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I searched here in this stack and the only question I found was this one, but I feel like it wasn't very well worded and it is also considered a duplicate. If this question is considered a duplicate of that same question, shouldn't a mod make an edit to that question and write something more akin to this one?


The combustion chamber generates an airflow of combustion strong enough to rotate the turbine blades and eventually the engine itself.

However, if you were to cut out the turbine and attach the combustion chamber to the compressor, would it work the same way?

The closest thing I found to this was this 3D animation of a hypothetical "turbineless jet engine", but no article/research which explored the practicality/efficiency of such an engine.

This is a firework-wheel by the way, the firework rockets are attached in an angle to the axis of rotation:

Image of a firework wheel

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    $\begingroup$ What do you mean exactly with "cut out the turbine"? You mean compressor + combustion chamber + exhaust instead of compressor + combustion chamber + turbine + exhaust? $\endgroup$
    – sophit
    Commented May 20 at 13:37
  • $\begingroup$ @sophit Yes, the combustion chambers would be angled in order to rotate the whole thing. $\endgroup$
    – mandiokai
    Commented May 20 at 13:40
  • $\begingroup$ @sophit The compressor would supply air because the combustion chamber would be attached to it like a turbine, but without the turbine. $\endgroup$
    – mandiokai
    Commented May 20 at 13:46
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    $\begingroup$ Well, there's your answer then. If you divert all thrust laterally you don't go flying. $\endgroup$ Commented May 20 at 14:48
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    $\begingroup$ Not to be rude, but highly doubtful. Reference the last two paragraphs of my answer. $\endgroup$ Commented May 20 at 16:55

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https://en.m.wikipedia.org/wiki/Tip_jet

The link describes the nearest (imho) working application of your idea in aviation.

While it generally works, it goes against important engineering tendencies tthat you need a good reason not to adhere to:

  • get the complexity out of the rotating parts.
  • get the mass out of the rotating parts
  • get the moment of inertia out of the rotating parts
  • get the asymmetry out of the rotating parts
  • get the extreme temperatures out of the rotating parts
  • use fewer moving parts
  • create as little as possible vortices anywhere
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    $\begingroup$ This answer also beautifully illustrates why designers, engineers, and mechanics seem to have such colorful commentary about helicopters. $\endgroup$
    – David S
    Commented May 22 at 14:12
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What you are proposing would certainly be capable of powering a compressor section, but don’t forget that the primary purpose of a jet engine is to propel an aircraft forward by producing thrust along the axis of rotation.

Jet engines happen to generate enough power to tap some energy to run the front compressor section and any fan section, but your suggestion to divert 100% of the combustion energy laterally would seem to be very wasteful.

In addition to all the other engineering problems mentioned in the other answer, there is simply no current problem that this would solve.

Realize that the best engineering minds in the business have been working on engine improvements for decades now, if this was a good idea someone would be using it now.

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    $\begingroup$ "your suggestion to divert 100% of the combustion energy laterally would seem to be very wasteful" the combustion chambers the OP is referring to are only slightly turned outward, the biggest part of the thrust would still be axial. $\endgroup$
    – sophit
    Commented May 20 at 19:13
  • $\begingroup$ @sophit, Ok, well... I considered the pinwheel example posted in the question and didn't take the time to research what else he might have meant. $\endgroup$ Commented May 22 at 0:40
  • $\begingroup$ Well, nobody else did but apparently the OP has found a satisfying answer anyway 😉 $\endgroup$
    – sophit
    Commented May 22 at 4:23
  • $\begingroup$ @sophit, well... the question did specify "like a firework wheel". I also asked where axial thrust might come from and the OP didn't have a clue, making no mention of being "only turned slightly outward." So, I just took the question at face value and answered what was asked... Thanks for the insight! ;) $\endgroup$ Commented May 22 at 15:14
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I believe you're suggesting having each combustor attached directly to the turbine shaft and using the hot gas output of each combustor to directly drive the main shaft instead of using the combined gasses flowing through vanes to turn the shaft.

I don't know the math and physics involved, but it just seems that each can individually wouldn't produce as much thrust as they all would together. Maybe I'm totally off on that one.

In your fireworks example, each "combustor" is a small firework that has a self-contained, predetermined amount of "fuel" in it. It burns for a few seconds, then the entertainment is over.

Even if the individual cans, when mounted tangential to the shaft, were to induce the exact same amount of "spin" to the main shaft as is generated by the current setup, the mechanics of getting fuel from the tanks, to a feed rail along the turbine shaft and into a ring spinning at many thousands of RPM to continually feed the combustor cans would be a very difficult feat of engineering. After all, you want the engine to run longer than your fireworks, right?

