Single vs twin engined fighters
Posted by picard578 on August 9, 2014
http://defenseissues.wordpress.com/2014 ... -fighters/
Single engined fighters have typically been favored due to their low procurement and operational costs, ease of maintenance and assumed better air-to-air performance. Yet there is also a belief that single-engined fighters are inherently less survivable and lower-performance than twin-engined fighters.
Fighter aircraft of World War I and II were invariably single-engined. Few twin-engined fighters – P-38 and Me-110 – had disastrous performance against single-engined fighters of the period (Me-109 and Spitfire, respectively), as they were too large, too heavy and too inferior in maximum g and roll performance; loosing one engine spelled doom for fighter as enemy fighters would down a straggler. In the end, P-38 had to withdrawn from Europe in fighter role (it did continue in photo reconnaissance role) due to heavy losses. For these reasons, twin-engined configuration was typically reserved for lumbering bombers, and night fighters whose only task at the time was bomber-hunting (Me-110 and Mosquito were notably successful in such role). Most successful Western fighters – F-86, Mirage, F-16 – have also been single-engined, and Soviet fighters designed after World War II (MiG-17, MiG-21) followed the suit. Only twin engined fighter of the era was English Electric Lightning, but it was not the most successful fighter despite being the first fighter aircraft to supercruise. Twin engined fighters only became popular when multirole requirements became standard, starting with F-4. But F-4 had disastrous performance in air combat considering its cost, and its ground attack performance was not stellar either.
Single engined fighters tend to be cheaper to buy and operate, easier to maintain and have lower basing requirements. Easier maintenance is primarly due to twice the number of engines means twice the work and twice the likelyhood of something going wrong. It also means that twice as many spare parts are needed, increasing aircraft’s logistical footprint. Even with a possible benefit of fewer peacetime losses (which, as shown later, is nowhere near certain), a fleet of single-engined fighters is still less expensive in both procurement and maintenance costs than a fleet of twin-engined fighters, and a force of single-engined fighters will almost always outperform a force of twin-engined fighters that costs same to operate and maintain.
This means that they provide advantage in two most important areas: 1) pilot training and 2) allowing larger number of combat sorties for the same cost. Further, small size tends to make them easier camouflaged on the ground. As a result, only twin-engined fighter ever to become the frontile fighter was the F-5, primarly due to its small size and weight.
Reasons why single engined fighters tend to have better combat effectiveness are several. Single engined fighters tend to be smaller, lighter, and better optimized aerodynamically, which automatically improves survivability in a dogfight. Having one engine means that mass is distributed closer to the centerline axis, which reduces roll inertia and improves roll onset rate. F-16 also has comparable roll rate to the F-22 despite latter’s thrust vectoring allowing it to use all control surfaces to roll. Wing loading is also typically lower for single-engined fighters. Comparing Western jet fighters up to 1980, only fighters with less than 500 ft area were single-engined ones (with sole exception of the F-5), and smallest modern Wester fighters are the F-16 and Gripen, both single-engined. Smaller size means that they have surprise advantage, as they are harder to acquire and track either visually or with optical (visual, IR) sensors; small size combined with better transient performance also means that they are more likely to slip out of sight after being acquired. Lower drag oftentimes (though not necessarily) means higher cruise speed despite often lower TWR – fastest cruising fighters of the 1950-1980 era were single-engined J-35, Mirage III and F-106. Mirage III was actually able to achieve Mach 1,3 without reheat, though its economic cruise speed was around Mach 0,92, as it was for J-35 and F-106. Gripen C similarly is capable of cruising at Mach 1,1 at dry thrust and with 6 missiles despite being underpowered, and its economical cruise speed is again Mach 0,92. F-16 can achieve Mach 1,1 at dry thrust and with two missiles, most likely due to added drag of horizontal tail, while the F-15 can achieve a cruise speed of only Mach 0,71 despite far higher thrust-to-weight ratio, primarly due to the high cruise drag. Single-engined F-104 could achieve cruise speed of Mach 1,1 and actually has better supersonic range than the F-22 (it could maintain Mach 2 for 15 minutes, whereas F-22 can maintain Mach 1,5 for about as long.*). An F-104A equipped with the -19 engine could maintain level flight at Mach 2 and 22.000 meters on a cold day.
Better thrust-to-drag ratio of single-engined fighters also allows better acceleration – oftentimes significantly so. F-16 has the best acceleration of all US teen fighters, and single-engined F-106 and J-35 have acceleration comparable to twin-engined F-4E, while similarly single-engined F-104A outperforms all three previous fighters by a significant margin. In fact, F-16Cs acceleration is better than that of the F-22A in transonic region (Mach 0,8-1,2 at 30k ft in 28 s vs 33 s for the F-22), though the F-22 has better supersonic acceleration. Both F-16 and the F-22 have significantly superior acceleration and endurance compared to the F-15 due to latter’s high tail-boat drag.
