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I was wondering why the F-35 engine is exposed. The F-22, F-16 and the YF-23 Black Widow have the exhaust nozzles of their engines blended flush with the fuselage.

(YF-23 Black Widow)

(F-35)

The YF-23 Black Widow clearly has two engines which are indented into the plane. However the F-35 has one engine which is exposed at the back. So I have two questions:

  1. Why does the F-35 have only one engine unlike the older models?
  2. Why is the F-35's engine exhaust nozzle clearly more exposed to bullets and rockets than older models?
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    $\begingroup$ The two questions are not really related to each other. $\endgroup$ – Deer Hunter Oct 31 '15 at 8:34
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    $\begingroup$ Re: #2, Being more exposed to enemy fire is not really relevant. Airplane skin is not armored; in fact it's as thin and light as possible, and does nothing to protect against bullets or fragments. Combat aircraft generally carry no armor whatsoever except for (sometimes) around the cockpit to protect the pilot; the cost in range and maneuverability far outweighs the benefits of increased survivability. $\endgroup$ – Phasma Felis Nov 1 '15 at 8:16
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The F-22 is intended (and the competing YF-23 was intended) to be an air superiority fighter to replace the F-15. The F-15 was a twin-engine model, as was the F-4 Phantom that it replaced. Twin engine planes have excellent "survivability," meaning they can take a lot of hits and still stand a chance of making it home (you can lose an entire engine and still stay airborne). They also are much higher-performance. The F-15 can takeoff, stand on its tail and climb vertically to over 40k feet. I've seen F-16s do something similar but they couldn't climb as high and they couldn't carry as much fuel and/or armaments while doing it.

The F-35 is intended to be a replacement for the single-engine F-16. The F-16 is a capable workhorse but it's more of a budget, multi-role aircraft, meaning it was designed, from the get-go, to do some air superiority AND some ground attack, operating on a tighter budget. We have considerably more F-16s in the Air Force inventory than we've ever had (or will have) F-15s and/or F-22s.

The F-15 used the same engine as the F-16 (two to choose from: either the P&W F100 or the GE F110), but two of them. When I was in the Air Force

  • F-15's cost about USD 50 million
  • F-16's cost about USD 35 million

Two engines and a larger, heavier airframe (albeit less sophisticated) added up to a significant price premium. In air combat, above 25k feet, my money would be on the F-15. At altitudes below 25k feet, my money is on the F-16. With an inherently unstable airframe and a computerized fly-by-wire system to tame it, the F-16 was wickedly maneuverable. The increased power of two engines on the F-15 working against the greater weight really loses the advantage at lower altitudes.

The "buried" engines in the F-22/YF-23 are trying to improve the stealth characteristics. The F-22's thrust vectoring, coupled with twin engines, make it more stealthy than the F-117 and faster and more maneuverable that the F-15 or the F-16. The YF-23 was arguably faster, but less maneuverable.

The F-35 is intended to cost less than the F-22 (debatable as to whether that will be the final case). And certain models of the F-35 are capable of a trick the F-22 simply can't match: STOVL. It can do a Short TakeOff and Vertical Landing. For the latter, the rear nozzle pivots downward 90% and a "lift fan" opens up in the forward fuselage. The F-22 was never intended to be able to do this. "Burying" the rear nozzle AND trying to do that would add a LOT of weight (and additional cost) to the aircraft.

