Introduction

This article will compare several engines used in modern fighter aircraft: EJ200 (Typhoon), M88 (Rafale B/C/M), RM-12 (Gripen A/B/C/D), F-135 (F-35A/B/C), F-119 (F-22A), F404-GE-402 (F-18C/D), F-414-400 (F-18E/F, Gripen E/F), AL-31F (Su-27, Su-30, J-11).

Thrust to drag

Since frontal area dominates drag, and engine frontal area dominates aircraft frontal area, thrust to drag ratio will take a form of thrust divided by the engine frontal area (inlet diameter used).

EJ200: 3.848 cm2, 90 kN, 23,13 N/cm2

M88-2: 3.805 cm2, 73,9 kN, 19,42 N/cm2

RM-12: 3.948 cm2, 80,5 kN, 20,39 N/cm2

F-135 (CTOL): 10.715 cm2, 191,35 kN, 17,86 N/cm2

F-135 (STOL): 10.715 cm2, 182,4 kN, 17,02 N/cm2

F-119: 6.136 cm2, 164,58 kN, 26,82 N/cm2

F404-GE-402: 3.959 cm2, 78,7 kN, 19,88 N/cm2

F-414-400: 4.745 cm2, 97,37 kN, 20,52 N/cm2

AL-31F: 6.433 cm2, 122,58 kN, 19,05 N/cm2

AL-41F: 6.433 cm2, 175 kN, 27,2 N/cm2

As it can be seen, EJ200 has the second best thrust-to-drag ratio after the F-119, while the F-135 has the lowest thrust-to-drag ratio. EJ230 has a ratio of 26,9 N/cm2, while the F-414EPE will have a ratio of 24,62 N/cm2. M-88ECO will have a ratio of 23,65 N/cm2.

(This is one of reasons why single engined fighters typically have better peformance than twin engined fighters despite lower thrust-to-weight ratio. Engine frontal area is one of major contributors to drag in all “normal” flight conditions. Taking two engines that use same technology and general design, frontal area – and drag – will increase with square of dimensions’ increase, while weight – and thus thrust – will increase with cube of dimensions’ increase. Engine that is 20% larger in all three dimensions will have 44% greater frontal area and 72,8% more weight and thrust – thus its thrust-to-drag ratio will be 20% greater than that of the smaller engine. If engines are of the same size and characteristics, then twin engined aircraft will be larger and have higher inertia and inferior transient performance. This of course assumes identical design goals and avaliable technology. For example, F-119 is 239% larger in volume than the EJ200, has 59% greater frontal area and 15% better thrust-to-drag ratio.).

NOTE: M88-2 has been tested at 18.700 lbf in 1990, which would give it 21,86 N/cm2.

Thrust to weight

Engine thrust to weight ratio is an important (though not the only) factor in determining aircraft’s thrust-to-weight ratios, just as engine’s thrust-to-drag ratio is an important factor in determining aircraft’s thrust-to-drag ratio.

EJ200: 2.180 lbs, 20.250 lbf, 9,17:1

M88-2: 1.977,5 lbs, 16.620 lbf, 8,40:1

RM-12: 2.326 lbs, 18.100 lbf, 7,78:1

F-135 (CTOL): 6.444 lbs, 43.000 lbf, 6,67:1

F-135 (STOL): 10.342 lbs, 41.000 lbf, 3.96:1

F-119: 3.900 lbs, 37.000 lbf, 9,49:1

F404-GE-402: 2.282 lbs, 17.700 lbf, 7,76:1

F414-400: 2.445 lbs, 21.890 lbf, 8,95:1

AL-31F: 3.460 lbs, 27.560 lbf, 7,97:1

AL-41F: 1.850 kg, 17.845 kgf, 9,65:1

EJ230 has a thrust-to-weight ratio of 10,60:1 and F-414EPE will have a thrust-to-weight ratio of 10,74:1. M-88ECO will have a ratio of 9,32:1.

NOTE: F-135 weights were provided by Bill Sweetman here (comments section), citing Erin Dick. They are also listed here.

NOTE 2: M88-2 has been tested at 18.700 lbf in 1990. This would give it a TWR of 9,17.

Fuel consumption

Fuel consumption depends on both thrust and thrust-specific fuel consumption. Since aircraft with higher TWR can reduce thrust and still match performance of lower-TWR aircraft, both thrust-specific and total fuel consumption, at dry thrust and afterburner, will be compared.

Dry thrust:

EJ200: 21-23 g/kN s, 60 kN = 4.536-4.968 kg/h

M88-2: 0,8 kg/daN h, 48,8 kN = 3.904 kg/h

RM-12: 23,9 g/kN s, 54 kN = 4.646 kg/h

F-135: 0,89 kg/daN h, 124,6 kN = 11.089 kg/h

F-119: N/A (est: 0,8 kg/daN h, 116 kN = 9.968 kg/h)

F404-GE-402: 82,6 kg/kN h = 4.039 kg/h

F414-400: 0,84 kg/daN h, 57,8 kN = 4.855 kg/h

AL-31F: 0,87 kg/kgf*h, 7.575 kgf (74,5 kN) = 6.590 kg/h

AL-41F: 19,18 g/kN s, 113,9 kN = 7.865 kg/h

EJ200 consumes 82,8 kg/kN h, M88-2 consumes 80 kg/kN h, RM-12 consumes 86 kg/kN h, F-119 consumes 80 kg/kN h, F404-GE-402 consumes 83 kg/kN h, and F414-400 consumes 84 kg/kN h. F-135 is not capable of supercruise, but for completeness’ sake it does consume 89 kg/kN h. AL-31 consumes 88,5 kg/kN h and AL-41 consumes 69 kg/kN h.

