you agree that the combustion in the cans has a central point suspended in the cans where each force vector is emanating from? vyVY I don’t have a problem with that, it’s just when you start talking about the exhaust being “wasted” that things become unclear. Using the word “wasted” implies that it is undesired. But the very principle of the jet engine is to create as much exhaust as possible. The more the better! ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 17, 2013, 2:19am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/86 "2013-01-17T02:19:29Z")
> [@virtually\_yours](#): > > That is not my analogy. mine is that _I_ am standing in between two people.(the combustion in the middle of the engine). because of the nature of combustion I have to push in both directions but I really only want to push the man to the north. but because I am combustion I have to push south also. so the north man goes north where I want it to go and the south man goes south where I am not interested. even if the south man wasn’t there the force would still go that way, from MY center MY south side, not from the side of the north man who is getting his push from MY north side (my expansion in that direction so to speak) . so the southward force from my center does nothing I need and the northward force from my center is used in the way I want. to push him north. No! Combustion is a process, not a physical entity. There is no physical entity corresponding to “you” in this analogy. There is just the jet engine, and the exhaust gases. The exhaust gases push on the jet engine. The jet engine moves one way, the exhaust gases move the other - as in the analogy I gave. There is no physical object in the “center of combustion” throwing shit to either side. ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 17, 2013, 2:21am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/87 "2013-01-17T02:21:18Z")
Here, please go read this: > **[Reaction engine](https://en.wikipedia.org/wiki/Reaction_engine)** > > A reaction engine is an engine or motor that produces thrust by expelling reaction mass (reaction propulsion), in accordance with Newton's third law of motion. This law of motion is commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force." > Examples include jet engines, rocket engines, pump-jets, and more uncommon variations such as Hall effect thrusters, ion drives, mass drivers, and nuclear pulse propulsion. > The discovery of the reaction engine has been... > [@](#): > > A reaction engine is an engine or motor which provides propulsion (thrust) by expelling reaction mass, in accordance with Newton’s third law of motion. This law of motion is most commonly paraphrased as: “For every action force there is an equal, but opposite, reaction force”. > > Examples include both jet engines and rocket engines, and more uncommon variations such as Hall effect thrusters, ion drives, mass drivers and nuclear pulse propulsion. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:23am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/88 "2013-01-17T02:23:08Z")
> [@virtually\_yours](#): > > no, whats used to drive the turbine is absorbed converted, none of that energy comes out the back, the energy was converted, so you find more power before the turbine than after it. otherwise it would be kind of perpetual energy thing going on.something for nothing. > vyVY maybe I should start using unused energy or non converted energy than wasted.. I think I understand what you mean: in essence it is necessary to have all that power available. But because of friction and mass etc.. a lot of it is not used to it’s full potential. actually I think out of all the power produced in an engine only about 25% or so is actually used to move it . the rest is used to turn the turbines.. vyVy ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:29am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/89 "2013-01-17T02:29:12Z")
> [@Absolute](#): > > No! Combustion is a process, not a physical entity. There is no physical entity corresponding to “you” in this analogy. There is just the jet engine, and the exhaust gases. The exhaust gases push on the jet engine. The jet engine moves one way, the exhaust gases move the other - as in the analogy I gave. There is no physical object in the “center of combustion” throwing shit to either side. so the combustion doesn’t have a center? anyone knows expansion is not one sided, it goes in all directions away from a central it couldn’t go the other way, implode, unless some other type of weird physics is going on, because then there would be no expansion., otherwise why do you feel force in all directions away from the initial combustion? vyVY ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 17, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/90 "2013-01-17T02:36:54Z")
> [@virtually\_yours](#): > > I don’t wanna go south I wanna go north vyVy No you don’t, you are the expanding gasses from the combustion, you want the aeroplane to go north, in order for that to happen you must go south, therefore you want to go south, therefore it is not “waste” when you move south. > [@](#): > > maybe I should start using unused energy or non converted energy than wasted.. I think I understand what you mean: in essence it is necessary to have all that power available. But because of friction and mass etc.. a lot of it is not used to it’s full potential. actually I think out of all the power produced in an engine only about 25% or so is actually used to move it . the rest is used to turn the turbines.. No that would still not be right. For every bit of energy that you get from the combustion gasses before it is ejected from the engine, by turning turbines, driving the hydraulics, generators, fuel pumps, oil pumps etc. All of that energy being extracted from the exhaust is actually reducing the thrust of the engine. The exhaust is not “unused” it is necessary for the thrust to exist, it is being used to drive the aeroplane forward. Whenever you put something into the exhaust to try and extract something extra out of it, you are reducing the thrust, and wasting