Getting the F-35 to supercruise

The F-35 may do Mach 1.1~1.25 as is at optimum altitudes (40~50K feet). That is within the realm of the plausible given the wing plan form, cross section, installed thrust and, to a lesser degree of importance, mass. This will also be consistent with the performance of late model sixteens when flying clean with the -29 and -32 engines. How useful such marginal "supercruise" really is tactically is dubious. It may be better to simply conserve fuel and fly at Mach 0.92 then light the burners when it matters.

However, let's assume that we want to get some future variant of the F-35 to go faster, get into the Mach 1.3~1.4 range and have some tangible "supercruise" capability -- not quite the Mach 1.5~1.7 bracket of the F-22, but enough to actually alter the tactics surrounding the platform. How do we get there?

I believe that this can be addressed in several ways:-

• Re-engine it
• Add an auxiliary propulsion unit.
• Modify the intakes
• Modify the wing

A proper re-engine for effective supercruising means an all new engine not an uprated version of the F135 or F136. The 51" engine envelope is actually quite generous, and if you want an engine with as much thrust as possible (sans fuel economy, noise, weight and everything else) you'll try to stuff as big a core in that 51 inches as possible. Taken to the extreme this means a turbojet -- like a modern day J75 if you will. Such an engine will most likely produce about 50,000 lbs of thrust. More importantly, it should make about 3/4th of that, or about 38,000 lbs dry. Like the J79 and 75 it'll probably run pretty hot in terms of case temperature which may be a problem. Assuming the heat issues are surmountable it represents a 35% increase in dry thrust.

The wedge shaped area under the F-35's body where the gun pod goes is an area which can be capitalized on for an auxiliary propulsor to aid in high speed cruise. By this I mean the addition of a small 2D ramjet with doors that open at high speeds. The unit has no turbo compressor and should be relatively low profile and light. It probably won't make a lot of thrust and won't have good TSFC, but it'll add 3000~4000 lbs of supersonic thrust which makes all the difference.

Diverter-less or not, the F-35's intakes will present a problem at higher speeds in that it is fixed. This hurts efficiency and at the limit can cause engine problems with over pressure and near supersonic flows hitting the compressor face. The least signature compromising fix and also the easiest from a structural standpoint is to add a set of saw toothed over pressure vent doors ahead of the engine in the rough position auxiliary STOVL intakes are just ahead of the engine. The F-22 has two sets of overpressure doors on the upper surface too in similar locations. This require changing the air frames's internal structure, but probably not by very much.

Lastly, you may want to re-wing the F-35. The plan form itself actually isn't so bad, but the wing can certainly be made thinner especially if you go for a dry wing and dump any accommodations for the roll posts. But since we are going for a new wing anyway we may as well pick a planform which gives a greater area for a given frontal cross section. A rhombic wing (ala YF-23) or a delta comes to mind. With a rhombic wing, the cL issues associated with the forward concentration in wing area (compared to the current wing) can be addressed with the selection of a significantly aft loaded airfoil. This also happens to be most super critical foils which helps with drag rise in the transonic and low supersonic regimes.

The end result will be an F-35 that will go some 25~30% faster with no RCS compromise. The penalty will be a notable reduction in range due to the thirstier engine and reduced fuel load from the loss of the wing's bunkerage. Combat radius on internal fuel will probably drop by about a third to roughly 450 nm. This is actually not bad and roughly on par with the F-22. In addition, you may lose some of the wing pylons -- probably the outer pair and possibly the mid-wing ones. What's left will also probably have reduced load carrying capacity. Because we left the weapon bays alone, it'll carry the same 4~6 AAMs or 2AAMs + 2 bulky ordnance. In short it'll be an air defense F-35 with a secondary A2G capability. The cost? Probably $2~3 billion for the new engine, maybe another billion if you create an auxiliary ramjet for it, not too much for the over pressure doors maybe 1/2 a billion to get it resigned, tested and refined. Probably 2~3 billion to do a new wing. That's about 6.5~10 billion. About 1/4 the cost of a new aircraft design or roughly how much a new commercial airliner costs Boeing or Airbus. Buy enough to replace the F-15s (about 400 units worth) and each will carry an additional $25 million on top of what an F-35A costs. Not a super deal, but not too bad really.