Afterburner:

EJ200: 47-49 g/kN s, 90 kN = 15.228-15.876 kg/h

M88-2: 1,7 kg/daN h, 73,9 kN = 12.563 kg/h

RM-12: 50,6 g/kN s, 80,5 kN = 14.664 kg/h

F-135: 1,92 kg/daN h, 191,35 kN = 36.739 kg/h

F-119: N/A (est: 1,85 kg/daN h, 164,58 kN = 30.447 kg/h)

F404-GE-402: 177,5 kg/kN h, 78,7 kN = 13.969 kg/h

F414-400: 1,85 kg/daN h, 97,9 kN = 18.112 kg/h

AL-31F: 1,92 kg/kgf*h, 12.501 kgf (122,58 kN) = 24.002 kg/h

AL-41F: 54,11 g/kN s, 175 kN = 34.089 kg/h

Neither EJ230 or M88ECO offer improved SFC over basic variants. F414EDE/EPE could reduce SFC to 0,81 kg/daN h and 1,78 kg/daN h, comparable to the M88.

In the afterburner, EJ200 consumes 169,2-176,4 kg/kN h, M88-2 consumes 170 kg/kN h, RM-12 consumes 182 kg/kN h, F-135 consumes 192 kg/kN h, F-119 consumes 185 kg/kN h, F404-GE-402 consumes 177,5 kg/kN h, F414-400 consumes 185 kg/kN h, AL-31 consumes 195,8 kg/kN h and AL-41F consumes 194,8 kg/kN h.

Overall, M88 is the most fuel-efficient engine, followed by the EJ200 and F119, though all are less fuel efficient than AL-41 at subsonic regime. AL-31 is the least fuel-efficient engine.

Bypass ratio

Main function of low bypass ratio is to enable the engine to achieve high thrust-to-weight and thrust-to-drag ratio at dry thrust; both these qualities are required for supercruise.

EJ200: 0,4:1

M88-2: 0,3:1

RM-12: 0,31:1

F-135: 0,57:1

F-119: 0,3:1

F404-GE-402: 0,34:1

F414-400: 0,25:1

AL-31F: 0,59:1

AL-41F: 0,59:1

M88-2 has, interestingly enough, lower bypass ratio than the EJ200, indicating greater focus on supersonic performance. F-135 is quite obviously optimized for subsonic/transonic performance, and combined with unaerodynamic airframe (too fat for proper area ruling) and engine’s own low thrust-to-drag ratio, it is unrealistic to expect the F-35 to achieve any kind of sustained supersonic cruise. AL-31 is also optimized for subsonic-supersonic performance, but is paired to the far superior airframe. F-414 is the closest to being a turbojet out of all engines listed.

Percentage of maximum thrust achievable on dry power:

EJ200: 67%

M88-2: 66%

RM-12: 67%

F-135: 65%

F-119: 70%

F404-GE-402: 62%

F414-400: 69%

AL-31F: 61%

AL-41F: 65%

F-119 is the best while most other engines trail very closely behind it. AL-31 is the worst, and the F404 is the second worst.

Mechanical reliability and maintainability

Mechanical reliability depends in part on mechanical complexity. While most engines use the same basic architecture, there are things they very clearly differ in.

EJ200 has 8 compressor and 2 turbine stages

M88 has 9 compressor and 2 turbine stages

RM-12 has 10 compressor and 2 turbine stages

F-135 has 9 compressor and 3 turbine stages

F-119 has 9 compressor and 2 turbine stages

F404-GE-402 has 10 compressor and 2 turbine stages

F414-400 has 10 compressor and 2 turbine stages

AL-31F has 13 compressor and 2 turbine stages

AL-41F has 13 compressor and 2 turbine stages

While this is a vast oversimplification, going by number of stages alone, EJ200 would be the most reliable and easiest to maintan, while AL-31 would be the least reliable. EJ200 also has the fewest 1st stage fan blades of any modern fighter aircraft engine. F-119 has an additional failure point in form of the thrust vectoring nozzle, and the F-135 variant used on the F-35B has two additional failure points – TVC nozzle and a lift fan, plus a third failure point in form of doors for the lift fan which techically are not part of the engine. In the F-135s case, several weight reduction measures also made it far more vulnerable to the combat damage.

Many of these engines also use modular design to simplify maintenance. Number of modules is as follows:

EJ200: 15

M88: 21

RM-12: 6

F-135: 5

F-119: 4

F404-GE-402: 6

F-414-400: 6

As it can be seen, M88 and EJ200 would be easiest to maintain, especially the M88.

