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Why do VTOL aircraft (F-35 or Harrier) have only one engine? Is it because it is simpler and less expensive? Or what are the reasons?

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    $\begingroup$ In aviation, the answer to the question "why is there only one" is nearly always because "that's all it needs". $\endgroup$ – Simon Sep 20 '15 at 9:26
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    $\begingroup$ In government, the answer to the question "why is there only one" is nearly always because "that's all there was money for". Many F-16's were lost to single-engine failures. Canada expected this and bought the twin-engined CF-18 instead, but not all countries could spare the up-front expense. $\endgroup$ – MSalters Sep 21 '15 at 13:46
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    $\begingroup$ I'll make this a comment since it's not enough info for a full answer, but a single-engine craft is much more stable in an engine failure. If the single engine fails, the craft remains right-side up allowing easy ejection. In a twin-engine craft, the most obvious arrangement of lift vents is similar to the Harrier's, but one front and one back per engine. If an engine fails in that configuration, the aircraft will roll, hard, making ejection risky or fatal. Similar issues of redundancy plagued the Osprey's designers and contributed to the most complex rotor-driven aircraft in the U.S. arsenal. $\endgroup$ – KeithS Sep 21 '15 at 15:02
  • $\begingroup$ The nozzle mechanism looks kind of heavy? $\endgroup$ – user3528438 May 1 '17 at 5:17
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VTOL aircraft can have more than one engine. It is just that the design process of the Harrier and F-35 led to a situation where only one engine was required.

The main challenge in any VTOL aircraft is to have the engine produce thrust in two different phases of flight: during cruise and during takeoff/landing. This basically requires the powerplant on the aircraft to have two different characteristics at those two phases, because

  • The direction of the thrust vector is different in TO/Landing and cruise
  • Amount of thrust required is different; for vertical TO, the thrust should be more than the weight while in cruise, the required thrust is (comparatively) pretty low.

A number of approaches have been tried to solve this problem:

  • Takeoff the aircraft vertically (aka the tail sitter), like Ryan X-13 Vertijet, and then transition to horizontal flight. However, one major problem is the pilot has severe disorientation problems during transition
  • Put extra fans, like the Ryan XV-5 Vertifan. The problem was the large (wing) area taken up by the fan and weight penalties. Ironically, a similar concept has been used in F-35
  • Put extra engines specifically for vertical lift (aka the 'Rolls Royce Concept'), used in Dassault Mirage Balzac. Though this offered (theoretically) better safety, the weight penalty was too much.
  • Optimise the main engine for cruise and deflect the thrust for takeoff and augment with (small) lift engines (aka the tiltjet concept), like the German EWR VJ 101. This reduced the weight penalty and complexity, but reduced safety in the event of an engine failure.

In order to avoid the weight penalty, the best approach is to have no extra engines at all, with the thrust deflected down at take off and landing. This was called 'vectored thrust' (a term attributed to Theodore Von Karman). This concept was tested in the Bell X-14, which had fixed eingines and thrust vectoring vanes that deflected the exhaust to achieve vertical lift.

Bell X-14

Source: history.nasa.gov

In case of the Harrier, the engine, the Bristol Siddeley Pegasus was first developed (as BE.53) and the Hawker aircraft decided to develop an aircraft around it.

During the design proces, the aircraft was redesigned around the Hawker modified BE 53 which the rear jet was split into two, like the Hawker Sea Hawk, which had a similar arrangement. This lead to the 'four poster' thrust vectoring system in the Harrier.

Four poster thrust vectoring system

Source: www.aerospaceweb.org

This is the reason, the Harrier had a single engine.

The case of F-35 is different. The F-35 was designed to replace aircraft operated by a number of US services, and as such had conflicting design requirements. At one end, the USAF wanted a conventional fighter while the USMC wanted a VTOL aircraft. The USN requirements were somewhere in the middle.

Lockheed designers decided that the best way to meet these requirements is to have a separate fan for lifting the aircraft in VTOL, which can simply be eliminated in the USAF and USN variants. This led to the lift fan arrangement (eerily similar to the Vertifan arrangement).

F-35 Lift fan

"F-35B Joint Strike Fighter (thrust vectoring nozzle and lift fan)" by Tosaka - Own work. Licensed under CC BY 3.0 via Commons.

So, in case of F-35 the use of a single engine was the best way to meet the requirements. The engine used, the Pratt & Whitney F135 is the most powerful fighter jet engine in service, which helps in making F-35 the first supersonic VTOL aircraft.

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  • $\begingroup$ Form your answer I can summarize that there was 2 reasons: weight and size. But F-18 has 40.4 fit in width, and F-35 only 35 ft, and 2 engines probably means 2x vertical thrust. So I can assume that a fuel consumption because of weight is the real reason. $\endgroup$ – R S Sep 20 '15 at 11:36
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    $\begingroup$ @RS Are you talking about the wingspan? In that case, please note that the F-35 has heavier than the F-18 and the thrust produced by the F135 in F-35 is greater than the thrust produced by both the F404 engines of F-18 combined $\endgroup$ – aeroalias Sep 20 '15 at 12:03
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    $\begingroup$ @RS not only weight: keep in mind that F-35Bs are designed to land vertically on amphibious assault ships, so, unless the F-35Bs were designed with two engines, each one capable of lifting the entire weight of the aircraft, having two engines is completely pointless. $\endgroup$ – Marco Sanfilippo Sep 21 '15 at 9:22
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Many VTOL aircraft have two engines: Bell 212, EC135, MH-53. There is even one with three engines, the CH-53E. Yes helicopters are aircraft too. These have all engines drive one propulsion device, and that conveniently means there is no change in the location of the thrust point if an engine fails.

As @Ryatt Broach mentions, the V-22 has a complicated system that allows one engine to drive two rotors, and so does the CH-46.

enter image description here

The Harrier could have two engines blow in the one propulsion manifold, and enjoy a bit of redundant propulsion safety as well. But there are a lot of tradeoffs in aircraft design, and the Harrier did pretty well.

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  • $\begingroup$ The mighty Phrog lives, if only in our hearts ... $\endgroup$ – KorvinStarmast May 1 '17 at 14:46
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Don't forget about the V-22. The V-22 has had problems in the military because two engines may cause different amounts of thrust in each engine, So it may cause a problem.

For example a sudden loss of thrust on one side due to a mechanical issue, surge, FOD, etc. The attitude may end up in a position that does not favor an ejection.

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    $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – J. Hougaard May 1 '17 at 7:03
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    $\begingroup$ @J.Hougaard I don't quite understand your flag here. Why don't you think this answers the question? As I read it, the answer here seems to be "because asymmetric thrust can cause big problems during VTOL operations, as the V-22 has demonstrated." $\endgroup$ – reirab May 1 '17 at 15:12
  • $\begingroup$ The V22's two engines are connected together via a driveshaft that runs through the wing, to address that very situation. Doesn't matter if the power output from one engine varies from the other, the single driveshaft connecting them insures that there will be no asymmetric thrust. Also, one engine can supply enough power to operate both rotors. $\endgroup$ – tj1000 Dec 6 '17 at 1:32

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