Turbofans look a lot like the jet engine in this video but, as you point out, only the core is the turbine and most of what’s inside the cowling is a fan for pulling air past the engine, which pulls the plane forwards.
Turboprops do this but, to a lay person, in a different way. Four propeller blades instead of hundreds, and no cowling. Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance?
A turbofan however is typically installed on planes that want to fly around 80% of the speed of sound. That's really close to the limit, and there isn't enough leeway there for a propeller to get away with it. A turbofan manages by using lots of blades to more gently push the air back, and then the cowling helps by preventing blade-tip vortices, and by constricting the airflow slightly, which increases the air's backwards speed.
So a propeller and a turbofan are suitable for different plane speed regimes. When you're travelling slowly, it is most efficient to push a very large amount of air back just a little bit faster than the plane is moving, and a propeller is great at doing that. As you approach the speed of sound, that becomes infeasible, and a turbofan becomes more efficient. If you're trying to break the speed barrier, then you need a lower bypass ratio, or you may even need to switch to a turbojet.
In this video of an Iran Air 747SP the buzzsaw effect is particularly isolated/exaggerated due to the camera/mic placement, the use of 1970's design (louder) JT9D engines, and the shorter length of the 747SP. https://youtu.be/3lhHwKK-6ms
On a 737 the pitch is higher due to the smaller diameter fan (higher max RPM), while a 777 has the lowest buzzsaw pitch due to having the largest fan diameter.
The most recent jet engines (like on the 747-8) have highly suppressed this sound with contoured fan blades.
Now, for a given energy, you can push some mass of air with speed v or 4 times as much with speed v/2. The momentum of the air in the latter case is double the first case: 4 m v/2 = 2mv. You always want to move more air at lower speed.
You achieve that with higher and higher bypass ratios. Of course, with a higher bypass ratio the engine becomes larger and heavier (and draggier), so there are limits. But if you could increase the bypass ratio without increasing the engine weight and drag, you would always do it. As a bonus, with lower air speed come lower vibrations and noise.
glider wing area ~10m2, F16 ~30m2, F22 ~80m2
This is a complicated area, but in general basically: Supersonic flow is bad. At best you get a ton of noise (like the Russian Tu-95). Efficiency also goes to hell.
This is also why the SR-71 had those crazy moving brake cones… they manipulated the shockwave so the air actually entering the engine was subsonic.
The fuel in an airplane is also exhausted but I'm not sure how much of momentum is gained due to this rocket fuel effect, in the traditional working regime.
For lower stages, you can inject water into the nozzle and ISP (exhaust velocity) goes down, but thrust goes up, more than compensating for the weight of the water tanks.
But you wouldn't want to do that for an upper stage, as all the rocket below that would need to carry the water tank to a higher velocity.
Saturn V second stage had variable mixture ratio, using a leaner mixture with lower ISP (but better density and thrust) at lower altitude and richer mixture with higher ISP at higher altitudes. https://yarchive.net/space/launchers/saturn_variable_mixture...
So, ISP is clearly not everything. At low altitudes, you're looking at basically maximum thrust and thrust to weight. At some level you're also looking at volume and thus density, to minimize cost.
My upper point was more about asking about the regime in which we can consider an airplane a constant mass system, and could totally neglect the mass of the consumed fuel compared to the momentum of the air moved.
One of the main difference between a jet engine and a rocket engine is that the rocket engine carry its own oxygen, but I'm also not quite sure how much the momentum of the surrounding air is relevant, in helping push the plane forward.
There is probably some kind of transition where either when air density get lower, or when speed get higher, than your jet engine behave more like a rocket motor than a blowing-fan (like the turbo-prop vs turbo-fan in the other comments).
The other day I saw the TechIngredients video about afterburner https://www.youtube.com/watch?v=EP4Hf63zR6w that I found interesting.
Edit: The wikipedia page about specific impulse https://en.wikipedia.org/wiki/Specific_impulse and in particular its graph https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Sp... helped me quantify the importance of air in the propulsion efficiency of a jet engine with respect to a rocket engine.
For rockets, there's Rocket Lab's Electron that uses electric pumps to pump the regular propellants to the combustion chamber. This was much easier to develop than a turbine for the turbopump. It drops some spent batteries along its flight.
Could an electric airplane drop batteries at certain points? Like after climbing to target altitude. A lot of operational headaches there, so not likely. There have been some military planes that have used takeoff trolleys or takeoff rockets so there is some precedence there.
Rocket engines are extremely inefficient, they just have to be used because of the very wide speed range they support. So it wouldn't really make sense to eject water with an electric pump. Batteries have such low energy density and the pumps are heavy too. It's the worst combination to carry your own reaction mass and propel it with battery stored energy!
A rocket engine has tremendous trust to weight and power density, it can be gigawatts in a car sized package. This is enabled by the very energy dense liquid fuel and liquid oxidizer. Even a jet engine pales in comparison, as it has to use gaseous air as oxidizer which is not dense, and which is 80% "fluff" nitrogen. Hence the jet engine compressors have to be large, and the compression ratios are meager 10 to 20 bars. A rocket engine can run at 200 bars main chamber pressure. The compressors (pumps) can operate at hundreds of megawatts (this is actually not far from big jet engines). The thrust to weight ratio can be 100 to 1, while for jets it's 10 to 1.