EXCLUSIVE!

F-35 supercruise modification in test.

Dwight, dont forget thrust rate at test bench says nothing for the regimes your contemplating. The F135 engine has a higher bypass ratio wich means the fan will be useless at high altitude and mach numbers. Worse may infact impair engine perfomance to a point it simply wont supercruise (ie. even above just 1.1). We do not know if sticking an F119 engine would do it. The inlet ducts would have to be changed, and that could compromise stealth.

Unfortunatly this disscussion would have to be postponed to a date after more perfomance stats are released.

Your comparison between the F-35 and the "supercruising" F-16 will work against your argument at best. Thats because the F-16 probably has the best smallest frontal profile and slender design but has to carry weapons externaly (thus loosing the mach 1.1 dry ability) while the 35 has a monolithic design to cram the lift fan and weapons bay inside.

They are just too dissimilar to extrapolate perfomance figures.

triple post sorry

tripple post sorry

Seriously - thinner-section wing, pure-jet engine...

That looks better.

And it's CV-compatible!

wow, that's a great find!!!!!!!

but that never landed on a CV

Neither has an F-35.

Big What If: Where would engine technology would be today if the ATF project had been allowed to go forward with GE's variable cycle engine, instead of the more conservative F-119. We would have had to wait a few years beyond 2005 for a 5th generation fighter squadron to go IOC while that engine went through its teething pains, and definitely a lot more development money. But in return, a variable-cycle engine might then have been available for the JSF project, allowing the F-35 to supercruise. Would the wait have been worth it? I can hear people already saying "we haven't needed the F-22 so far, so it would have been worth it." Thoughts, anyone?

Jetnerd

Edit: I suppose we should also add to the "pro"s for variable cycle engine choice that the F-22 would likely be more fuel-efficient at subsonic speeds than it is now.

dwightlooi wrote:A proper re-engine for effective supercruising means an all new engine not an uprated version of the F135 or F136. The 51" engine envelope is actually quite generous, and if you want an engine with as much thrust as possible (sans fuel economy, noise, weight and everything else) you'll try to stuff as big a core in that 51 inches as possible. Taken to the extreme this means a turbojet -- like a modern day J75 if you will. Such an engine will most likely produce about 50,000 lbs of thrust. More importantly, it should make about 3/4th of that, or about 38,000 lbs dry. Like the J79 and 75 it'll probably run pretty hot in terms of case temperature which may be a problem. Assuming the heat issues are surmountable it represents a 35% increase in dry thrust.

Turbojets have their own issues; I'll spell out 2, 1-they are hot, externally they often require heat-shields/protection inside the fuselage and engine components operate at a much higher temp which ages them more quickly; 2-the afterburners are very inefficient due to the lack of oxygen in the exhaust stream (no fan air) and little cooling from engine's exhaust.

A 'near-turbojet' like the PW-229 or the PW1120, both lower bypass F100 'types' or similar very low BPR engines offer better performance than if they were 'pure' turbojets. The fan air cools the engine overall, and feeds fresh/cool O2 into the augmentor for optimal performance.

Like the PW-229 'modification' to the F100, a similar core change to the F135 could happen someday? Perhaps an F135-PW-253 (2nd generation, 53K lbs of thrust?) Enlarge the current core, leaving the common/older/smaller core of it's sister F119 behind. As technology advances, new methods/materials/technology could be used in the new PW-253's core; increasing it's pressure ratio, increasing it's turbine inlet temperature, decreasing specific fuel consumption, etc. This is much the same fashion that PW increased the F100's performance with the PW-229 and PW-232, both of which have a larger core (smaller by-pass ratio) than previous F100s. The later used advance airflow fan technology from the 'new' F119 design. If PW gets involved with the 'new bomber' contract and sees the need for a higher power/speed engine with a bigger core/fan, the F135 could benefit. The 'new' bomber engine F140? could use the 'common' core and low pressure turbine of the F135-PW-253, but perhaps the larger fan to further increase thrust if needed for a 'subsonic' or 'trans-sonic' bomber with more loiter time/range. If not the F135-PW-253 could be used it's self to power the bomber, 2, 3 or 4 engines per aircraft! Giving it super-cruise and outstanding fast-reaction.

