In the question:Why do VTOL aircraft (F-35 or Harrier) have only one engine? Aeroailas answered why mature STOVL fighter designs only use one main engine, but did not answer why there are no mature twin-engine STOVL fighter designs.

Also in the question Is it possible to modify an AV-8B Harrier II to a supersonic aircraft by applying some modification? He has shown the BS100 engine, adding an aftburner to plenum chamber for supersonic flight.

Carlo Felicione, in his answer to Why does the Harrier jet have four landing gears? , said that the two aft nozzles forced Harriers to adapt bicycle landing gears.

enter image description here Close-up of P.1127 Kestrel (Harrier prototype) shot by Alan Wilson under CC-SA 2.0 licence.

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Layout of the Pegasus. Public domain.

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Power system of Boeing X-32, archived by globalsecurity.org.

The only other mature STOVL system utilizing direct lift is that from X-32. enter image description here Hawker Sea Hawk shot by Smudge 9000 under CC-SA 2.0 licence.

Harriers seem to inherit elements from Sea Hawk, that the exhaust from a single engine is diverted to two side exhausts.

However, now I am even more confused by the design. The following are my questions.

  1. Why does Pegasus engine have two widely seperated aft (hot) nozzles instead of one straight TVN? Even if the thrust loss is negligible, isn't the extra need for strength and heat management increasing weight?

  2. Cont. to Q1, why Harriers place their tail assembly between such nozzles? From the pic above, anything in that tail has to be protected by extra panels, and the width of the it is limited by the separation of the nozzles. Wouldn't it be better to have aft exhausts at side-bottom in the further aft like Yak-38? Between two tail booms like P.1216, Yak-141 and (X)F-35? Sheath the exhaust in a duct like X-32? Or elevate the tail assembly like Phantoms? Any of them will eliminate the need for bicycle landing gears.

  3. Again, why are the two bow (cold) nozzles so widely separated? The air is forced through 90° bypasses after leaving LP compressor, and another 90° before exit. What would be the thrust loss and strength requirement?

  4. Cont. to Q3, why are the two bow nozzles in the same height of aft nozzles? As I see from the pic, the cold exhaust directly blows onto the aft nozzles (or their farings). If it is fine for cold exhaust to zig-zag through the bypasses, wouldn't it be better to travel smoother bypasses (like 135°-135° bents) to the side-bottom, at least to a lower height than the aft nozzles?

  5. Cont. to Q4, or is it possible to just bleed cold exhaust into one TVN at the bottom of fuselage? (The Harriers' gun and ammo pods are located at two sides of the fuselage, so they shouldn't be a hurdle.)

  6. Cont. to Q3, when in level flight, why not retract the bow nozzles, and let cold exhaust flow into a straight bypass around engine core, and mix with hot exhaust to be expelled from the aft? This will streamline the fuselage to better suit supersonic flight. Also this will ease the placement of aftburners, rather than in the cramped plenum chambers.

  7. Why X-32 still uses two awkward-shaped lift nozzles for vertical landing, while it has (unlike Harriers) plenty of space as well as heat-proof linings in the exhaust duct to transform into a single, central, downward TVN?

  8. Harriers have their Pegasi in their CoM to balance when hovering, making two pairs of nozzles rather close to CoM and to each other. Is it possible to have larger separations? E.g. Place engine core and aft nozzle to further aft, and LP compressor and bow nozzles to further bow, only to be connected with a central shaft, so that the extra space around the shaft can be streamlined and holds more fuel and ordnance.

  9. What are the blades in the nozzles called? Are they necessary, as they aren't seen in later generations' designs?

  10. Is it possible to go shaftless in transmitting power from the core engine to the front lift component in any STOVL configurations (as proposed in McDonnell-Douglas' JSF)?

  11. Is it possible to have twin-engine STOVL jets? E.g. combine two Pegasi to build a heavy fighter, somewhat a supersonic Do-31.

  • $\begingroup$ that's a lot of questions in one. please consider splitting this in multiple posts so that the questions can be answered properly without needing to write a book. $\endgroup$
    – Federico
    Jul 10 '21 at 15:33
  • $\begingroup$ @Federico Would splitting questions into many posts be considered as spamming? $\endgroup$ Jul 10 '21 at 16:18
  • $\begingroup$ not if made in a sensible way. you can post a question every x days instead of all at once, for example $\endgroup$
    – Federico
    Jul 10 '21 at 16:27

How about a six-engine supersonic VTOL jet?

