History chewed out and spat out some incredible aeroplanes. We drag these rotting morsels out of the compost mulch of history and drag them to our laboratory/fight-club for autopsy. To assist us in our morbid analysis is Hush-Kit’s tamed scientist and engineer Jim ‘Sonic’ Smith (a key figure in the Typhoon and UK JSF programmes among others). To further our thrills we shall pit these dead aeroplanes against each other! Round 1 is supersonic vertical and landing aircraft: the US’ ‘Big Mouth’ Boeing X-32 (that lost out to the X-35) takes on the ‘Last Red’, the Soviet Yakovlev Yak-141. FIGHT!
“Aerospace is one of the fields where technology and innovation, particularly in times of conflict, have resulted in radical advances in development, as well as the pursuit of some spectacular evolutionary blind alleys. Hush-Kit have asked me to consider some of the aircraft that fell by the wayside in the development of warplanes from the Wright Military Flyer of 1909 to today’s emerging 6th generation fighters.
This developmental saga is often told through the path of spectacular successes, generally perceived through quite narrow National perspectives. There are also, it is true, whole books about ‘Awful Aircraft’ documenting well known failures. But there are many interesting possibilities to explore in the world of the not quite successful. The aircraft that lost out in competitive evaluation, or seemed like a good idea at the time, but never got into service.
The possibilities seem endless, and, to limit the size of this article, we are going to look at five pairs of aircraft that are broadly contemporary, that flew successfully, but that never quite made it into operational service. The plan is to look at these aircraft in pairs, explore why they were designed, how they compare, and why they missed out. All the aircraft were seriously intended to be operationally capable, and hence research aircraft are excluded. The periods chosen are broadly the 90s (one pair), the Cold War (two pairs), and the Second World War (two pairs). If these prove interesting, I am fairly confident that other pairs could be constructed, with the early years, the First World War, and an almost limitless array of VSTOL aircraft as possible examples.
In making my selection, I have decided, for artistic reasons as much as anything else, that the pairs should consist of aircraft which share the appeal of the mythical designs of Gerry Anderson, whose creations include the Angel Interceptor, which I analysed recently for Hush-Kit. Anderson’s designs manage to be both plausible and yet somehow unlikely. For real analogues, which might have been potential examples for this article, but did not quite fit the brief, consider a couple of British efforts, the Saunders-Roe SR 53, and the Armstrong-Whitworth AW 52. Both aircraft flew successfully, both were at the edge of the then-achievable technology, both had that futuristic look, and neither went on to become operational.
Hush-Kit have asked me to consider possible outcomes had the selected pairs met in air combat. This is not an easy or straightforward process, and rests to an undesirable degree on personal judgement, since adequate information is not available on most of the aircraft concerned. Hush-Kit’s specific interests for Round 1 are WVR (within Visual Range) and BVR (Beyond Visual Range) air combat, Stealth (for the X-32B and Yak-141 – it is not relevant for the other aircraft), and sortie rate.
Of course, both BVR and WVR combat outcomes are very dependent on total system capabilities – not just platform capabilities. The outcome of a BVR combat will very much depend on what off-board systems are providing situational awareness; whether or not 3rd-party targeting is available; and the size of the ‘No Escape Zone’ of the weapons carried. Additionally, it is worth noting that the result of a WVR combat with anything resembling modern weapons is likely to be a mutual kill.
Taking all this on board, I am going to loosely interpret WVR combat as manoeuvring air combat where the objective is achieving a guns kill. Any other outcome becomes too dependent on such features as whether or not helmet cueing of the missile is available; missile seeker detection and guidance range; missile fly-out range and so on. I’ve taken this approach so that the discussion can focus on platform performance and manoeuvrability, rather than system issues.
Similarly, I am going to consider BVR combat as a contest between platform sensors, and aircraft signature where appropriate. I will assume missiles to have broadly similar capability, as are available countermeasures and protection systems. The issue becomes one of numbers of missiles carried, capability of on-board sensors, and signature.” Before we compare the air combat capabilities of these two aircraft types let’s take a deep dive into their histories and designs.