jetnerd wrote:Big What If: Where would engine technology would be today if the ATF project had been allowed to go forward with GE's variable cycle engine, instead of the more conservative F-119. We would have had to wait a few years beyond 2005 for a 5th generation fighter squadron to go IOC while that engine went through its teething pains, and definitely a lot more development money. But in return, a variable-cycle engine might then have been available for the JSF project, allowing the F-35 to supercruise. Would the wait have been worth it? I can hear people already saying "we haven't needed the F-22 so far, so it would have been worth it." Thoughts, anyone?

Edit: I suppose we should also add to the "pro"s for variable cycle engine choice that the F-22 would likely be more fuel-efficient at subsonic speeds than it is now.

Variable cycle engines were investigated by all major engine manufactures, but it seems they are too complex/expensive for anyone to field. Some changed cycles, some varied core by-pass, etc. None have seen use by the military or civil operators. IF the YF120 had been fielded it may have broken the barrier, but if the engine took that long to 'mature' into something reliable, I highly doubt that we'd have the 187 Raptors we got. We 'didn't need' the F-15/F-16 until Desert Storm either... (dumb public perception of military needs versus deterrence)

Always better to have something for deterrence, than 'need' something to fight off an adversary and not have it until it's too late.

You may notice the F136 (if it survives the current QDR/budget cuts again) does not include the 'variable cycle' of the YF120. GE is trying to play it safer this time around. Perhaps if the YF120 had won, it may have morphed into the F136, but if the F120 had 'issues' being initially fielded I highly doubt GE would have come back to the table with a newer version of it. (Looks bad to the people writing the contracts/checks)

Something PW is working on currently is a 'geared fan' that could be utilized in a military application in future generations. GE is working on PDE combustion chambers/cores for engines, and PW is working on pure PDE engines. PW is developing hydro-carbon fueled SCRAM jet engines... (neither are currently working variable-cycle program as far as I know)

Let's not forget the F135 should have lots of 'growth potential' built into it. PW's not going to let GE come in 7 years into the F-35 program and say "our engine can make more power than the F135 you should buy it now" PW will play it's cards close to their chest until they need to play them. Simple changes like blades/stators to increase the OPR of the fan, or increasing the TIT in the core to add super-sonic (non-AB) performance. Maybe changing the nozzle area, or control schedule to squeeze a little more exhaust velocity without getting into the engine's stall margin?

Keep 'em flyin'
TEG

A few things...

(1) I have said this before, but let's recap the whole bypass issue... when it comes to supercruising in the lower mach 1 regime, the difference in engine performance with a bypass ratio of 0.56 or 0.20 is not significant. Both are basically leaky turbojets -- most fighter engines are in any case. You can go to about 2:1 bypass and still supercruise at Mach 1.5~1.6 drag of the airframe permitting, in fact that is what most of the supersonic executive jet proposals envision. Mach 1.4~1.6 cruise using the well aged JT8D-200 series low bypass turbojet (2:1 bypass).

(2) The reason they go to a bigger fan on the F-135 is because this is a much cheaper and easier way to get another ~4000 lbs of thrust. It's defnitely much cheaper (and lower risk) than developing a new core.

