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In aeronautics, redundancy is of primary concern. Having at least two engines is common for civilian aircraft. I expect the military to have at least the same level of redundancy. Moreover military aircrafts are prone to other failure modes such as bullet in the airframe over a territory in which they cannot easily land (e.g. mountains or desert in which pilots can be captured).

I will restrict this question to jet fighters but I think it can be extended to any military aircraft.

I expect modern fighters to have at least two jet engines for redundancy. This is the case for many of them (such as the F-22 Raptor, the Dassault Rafale, the Eurofighter Typhoon, the Su-27 family, the F/A-18 Hornet, and others).

Yet, some modern fighters have only one jet engine (for instance the F-16 Fighting Falcon, the Saab JAS 39 Gripen, the F-35 Lightning II, and the Dassault Mirage 2000). Thus I imagine there are good reasons to abandon engine redundancy, but I fail to see what reasons can outweigh such a feature, especially for military aircraft.

What are the reasons behind the design of a modern jet fighter with only one jet engine?

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    $\begingroup$ There simply aren't many engine options for the class of the airplane. Like, if at the time of F16 there were a good engine with half a thrust, then YF17 would have won the bid. Does F35 have a choice, with F404 derivatives maybe? Not really. Could Chengdu J10 be made with 2 RD33s? No without throwing away a lot of performance. Reality is, the twin engine heavy fighters are the top performers and consumes huge amount of resource to develop, so if you want to develop a lighter fighter afterwards, you have a better bet by reusing its engine (core). $\endgroup$ – user3528438 Nov 24 '19 at 17:33
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    $\begingroup$ The safety "redundancy" of jet fighters is covered by the ejection seat $\endgroup$ – slebetman Nov 25 '19 at 2:16
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    $\begingroup$ @slebetman a fifth gen fighter is an expensive thing to lose because of a ‘simple’ engine out scenario.... therefore it’s more about the trade offs (reduced purchase costs, reduced maintenance etc) than “oh, redundancy is covered because we can save the pilot” ;) $\endgroup$ – Moo Nov 25 '19 at 5:56
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    $\begingroup$ @vsz The Netherlands have used F16 as ground attack aircraft (it was their only jet fighter until very recently) and are used offensively abroad, even in mountainous areas (including in recent Middle East conflicts). It's dangerous, sure, but your comment seems to indicate it's simply not done. It is. Often enough. $\endgroup$ – Mast Nov 25 '19 at 18:31
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    $\begingroup$ @vsz That's just untrue. The F-16 even had a close support variant that got cancelled in favour of retaining the A-10 wings instead. There have been other close support variants of the F-16 at various times, and they've certainly been employed in close support roles by various air forces at various times (including by the US during Desert Storm). $\endgroup$ – J... Nov 25 '19 at 20:43
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Fighters don't carry passengers.

The figure of merit for combat aircraft isn't passenger-miles flown between accidents, it's objectives completed (like enemy targets destroyed) per billion dollars spent. Adding redundant engines only improves this figure if single engine failures are very common.

Today, military engines have gotten much closer to civilian ones in reliability than before, and their failures account for only a small proportion of total losses. Single engines are more efficient, cost-effective, and cheaper to maintain.

In combat, for a fighter-typical near-centerline engine layout, a single destroyed engine is likely to also damage the other engine. This can be avoided, as has been done in the Su-27, by spacing the engines far apart and placing an armor plate between them, but that's even more weight.

Finally, jet fighters have one more safety feature. If an engine fails on takeoff in an airliner, everyone on board needs the plane to either take off or successfully abort the takeoff. A fighter pilot has a third option, eject and hopefully survive. The cost of this is, of course, destroying the aircraft.

This comes down to comparing two costs: that of total engine failure and that of preventing this failure. With modern engines, failures are rare enough that the occasional hull loss due to them is much cheaper than maintaining a second engine in every fighter just in case. This is very different from the 1960s-1970s, when engines were prone to malfunction.

For the most part, twin engines in fighter aircraft are becoming a fallback for when a single engine of sufficient thrust and reliability is not available.

Getting such an engine is a concern of its own - designing an engine powerful enough for a modern top of the line fighter is difficult. Since nothing except a heavy fighter needs that much thrust with supersonic capability, that engine will then only power a single aircraft type (in contrast to smaller engines, which find a variety of uses). So only the largest projects like the JSF can afford a custom high-end engine.