Yakovlev 141 and Boeing X-32B
“Both of these aircraft were designed to have ASTOVL capabilities. ASTOVL stands for Advanced Short Take-Off and Vertical Landing, and the word Advanced is code for being supersonic. It is worth noting also that the STOVL part of the acronym reflects operational experience with aircraft like the Harrier. An aircraft which can transition from wing-borne flight to hover and land vertically, using its engine to provide lift, would, in general, be also able to take-off vertically, just as the Harrier can. However, to do so would unnecessarily reduce the available payload and range, and it is much more effective to perform a rolling take-off, especially if a ski-jump is available, as this enables operation at much higher weights than can be achieved with a vertical take-off.
ASTOVL design considerations
When considering alternative configurations for ASTOVL capability, the arrangement of the propulsion system in the vertical landing part of the flight is often the key. This is because the vertical lift available in this phase will determine the maximum landing weight, and because the engine exhaust management is likely to be critical in determining the vertical lift available.
Three aspects are critical – hot gas ingestion (re-ingestion if you are an American reader); Aerodynamic suck-down; and ground erosion. In addition, aircraft controllability, thermal heating effects, and acoustic aspects may also be important. We will have a look at what these are, and then consider how well the X-32B and Yak 141 deal with the problems that arise.
Hot-gas ingestion means the air flowing into the engine intake contains hot gases which have passed through the engine and have then been sucked into the intake. This is undesirable because the hotter air has lower density, reducing the thrust of the engine. But also, the exhaust gases will be only partially mixed with cooler, ambient temperature, air entering the intake, resulting in an unsteady and distorted intake flow, which can cause major problems with engine operation and stability. Avoiding this problem generally means keeping hot gases as far away as possible, containing forward flowing exhaust gases with under-fuselage dams or fences, or reducing exhaust gas temperature.
Aerodynamic suck-down results as the vertical jets from the propulsion system reach the ground and spread outwards at high speed under the aircraft. This high-speed outflow results in reduced pressure under the aircraft wings and fuselage, pulling the aircraft towards the ground. Measures to reduce this include reducing the exhaust jet velocity, using a high-positioned wing, and manipulating the supporting jects to produce an upward-pointing ‘fountain effect’.
Ground erosion, acoustic and temperature effects all increase dramatically with increased jet velocity and operating temperature. ‘Acoustic effects’ refers not only to the potentially damaging environment for ground crew, but also to fatigue damage caused by the very high noise levels resulting from high-speed jets in close proximity to the ground.
Unfortunately, the obvious ways to reduce these impacts – lower exhaust jet velocity, and reduced jet temperature, also reduce the thrust available, which impacts directly on the maximum achievable landing weight.
The Yak-141 looks every inch a supersonic fighter, with a variable-ramp intake system, small, thin, swept wings, large engine, and twin tail fins. At first glance, a single engine F-15 with half the wing area. In the vertical lift mode, the Yak 141 uses two 9,300 lb thrust turbojet lift-engines, located behind the cockpit, to balance the thrust of the Soyuz R-79 turbofan main engine, rated at 19,840 lb dry thrust or 34,170 lb in afterburning. For vertical lift, the engine exhaust is vectored via a circular nozzle, which uses rotating segments to achieve a vectored angle of up to 95 degrees, to balance the lift engines. The afterburner can be used with the engine vectored up to the full 95 deg.
Hot gas ingestion is partly managed by the separation of the lift engines from the rear nozzle, and partly by the convergence of the forward and rear jets which helps to prevent hot exhaust from the main engine migrating forward to the engine intakes. The lift engine thrust is directed 5 degrees aft of the vertical, and the main nozzle 5 deg forward, and this will result in convergence of the jets to form a fountain effect, reducing suck-down, along with the use of a high wing location.