This suggestion is very impractical from a purely engineering standpoint. While I'm sure the problem could be solved, what would be gained?

Additionally, had the combustor cans been mounted tangentially to the spin of the shaft, the original turbo jet engine would have never worked, since it relied on the exhaust gasses being ejected from the rear of the engine to provide thrust.

A modern, high-bypass ratio fan-jet might be able to work this way, since the vast majority of its thrust is generated by the bypass air going around the jet core. If this were able to spin the fan shaft at sufficient speed (without the fuel lines disintegrating at the required RPM), the fan itself would create a pretty significant amount of thrust, but you'd still lose the thrust of the exhaust gasses themselves.

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  • $\begingroup$ "what would be gained?" Well, the turbine needs to be made of superalloys like inconel with ceramic coatings and air vents to cool it off which costs a lot. Maybe a turbineless jet engine would be a cheaper alternative? $\endgroup$
    – mandiokai
    Commented May 20 at 14:24
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    $\begingroup$ There might be cost savings, however, there would be additional expense in designing, certifying, and maintaining a new system too, @mandiokai. CAAs, airlines and manufacturers are a bit on the risk-adverse side, so unless there are overwhelming benefits, they're a "stay the course" kinda group. $\endgroup$
    – FreeMan
    Commented May 20 at 14:48
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The closest thing I found to this was this 3D animation of a hypothetical "turbineless jet engine", but no article/research which explored the practicality/efficiency of such an engine.

This is a picture of the "turbineless jet engine" as taken from the link in your question:

enter image description here

Even if you could theoretically dispose of the turbine assembly, at the end that design would be more dangerous and less efficient than a standard turbojet. In particular, I see the following limitations (and for sure I'm missing something):

  • The rotary screw compressor used in the animation is nothing new and is for example normally used to provide compressed air to the jack hammers which are seen on construction sites. It is very good in continually providing a lot of compressed air but as soon as the pressure needs to be high as well (like in a turbojet), then leakages between the spirals and the case would be no more acceptable. Even with a very precise (and costly) manufacturing, the tightness couldn't be ensured due to deformations under load.

  • Supply fuel to the rotating combustion chambers would need some sort of rotating distribution system. As already said in the other answers, this system has already been tried out in the helicopter world and its tightness has proven to be difficult to achieve and/or unreliable. In case of malfunction, fuel would be dangerously sprayed around.

  • In a "standard" turbojet, air from the compressor flows straight inside the combustion chamber. Due to the rotation, the air in that design would be highly swirling and getting it inside the combustion chambers in a straight way would be problematic if not impossible. The whole rotating combustion part should also be tight on the left end otherwise the air from the compressor would simply escape from there instead of going through the combustion chambers.

  • A combustion chamber typically reaches temperatures which can be as high as 1700°C. This value is limited by the thermo-mechanical properties of the material used. This temperature is very important because the higher it is, the more efficient the whole process is. Fancy materials and ways of constructing the blades are continuously being researched exactly to increase that temperature. Making a combustion chamber rotate goes exactly the opposite way: having to resist not only the high temperatures but also the high centrifugal forces, the combustion temperatures should be lowered in order to not overstress the materials. A lower whole efficiency can therefore be expected.

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  • $\begingroup$ The rotary screw compressor that you linked is a device with *two counterrotating screws", not a single screw with a helical blade like the one in the image. Rotary screw compressors work efficiently, because they trap air in a (moving) chamber that shrinks as it moves towards the exhaust. The helical blade screw, however, relies solely on friction between the outer wall and the pumped medium. As such, it is always highly inefficient, and only really useful when the main goal is to heat the medium, rather than compressing it. And it works abysmally with low viscosity media like air... $\endgroup$ Commented May 21 at 12:04
  • $\begingroup$ @cmaster-reinstatemonica thanks for expanding upon the working principles of the helical screw, which makes it a definitely useless choice for a turbojet $\endgroup$
    – sophit
    Commented May 21 at 12:16
  • $\begingroup$ @cmaster-reinstatemonica The helical screw combines heating with compression. That is precisely why it is used in injection molding machines. $\endgroup$ Commented May 23 at 5:40
  • $\begingroup$ @PeterKämpf Exactly what I was thinking about. Fill in cold plastic pellets, get out a warm, homogenized plastic fluid that's exactly at the temperature required for a given viscosity. Ready to fill a mold, or extrude a sheet like in foil/film production. $\endgroup$ Commented May 23 at 9:36

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