RogerBW's Blog

Yakovlev Yak-38 11 May 2014

The Yak-38 (NATO reporting name "Forger") was the Soviet carrier-borne fixed-wing aircraft of the Cold War.

Its story actually begins in 1960, when the Yakovlev design bureau first proposed the use of two small lift engines coupled with a thrust-vectored conventional main engine. Other engine layouts were tried (including something like a Harrier, a single engine with rotating nozzles, and the Yak-36 demonstrator, with two engines each driving a vectored nozzle on one side of the aircraft), but eventually Yakovlev came back to the idea of separate lift jets.

Enthusiasm for a full order wasn't high until the Kiev-class ships were being planned. To a Western eye these look quite odd: they are externally similar to helicopter carriers, but the foredeck is filled with a heavy surface-to-surface missile armament. Western doctrine says that the carriers should be kept out of harm's way and send their aircraft forward to do the fighting, so their own weapons fit would be minimal and defensive; the Soviet approach was that the carrier should be a major surface combatant which happened also to have an aviation capability. (Even their full-deck carrier design, the Admiral Kuznetsov, carries twelve supersonic P-700 Granit missiles with a range of over 300 miles.)

However, while it had originally been hoped that the Kiev would be fitted with catapults and able to conduct full carrier operations, it rapidly became clear that this could not be afforded; the need for some more capable form of air wing than could be provided by helicopters rekindled interest in the Yakovlev VTOL designs.

The aircraft eventually built, first as the Yak-36M in pre-production form and then with modifications as the Yak-38, had three engines: two small Rybinsk jets mounted (almost) vertically within the fuselage, and one larger Tumansky jet with a pair of rotating nozzles at the rear. Supersonic capability was considered, but deferred to a possible later improved model. Indeed, the Yak-38 was a pretty basic aircraft: no radar (it was to be guided to its target by ground controllers, in keeping with standard Soviet doctrine), no internal gun, and the only air-to-air missiles it could carry were short-range IR-homers. An airborne radar, and radar-guided missile capability, were also deferred to a later version.

Its primary role was to be as an attacker going against surface targets, but the need for VTOL capability kept payloads very light. Indeed, when Minsk sailed off the coast of West Africa and in the Indian Ocean, high temperatures reduced performance to the point that the aircraft couldn't carry any external stores at all. Rolling takeoffs were possible, and helped the payload capacity a bit, but this was always the aircraft's major problem; the need to carry the dead weight of the two lift engines even when not flying vertically cut directly into the payload. High fuel consumption and the need to keep a large landing reserve gave it short ranges and loiter times.

All three engines were needed in order to land, and this introduced a reliability problem compared with the Harrier, especially since the lift engines had a reliable life between maintenance of around twenty hours. The designers realised this, and included an automatic system which would fire the pilot's ejection seat if any one of the engines failed while in vertical flight mode. (On at least one occasion this fired without an engine failure, with consequent loss of aircraft.) The system also monitored other parameters, such as aircraft attitude and descent rate, and would fire if it sensed that a landing attempt was going wrong. As one would expect, this was vastly unpopular with the pilots, who liked to make their own decisions about when to abandon the aircraft.

They flew very briefly over Afghanistan in 1980 in a fifty-day "near-operational" evaluation, possibly dropping small numbers of bombs in combat (it's still not clear), but with very limited capability in the hot thin air; this was the closest they ever got to war.

The Yak-38M variant upgraded all the engines and increased payload capacity, though it didn't add any new capabilities; it entered service in 1987. But the aircraft was never terribly effective, nor was it popular, and the collapse of the USSR led to its rapid retirement. Unlike many other Soviet aircraft, no attempt was ever made to sell it to overseas customers, even when India bought the Kiev-class Baku (later Admiral Gorshkov, now Vikramaditya).

The Yak-141 (Yak-41M) was a development of the Yak-38, finally giving the supersonic capability that had eluded other VTOL aircraft designers, and adding back in features like radar and guns. Prototypes first flew in 1987, but it did not enter service before the end of the USSR and in spite of efforts by Lockheed the project seems to have been completely abandoned.

  1. Posted by Owen Smith at 12:08pm on 11 May 2014

    The parasitic weight of the lift engines just reminds me so much of the parastic weight of the lifting fan and doors on the F35-B. Do we know how the weight of the two systems compares?