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    $\begingroup$ There are meritorious duties for a combat aircraft besides just destroying enemy targets. Reconnaissance and power projection don't always have measurable results, especially in peace time, but are still things militaries think are worth spending money on. ;) $\endgroup$ – Kapten-N Nov 25 '19 at 11:23
  • $\begingroup$ Military mentality is closer to: maximize targets that don't need to be destroyed because the enemy knows not to mess with you. The word for is capability. $\endgroup$ – candied_orange Nov 25 '19 at 22:28
  • $\begingroup$ Now I'm curious about how are costs counted. Fuel, weapons and aircraft purchase and spare parts is the easy part. Training of all crews and people involved in making the aircraft and airbase work is hard to split between each aircraft type (and even more if you have both single and multi engines fighters operating from a single airbase) $\endgroup$ – Manu H Nov 26 '19 at 7:18
  • $\begingroup$ Ejecting during takeoff? Didn't think it was possible under x thousand feet? $\endgroup$ – Cloud Nov 26 '19 at 10:13
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    $\begingroup$ @Cloud Depending on the ejector seat, x can be as low as zero. See aviation.stackexchange.com/questions/1439/… $\endgroup$ – Martin Bonner supports Monica Nov 26 '19 at 10:38
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Because single engine fighters are substantially cheaper to purchase and operate. Exact figures are hard to obtain, but as an example, an F-15 squadron will spend about 25,000 USD per flight hour whereas an F-16 Squadron spends about 15,000 USD per flight hour. Effectively, you can purchase and operate twice the fighter force with single engine fighters than with multi-engine fighters. This becomes very attractive for land based air forces belonging to countries with small defense budgets to operate the maximum number of combat aircraft per unit of currency spent.

Another major advantage to single engine fighters is maximized performance in a minimal size and weight package. Adding a second engine will produce more thrust but at the cost of greater weight and fuel consumption. This in turn requires more fuel be carried with a heavier airframe, creating a vicious circle for the design of these airplanes. Typically in fighter design you want a specific weight ratio $W_a = W_{af}/(W_{af} + W_f)$ as low as possible. This translates into an airplane which is a nimble, high capacity gas can with as low an empty weight as possible for a given range profile. This is extraordinary difficult to do with a multi engine airframe.

The major advantage to a multi engine fighter is the same advantage that any other multi engine airplane has: they can carry larger payloads with greater system redundancy. The large airframes accommodate big powerful fire control radars for detecting and tracking targets at very great distances, etc. Certain units, particularly the United States Navy, prefer multi engine airplanes for overwater operations for the redundancy of having an additional engine to get you back home. So too is this a design feature of the A-10 attack jet which anticipates enduring battle damage and the additional engine offers a better chance of limping it home if it gets shot up.

While the redundancy of two engines may seem ideal for combat ops, keep in mind an engine failure combined with the compressor and boat tail drag from the dead engine results in an airplane which has lost 50% of its total available engine thrust and 80% of its combat effectiveness. All you could do after an engine failure would be to limp the airplane home to a friendly base - provided the enemy is gracious enough to let you do so without jumping your six and blowing you out of the sky!

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  • $\begingroup$ How about fuel efficiency? $\endgroup$ – bogl Nov 24 '19 at 13:49
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    $\begingroup$ Fuel efficiency isn't often the forefront of military concerns @bogl. $\endgroup$ – GdD Nov 24 '19 at 13:52
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    $\begingroup$ @GdD, you probably refer to cost. How about range? $\endgroup$ – bogl Nov 24 '19 at 14:04
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    $\begingroup$ I agree @bogl, range is a very big deal to the military, and fuel efficiency creates range. $\endgroup$ – Harper - Reinstate Monica Nov 24 '19 at 21:37
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    $\begingroup$ In addition to the valid points noted above, it's worth pointing out that at the relatively low failure rates for aircraft engines (even for military aircraft), adding a second engine just about doubles the probability of an engine failure. Also, a failed engine creates drag, so while actual thrust goes down by 50%, drag is higher so the effective thrust is even lower than that. $\endgroup$ – Peter Duniho Nov 25 '19 at 6:12
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The other answers address the question "Why 1 engine?" but this begs the question: "Why do some fighters have 2 engines?" as actually, 1 engine is the norm for fighter aircraft. Note the list of 2 engine aircraft in the OP:

F-22 Raptor, the Dassault Rafale, the Eurofighter Typhoon, the Su-27 family, the F/A-18 Hornet

With the exception of the F/A-18 these are air superiority fighters designed to outmanoeuvre, outrun and intercept enemy aircraft. Performance is more important than cost effectiveness here, reliability is not a major concern. The F/A-18 is a different story with the twin engine design chosen by the US Navy to support a wider variety of missions and higher level of readiness. A second engine normally reduces reliability but this was factored into the design of the GE F404 engines which were built for maintainability and reliability rather than performance.

