Bah, nm. Too drunk to make sense right now.
Cheers!
Wooo!
I got about halfway through my post when I realized I had no idea what I was actually responding to.
Edit: now if I could only do that while sober…
Not exactly sure here, but I would guess a turbofan, being ducted, uses the thrust fan to also pull in the non-bypass air that feeds the turbine, whereas the turboprop is a propeller driven by a turbine engine that are not directly flow-integrated with each other and the turbine may or may not be designed to produce significant thrust.
Said another way, pure turbojets and pure turboprops started out very different once you got past the identical way they make the hot fire.
Ever since they’ve slowly gotten closer and closer together through a process of convergent evolution. Here’s a continuum to consider:
Turboprops mutating towards the middle:
The eventual future common descendant of both will be similar to:
Turbojets mutating towards the middle but shown in reverse order:
Turbines produce more than the minimum power necessary to drive a compressor. All of them do, if they didn’t pump some additional air for cooling they’d melt. A turbofan is just a descriptive term for engines that produce lots of additional air using a ducted fan. The blades you see in the front of a commercial airline jet engines are the fan. The outer part of the fan produces high speed bypass air that help cool the engine and produce additional thrust. The inner portion of the fan begins the acceleration of air into the compressor.
I think that’s it- basically turboprops are propeller driven planes that happen to use a gas turbine to drive the propeller, but that don’t add significant thrust from their exhaust. It’s similar in concept to the way that turboshafts power helicopters with gas turbines- the forward thrust is from the rotor, but the rotor is powered by a gas turbine, whose exhaust isn’t relevant.
Turbofans are basically a turbojet driving a large ducted fan, but a significant amount of thrust is still derived from the exhaust.
Jet engines are pretty simple, mechanically. Not simple to make.
Or rather they are depending on the use.
A jet engine can be made in a workshop. (See the mechanical simplicity bit). Now it won’t be very useful for much (except maybe small fast suicide drones, which is a concern), but it will do some real work.
A middle-level industrial power can make a jet engine good enough to power a cruise missile. One time use only, so its not as difficult. We see real world examples of this.
If said power spends a bit more time and effort it could probably make a jet engine with performance equivalent to modern ones…except there will be a trade-off somewhere. Such a high-performance engine made will probably need an overhaul every 100 hours or so and have a useful life of 500 hours (about equivalent to early 50’s engines), compared to about 3-4000 hours for modern engines. Anything with longer-lasting ability will have lower performance. Maybe if you are inclined and are under sanctions you might take it for your military jets, especially fighters but that would be horribly inefficient for commercial airlines.
The old saw about “the devil is in the details” applies in abundance here with respect to whether it’s “easy” or “difficult” to build a turbine or jet engine, and involves complicated tradeoffs of cost, durability, and efficiency. @LSLGuy provided many excellent references in post #45. The GE36 is a particularly interesting example that truly blurs the distinction between a turboprop and a jet. Those peculiar blades are definitely a cross between a turbine-driven propeller and the fan and initial compressor stage of a turbojet. That type of engine has been variously known as an “unducted fan” (UDF), propfan, or ultra-high bypass (UHB) turbojet.
I was long under the impression that turboprops were widely used in passenger aircraft for a time as piston-engine replacements because the technology just wasn’t there yet for pure turbojets. In some respect this must have been true in terms of important criteria like those three I mentioned, but jet engines nevertheless could definitely be built at the time. An interesting case in point is the Vickers Viscount, the first commercial turboprop airliner – but by no means the last. It was the first plane I ever flew in as a kid – Air Canada had many of them, and later also had the larger and newer Vickers Vanguards, also turboprops. Yet it turns out that even back when the Viscount was a turboprop innovation and most airliners were still piston-powered, there was a pure turboject version of the Viscount, the Type 663 Tay Viscount (scroll down in the above link), but it was only built as a demonstration prototype and never went into production.
At one point, Chrysler believed that turbines could be cheap enough to be cost-effective and reliable replacements for piston engines, hence the Chrysler Turbine car of 1963-64. It was more than just an ordinary prototype as a total of 55 were built, and 50 were lent out for public use. It was a bit of a weird turbine design, with ignition in the combustion chamber accomplished by a single spark plug instead of the traditional “flameholder”, but it was a true turbine in every sense. There was further development of the technology but it never went into production for a variety of reasons, efficiency being a big one. Turbines are well-suited to high power requirements but generally don’t scale down very well. The Chrysler turbine had a number of deficiencies including poor mileage and acceleration lag – the classic “spooling up” delay of turbines and jets.
Continuing the bit about convergent evolution …
The GE36 was purely an experiment, but it was indicative of where they’re going.
The Russians arrived in almost the same place from the other end of the pipeline with the Progress D-27 which was a fully production engine not just an experiment. The engine seems to be a good product by Russian standards, but sadly the aircraft it was mated to is a bit of a casualty of the general post-Soviet Russian /Ukrainian quagmire. SO it remains an oddity in aviation.