(3) It has been said that thrust falls off with higher bypass engines at supersonic speeds because the exhaust velocity is lower on such engines. This is not strictly speaking true. Newtonian physics say that force = mass x acceleration. The relative velocity of the exhaust to the velocity of the aircraft has zero effects on the thrust the engine produces. A 10:1 bypass and a 0.2:1 bypass engine will make the same thrust they did at Mach 2 as they did at Mach 0.7. The PROBLEM is that a 10:1 bypass engine has a big fat intake. This big fat intake is not just a hole, it also needs to be constructed to brake intake air to subsonic velocities before hitting the compressor face. So this big fat intake really acts like a big parachute adding drag. Hence, a low bypass engine has the advantage of being able to provide the same thrust wile incurring less ram drag. Because of this, the net thrust is higher -- net thrust of an engine being the thrust minus the drag associated with the engine. Having said that, the differnce in intake cross section between an aircraft using a 0.56:1 engine and one using a 0.20:1 engine (of the same output) is quite small when you take into account the overall cross section of the aircraft.

So, all the technical mumbo jumbo aside, it's possible to make the F-35 supercruise by replacing the engine or upgrading it?

if the engine took that long to 'mature' into something reliable, I highly doubt that we'd have the 187 Raptors we got.

I forgot about the political ramifications of trying to bring TOO big a step-change in technology to front line service: bigger delays and even bigger cost-overruns from trying to incorporate un-matured technology could eventually bog a project down completely. The Navy's DDG-1000 and the Army's Land Warrior, and maybe the Comanche are projects that come to mind. If the F-22 had been even a few years later, I think the program would have been killed by now and that would be a crime, let alone the 187 cap.

On the topic if re-engining F-35 for supercruise, it would more likely be from an evolved F135 or 136 than anything else.

For me, I'd really like to see big step-changing tech like pulse detonation / multi-cycle go operational on a frontline platform, because it would scare the #*%#%@# out of our enemies [from reading crazy expectations of supercruise + Mach 2.5+ dash + BIG combat radius + responsiveness for subsonic WVR knife fights]. But I guess we won't see something like thatfor F-35. Even if it were soon enough, something that radical would probably require new airframe and systems design from the ground up.

dwightlooi wrote:A few things...


(3) It has been said that thrust falls off with higher bypass engines at supersonic speeds because the exhaust velocity is lower on such engines. This is not strictly speaking true. Newtonian physics say that force = mass x acceleration. The relative velocity of the exhaust to the velocity of the aircraft has zero effects on the thrust the engine produces.

Dwight, I think you are wrong about lower exhaust velocity having no effect on thrust at high speed conditions. Take your Newton equation, where thrust equals mass times acceleration. Consider the airplane speed as the initial velocity of the air entering the inlet. If the air in the inlet/engine combination is accelerated, it will have a higher velocity than when it entered. The difference in velocity is a measure of the acceleration, thus is also a measure of the thrust (according to Newton). More delta velocity equals more thrust. So the exhaust velocity must be higher than the airplane velocity to have positive thrust. In fact the equation for thrust is mass flow times delta velocity, with appropriate unit conversion to result in force units. As the airplane speed increases, the delta velocity is reduced. At lower speeds, increased mass flow is greater than reduced delta velocity, so thrust goes up. At higher speeds, mass flow stops increasing and reduced delta velocity results in lower thrust.


Because the F-35 engine has a higher bypass ratio, its exhaust velocity is lower than the F-22 engine, making supercruise more difficult.

dwightlooi wrote:(1) I have said this before, but let's recap the whole bypass issue... when it comes to supercruising in the lower mach 1 regime, the difference in engine performance with a bypass ratio of 0.56 or 0.20 is not significant. Both are basically leaky turbojets -- most fighter engines are in any case. You can go to about 2:1 bypass and still supercruise at Mach 1.5~1.6 drag of the airframe permitting, in fact that is what most of the supersonic executive jet proposals envision. Mach 1.4~1.6 cruise using the well aged JT8D-200 series low bypass turbojet (2:1 bypass).