the exhaust’s energy. > [@](#): > > no, whats used to drive the turbine is absorbed converted, none of that energy comes out the back, the energy was converted, so you find more power before the turbine than after it. otherwise it would be kind of perpetual energy thing going on.something for nothing. > vyVY That does not address what I said. Ideally you don’t want to get power out of the exhaust in that way. By using some of the exhaust gasses to power the compressor stages you are reducing the thrust of the engine. The ideal reaction engine doesn’t have the exhaust doing anything other than shooting out the back. There are no turbines or anything like that, just compression by ram effect followed by combustion and expansion. What I meant by my statement was, do you consider the energy in the exhaust that has not been used to drive the turbine to be “wasted”? Because the opposite is true, it is actually the energy being used to drive the turbine that is undesirable and in a way “wasted”. Think of it this way, if we could design the turbine out of a turbojet engine, we would, but we would never try to design the exhaust out of a turbojet engine. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:37am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/91 "2013-01-17T02:37:10Z")
> [@Absolute](#): > > Here, please go read this: > > [Reaction engine - Wikipedia](http://en.wikipedia.org/wiki/Reaction_engine) A reaction engine is an engine or motor which provides propulsion (thrust) by expelling reaction mass, in accordance with Newton’s third law of motion. This law of motion is most commonly paraphrased as: “For every action force there is an equal, but opposite, reaction force”. > > Examples include both jet engines and rocket engines, and more uncommon variations such as Hall effect thrusters, ion drives, mass drivers and nuclear pulse propulsion. once again the opposite reaction is not the plane moving opposite to the force, that would mean the force itself was one sided. The opposite reaction to the exhaust-bound force is the equal force going in the opposite direction of the exhaust bound force. if the plane wasn’t there you would feel that force in the opposite direction of the exhaust. you are missing a step. For the exhaust-bound force there has to be an equal opposite front-bound force, and THAT force is what pushes the aircraft away. the aircraft can generate no force by itself, it wouldn’t need an engine then. vyVY ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 17, 2013, 2:42am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/92 "2013-01-17T02:42:06Z")
> [@virtually\_yours](#): > > so the combustion doesn’t have a center? anyone knows expansion is not one sided, it goes in all directions away from a central it couldn’t go the other way, implode, unless some other type of weird physics is going on, because then there would be no expansion., otherwise why do you feel force in all directions away from the initial combustion? > vyVY Combustion isn’t a thing. The only thing being considered is the airmass and the air is constantly moving through the engine, so any centre you choose is not the centre for long. It’s very simple if you consider the engine to be a device the throws a heap of hot air backward at high speed. It becomes much more complicated if you try to think of all the pressure gradients through the inside of the engine. It becomes impossible when you mix these two ideas together (which is what you seem to be trying to do.) ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 17, 2013, 2:44am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/93 "2013-01-17T02:44:51Z")
> [@virtually\_yours](#): > > so the combustion doesn’t have a center? anyone knows expansion is not one sided, it goes in all directions away from a central it couldn’t go the other way, implode, unless some other type of weird physics is going on, because then there would be no expansion., otherwise why do you feel force in all directions away from the initial combustion? > vyVY No, it doesn’t have a center, not in the way you are thinking. Despite your claims otherwise, you are still thinking of this as an explosion, in which an expanding pressure wave or shockwave moves outward in all directions from an ignition point, and reflects off solid surfaces, etc. If this were what propels a jet engine, you would be correct - a large fraction of energy would be lost as a shockwave moving out the back of the engine without ever transferring any forward energy to the engine itself. But this is not what happens, and you are wrong. I understand now what you are talking about, but this is not how a jet engine works. A wave is a disturbance that propagates through material, without carrying any material along with it. What propels a jet engine is that the continuing process of combustion creates high pressures inside the combustion chamber, and this high pressure causes the air/fuel/exhaust mixture to jet outwards at high speed. The reactive force of the exhaust moving outwards propels the jet forwards. ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 17, 2013, 2:47am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/94 "2013-01-17T02:47:46Z")
> [@virtually\_yours](#): > > once again the opposite reaction is not the plane moving opposite to the force, that would mean the force itself was one sided. No! This is the _whole point_! In order for the plane to move in one direction, a volume of air must be accelerated in the opposite direction. How do you think rockets work, in space? They are not pushing against anything. A turbojet engine is fundamentally just a rocket engine that gets some of its fuel from the atmosphere as opposed to carrying it all internally. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 2:56am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/95 "2013-01-17T02:56:53Z")
