Turbofan

Ever hear about how the Wright brothers didn't publish a book and didn't go to university?

 

The A-10 Warthog demonstrates that turbofans can operate perfectly well down here in the thick air.

http://www.youtube.com/watch?v=RTpPxwjVLg8

 

Film of the now retired English Channel hovercraft. These craft were retired not because they became obsolete as boats, but because TRAINS made them obsolete as a transportation system.

http://www.youtube.com/watch?v=Cyq0M4WhW9s

http://www.youtube.com/watch?v=UhFIUL4173A

Marine Corps hovercraft continue to demonstrate that waterscrews are not the only means of providing sufficient motive power for a large craft.

http://www.youtube.com/watch?v=1m920TRWywU

 

Posts containing insults directed towards other members have been removed - let's please keep the forum polite to other members and not escalate the insults and personal attacks. Thanks.

 

The comment above about "engines designed to operate at lower altitude" indicates a fundamental lack of understanding about the design of turbine engines. I've been in the turbine engine design business for over 40 years and I can tell you absolutely that there is no such thing as an engine designed for low altitudes. Gas turbine engines commonly operate from sea level to altitudes up to 40,000 ft without any real concerns.

The density of the air entering the engine obvioulsy is reduced as the altitude increases, but the engine operates the same (with the exception of Reynolds number effects, which is beyond the scope of this conversation), it only produces less thrust as you go up in altitude

Neither of those particular engines noted above are well suited for powering a low speed vehicle since they are several generations old and are not considered high bypass ratio engines compared to current technology.

The key element in consideration of a turbofan engine for lower speed duty is what is called the bypass ratio. That is, the amount of air bypassing the core of the engine compared to the amount of air passing through the core. The higher the bypass ratio, the better the PROPULSIVE EFFICIENCY, (there's that pesky term again). The reason higher bypass ratios result in better fuel consumption is that they are much more efficient because the move more air at lower velocity than a smilar engine with lower bypass ratio. Aircraft designed for higher speed operation (typically above Mach 1) will have lower bypass ratios, but for aircraft operating up to high subsonic speeds, like commercial aircraft engines, higher bypass ratios will improve fuel consumption. This is why the later generations of engines have much better fuel consumption than older engines, and the next generation of engines will have geared fans that turn slower, and move more air and consequently have better propulisive efficiency and burn less fuel.

The OP in his post (shown above) was really asking if it made sense to use a modern high bypass turbofan engine to push a SWATH (not a very high speed hull), as opposed to more conventional propulsion. His question was basically if he could derate the engine and save fuel, since the fuel consumption of these engines is very high.

Turbine engines actually have their best specific fuel consumpion (fuel burned per thrust generated) at high power. This is because the thermodynamic efficiency of these engines is highest at the the highest power because the engine is running at the highest pressure ratio and cycle temperature at these conditions. Part power fuel consumption is lower, but you burn more fuel per pound of thrust at those conditions, so derating the engine, or picking a bigger engine and running it at lower power is less efficient than picking the right size engine and running it at higher power.

The fundamental problem is that the propulsive efficiency of a turbofan is worse than a turboprop at low speed. You can think of a turboprop as much higher bypass ratio fan engine and you wouldn't be wrong. Up to about .6 or .7 Mach propellers are more efficient than fans, and above that turbofans are more efficient. This is why most air cushion vehicles use props as opposed to fans.

However, we've shown demonstrably that props are a poor way to propel a lower speed boat, and if props are bad, then in this speed regime, fans are even worse. So a turboprop would be a better choice than a fan, but the fuel consumption of a turboprop would likely be twice that of a turboshaft engine driving a propeller, and that would, unless you are talking about a very large and modern engine will burn a good bit more than a diesel. Moreover a turboprop engine has a gearbox, so the difference between a turboprop and a marine engine in terms of complexity isn't as great, and some of the advanges of the prop over the fan go away.

Bottom line is that you can propel a boat with a turbofan, but it's going to burn so much fuel that it doesn't make sense. If you put a turboprop on it, it's not as simple and elegant as a fan, and the prop makes more noise and has other safety drawbacks, and it's going to burn about twice the fuel of a water prop driven by the same engine, so an airscrew is also a poor choice.