    The swivelling nozzles on the Harrier weigh a lot less I'm told, not to mention introducing no extra failure modes on landing or takeoff since they're used for normal flight as well.

  2. Posted by RogerBW at 01:06pm on 11 May 2014

    Hard numbers are difficult to come by. I would guess, based on the thrust, that the Rybinsk lift jets weighed somewhere around 1500lb each, so a ton and a half for the pair of them, but I haven't been able to find any indication of the weight of the Rolls-Royce LiftSystem being built into F-35Bs.

  3. Posted by John Dallman at 05:34pm on 11 May 2014

    Well, the empty weight of the F-35B is 1400kg more than the F-35A, which gives us a rough figure. That will include extra weights like the doors for the LiftFan, but also includes anything that's been taken out, such as the smaller fuel tanks (which hold 6125kg of fuel as opposed to 8280kg). The A model seems to be the right thing to compare with the B, as their dimensions are very similar; the C has a bigger wing.

    This article quotes Rolls-Royce as claiming they're been able to shave 320kg off the LiftSystem: hollow titanium fan blades seem to have been significant.

  4. Posted by RogerBW at 05:51pm on 11 May 2014

    That suggests that the LiftSystem dead weight is somewhere reasonably close to what the Yak-38 carried, but the F-35's empty weight is something like 80% higher, and main engine dry thrust is around 85% higher, so it's a smaller proportion of the total.

  5. Posted by John Dallman at 07:45pm on 11 May 2014

    Looking at the specs for Rolls-Royce's 1950s specialised lift engine, the RB 108, suggests that you could probably get the same lifting thrust for less weight with lift engines, but there are drawbacks.

    Lift engines are going to be more expensive to develop, build and maintain. They'll have a hot exhaust, and the F-35B has enough problem with damaging landing surfaces and re-ingesting hot gas already. They're an inconvenient shape for the current style of fighter design, which wants to use the body for lift, while the LiftFan fits that well.

    Overall, the LiftFan is well-designed to appeal to the Pentagon decision-makers. It's a sexy new technology, rather than being old-fashioned, and it's also simpler and likely to last longer between failures. And since failures in your vertical landing system tend to write off expensive aircraft, that's appealing.

    I also realised why the LiftFan has to work with a 250 knot crosswind. Since you lose thrust on the main engine when you engage the fan, and also open some very draggy doors, that's the starting speed you need to avoid falling out of the sky during the transition.

  6. Posted by RogerBW at 07:56pm on 11 May 2014

    While the Pegasus is the only combat-proven V/STOL system (and gives you VIFFing, which lift jets and the big fan don't), there are clearly huge difficulties with building a modern and supersonic aircraft round a descendant of it.

    I wonder how well an F-35B can recover if something jams during the transition.

  7. Posted by John Dallman at 09:23pm on 11 May 2014

    I suspect it can probably make a conventional, if very fast, landing in most of its configurations. However, long runways are not going to be in plentiful supply in places where you need F-35Bs.

  8. Posted by RogerBW at 09:58pm on 11 May 2014

    Oh, sure, anything except an Osprey can get to a horizontal landing if it has enough height or airspeed. But if you drop the nozzle, open the doors, spin up the fan, and there's a nasty graunching sound from the latter... just how much speed/height do you lose before you get back up to a flying speed?

    The reason I'm thinking about this is that a Harrier that's been flying at treetop/wavetop height can transition into the hover without gaining height; and it can take a while over it, gradually shifting more and more to engine-borne flight. And once it's done it, it can land quite quickly, because it's already close to the deck. But if safety dictates that you need to enter the transition at a higher speed and altitude, you're left at zero speed but still substantial altitude, and you have to balance the plane down... which takes time. (I wonder if that's why the Yak-38's lift engine cover is slatted? That would mean that you could spin up the lift engines to idle before you slow down for the transition, i.e. you could make sure they were working before you committed to the transition, and thus enter it at a lower altitude because you didn't need that safety margin.)

  9. Posted by John Dallman at 10:31pm on 11 May 2014

    I'd hope you can spin up the fan before you drop the nozzle, since the latter is where you start loosing speed really fast. But I don't know, and I can't find an account of how to fly the thing in casual searching.

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