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Note: increased drag in twin-engined fighters is primarly a result of three factors. First is increased tail-boat drag due to shaping required to place two engines next to each other – there is an area between the engines which typically ends in a flat plate if engines are close together, and if not then additional fuselage required to separate engines still leads to higher drag. Second one is increased engine-face drag [F-16s F100-PW-229 has 129,7 kN of thrust and 88 cm inlet diameter (6.082 cm2 area), while F-18s F404-GE-402 has 78,7 kN of thrust and 79 cm inlet diameter (4.902 cm2 area). Thus, thrust-to-inlet area ratio would be 21,33 N/cm2 for the F-16 and 16,05 N/cm2 for the F-18. Both engines have same thrust-to-weight ratio and use similar technologies. For Gripen's RM12, ratio is 20,4 N/cm2 (80,5 kN, 3.944 cm2)]. This is a problem since engine drag accounts for a significant proportion of total fighter cruise drag. Third one is typically wider body leading to higher form and profile drag and inferior area ruling, which itself leads to higher wave drag.}
Single engined fighters also tend to have higher fuel fraction, and thus combat persistence, than twin-engined ones – only fighters USAF produced from 1950 to today with fuel fraction of 0,3 or above are single-engined F-101, F-8, F-16 and F-35. Combined with lower drag of single-engined fighters (greater range at same fuel fraction), this resulted in the F-8 and the F-16 having significantly greater persistence and range than their twin-engined counterparts (F-15 and F-18 for the latter; speaking of USAFs competence, USAF did not want a single-engine fighter to have a greater range than the F-15, but focused on total fuel capacity as opposed to the fuel fraction and thrust-to-drag). With modern European fighters situation is opposite, mostly due to differing requirements France, Sweden and Eurofighter consortium had for their fighters. That being said, Gripen E is expected to have almost as high fuel fraction as, and better endurance/range than, twin-engined Rafale.
And as counter-intuitive as it may sound, single-engined fighters have better combat survivability as well. Most modern Western fighters have engines so close together that any amount of damage taking out one engine is almost certain to take out another as well. Even if a twin-engined aircraft loses a single engine without another one getting taken out, it immediately looses 50% of the thrust and 81% of the performance, making it a sitting duck and easily killed by the opponent. One of reasons for that is large amount of assymetric thrust generated by only one working engine, and designs most likely to suffer loss of only one engine in combat are also ones that have widest engine spacing and thus greatest amount of assymetric thrust and roll inertia. Due to all above factors, twin-engined fighters are more likely to get hit in combat while not being any more likely to survive getting hit.
Twin engined designs do not necessarily have better peacetime survivability either. F-106, despite being single-engined, had 15 losses in first 90.000 hours, compared to 17 for the F-4. In the first 213.000 hours, it had 26 losses, compared to 44 for the F-4. It can be seen that the more complex F-4 had worse loss rate than the F-106 despite having two engines, and while F-106s loss rate improved, F-4s grew worse. Single-engined F-105 also had low peacetime loss rate.
Loss rate of the F-104 was 26,7 losses per 100.000 hours, compared to 12,2 for F-106 and 20,7 for the F-4. Main reason of losses was not the single engine, but rather USAFs bureocratic stupidity. Namely, once Lockheed gave USAF F-104 (a point defense interceptor), USAF ordered Lockheed to modify F-104 into a nuclear-capable bomber. Main cause of F-104 losses in German service was improper pilot training – in the same period Spain had no F-104 losses despite flying them in similar conditions. Specifically, German pilots received conversion training on Starfighters in United States – in desert, with clear skies and lot of room. Once instruction regiment was changed to something appropriate to Central European conditions, loss rate improved drastically, to the point that it was comparable to twin-engined fighters of the day. Pilots also often overrode aircraft’s AoA limiter despite F-104s propensity to pitch up and enter spin at high angles of attack. Further, Luftwaffe used F-104s in inappropriate fighter-bomber role, which led to major use at low altitudes as well as to installation of heavy ground-attack electronics and INS, to the point that it was considered “overburdened” with technology. In Canada, single-engined Tutor, T-33 and CF-104 were more reliable than twin-engined T-37, T-38 and CF-5. While CF-104s were dubbed “Widow Makers”, that did not have anything to do with number of engines but rather with the fact that CF-104s were pressed into a low-level bombing role despite being designed as high-altitude Mach 1,4 bomber interceptors. They had very small, razor-sharp wings that were suitable for high-speed supersonic flight; however, at low speeds high wing loading and bad separation characteristics (due to sharp edge) meant that they were prone to stall as soon as the fighter maneuvered, and low altitude did not leave any room for recovery.
