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Well, I know that the combustion chambers drive the compressors, and the reaction occurs, and all that stuff.

But let's imagine that somehow someone is converting a turbine engine (turbojet, turboprop, or whatever) into a Hero's engine "aeolipile" where the combustion chambers are used to rotate the entire device—and thus, the compressor—with its chemical reaction.

Illustration showing how a hero's engine works

Of course, all of this would be a thesis on its own, so let's just concentrate on the question in the title regarding combustion chambers.


If someone where to expose the combustion chambers of a turbine to the open air without the turbine, still suplying compressed air and fuel in the right measurements, would the expanding gases exiting combustion chambers produce any thrust?

illustration showing a combustion chamber of a turbine

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    $\begingroup$ It depends on what you mean by "by itself". The diagram provided in the link by @fooot below does a nice job of breaking down the forces per component in an engine -- but that is still an engine with everything working together. I.e. there is still a compressor providing lots of flow to the combustor. If a combustor really was 'by itself', it would be little more than a candle flame. $\endgroup$ Apr 7, 2023 at 21:43
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    $\begingroup$ You have basically invented the ramjet. $\endgroup$
    – tylisirn
    Apr 8, 2023 at 8:41
  • $\begingroup$ @tylisirn absolutely, just make the exhaust area slightly larger (to accommodate the heated gas) and it will generate thrust. But it needs an external power source to get it going. This one ran on economy grade gasoline. $\endgroup$ Apr 8, 2023 at 13:05

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Yes. The expanding gases from combustion is what does the work in an engine. Most of the thrust of a turbojet engine comes from the combustors. The rest of the engine is really just there to provide the stream of compressed air for the combustion process.

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Yes you can. However you would need a means to compress the air prior to it entering the combustion chamber. This can be accomplished with a turbo compressor as in a typical jet engine. Other methods are to use stagnation pressure at the air inlet, which is what a ramjet engine uses.

A third option, which has not been pursued since the early days of jet aircraft is the motorjet. This is a jet engine, which uses an auxiliary engine to drive the compressor. Very few aircraft or designers ever pursued this, but one exception was the Italian made Caproni Campini N.1 aircraft developed in the 1930s.

enter image description here

This design was effective, but with the advent of better metallurgical technology and the development of the turbo supercharger (which is, essentially a gas turbine engine sans the isobaric combustion chamber), the motorjet fell out of favor compared to turbojets due to an inferior thrust to weight ratio.

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Technically yes, BUT...

...The compressor of a jet engine is one of very few devices capable of actually supplying the required amount of air at the required pressure.

According to PEter's answer here - How much power is drawn by a turbofan to power its own compressor? - a 200 kN engine can be expected to draw 95 MW at the compressor at takeoff. Most of this power is then recovered, and the engine produces a peak output of 40-50 MW at the low-pressure shaft.

There are a few wind tunnels in the world that can produce this kind of power. A smaller engine, or the same engine at cruise, would require less. Still, you're looking at a major industrial installation to supply the air. Or at another jet engine.

Jet fuel does not ignite easily, it will not do so at ambient pressure. Commonplace air compressors will not provide enough pressure or enough flow. If you take a really small engine, like one used on radio-controlled jet models, which could be supplied, then the combustion chamber will produce some amount of thrust.

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supplying fuel and air in the right measurements, would the expanding gasses exiting the combustion chambers produce any thrust?

Exposed to the open air, no. Essentially you've built yourself a fireplace. The circulation would be at best be cooler denser air being drawn in from the bottom. Hotter, less dense air would rise. Adding fuel in the "right proportions" you have ... a Bunsen burner.

The key is raising pressure in the combustion chamber to increase both mass and velocity of exiting gasses in order to generate meaningful thrust:

$ mass × velocity/second$

Thrust is a Force = mv/s = ma = kg × meters/second$^2$

Back to Heros Engine. This is a steam engine. Pressure is raised by using an external heat source to expand liquid water into steam. The expansion ratio is 1 to 1700.

Heros engine spins by expelling steam vapor out its nozzles at high velocity. But significant pressure is required to do this. Action - Reaction. (Tea kettles won't fly, but steam boilers can explode).

Back to our jet engine, along with its rocket and reciprocating (piston) relatives. These are internal combustion engines, which provide thrust from a heat source inside a "combustion chamber", creating pressure.

One little problem is that generating extremely high pressures and expelling high velocity gasses can be terribly inefficient compared to converting pressure into mechanical energy to turn a wheel, or propeller.

the quantity of fuel required to produce a given amount of thrust for a given amount of time is call Thrust Specific Fuel Consumption

It's inverse is ... Isp or Specific Impulse.

Rockets are the worst, but will function in any environment. So will Hero's engine. If you are ever stuck 10 million miles from earth with a nuclear reactor and happen to find an icey asteroid, you're going home!

Reciprocating engines create cylinder pressure in power pulses to generate mechanical energy, jets create power continuously to generate mechanical energy by spinning a turbine.

Jets have Horsepower!

Jet need their "Horsepower" to increase incoming air pressure in the combustion chamber by turning a compressor. This is called a "turbojet". In spite of being less efficient, turbojets are able to generate thrust at speeds that mechanical devices such as propellers cannot match.

unfortunately, these days, fuel is expensive

So now jet engines use their "Horsepower" to turn not only compressors, but also their fans, which are able to generate thrust at higher speeds than propellers.

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