You’ll notice the blade assemblies almost look interchangeable between the two machines. IOW, the evolution has almost fully converged.
A subsequent experiment funded by the EU was done by the French manufacturer Safran. See a bunch of cool vids here:
https://www.youtube.com/results?search_query=safran+open+rotor
In very broad strokes there were two overlapping issues. As you say, pure jets = turbojets were technologically immature and very inefficient.
But the other issue was that pure jet engines need fast airplanes. They are double-stupid inefficient at pushing slow airplanes at lower altitudes. The immediate post-WWII airplane design repertoire was limited to straight-winged low speed designs. The small size & poor condition of the world airports didn’t help.
So a turboprop was a good engine to install on a low speed airplane. It had better performance and less vibration than a recip. At first the reliability of early turboprops wasn’t real stellar, but neither was the reliability of the very last stretched-to-the-limits recips.
But in just a very few years, the world learned how to design swept wings and operate safely in the atmosphere above roughly 25,000 feet.
Once those airplanes could be built and equipped with equally (un-)reliable turbojets, and as bigger & better major city airports were built, the turboprops were doomed to short-haul and lesser markets. Too slow, too vibrational, and too much cruising in the weather to compete.
Which is why turboprops today are still found in the ranks of the “regional airliners” and especially so in second & third tier countries.
IOW, they were perfected just in time to be quickly obsoleted. A fate shared by a lot of V2.0 inventions in areas with fast-moving progress. V3.0 eats their lunch before V2.0 knows what hit them.
Do you have any comparisons to link to on this? I would think early turboprops were less prone to compressor stalls by their nature than jet engines. Recips at their finest were always going to be low time TBO engines. A B-29 Wright 3350 had a TBO time of 3,500 hrs. That’s the engine used on the Lockheed Constellation. I can’t find any numbers for early turboprops but an overhaul time of 3500 hrs would be the time to beat.
Turbo fans have a shrouded many-bladed fan.
Turbo props have an unshrouded, low-blade-count propeller
Turbo rockets have a proportion of the the thrust provided by expelling hot air and fuel out the back: all turbo engines do this to a small extent, but turbo fans and turbo props use most of the energy to turn the turbine.
Turbo jets are named, AFAIK, from the idea that early turbo jets were turbo rockets, not from using jets in their turbines, but I could be wrong: (I don’t know much about turbines but some of them use internal jets that are called “jets” in the component list). However, my understanding is that for normal passenger jets, the amount of thrust from rocket propulsion is small, both for turbo-fans and turbo-props.
Can’t say that I’ve ever heard the term “turbo-rockets” before. Just turbojets, turbofans, turboshafts, and turboprops.
Turbojets get all their thrust from the turbine exhaust, turbofans have a non-trivial amount of thrust (15-25%) from the turbine exhaust since it’s basically a turbojet powering a larger ducted fan, but turboprops and turboshafts don’t get an appreciable amount from the turbine exhaust. In many cases, the exhaust is ducted out the side or something like that.
Some turboprops still get a non-trivial amount of thrust from the exhaust. I’m struggling to find much in the way of specific cites, but I was told it’s around 10% for the PW123 fitted to Dash 8 aircraft and the cites I’ve found seem to back that up. You are correct that some turboprops have the exhaust pointing sideways, I assume these have a much smaller amount of exhaust thrust.
And from here:
The propeller of a typical turboprop engine is responsible for roughly 90 percent of the total thrust under sea level conditions on a standard day.
One turbine engine design feature is recuperation, in which the intake air is pulled across the exhaust pipes to recapture the waste heat (hot air compresses more readily, IIUC. Recuperation can double the fuel efficiency.
I didn’t know that. Thanks.
About the closest thing to what you’re describing is the J-58 engine in A-12 and SR-71 aircraft. It’s something of a hybrid that is sometimes described as a turbo ramjet. At high speeds it takes bleed air from the compressor side and bypasses the turbine side altogether and feeds it directly to the afterburner.
The early turbofans weren’t much wider than turbo jets and can be visually hard to tell them apart. Look at the 707 and DC8. What’s interesting are the engines on the Convair 880 and 990. The fan section is in the back of the engine and not the front.
That could make sense if they wanted to gear the compressor lower (faster) than the turbine and the turbine speed was more suitable for the fan.
The engine on the Convair 880 is a turbojet, but the 990 had a turbofan with the fan at the rear.
The Convair 990 used CJ805-23 engines. These were a modified version of the CJ805-3 turbojet that powered the Convair 880.
It’s not so much that they wanted the compressor stage to be faster, rather that it happened to be too fast on the unmodified engine. Their solution was to bolt on an extra turbine stage with the fan section at the back as you correctly surmised.