However, little attention appears to have been paid to ground erosion and acoustic effects. The main engine uses afterburning in the vertical lift, and the lift engines are straight turbojets with a turbine entry temperature of 1480 deg C. As a result, the exhaust gases are hot and high-speed. The aircraft’s 1991 flying at Paris was curtailed due to damage caused by the aircraft exhaust to the runway, and at Farnborough in 1992, hovering flight was demonstrated, but at 500 feet rather than by a vertical landing.
The Boeing X-32 competed with the Lockheed-Martin X-35 in the Joint-Strike Fighter Concept Demonstration Phase, losing out to that aircraft, which went on to become the F-35 Lightning II. The JSF programme was extremely challenging, calling on manufacturers to provide configurations from the same basic concept suitable for ASTOVL, conventional carrier operations, or use as a land-based fighter and strike aircraft. In addition, the aircraft was required to be stealthy, supersonic and have manoeuvrability similar to an F-16. The X-32B and X-35B were, respectively, the Boeing and Lockheed-Martin ASTOVL concept demonstrators.
The Boeing response was a remarkably chubby aircraft with a small delta wing and a chin intake. The propulsion system took air from the exhaust system of its F-119 turbofan engine, and ducted this forward to a pair of vectoring nozzles located near the centre of gravity. No afterburning or other thrust augmentation system was used to increase thrust, and the vertical lift available would thus have been limited to perhaps 28,000 lbs. In comparison, the Lockheed-Martin F-35B STOVL variant of the JSF has a maximum vertical lift of 39,700 lb.
The Boeing X-32B design was focused on simplicity and low risk, using a mixed-flow vectored thrust arrangement which relied only on changing the exhaust path from the rear nozzles to the lift nozzles for VL operation. The Lockheed-Martin X-35B design relies on the clutch-driven lift fan to greatly increase the mass flow through the propulsion system, as well as a three-bearing vectoring rear nozzle like that first flown on the Yak-141, but derived from earlier Pratt & Whitney and Convair design work.
The use of mixed fan and core air at the lift nozzles reduces the temperature of the exhaust, but the location of the vertical lift nozzles at the centre of gravity, combined with the chin intake system, increases the probability of hot gas ingestion. However, a portion of the fan air is brought forward ahead of the main lift nozzles to act as a cold-air dam, helping to prevent hot gas ingestion. The high wing, cold-air dam and convergence of the lift jets were all intended to reduce aerodynamic suck-down, and the mixed-flow exhaust system was also intended to reduce ground erosion and airframe heating issues.
Notwithstanding the risk, weight, and simplicity benefits of the propulsion concept, it required positioning the engine vertical landing nozzles at the centre of gravity, with an extended jet pipe leading to the rear propulsion nozzle. Overall size constraints and weight and balance requirements lead to the relatively short forward fuselage, and the chubby appearance noted earlier. Moreover, changes in the manoeuvre requirements for the aircraft meant that a larger wing and a tailplane would be required for the production design, which could not be demonstrated by the X-32B.
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Killing the Yak and the end of Bigmouth
The Yak-141 seems to have been able to deliver the performance required for an ASTOVL fleet defence fighter. It was supersonic and could carry sufficient fuel and weapons to fulfil that role. A fully mission-capable aircraft, had however, not been demonstrated at the time of its cancellation. However, it was a single-role aircraft, and would only be suitable if more capable aircraft were not available, or if the Russian Navy were only to operate small aircraft carriers. The development of maritime variants of the MiG-29 and the Su-27, coupled with the continued use of large aircraft carriers, has meant that more capable and flexible aircraft were readily available to Russian maritime forces, and hence the requirement for the Yak-141 evaporated.
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The X-32 lost out to the X-35, which has gone on to be developed into the F-35 Lightning II. The X-35 was able to demonstrate suitability for the USAF land-based strike fighter requirement, US Marines ASTOVL Strike capability, and USN carrier-based strike fighter capability more convincingly, and with greater flexibility, aided by its use of a lift fan to increase VL landing weight, and hence operational flexibility.