> [@Absolute](#): > > Here, please go read this: > > [Reaction engine - Wikipedia](http://en.wikipedia.org/wiki/Reaction_engine) A reaction engine is an engine or motor which provides propulsion (thrust) by expelling reaction mass, in accordance with Newton’s third law of motion. This law of motion is most commonly paraphrased as: “For every action force there is an equal, but opposite, reaction force”. > > Examples include both jet engines and rocket engines, and more uncommon variations such as Hall effect thrusters, ion drives, mass drivers and nuclear pulse propulsion. once again the opposite reaction is not the plane moving opposite to the force, that would mean the force itself was one sided. The opposite reaction to the exhaust-bound force is the equal force going in the opposite direction of the exhaust bound force. if the plane wasn’t there you would feel that force in the opposite direction of the exhaust. you are missing a step. For the exhaust-bound force there has to be an equal opposite front-bound force, and THAT force is what pushes the aircraft away. the aircraft can generate no force by itself, it wouldn’t need an engine then. vyVY ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 17, 2013, 2:59am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/96 "2013-01-17T02:59:53Z")
> [@virtually\_yours](#): > > once again the opposite reaction is not the plane moving opposite to the force, that would mean the force itself was one sided. The force is one sided. By creating a hole for the exhaust to exit from you are making the force one sided. We call that force “thrust.” It has magnitude and direction equal and opposite to the exhaust. ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 3:53am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/97 "2013-01-17T03:53:38Z")
> [@Richard\_Pearse](#): > > The force is one sided. By creating a hole for the exhaust to exit from you are making the force one sided. We call that force “thrust.” It has magnitude and direction equal and opposite to the exhaust. If the metal of the plane itself was burning (rapidly), using up its own _stored_ energy,you can say that, so to speak. but the plane would dwindle away. Its the fuel and air introduced into the empty chamber that causes the combustion/expansion in all directions. one force end high tailing it toward the exhaust and its equal force hightailing it the other way , hitting the _closed_ end of the cans hence pushing the aircraft as best it can. Suspend an independent (remember the combustion is independent of the plane, it pushes it not attached to it) hose in the combustion chamber with a T junction attached. one side pointed at the exhaust and the other in the opposite direction towards the metal of the plane. turn the water on. water comes out in opposite directions. That is your equal and opposite forces in a linear example. The force goes forward and backward from a central point. the water hitting the plane begins to push the plane. The water out the back is unused. If you closed the back, the forward and backward push would counteract itself hence no movement.. Like I said that’s what i see in my head.. vyVY ---
**Author:** ![virtually\_yours](https://avatars.discourse-cdn.com/v4/letter/v/a587f6/32.png) [@virtually\_yours](https://boards.straightdope.com/u/virtually_yours)\ **Post date:** [January 17, 2013, 4:01am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/98 "2013-01-17T04:01:44Z")
> [@Absolute](#): > > No! This is the _whole point_! In order for the plane to move in one direction, a volume of air must be accelerated in the opposite direction. > > How do you think rockets work, in space? They are not pushing against anything. A turbojet engine is fundamentally just a rocket engine that gets some of its fuel from the atmosphere as opposed to carrying it all internally. They (the forces)are pushing against the rocket on the inside.against the firewall so to speak. when the combustion happens there is force in ALL directions towards the rocket and away from it out the back. the key word you are missing is opposite. the opposite happens in the combustion chamber.. there is where you have the opposite directions. one towards the rocket and one away from it.. you seem to think the rocket is the combustion..the combustion is not attached to the rocket it is separate from the rocket, otherwise the rocket would burn up vyVY ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [January 17, 2013, 4:12am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/99 "2013-01-17T04:12:01Z")
Again, there is no such entity as “combustion”. Combustion does not “push”. “Combustion” is not separate or independent from anything, because it is not a part of anything. Your hose example does not make any sense. _There is no “independent hose”!_ There is only the gas that is combusting, and the engine. Combustion is just a physical process that occurs in the gas and raises its pressure. The pressurized gas in the engine pushes on the engine. So the engine moves one way and the gas moves the other way. Conservation of momentum. The statement “the combustion is not attached to the rocket it is separate from the rocket” is meaningless. It is neither true, nor false. It is, to borrow a phrase, _not even wrong_. Combustion cannot be “attached” to anything. It is not a physical object, or force. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [January 17, 2013, 4:20am UTC](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758/100 "2013-01-17T04:20:49Z")
**virtually yours** , how is the combustion product (exhaust) not touching a jet or rocket engine? Is there some magical force field between the combustion product and the walls of the combustion chamber? Does it exhaust somehow magically through a nozzle without touching the nozzle? It is all a bunch of molecules which are interacting with each other. Nothing is happening in isolation. There is a pressure differential between the front and the back of the engine, so it is propelled forward as it propels the exhaust rearward. It is an equal and opposite reaction. Do you think that the opposite is wasted? It is necessary because it is the only way to move something mechanically. [Previous page](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758.md?page=4) [Next page](https://boards.straightdope.com/t/why-does-reverse-thrust-work/646758.md?page=6)