It's the old saw of "just because you can do something, it doesn't mean you should".

 

FR,

Please don't send me rude PM's.

If it continues, I'll have to report them to the moderator.

 

Darryl Greenameyer went 1,000 mph (ONE THOUSAND MILES PER HOUR) with a J-79 turbojet engine flying 100 feet (ONE HUNDRED FEET) off the ground. So cut the happy ********** about gas turbine engines giving a crap about what altitude at which they're flying. Up in the thin air the J-79 is a Mach 2 engine. The A-10 Warthog is designed specifically for GROUND ATTACK using TURBOFAN engines.

 

The moderator ALREADY deleted YOUR rude *********. Get your head out out of the weeds and take a look around at the field in which you're playing.

 

214 mph (TWO HUNDRED FOURTEEN MPH) in a quarter mile with an 8,000 lb (EIGHT THOUSAND POUND) vehicle using a J-79 turbojet designed for Mach 2 up in the thin air. http://www.youtube.com/watch?v=mgY6GbWhHCI

 

Modern Marvels episode about the A-10 GROUND ATTACK aircraft using twin TURBOFAN engines. http://www.youtube.com/watch?v=PECNBHphIOc

 

"The key element in consideration of a turbofan engine for lower speed duty is what is called the bypass ratio." Bypass ratio doesn't have shyte to do with limiting a turbofan to low speed duty. A Harrier fighter has a HIGH bypass turbofan engine and can hit 662 mph at altitude.

 

I'm sorry that you don't understand the fundamental relationship between aircraft speed and altitude. Despite producing more thrust at low altitudes, because the aerodynamic drag is also higher, you can't fly as fast at low altitude with the same engine. Moreover, high speed at low altitude results in high engine inlet pressure and higher engine inlet temperatures, which can effectively limit the aircraft speed at lower altitudes. While the A-10 is a ground attack aircraft it is also a LOW SPEED (relatively) aircraft compared to combat jet, and therefore is equipped with a (moderate bypass ratio) fan engine. Faster aircraft use turbojets, and slower aircraft use fans. It has nothing to do with the altitude at which they fly.

 

I'm sorry you don't understand I don't give a crap about your COMPLETE inability to demonstrate the LEAST comprehension of the fact efficiency has nothing to do with the capability to generate brute power and the capability to generate brute power has nothing to do with efficiency. For example, ion rockets are VASTLY more efficient than chemical rockets. A chemical rocket cannot remotely approach the amount of thrust produced per the pounds of propellant consumed per second attainable with an ion rocket. However, an ion rocket cannot lift itself off the ground, let alone lift a payload either out of the atmosphere or into orbit.

 

You are confirming exactly what I have said earlier, this is getting quite exasperating. You continue to not read what I have said, and then interject something that makes no sense. I said you want high bypass ratio engines until you get near Mach 1, and then you want lower bypass ratios if you want to go faster than that. It is not coincidental that the maximum speed of the Harrier is Mach 1.0.

Higher bypass engines have higher inlet drag and are less efficient as the speed increases. The Harrier has a maximum speed of Mach 1 for the simple reason that the drag of the high bypass engine is holding it back at high speed. The Harrier has a thrust to weight ratio of greater than 1, but it isn't nearly as fast as a Phantom that has a much lower thrust to weight ratio. The reason is that the Phantom has turbojets whose thrust increases with increasing speed due to ram effects. The thrust of the higher bypass fan does not increase due to increasing speed due to inlet drag effects.

So, again, you are confirming exactly what I have said earlier, because you don't understand basic propulsion concepts. Higher bypass engines have high drag at high speed, so if you want to go faster than Mach 1, you want a lower bypass ratio engine, or an engine with no bypass, like a turbojet.

 

Up until the new generation of engines used in the F-22/F-35 fighters, pretty much NOBODY went supersonic without using an afterburner -- regardless of whether it was turbojet or low bypass fanjet.

The top speed of a Pegasus-powered high bypass fanjet fighter such as the Harrier could just as easily have been boosted with a technique called duct burning, in which fuel is injected into the cold air behind the fan and ignited.