VJ 101 in hover

Second prototype of the VJ 101 in hovering flight (picture source).

By using swivelling wingtip-mounted engines, a twin-engine STOVL jet is feasible and it will be much easier to swivel the whole engine, so avoiding the complexity and efficiency losses of sviveling exhausts. The downside is twice the likelihood of an engine failure (which, when happening in hover, will translate into an immediate and unrecoverable roll) plus a much higher roll inertia which results in reduced agility. Wingtip engines were chosen in the hope that the hot exhaust from their afterburning version would be far enough from the landing gear.

Another VTOL jet with more than one engine was the Yakovlev 36 which flew first in 1963. It had two fuselage-mounted engines because no single one at that time was powerful enough to lift an airframe with the desired specifications. Since engine failure in hover meant unrecoverable roll, the series version Yakovlev 38 used a single engine, the development of which had started with the Yak 36. Like the Harrier, it used jet exhausts situated at the sides of the fuselage since a single exhaust would be placed too far away from the center of gravity. Like the Harrier, it could only fly at subsonic speeds because an afterburner in combination with mid-fuselage nozzles is impossible.

Yak 36 in flight

Yak 36 in flight (picture source)

The Kestrel/Harrier concept started with the most efficient engine at its core, adding the remaining parts of the airframe later. This meant a high bypass turbofan (which explains the width of the engine and the spacing of the exhausts). Using two engines of half the size would incur a weight penalty. Remember that the P1127 prototype was only just capable of hovering, with fuel for only 3 minutes in the tanks and 700 lbs of equipment stripped for the first hover tests.

The Pegasus engine started with Wibault's Gyroptère concept, a brochure of which he left with USAF colonel Chapman in Paris in 1956 who subsequently got Stanley Hooker interested in this idea. This meant a VTOL-capable engine was already running in 1959, so the P1127 / Kestrel had the basis for its design already laid out when development started.

The reason why the VJ 101 was never developed into a mature design is politics, plain and simple. While NATO doctrine in the late Fifties and early Sixtes demanded VTOL airplanes (including the planes for supplying forward bases, which explains the Do-31), this was abandoned in the mid-Sixties, leaving the engineers and their prototypes stranded in the middle of the development program. By that time only the Harrier was far enough into its development to make a continuation to a series airplane viable.

I'm sure you will understand that I consider a single answer sufficient to your 11 questions.

  • $\begingroup$ You explained that: 1. It is better to have single large engine instead of two smaller engines; 2. The spacing of cold exhausts; 3. Kestrel/Harrier is built around Pegasus; 4. Other different designs were cut midways. Yet, I believe some questions remain unanswered. $\endgroup$ Jul 10 '21 at 15:45
  • $\begingroup$ Pegasus was inspired by Gyroptere, but Gyroptere had a different principle, closer to (X)F-35 lift fan config. It doesn't answer my questions about: 1. Zig-zagged cold bypass, as the blowers were not fed by front intakes; 2. Split hot exhaust nozzles, as it has a straight one to the aft; 3. The placement of Pagasus around the CoM, as Gyroptere can have lift components widely separated and linked by shafts. $\endgroup$ Jul 10 '21 at 15:57
  • $\begingroup$ I have been avoiding mentioning tilt-engine VTOLs as they are more engineeringly nightmarish. Even wet, the six turbojets of VJ101 cannot surpass one turbofan of Harrier in thrust, and their max VT weight are not comparable. What I asked was a heavy fighter equiped with two Pegasus-like engines and having weight and thrust comparable with (Spey) Phantom. $\endgroup$ Jul 10 '21 at 16:04
  • $\begingroup$ @CrystallizedRefresher Nobody in their right mind would ever use two Pegasus engines if it can be avoided by any means. The Do-31 had no other chance, but fighters would be limited by the largest available engine. They can be twice the weight of a Harrier if this engine is an F-119. $\endgroup$ Jul 12 '21 at 22:18

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