Both aircraft feature improbably small wings and large engines, and clearly satisfy the ‘plausible but improbable’ ethos of Gerry Anderson designs. Of the two, my choice of the better loser goes to the Yak-141, which succeeded in meeting its design aims, and, indeed, has the distinction of being the first aircraft to fly with a rotating wedge vectoring rear nozzle, similar to that used by the F-35 Lightning II.
From a configuration standpoint, the layout is quite similar to the Lockheed-Martin tandem-fan design considered in the 1988 UK-NASA JART (Joint Assessment and Ranking Team) assessment, although that aircraft had a rectangular vectoring nozzle and used a clutch-driven forward fan rather than separate lift engines.
The Yak-141’s rotating wedge vectoring rear nozzle is similar to that used by the F-35 Lightning. The three-bearing swivel duct nozzle, as it is known in the US, was first built and tested back in the mid-sixties by Pratt & Whitney, and proposed for application to the Convair Model 200 Sea Control fighter design for the USN.
On sortie generation, on the one hand we have the Yak-141, with a main engine and two lift engines to look after. On the other, noting the experiences in developing the F-35, avionics reliability, software and hardware integration, and looking after low signature materials in a maritime environment are all potential problem areas for a developed F-32. While at first glance, you might think the X-32B would be simpler and quicker to turn around than the Yak 141, I don’t think there would actually be much in it.
Yak 141 and Boeing X-32B – Air Combat Comparison
Analysis here is handicapped because neither aircraft flew with an integrated weapons system, and because there were clear difficulties for Boeing in fielding an aircraft able to meet the requirement to demonstrate that one configuration could satisfy the needs of the USAF, US Navy and US Marines.
Beyond-Visual-Range Combat and the merge
For BVR combat, low signature is an important enabler, as it offers the possibility of achieving a missile launch against opposition aircraft before they are aware of your presence. In a fully swept-up system, where stealthy fighters are supported by AEW&C and co-operative electronic warfare and targeting systems, this ability may be quite dominant.
However, we are looking primarily at inherent platform capability, rather than the full future air dominance system-of-systems approach. There is little doubt here that the X-32B would have a significant first shot advantage over the Yak 141, which would have pre-dated an operational F-32 by at least 10 years, and perhaps 15.
It is prudent to expect that once the first missiles have been fired, opposition launch detection systems, and the increase in signature associated with launching the weapon from an internal bay, would allow localisation by the Yak-141. If, after the initial missile firings, aircraft on both sides survive, they are likely to be manoeuvring hard to defeat opposition missiles, and to position for a follow up, or a counter, missile firing.
At this point, numbers of missiles available also matter. The X-32B claimed to be able to carry six AMRAAM; the Yak-141 could carry four missiles on external pylons. With the fragile weight margin evident for the X-32B, and the need to develop the aircraft as a weapons system, as well as to meet full USN and Marines requirements, I am doubtful that an operational aircraft would carry this loadout. However, there is no real doubt that the X-32B would have had a major advantage over the Yak-141 in BVR combat. Well, perhaps a small tinge of doubt, depending on the effectiveness of whatever magic is used to conceal the engine from generating a high radar signature, behind such a short intake duct.
In WVR combat, the Yak-141 has an advantage in maximum speed, and, because of its higher aspect ratio, may well have an advantage in a sustained turning engagement. The X-32B has a big wing, and a big engine, and would be likely to have an advantage in instantaneous turn rate. However, this would come at the cost of significant loss in energy. The X-32B would be advised to be cautious about committing to a WVR engagement of any prolonged duration because hard manoeuvring would be likely to leave it at a disadvantage, and possibly unable to break off combat easily.
It is worth pointing out, however, that to get to a turning WVR combat, the Yak-141 would first have to survive a BVR engagement, and then avoid a likely mutual kill in the merge.”
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