# Why do turbine (jet) engines cost so much more than piston engines? **URL:** \ **Category:** Factual Questions\ **Created:** [September 30, 2020, 5:04am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867 "2020-09-30T05:04:58Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [October 3, 2020, 6:09am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/41 "2020-10-03T06:09:55Z")
Bah, nm. Too drunk to make sense right now. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 3, 2020, 6:17am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/42 "2020-10-03T06:17:31Z")
Cheers! ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [October 3, 2020, 6:34am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/43 "2020-10-03T06:34:13Z")
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… ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 3, 2020, 2:41pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/44 "2020-10-03T14:41:12Z")
> [@Whack-a-Mole](#): > > I’ve been curious as to the differences between a turbofan and a turboprop engine. 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. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 3, 2020, 2:52pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/45 "2020-10-03T14:52:47Z")
> [@Whack-a-Mole](#): > > In the end they seem variations on the same theme despite them look so different. 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: > **[Allison T56](https://en.wikipedia.org/wiki/Allison_T56)** > > The Allison T56 is an American single-shaft, modular design military turboprop with a 14-stage axial flow compressor driven by a four-stage turbine. It was originally developed by the Allison Engine Company for the Lockheed C-130 Hercules transport entering production in 1954. It has been a Rolls-Royce product since 1995 when Allison was acquired by Rolls-Royce. The commercial version is designated 501-D. Over 18,000 engines have been produced since 1954, logging over 200 million flying hours.... > **[Rolls-Royce AE 2100](https://en.wikipedia.org/wiki/Rolls-Royce_AE_2100)** > > The Rolls-Royce AE 2100 is a turboprop developed by Allison Engine Company, now part of Rolls-Royce North America. The engine was originally known as the GMA 2100, when Allison was a division of former corporate parent General Motors. > On July 11, 1989, Saab-Scania A.B. selected the GMA 2100 to power its new Saab 2000, a 50-seat stretch of the Saab 340 turboprop, in a US$500 million deal. In July 1990, Industri Pesawat Terbang Nusantara (IPTN) of Indonesia picked the GMA 2100 as the engine for... > **[Europrop TP400](https://en.wikipedia.org/wiki/Europrop_TP400)** > > The Europrop International TP400-D6 is an 11,000 shp (8,200 kW) powerplant, developed and produced by Europrop International for the Airbus A400M Atlas military transport aircraft. The TP400 is the most powerful turboprop in service using a single propeller; only the Kuznetsov NK-12 from Russia and Progress D-27 from Ukraine, using contra-rotating propellers, is larger. > The TP400 was originally selected by Airbus Military to power the A400M in December 2000. However, Airbus reopened the engine... The eventual future common descendant of both will be similar to: > **[General Electric GE36](https://en.wikipedia.org/wiki/General_Electric_GE36)** > > The General Electric GE36 was an experimental aircraft engine, a hybrid between a turbofan and a turboprop, known as an unducted fan (UDF) or propfan. The GE36 was developed by General Electric Aircraft Engines, with its CFM International equal partner Snecma taking a 35 percent share of development. Development was cancelled in 1989. > General Electric (GE) started performing studies and component test work on the concept that would become the UDF in 1981, based on the initial results of early Na... Turbojets mutating towards the middle but shown in reverse order: > **[Pratt & Whitney PW1000G](https://en.wikipedia.org/wiki/Pratt_%26_Whitney_PW1000G)** > > The Pratt & Whitney PW1000G, also called the GTF (geared turbofan), is a high-bypass geared turbofan engine family produced by Pratt & Whitney. > After many demonstrators, the program was launched with the PW1200G on the Mitsubishi SpaceJet (later cancelled) in March 2008, first flight tested in July 2008. > The first variant to be certified was the PW1500G for the Airbus A220 in February 2013. > The program cost is estimated at $10 billion. > The gearbox between the fan and the low-pressure spool allow... > **[CFM International CFM56](https://en.wikipedia.org/wiki/CFM_International_CFM56)** > > The CFM International CFM56 (U.S. military designation F108) series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of 18,500 to 34,000 lbf (82 to 150 kN). CFMI is a 50–50 joint-owned company of Safran Aircraft Engines (formerly known as Snecma) of France, and GE Aerospace (GE) of the United States. GE produces the high-pressure compressor, combustor, and high-pressure turbine, Safran manufactures the fan, gearbox, exhau... > **[Pratt & Whitney JT9D](https://en.wikipedia.org/wiki/Pratt_%26_Whitney_JT9D)** > > The Pratt & Whitney JT9D engine was the first high bypass ratio jet engine to power a wide-body airliner. Its initial application was the Boeing 747-100, the original "Jumbo Jet". It was Pratt & Whitney's first high-bypass-ratio turbofan. > The JT9D program was launched in September 1965 and the first engine was tested in December 1966. > It received its FAA certification in May 1969 and entered service in January 1970 on the Boeing 747. > It subsequently powered the Boeing 767, Airbus A300 and Airbus... > **[Pratt & Whitney JT8D](https://en.wikipedia.org/wiki/Pratt_%26_Whitney_JT8D)** > > The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered the US Navy A-6 Intruder and A-4 Skyhawk attack aircraft. Eight models comprise the JT8D standard engine family, covering the thrust range from 12,250 to 17,400 pounds-force (54 to 77 kN), and power the 727, 737-100/200, and DC-9. The updated JT8D-200 family, c... > **[Pratt & Whitney JT3D](https://en.wikipedia.org/wiki/Pratt_%26_Whitney_JT3D)** > > The Pratt & Whitney JT3D is an early turbofan aircraft engine derived from the Pratt & Whitney JT3C. It was first run in 1958 and was first flown in 1959 under a B-45 Tornado test aircraft. Over 8,000 JT3Ds were produced between 1959 and 1985. Most JT3D engines still in service today are used on military aircraft, where the engine is referred to by its US military designation of TF33. > Aware of the competition from the Rolls-Royce Conway turbofan, Pratt & Whitney decided to develop the JT3D turb... > **[Pratt & Whitney J57](https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J57)** > > The Pratt & Whitney J57 (company designation: JT3C) is an axial-flow turbojet engine developed by Pratt & Whitney in the early 1950s. The J57 (first run January 1950) was the first 10,000 lbf (45 kN) thrust class engine in the United States. The J57/JT3C was developed into the J52 turbojet, the J75/JT4A turbojet, the JT3D/TF33 turbofan, and the XT57 turboprop (of which only one was built). The J57 and JT3C saw extensive use on fighter jets, jetliners, and bombers for many decades. > The J57 was a ... ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 3, 2020, 3:15pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/46 "2020-10-03T15:15:46Z")