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  • $\begingroup$ debatable whether [the F35 will cost less than the F22] - Just FYI, the incremental cost for an F22 was about \$150 million. F35 prices are still falling. The latest A-models in LRIP lot 9 cost \$98 million flyaway (including engine). It's on track to fall to <\$75million by 2019. Average cost: <\$80 million per F-35A over the entire production run. All prices in FY15\$. $\endgroup$ – Hephaestus Aetnaean Jan 3 '16 at 6:22
  • $\begingroup$ What about the F35-A and -C? The F35-B is only capable of performing STOVL. $\endgroup$ – Hugo Woesthuis Nov 23 '16 at 11:04
  • $\begingroup$ True, only one of the three does STOVL. But, like the F-16, it's intended to be the "lower-cost" model. Adding extra fairings to "bury" the engine would add weight (hurting the performance) and increasing the cost. $\endgroup$ – Meower68 Nov 23 '16 at 15:05
  • $\begingroup$ @HugoWoesthuis - A model doesn't need STOVL. It operates from conventional strips. The C model doesn't need STOVL because it operates from CATOBAR carriers. Neither model should have STOVL. $\endgroup$ – Hephaestus Aetnaean Nov 26 '16 at 6:38
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The answer to both of your questions lies in the vertical takeoff and landing (VTOL) capability of the F-35.

In order to provide vertical thrust, the rear part of the engine rotates downward. This is much easier to accomplish if the engine exhaust is not kept on top of the plane, as in your YF-23 example. This rotating ability is also much easier to accomplish if there is only a single engine. With two engines, they would both have to rotate downward and drive the lift fan together. The Boeing X-32, which was competing against the X-35 for the Joint Strike Fighter contract, also opted for a single engine design for similar reasons.

Your impression that the YF-23 protects the engines more doesn't really apply to things like bullets. Although the engines on a fighter are buried more in the fuselage than a typical modern airliner, they are still not protected much from the top or bottom, regardless of the configuration. If shrapnel is hitting the plane, protecting the engines will not solve all of your problems. You will notice that planes like the A-10 that specifically are designed for withstanding damage do not go to great lengths to hide the engines. Planes like the B-2 and YF-23 hide the engine exhaust for stealth reasons.

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    $\begingroup$ Given man portable heat seeking missiles, reducing the heat signature seen from the ground is how you go about protecting the engines. People don't shoot down planes with bullets that often. $\endgroup$ – Pete Kirkham Oct 31 '15 at 9:06
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Why does the F-35 have only one engine unlike the older models

  • The USMC wanted the aircraft to have S/VTOL capability. This meant that the aircraft should have a single large engine as the best configuration would be to have a single engine with swiveling nozzle aft of center of gravity for supersonic flight.

    Having multiple engines in a VTOL aircraft is problematic- The same reason Harrier and Yak-36 has single engines (Yak-36 had smaller engines purely for takeoff, but had an automatic ejection mechanism). Loss of a single engine in a multiple engine configuration will result in uncommanded roll during (vertical takeoff and landing).

  • The F-35 engine is one of the most most powerful military engines out there. Improvements in engine technology have made the aircraft engines more reliable compared to the older models, so dual engines are not a requirement anymore except in some special cases.

Why is the F-35's engine clearly more exposed to bullets and rockets than older models

  • I think you're talking about the nozzle being visible. All modern combat aircraft engines are buried in the fuselage for various reasons. The nozzle is exposed in case of F-35 as it has to swivel for VTOL.

F-35 VTOL

Source: jeffhead.com

Note that this configuration is similar to Yak-141 (which again had two lift engines), another VTOL aircraft (Lockheed Martin worked with Yakolev during the JSF competition).

  • F-22 nozzle is exposed- This is a result of the thrust vectoring. YF-23 and other stealth aircraft (like B-2) had nozzles above the fuselage to reduce IR signature from below.
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  • $\begingroup$ "Loss of a single engine in a multiple engine configuration will result in uncommanded roll during (vertical takeoff and landing)" - Not if the two engines exhaust through a single nozzle. $\endgroup$ – Sean Sep 22 at 1:04
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The exposed engine outlet nozzle of the F35 is a serious shortcoming of the design, it makes the F35's IR signature much greater than that of the F22 and significantly increases it's vulnerability to IR guided weapons. Cost and weight factors probably played a part but in engineering terms it would be difficult to fully enclose the vectoring nozzle of the F35B version, necessary for VTOL operation.

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