Comments by "Keit Hammleter" (@keithammleter3824) on "See Thru Jet Engine" video.

@Hawk7886 : No field of engineering is 100% science. Confine yourself to only science and you are a mere technician and cannot invent or innovate, only replicate or copy. Real engineers are creative - just as creative as any artist.
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@craigstephens93 : You are exactly right - it does depend on the practitioner. And no actual practitioner is 100% one or the other. What does a scientist do? Discovers or deduces some logical principle. How does he decide what possible principle to investigate? By using his imagination - that's art. What does an engineer do? Uses in an application logical principles discovered by scientists. How does he know what logical principle can be used in some useful way? How does he do that? By being creative, using imagination - that's art. Incidentally, I am an engineer who has worked in electronic product research and development. Such people work with "product designers", who are a type or artist, to devise how to make products look attractive and have good ergonomics - just as civil engineers work with architects - the engineer ensuring the building is sound and safe, the architect, a type of artist, ensuring it will look nice and be functional and human-friendly. In both cases, for this team-up to work, each (the engineer and the designer/architect) must have some expertise in the other's field.
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@ron25-33 He has the intake air from the compressor, which goes through in considerable quantity, pass between the transparent cylinder and the combustion chamber, before entering the combustion chamber via the holes you see - the intake air thus isolates and cools the transparent cylinder. The cylinder is presumably some sort of Pyrex (borosilicate) glass, which has a low coefficient of expansion and can thus take high temperatures anyway.
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At 8:59 while talking about the amount of air going through vs the thrust produced, Warped Perception states "most of the energy comes from the fuel." That's sort of right, as the fuel notionally supplies chemical energy though combustion, but he doesn't say that, and in his context it is misleading and confusing. Jet engines operate stochiomentrically - that is the mass of air consumed must just equal the amount required to completely combust the fuel and no more. For hydrocarbon fuels, the mass of air must be about 15 times the mass of the fuel consumed. (Only the oxygen is used chemically - air is about 80% nitrogen which passes through chemically unchanged. The actual oxygen mass is about 3 times the mass of the fuel.) What produces the net thrust in a jet engine is that combustion raises the volume of air/fuel mix such that the burning gasses can push against a greater area of the forward inside engine surface than the combustion chamber air inlet area. The burning gas pressure in the combustion chamber must be approx equal or a bit less than the incoming air pressure to the chamber as otherwise burning gasses would come out the front. Thus forward thrust is produced as the exhaust orifice, always much larger than the air inlet orifice size, cannot offer much more than surrounding air pressure. Jet engines are most easily understood properly by first considering a ram-jet engine, which has no moving parts. Intake air is compressed by the forward motion forcing the air through a funnel, so reducing the area and raising the pressure. Again, as with a turbojet, the forward part of combustion chamber surface area must be greater than the chamber hole for the incoming air, and is much greater than the exhaust orifice back pressure. Understand all that and you will not only understand that the mass of air is more important than the mass of fuel (both supply the pressure, and there's much more air than fuel), you will also understand that the efficiency of a jet engine (ram or turbo) is proportional to the compression ratio. In World War The British Air Ministry and the Royal Air Force famously took no notice of Frank Whittle and his jet engine, because his design and available materials permitted only a very low compression ratio, and thus they knew the fuel consumption would have to be horrendous without needing to test it. The jet became practical when others (eg at Rolls Royce) redesigned it to have a better compression ratio).
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@brianb-p6586 : You are correct that efficiency is not DIRECTLY proportional to compression. I didn't include the word "directly." I perhaps should have included clarifying words but my post was already quite long. You are also correct in that the term compression ratio is not used in jet engineering texts, but for the purpose of explaining jets to lay persons, it is clear enough. For that matter, what matters in piston engines is not the volume ratio either but the pressure ratio, which depends on volumetric efficiency, and the sum of compression ratio and external compression, eg turbo charging if used. Thinking about a jet engine in terms of push against surfaces is correct. It works just as a balloon with a hole in it darting about when you let it go without typing off, except that a jet engine can keep going due to continuous intake of air and fuel through small orifices. However, its also correct to analyse in terms of brayton cycle and reaction - both are correct. Its the same with piston engines - you can think about intake, compression, power, and exhaust strokes (or more correctly, intake, compression, combustion, expansion, blow down, equalisation) or in terms of something called the "standard air cycle" based on Carnot, as mechanical engineers are taught in university. I've never seen a text book intended for mechanics go into Carnot and the standard air cycle. They need a simpler view without math. Same with jet engines - you can talk about Brayton and reaction but it doesn't help the ordinary person much. Thinking in terms of push against surfaces does, and makes the importance of things like compression quite clear. It's all very good to talk about reaction al la Newton, but just where does the push on the airframe come from? Answer - directly transmitted from the forward surfaces of the combustion chamber? Where does that pressure come from? Answer - from the acceleration of that gas mass out the back. What will happen if you confine the exhaust orifice size? Answer: you will drop efficiency due to back pressure.
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@WarpedYT : I understand the need to keep it simple, but the way you put it might lead to viewers thinking more fuel than air is used, even though you previously emphasised air quantity. Keeping explanations simple and without using math, without going astray in translation is hard. I am a part time technical journalist and service manual author, and looking and testing simple explanations is what takes most of my time.
