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Why don't most military transports use modern commercial engines?

More specifically planes like C-17, the Antonov's use older less powerful engines while modern efficient and powerful engines are available in the commercial sector.

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    $\begingroup$ mainly because their missions are different (cruise and landing speed constraints, runway pavement, reliability after being shot,... are not the same) $\endgroup$ – Manu H Mar 26 '18 at 11:52
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    $\begingroup$ Are you going to fund the upgrade? $\endgroup$ – KorvinStarmast Mar 26 '18 at 12:35
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    $\begingroup$ Not sure about US military but to some other countries' military, acquiring commercial engines for military use, with a reliable supply of both engines and parts, is hard. E.g. if China and Russia can source modern commercial engines independently and reliably, then then they would very much like to use them on their military transports, or even bombers. $\endgroup$ – user3528438 Mar 26 '18 at 12:47
  • $\begingroup$ @KorvinStarmast Upgrades of that sort tend to be self-funding... $\endgroup$ – Harper Mar 26 '18 at 19:24
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Some do; the C-17 you mention uses the Pratt & Whitney PW2000 which is also used on the Boeing 757. The USAF and USN also fly the C-40 which is the military variant of the 737 and carries the same engines.

Generally the answer is that military aircraft are built on spec for the military which is a bit different than the way the civilian market works. Civilian builders shop a design around to airlines to get some deposits then build the airframes (the notable exception being the 747 which was largely the idea of Pan-Am CEO Juan Trippe). This often causes there to be new aircraft every few years on the civilian side as Boeing needs to stay in business... On the contrary the military will put out a request for designs to meet their needs, evaluate the submissions and chose what is to be built. They can do this as often or as little as they so please (or as budget allows). If there is no active conflict or need for new tech then the military will keep flying what they have. Many military aircraft have cutting edge tech from the era they were designed in, they may however be dated by todays standards.

Much like the FAA here in the US the various military branches have their own certification programs and regulations thus upgrading and bringing in new gear may require something to be approved this can unfortunately stifle new technology being included due to high time and cost of approval. Similarly military requirements may be different, while a civilian airframe may suffice they may have takeoff length requirements or something similar that necessitate more thrust than their civilian counterpart thus variances in outfitting may occur.

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    $\begingroup$ Is it relevant to bring in differing cost pressures? My (perhaps mistaken) understanding is that militaries have much less problems with regular operating budget than extraordinary budgets, such as for upgrading or new acquisitions; whereas commercial airlines are much more sensitive to operating costs, and consequently, commercial airlines are more willing to pay for fleet upgrades more regularly than militaries. Militaries have a harder time justifying new acquisitions if their expected threat/mission profiles haven't changed much, as you pointed out. $\endgroup$ – aerobot Mar 26 '18 at 15:26
  • $\begingroup$ Nitpick. Is the 757 "modern"? It first flew nearly 40 years ago and has been out of production for nearly 15. $\endgroup$ – David Richerby Mar 26 '18 at 16:01
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    $\begingroup$ @DavidRicherby It was reasonably modern when the C-17 was built. The C-17 first flew only 8 years after the 757 entered service. $\endgroup$ – reirab Mar 26 '18 at 16:21
  • $\begingroup$ The 747 was actually based on a design for a military aircraft (it came out of Boeing's failed entry in the contest to design and build the strategic airlifter that ended up being Lockheed's C-5). $\endgroup$ – Sean Aug 15 at 22:46
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Most military transports use engines that were modern when the transport was developed. 30 years later, the engines are outdated, but buying 4 new engines costs in the region of $100 million (and that's not counting the redesign and recertification) so that's rarely, if ever, done.

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  • $\begingroup$ @DavidRicherby It's the redesign and recertification that makes it cost-prohibitive. This is even worse for a small air force with only 5 planes, as the fuel savings for the 5 frames would never come close to the up-front costs. The only way it would ever make sense for a small air force would be if a larger one had already absorbed the up-front R&D and recertification costs. $\endgroup$ – reirab Mar 26 '18 at 16:47
  • $\begingroup$ @reirab But it turns out that the $100M claim is actually per-plane, as the answer has now been edited. $\endgroup$ – David Richerby Mar 26 '18 at 17:01
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There are several reasons, although all of them can be reduced to a single one: they have different requirements:

  • Military aircraft are most of part of their life on idle, in a hangar. We do not want to use military aircraft; we only use them when there is a conflict. That kills any economical trade-off of fuel savings for example.
  • Mission of military aircraft is extremely different. No discussion about fighters as they are flying at higher Mach number (where commercial airplanes engines are not useful), concerning transport aircraft the commercial one are optimized for fuel saving that creates an expensive engine that is amortized with fuel savings along the life of the airplane and transport military airplane fly very often.
  • There are requirements from military aircraft not satisfied by commercial engines, like for example, being able to land in sand or bumpy terrain. Engines from military aircraft are protected against that.

Saying that, there are some commonalities that engine manufacturers try to achieve, but that can be found at component level (i.e. having the same turbine for some stages).

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    $\begingroup$ "we only use them when there is a conflict" ahem, if you would do that, then the conflict would be a bit one-sided, since your side would not know how to use those aircraft. $\endgroup$ – Federico Mar 26 '18 at 13:05
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    $\begingroup$ Military planes don't just sit in hangars while there's no war on. They're used for training. A lot. $\endgroup$ – David Richerby Mar 26 '18 at 17:04
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    $\begingroup$ Can confirm, I live near an air force base, they're out there almost every day. $\endgroup$ – C_Elegans Mar 26 '18 at 17:27
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    $\begingroup$ @DavidRicherby: And for humanitarian missions and general heavy airlift, in the case of military transports (i.e., the ones that start with a C). $\endgroup$ – Sean Mar 26 '18 at 18:08
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    $\begingroup$ Yeah, military transports definitely don't just sit in a hangar until a conflict. However, it's true that they don't accumulate cycles and time as quickly as their civilian counterparts. Airliners are typically kept in the air as much as reasonably possible, as airliners sitting on the ground generate no revenue. $\endgroup$ – reirab Mar 26 '18 at 21:22

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