True, most are leaky turbojets, but there is still a significant difference between an engine with a .76 BPR compared to a .32 BPR when it comes to fighter performance and supercruise. Don't forget the acceleration (spool up response) of a fighter engine is much different than an SSBJ's commercial engine; so too are the thrust/weight ratios. (JT8D weight = 4,612 lbs while the PW-229 = 3,795 lbs) Stall/Performance margins in a fighter engine run very close together. The JT8D is hardly a 'high-performance' motor, more like a 'tried and true workhorse'. While the JT8D is a low bypass engine, it still has quite an exhaust velocity at MIL power. The older JT8Ds had a 1450ft/sec exhaust (about MACH 1.2) and the newer models JT8D-2xx have higher OPR fans with larger BPR, and advanced mixers; should be no huge problem getting specific thrust to fly MACH 1.6.

Important thing to remember, when the thrust (Mass * Acceleration) of the exhaust exceeds the negative drag (mass * deceleration) of the inlet, the thrust is positive, IE - you have a net forward thrust. When that net thrust is equal to aircraft velocity (even) your thrust will fall to nothing. Throttle back and your engines become speed-brakes.

dwightlooi wrote:(2) The reason they go to a bigger fan on the F-135 is because this is a much cheaper and easier way to get another ~4000 lbs of thrust. It's defnitely much cheaper (and lower risk) than developing a new core.

Agreed on all accounts, also looks great for long-term logistics and support contracts. (adds cheapness to all programs involved)

dwightlooi wrote:(3) It has been said that thrust falls off with higher bypass engines at supersonic speeds because the exhaust velocity is lower on such engines. This is not strictly speaking true. Newtonian physics say that force = mass x acceleration. The relative velocity of the exhaust to the velocity of the aircraft has zero effects on the thrust the engine produces. A 10:1 bypass and a 0.2:1 bypass engine will make the same thrust they did at Mach 2 as they did at Mach 0.7. The PROBLEM is that a 10:1 bypass engine has a big fat intake. This big fat intake is not just a hole, it also needs to be constructed to brake intake air to subsonic velocities before hitting the compressor face. So this big fat intake really acts like a big parachute adding drag. Hence, a low bypass engine has the advantage of being able to provide the same thrust wile incurring less ram drag. Because of this, the net thrust is higher -- net thrust of an engine being the thrust minus the drag associated with the engine. Having said that, the differnce in intake cross section between an aircraft using a 0.56:1 engine and one using a 0.20:1 engine (of the same output) is quite small when you take into account the overall cross section of the aircraft.

Have to disagree with you on this one. Take an 80" High BPR engine, say 10/1, that engine will have a much lower specific thrust, and higher thrust lapse than the same 80" fan with a larger core/lower BPR, say 2/1. The latter will fly faster/higher than the same 80" fan, frontal area and drag being equal. It's higher core/fan exhaust ratio will impart more velocity on the same amount of airflow and provide more thrust (same 80" fan remember but now getting more velocity) Fuel consumption will suck compared to the former, and noise will be much higher, but that's the price you'd pay to gain the top-end performance.

I doubt you'd ever get a high-BPR engine to MACH 2, the OPR of the fan would need to be too high, and then the OPR/TIT of the core would need to be even higher to drive the fan under those demands. RPMs and tip speeds will get too high, or the cores would get too small. If it WERE true, then fighter engines would be high-BPRs to save fuel, but still allow super-cruise. The trend of late seems to be lower BPRs (or steady) when it comes to fighter applications. The largest BPR 'fighter' engine I can think of is the F101 (BPR of 1.9/1) which could fly at MACH 2 in the B-1A, but that was with variable inlets and afterburners lit. The F101 in MIL would never support MACH 2 without burners, nor would the TF30, F100, F110, or F404 for that matter.

discofishing wrote:So, all the technical mumbo jumbo aside, it's possible to make the F-35 supercruise by replacing the engine or upgrading it?

Possible, but more likely would need to be accompanied by some aerodynamic 'tweaking' to reduce super-sonic drag and increased inlet performance to feed the engine 'better' air.

Keep 'em flyin'
TEG