Until then we have to stay tethered to the sea with something like Zapata's Flyride / Flyboard https://www.youtube.com/watch?v=PCAvIHGylKo
Edit : (We just have to put an electric jetski inside a fire-fighting plane, and have the Flyride pull the plane forward until it reach lift-off speed, (no expensive fuel needed :) )
In outer space, the rocket equation says that the delta-v is only a function of propellant mass and exhaust velocity. It is always better to have higher exhaust velocity, no exceptions.
But until you get to orbit, you experience the “gravity drag”. Gravity acts in the opposite direction of your motion, and reduces your speed, so you end up needing overall more delta-v.
In order to reduce the effect of the gravity drag, you want to accelerate as quickly as possible. That’s why astronauts have to be able to withstand high Gs for a few minutes. Ballistic missiles, which are not limited by the human limits can (and do) acceletate much faster. A Trident II gets to about 50 Gs.
To get to these insane accelerations, you need huge thrust. You achieve that at the cost of lower exhaust velocity (like in the case of the Space Shuttle). Lower exhaust velocity means lower delta-v, but higher thrust means lower delta-v losses due to the gravity drag, so in many cases this tradeoff makes sense.
The problem for first stages is that some high ISP propellants are very non-dense. Liquid hydrogen is 70 kg per cubic meter. Kerosene is about ten times as dense. So the tanks need to be a lot bigger for the same payload. The same holds for pumps and turbines too. So the engines need to be bigger for the same thrust. Hydrogen is also annoying to deal with as it's much colder than liquid oxygen, the molecule is also much smaller and leaks much more easily.
Technically the fueled all-up hydrogen rocket might be lighter, so the engines should be smaller, but the density disadvantage eats back all that advantage. The dry mass, which is the actual cost driver, is about the same.
For upper stages, it makes sense because the cost driver is not the dry mass but the wet mass. Also they need less thrust so engine mass is a smaller proportion of stage mass.
(This ignores potential energy from changes in altitude.)
This is one reason why it's better to use a lower Isp propellant on the first stage of a multistage launcher (also, those propellants are much denser.)
Turbo props and turbo fans, both produce thrust, but very differently.
In a turbofan, the turbine spends most of its effort turning the compressor blades. In a modern turbofan (TF) engine like GE 9x, you are looking at a 80-20 bypass ratio. This means, 80% of the air drawn from the front just flows through. Now, as the "FAN" (duh!) rotates, a huge volume of air is drawn in and compressed. As the volume moves deeper into the engine, the air pressure increases because the engine tapers down, as well as the compressor works in "compressing" the voume of air. As the air pressure increases, so does the speed and temperature. Now, as this compressed air exits from the back of the engine, it provides a forward thrust. The 20% air that DID get into the core, carrys on the combustion in the engine that provides the energy required to rotate the compressor blades. Now this is definitely simplified as there are many additional pieces, like stator vanes, equipment to shape and correct the velocity of the air, fuel injectors, igniters, afterburners (not in civilian airplanes of course) and many more contraptions.
While not completely comparable, a turbo prop (TP) behaves a lot like its piston counterpart. The turbine is simply replacing a traditional 4 stroke IC engine in this case and just tasked with rotating the blades. Each of the blade is a mini (well not so mini) wing that has the shape of an airfoil. As these rotate, they generate lift but since the rotational axis is parallel to the fuselage, it is actually thrust.
> Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance?
It is definitely cheaper to build a TP. And TPs can take a lot of abuse and needs miniscule maintanance compared to TFs. But if you are talking of raw performance, turbojets (TJ) actually run (fly?) around in circles around TFs. TJs are just incredibly wasteful, inefficient and needs a lot of maintenance.
> Now, as this compressed air exits from the back of the engine, it provides a forward thrust.
This is something you generally don't want to happen in a reaction engine. If you eject above-atmospheric-pressure out of your engine, the expansion takes place downstream, in the wake, and that energy is mostly wasted (some pressure thrust is still created, but it's minuscule). You always want all the expansion to happen inside your engine, in a properly designed propelling nozzle, which should always result in a flow of low-pressure high-speed air leaving the engine. If the nozzle fails to expand the air (choked flow, nozzle too short, wrong shape), you're losing thrust (and efficiency).
- Treat TJ as base model
- TF uses the bypass section to provide additional power to the compressor
- TP generates its thrust from a prop on the shaft, vice the core's output (is this called exhaust?). TJ and TF both use the core's output for thrust.
https://en.m.wikipedia.org/wiki/Propulsive_efficiency#/media...
By contrast, the cowling around a turbofan prevents such vortices from being created. Ducted fans are, in general, a more efficient design because of this.
Figure of merit is the bypass ratio. Props are 50-100, fans are 3-10.
So big advantage of a turboprop is a much higher bypass ratio than fans. Downside is props can't spin as fast and so need a high speed gearbox to drive them. And props loose efficiency at high speeds.
I think for large fast aircraft turbofans are a win. For medium sized slower aircraft turboprops are a win.