Swedish JAS-39 has a better safety record than the F-18 despite having one engine less – 13% of Canada’s CF-18s have been lost in crashes compared to 2% of Gripens; a loss rate of 0,36% per year versus 0,08% per year for Gripens. Rafale suffered 4 crashes in 64.000 hours, 3 were due to the pilot error. F-16 fleet logged 11 million flight hours by 2004, with 493 losses. Less than quarter of the F-16 losses were due to the engine failure, with leading cause of losses being FCS issues and human mistake. Comparing Gripen with Eurofighter Typhoon, Gripen suffered 5 crashes total in 203.000 flight hours. None were related to either engine or aerodynamic configuration of the aircraft: 2 were due to underdeveloped FCS, 2 were due to the pilot error and 1 was due to ejection seat issue. Typhoon suffered 3 crashes total in 240.000 flight hours. One was due to double engine flameout and two due to unexplained reasons. F-22 reached 100.000 flight hours on 11.9.2011., and by that time had 4 losses.
Overall, F-15 had a crash rate of 2,36 per 100.000 hours and F-16 of 4,48 per 100.000 hours. F-18 crash rate is 3,6 per 100.000 hours, and Gripen’s is 2,46 per 100.000 hours, compared to 1,25 for Typhoon and 6,25 for Rafale. F-16s safety has improved over time, with cumulative loss rate with 11.000.000 hours being 4,48 losses per 100.000 hours, cumulative loss rate at 12.000.000 hours being 3,55 per 100.000 hours and non-cumulative loss rate at 12.000.000 hours being 1,59 per 100.000 hours. F-22s loss rate is 4 in first 100.000 hours. As already mentioned, however, most losses were not engine-related: engine-related loss rate is 0,00 per 100.000 hours for Gripen and 0,42 per 100.000 hours for Typhoon.
MiG-21 is much maligned in India due to its high crash rate. However, many crashes are not a result of the single-engined design but rather of bad cockpit visibility and high landing speed. 40% of crashes are in fact result of the human error. Further, MiG-21s have high total crash numbers because they constitute 75% of the IAF fighter fleet. Other problems include lack of simulators and inadequate maintenance. Many MiG-21s, and majority of spare parts, were produced locally in India and were not up to Russian (let alone Western) standards. Croatian MiGs are nearing end of their service life, and many of them had to have their service life extended beyond production limits and have insufficient maintenance. Crashes were in 2010 (cabin fell out, hit second MiG and both crashed) and 2014 (pilot bailed out due to the MiG catching fire in middle of flight due to the landing gear problem), but not a single CroAF MiG-21 was lost due to the engine problem.
Overall, statistics show that single-role air superiority fighters tend to be safer than contemporary multirole fighters regardless of number of engines (ref. F-106 vs F-4, F-15 vs F-16, Typhoon vs Rafale vs Gripen). And while loss of engine in a single-engined fighter invariably means that the aircraft is lost, engine is not the leading cause of loss (especially today), and lesser reliability of some other systems can make survivability benefits of having a second engine irrelevant.
And while very rare, it is also very possible to land a single-engine fighter with engine out. More common are crashes of twin-engine aircraft due to a single-engine flameout.
In the end, theoretical superior peacetime survivability of a twin-engined aircraft is neither large or certain enough to offset lower combat survivability and performance, typically smaller fleet size, higher maintenance downtime and higher operating cost. That being said, aircraft has to be well designed aerodynamically in order to take advantage of a single-engined configuration (ref. Gripen); single engined F-35 is the worst-performing Western fighter, and one of most expensive ones, due to two factors: bad aerodynamic design and weight more typical of twin-engined fighters. It is also likely to have high crash rate – not due to its single-engine configuration, but due to its extremely complex hardware (overly complex engine and avionics) and software (24 million lines of code) design.
While ground attack aircraft do benefit from having a second engine, their mission is fundamentally different in its nature and cannot be used to draw conclusions about survivability of air superiority aircraft. Even there, however, a second engine may not benefit (or may even harm) survivability if it makes aircraft comparably large.
Notes
*Additionally, my own FLX design can cruise for 20 minutes at Mach 1,5 at distance of 372 km from base, on internal fuel only. Pierre Sprey proposed a design in 1980 which could cruise for 20-30 minutes at Mach 1,2-1,6 at distance of 322-483 km from base, again on internal fuel only. As it can be seen, both are single-engined yet both achieve better supersonic range and/or persistance than the F-22.
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O avião foi cumprir a missão que foi determinada... Vai levar 10 bombas de 400kg para satisfazer o público da Internet? Não mesmo! Mas se a missão fosse para levar mais bombas muito provavelmente ele faria. 
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