Service life is as follows:

EJ200: 6.000 h

M88: ??

RM-12: 4.000 h

F-135: 2.000 h

F119: 6.000 h (?)

F404-GE-402: 4.000 h

F414-400: 6.000 h

AL-31F: 1.500 h

AL-41F: 4.000 h

Overall, EJ200 is the most user-friendly engine.

IR signature

Engine inlet temperature can be used to approximate IR signature when combined with thrust. It is not a perfect measure as there are other factors influencing IR signature as well.

EJ200: >1.800 K, 20.250 lbf

M88: 1.850 K, 16.620 lbf

RM-12: >1.717 K, 18.100 lbf

F-135: 2.255 K, 43.000 lbf

F-119: ?, 37.000 lbf

F404-GE-402: 1.717 K, 17.700 lbf

F414-400: ?, 21.890 lbf

AL-31F: 1.685 K, 27.560 lbf

AL-41F: 1.887 K, 39.340 lbf

M88 has an additional cooling channel beyond one typically present, as well as second set of nozzles which partly hide the afterburning plume and inner nozzle. F-119s nozzles, meant to reduce RCS, also reduce IR signature of the exhaust plume by increasing its area/volume ratio. F-135 on the other hand has relatively thin skin, and the F-35 is thin-skinned itself, thus increasing IR signature. EJ200 is another engine that had its skin thinned in order to save weight.

Combination of thin skin, high thrust and very high inlet temperature means that the F-135 has the highest IR signature, while the M88 has the lowest IR signature due to low thrust and IR signature supression measures; F404 should have the second lowest IR signature. Further, higher operating temperature means greater stress on components and thus more frequent maintenance, other things being equal.

Conclusion

Overall, the F-119 is the best engine where performance is concerned, followed rather closely by the EJ200 and F-414-400. EJ-230 is better than the F-119. RM-12 is the second worst and F-135 is the worst Western engine while the AL-31F is the worst engine overall (not surprising considering its age; AL-41F does match modern Western fighter engines in at least some performance parameters, but at cost of the service life).

All problems with the F-135 are connected to the fact that the F-35 is a strike fighter by design, and not a proper multirole fighter; on the other hand, EJ200, M88 and the F-119 are designed for fighter aircraft whose primary role is air superiority. As a result, F-135 is optimized for different operational conditions and regimes compared to other engines listed here, and it is unrealistic to expect the F-35 to achieve even marginal supercruise performance. On the other hand, it can be seen from the article, and notes below, that the F-119s advantages stem mainly from its large size.

That being said, pure performance is not the only important factor. Just as important, if not more so, are reliability and ease of maintenance in the field. EJ200 is likely the most reliable engine, while the M88 is easiest to maintain. When all factors are taken into account, EJ200 would be the best choice for a fighter aircraft – assuming that thrust is sufficient, of course.

Notes

Turbojet J85-GE-21 with 5.000 lbf / 22 kN / 2.243 kgf of afterburning thrust would have a thrust-to-weight ratio of 11,74 and thrust-to-drag ratio of 13,84 N/cm2. J97-GE-100 has 8.000 lbf / 35 kN /3.629 kgf has a thrust-to-weight ratio of 11,5:1 and thrust-to-drag ratio of 14,27 N/cm2. Low thrust-to-drag ratio despite these engines’ high thrust-to-weight ratio and lower frontal area than that of the comparable turbofan can only be explained by their small size, confirming the conclusion about single vs twin engines from first section of the article. For comparision, J79-GE-17 turbojet (J97 was/is used on the F-104, F-5, F11F-1F, IAI Kfir, A-5 and F-16/79) has 17.835 lbf / 79,3 kN / 8.090 of afterburning thrust, thrust-to-weight ratio of 4,6:1 (40% of the J97-GE-100) but thrust-to-drag ratio of 10,67 N/cm2 (75% of the J97-GE-100).

If the F119 is reduced to the EJ200s size, it would be 4 meters long and 93 cm in diameter, compared to 74 cm for the EJ200. Inlet diameter would be 68,5 cm, dry weight ~1.000 kg, and thrust 91,4 kN (9.320 kgf). Thus it would have a TWR of ~9,32:1 and thrust-to-drag ratio of 24,8 N/cm2, or 92% of the current value, again confirming that larger engine offers better performance than two smaller engines.

Categories: technology

Think you’ve been a bit hard on the single engined types.

Twin engines allows the designers to run each one a bit hotter, to get more performance out of them. Single engined types have to trade a bit of performance for reliability, as losing your one engine is often regarded as being a bad thing.

Interesting though to plug new engines into old airframes. Take the F-104, which could supercruise anecdotally for about 15 minutes. Replace it’s J-79 with an R88 which has remarkably similar power and you free up enough space for about 5000lbs of internal fuel. Course you could also make the airframe much thinner too.

Anyone familiar with Breguet able to calculate the range of speed of an F-104 with EJ200, R88, F119 etc? Assume the fuselage remains the same width at first I guess, just as an example of the advancement of engines.

It wouldn’t surprise me if modern engines with a purpose designed airframe couldn’t supercruise above mach 2 if you ditch stealth as a requirement.