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@brianb-p6586 Ideally, operation is stoichiometric. In practice, just as with gasoline piston engines, for various minor reasons, they may best operate economically and cleanly somewhat lean., say up to 15% excess air. This is close enough to stoichiometric to estimate the approximate mass ratio air to fuel - which is what I wanted to point out here in order to clarify Warped's otherwise good talk. In the limit as air mass is increased well beyond stochiometric, net thrust must go toward zero.
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@brianb-p6586 Turns and counter flow of air in practical engines actually makes no difference to the basic principle. Actually, practical jet engines can have an inner chamber where combustion actually takes place, and this chamber has no significant structural strength (thus making and using a high temperature alloy easier) - the strength being in a cooler outer chamber. (See note below) That makes no difference to the basic principle either, as the pressure is transmitted from inner to outer chamber through holes. The basic principle being, as I said, combustion resulting in gas pressure, and this pressure is transferred to a greater area of forward-facing rigid surface than the area of any rear-facing rigid surface, and that pressure equal to or slightly less than the air pressure imparted by the compressor stage. It really doesn't matter where in the combustion chamber the air comes in - the centre of the front surface, from the sides, or even looping around so as to enter from the rear. What matters is the the air inlet orifice is smaller in area than the whole forward facing part of the chamber surface, so that the compressor can overcome the pressure resulting from combustion, but the aggregate force on the forward facing surface is still greater. If the air from the compressor takes one or more turns or even reverses direction, it just means the pressure from the combustion chamber is transmitted back to load the compressor via a path with turns. If you are going to deny gas pressure imparted on the forward facing part of the combustion chamber surface being transmitted to the airframe as the net thrust, then you are going to have to say just where the force pushing the plane forward comes from. It's not sufficient to airily say it's reaction of the gasses coming out the back. I suggest you try walking before you run - that is, simplify the thing down to its basic essentials - a ram jet engine which has no moving parts. It's somewhat like an odd shaped venturi. Obtain and/or draw a longitudinal cross section and think about how it in practice works. Not as overall governing theory about action and reaction, think about just how the force delivered to the airframe arises - how it gets transmitted there, and how come combustion gasses don't come out the front, despite no physical barrier. Once you understand just how a ram jet engine DELIVERS thrust to the airframe, you can then extend the principle to a turbojet, where inner chambers, compressors, turbines and whatnot complicate the engineering but the basic idea is the same. Note: An inner chamber where combustion actually takes place, with holes to communicate the pressure to a structural chamber, so that the inner chamber can run hotter without significant mechanical loading, was an early innovation on the path to make jets practical.
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@brianb-p6586 : Not at all. You have basically denied where the force applied to the airframe comes from. Nor have you given an explanation of where else it could come from. The only possible place is gas pressure on the forward facing component of the working (combustion) chamber. (The same pressure acts on the sides, but the side loading cancels out.) When you boil it down, your words are analogous to saying "mass is accelerated out the back. So there must be forward thrust - F = ma. " This is correct, but it is not an explanation of how the thrust actually arises. An analogy - you could say "in a clock, energy in the mainspring causes the hands to rotate." So it does, but most of us with enquiring minds interested in clocks would like to know that the spring applies a torque to a gearwheel which turns a gear train. You had better look at drawings of ram jets. You can start with the 2 drawings of ram jets in Wikipedia's article on ram jets. The first one is schematic; the second one is labelled "typical" and is a NACA design. In both diagrams the inlet orifice is very small and the exhaust orifice is about as large as it can possible be. It occurred to me shortly after posting before that you can simplify this even more in order to highlight the essential feature: consider a simple rocket. In its simple form, a rocket has three things: A source of fuel, a source of oxygen (which can be a chemical that releases oxygen when heated, this chemical being mixed with the fuel), and a burning chamber with a rear-facing hole in it. The fuel and oxygen burn, causing pressure in the chamber by forcing the combustion products out the hole. This pressure acts in all directions in the chamber, but SINCE THE HOLE CANNOT TRANSFER THE PRESSURE to the structure, and the FORWARD FACING PART OF THE CHAMBER CAN, there is a net forward force on the airframe. The ram jet has a complication - it gets its oxygen by forcing atmospheric air through a front facing orifice. This orifice must be small, or the combustion pressure load against the forward facing surface will not be greater than the load on the incoming air. Conveniently, it can be arranged that the intake orifice can be a funnel, thus trading air volume for air pressure, so that the combustion doesn't just simply exhaust out both ends. Practical rockets and turbo jets of all kinds usually have an exhaust nozzle of expanding diameter. This is a means of fine tuning so to speak - along the axial length of the nozzle the gas pressure reduces while gas volume increases - this is a means of converting SOME (it cannot be anything like all) heat energy into a pressure drop. It is not essential that the diameter is reduced before it increases, though it is often mechanically convenient.
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@brianb-p6586 : You're just being silly. Axial loads from compressors are just practical complications that do not invalidate the basic principle. Understanding the basics before considering complications is how we come to understand almost anything. I have not denied the loading from compressors etc - these just mean that the net force delivered to the airframe is a bit less than the force applied by the burning gas to the forward facing component of the combustion chamber. It still that forward component surfaces that furnishes what jets are good for. You still haven't provided an alternative explanation of just where the force on the airframe comes from. You can't, can you? Now, go look at some drawings of ram jets and then come back and explain it.
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