For each of these aircraft, cost is not the driving factor though an advantage of having the extra power available is that they perform well as multi-role jets reducing the need for buying multiple airframes and all of the additional costs that comes with that (such as support, training, facilities).

Single engine fighters are the norm and provide a good balance of cost/performance with twin engine fighters only used when performance is much more important than cost or if the alternative is a small number of multiple airframes. There are exceptions of course such as the F-35 but there the high cost comes from the technology rather than the hardware.

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  • $\begingroup$ You may rephrase your last paragraph. Is the hardware not a piece of technology? $\endgroup$ – Manu H Nov 25 '19 at 14:05
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    $\begingroup$ Most importantly, the EF-2000 or the Rafale are peace time aircraft. Designed for training in peacetime when the public will be highly critical of a high accident rate. Compare that with the attitude 70 years ago when a British Parliament inquiry in 1947 was answered along the line "a 10% loss rate per year in peace operations is perfectly normal". Things have definitely changed since then. $\endgroup$ – Peter Kämpf Nov 25 '19 at 19:48
  • $\begingroup$ @Manu H: What he means is development cost (technology) vs production cost (hardware). $\endgroup$ – Dakkaron Nov 26 '19 at 7:53
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The result of being shot up changed a lot for fighter planes over the years. Modern fighter planes rarely get hit by bullets, but rather by missiles. A missile hit usually takes out the whole aircraft, so there are far less instances where having a second engine would help the aircraft to get back home.

With this negating a lot of the reliability benefit of having two engines, the cost of having two engines outweighs the benefit in many cases.

I haven't expounded on the last sentence (cost/benefits) because others have explained that very well already. As to the ratio of bullets vs. missiles, that varies a lot depending on the type of aircraft and mission. Hard numbers will be very hard to come by. An A-10 is a plane that mostly flies slow and low in it's anti-ground missions. This plane will probably receive a lot more bullets, hence the dual engine. An F-16, which was primarily designed as a supersonic air superiority fighter will receive a lot fewer bullets since it flies fast and high. The faster and higher a plane flies, the harder it is to hit with a bullet (or any other ballistic projectile) and the more guided missiles need to be used to bring it down.

I googled a bit and every instance but one of an F-16 that got shot down was by some kind of missile. The only one I found of an F-16 that was shot down by bullets (https://www.popularmechanics.com/military/aviation/a27078461/dutch-f-16-flew-into-its-own-gunfire/) was one that flew into it's own gunfire. I am sure there are other instances of F-16 being brought down by gunfire, but from the sample I found, the overwhelming majority were missiles.

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    $\begingroup$ Also, a two engine design could actually worsen reliability here - twice as likely to have an engine problem, and in combat one engine down might mean you are effectively out of combat. $\endgroup$ – rackandboneman Nov 26 '19 at 21:00
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Two engines are better than one for safety, but one engile is cheaper to develop and maintain. And these days modern engines do give you more bang for your buck so you don't need as many. Plus you get more missiles/bombs/etc. per kilo with a single modern powerful engine.

US pilots initially hated the idea of the F16 because it was small and had one engine. Look at most fighters of the same era, big and twin engines. But the F16 has since proved to be a very capable fighter.

The primary concern is the mission, second the pilot and last the plane. As long as you can get in and bomb, shoot, blow up or generally give your enemy a bad day then its job done. Getting the pilot and/or plane back home is a lesser concern.

I don't really get the rationale about 5th Gen fighters. Because they simply aren't that good. I mean a 5th gen fighter is only good against and 4th (or less) gen fighter. You couldn't get into a fight (especially with a decent 4th gen fighter) because you could't afford to lose one, they are such an expensive asset. And they are no good in a turning fight against older type specific aircraft. They seem to be build to kill you enemy from a long way off without getting into a close-up turning fight. Surely drones would be a much better way to go?

It's well versed that most military pilots are interested in the next newest shiny thing rather that its capability as a true out and out fighter or anything else. I'm thinking that if ever an F35 (say) end's up in a serious conflict, pilots will find to their cost that its sadly lacking.

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  • $\begingroup$ You are correct my answer was a bit big to go as a comment so I added it as an answer but I will add to it to answer the question $\endgroup$ – djack109 Nov 26 '19 at 13:28
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    $\begingroup$ The first two paragraphs address the question; the remainder is off-topic editorial that doesn't belong here. $\endgroup$ – Ralph J Nov 26 '19 at 15:15

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