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. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [October 5, 2020, 3:10pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/47 "2020-10-05T15:10:21Z")
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. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [October 6, 2020, 6:16am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/48 "2020-10-06T06:16:43Z")
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. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [October 7, 2020, 12:59am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/49 "2020-10-07T00:59:36Z")
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**](https://en.wikipedia.org/wiki/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. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 7, 2020, 3:22pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/50 "2020-10-07T15:22:18Z")
Continuing the bit about convergent evolution … The GE36 was purely an experiment, but it was indicative of where they’re going. > **[General Electric GE36](https://en.wikipedia.org/wiki/General_Electric_GE36)** > > Pages for logged out editors learn more > > The General Electric GE36 was an experimental aircraft engine, a hybrid between a turbofan and a turboprop, known as an unducted fan (UDF) or propfan. The GE36 was developed by General Electric Aircraft Engines, with its CFM International equal partner Snecma taking a 35 percent share of development. Development was cancelled in 1989. > General Electric (GE) started performing studies and component test work on the concept that would become the UDF... 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. > **[Progress D-27](https://en.wikipedia.org/wiki/Progress_D-27)** > > Pages for logged out editors learn more > > The Progress D-27 is a three-shaft propfan engine developed by Ivchenko Progress. The gas generator was designed using experience from the Lotarev D-36 turbofan. The D-27 engine was designed to power more-efficient passenger aircraft such as the abandoned Yakovlev Yak-46 project, and it was chosen for the Antonov An-70 military transport aircraft. As of 2019, the D-27 is the only contra-rotating propfan engine to enter service. > The engine was lau... 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](https://www.youtube.com/results?search_query=safran+open+rotor) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 7, 2020, 3:33pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/51 "2020-10-07T15:33:00Z")
> [@wolfpup](#): > > 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 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. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [January 10, 2021, 10:05pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/52 "2021-01-10T22:05:38Z")
> [@LSLGuy](#): > > 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. 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. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [January 10, 2021, 11:53pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/53 "2021-01-10T23:53:02Z")
> [@bump](#): > > 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 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. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [January 11, 2021, 1:39am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/54 "2021-01-11T01:39:42Z")
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. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 11, 2021, 4:08am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/55 "2021-01-11T04:08:21Z")
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. > **[Turboprop | Technological aspects](https://en.wikipedia.org/wiki/Turboprop#Technological_aspects)** > > Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (beyond that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low. Consequently, the exhaust jet produces about 10% of the total thrust. A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds. > Turboprops have bypass ratios of 50–... And from [here](https://www.skybrary.aero/index.php/AP4ATCO_-_Turboprop_Engine): > 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. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [January 11, 2021, 4:30am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/56 "2021-01-11T04:30:11Z")
> [@Richard\_Pearse](#): > > some turboprops have the exhaust pointing sideways, I assume these have a much smaller amount of exhaust thrust 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. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 11, 2021, 6:58am UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/57 "2021-01-11T06:58:26Z")
I didn’t know that. Thanks. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [January 11, 2021, 7:17pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/58 "2021-01-11T19:17:55Z")
> [@Melbourne](#): > > Turbo _jets_ are named, AFAIK, from the idea that early turbo jets were turbo rockets, 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. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [January 11, 2021, 7:58pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/59 "2021-01-11T19:58:08Z")
> [@Magiver](#): > > 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. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [January 11, 2021, 9:39pm UTC](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867/60 "2021-01-11T21:39:09Z")
> [@Magiver](#): > > 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. The engine on the Convair 880 is a turbojet, but the 990 had a turbofan with the fan at the rear. > [@eschereal](#): > > 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 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. > **[General Electric CJ805 | CJ805-23](https://en.wikipedia.org/wiki/General_Electric_CJ805#CJ805-23)** > > Several airlines asked Convair for a larger version of the 880 with potential transatlantic range. Such a design would be larger to hold more seating, as well as having to carry more fuel. To power it, a more powerful engine would be needed. By this time, the Rolls-Royce Conway was entering service, and the Pratt & Whitney JT3D was following close behind. These designs both had twin-spool compressors, as opposed to using variable stators, and the lower speed of the front, low-pressure, spool made... [Previous page](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867.md?page=2) [Next page](https://boards.straightdope.com/t/why-do-turbine-jet-engines-cost-so-much-more-than-piston-engines/921867.md?page=4)