Fundamentals of supercruise

CFE

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I recall reading somewhere (I believe it was James P. Stephenson's "$5 Billion Misunderstanding") that an aircraft capable of supercruise had to possess a high fuel fraction. This statement seems to fly in the face of reality. I would think that supercruise would be dependent on a high lift/drag ratio at the supersonic speed desired for cruise. Would anybody care to elaborate?
 

overscan (PaulMM)

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I would venture that supercruise

1) requires a low bypass ratio engine, which generally burns fuel faster than a high bypass one.
2) requires a higher level of engine thrust compared to subsonic economic cruise, thereby burning more fuel even if the SFC was comparable.
 

Abraham Gubler

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Drag, thrust at military power, etc are important to get the aircraft to fly supersonically without reheat but fuel fraction is what keeps it there. It depends on the aircraft but Stephenson was referring to the ATF/F-22. It needed a high fuel fraction to get some range out of the supercruise capability because its engines burn a lot of fuel at military power. The F-22A burns about half a tank of gas for the 200 NM of super cruising and the other half for takeoff, climb, cruise out 310 NM, cruise back 310 NM and land. The ATF was conceived to have a lot more fuel on board so it could do a lot more super cruising. That 200 NM of 'supercruising' lasts only 14 minutes.
 

KJ_Lesnick

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CFE,

There are a number of variables that determine good supercruising capability. They include both the engines and the airframe

For the engines, the qualities you'd want would include high exhaust-velocity (which generally requires a low bypass ratio, but not necessarily if you have a good pressure ratio), high pressure-ratio, good turbine-materials selection and fabrication, and air-cooling, for the airframe you'd want highly efficient inlets, a high supersonic L/D-ratios, and an overall good T/W ratio.


KJ Lesnick
"Them thats gots the gold makes the rules..."
 

AeroJadeXG

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Supercruise isn't actually that hard if it is the design point for the aircraft. The trick is getting good performance when 'off design'. The engine design needs to efficiently develop a shock train in front of the engine that doesn't cause major losses in flow total pressure. After slowing down the flow, you compress and burn it, then run through the turbo. At this point, the flow is still subsonic, so you run it through a C/D nozzle, and expand it out the back. All this is fairly easy for 1 specific velocity, and doesn't carry and particular fuel considerations. The other crucial element to supercruise is lowering the plane's overall drag. I should mention as well, and this is the part I can't speak to in regards to the F-22 as I know very few of its engines actual specifications, that the closer the exhaust velocity is to the velocity of the plane, the higher the efficiency, but the lower the thrust. I.E. since thrust goes like: T=Mmassflow*Vexit-Vinlet the massflow through the jet becomes very important. I imagine that there is a LOT of massflow for the F-22 if the supercruise is as efficient as some seem to suggest.

The nasty bit is getting a supercruise inlet that won't blow up the compressor in transonic flight. If a shock makes it into the compressor, you're toast.
 

KJ_Lesnick

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AeroJadeXG,

Supercruise isn't actually that hard if it is the design point for the aircraft. The trick is getting good performance when 'off design'.

Actually that's a very good point and is what makes designing high-performance aircraft so difficult. It has to perform well across the whole speed-range.

With that said, a good supercruise design from an airframe and aerodynamic-standpoint requires good L/D ratios at subsonic and transonic speeds, as well as a L/D ratio well suited to supersonic flight. It also would require inlets that not only would deliver an optimum pressure-recovery at cruise-speed, but would work well at all speeds from sitting still to mach numbers higher than the plane's cruise speed (for dashes). A good supercruise design should also have low-trim drag which is highly important for sustained supersonic flight.

The engine requires a high-exhaust velocity, and a pretty good overall thrust both at sea-level sitting still and at high altitude and high mach-numbers, fuel-efficiency at high-speed is very important, but a good all around SFC is also generally desired. It is also generally a good thing for the engine to be relatively lightweight and compact. This generally results in a low-bypass high pressure-ratio turbofan these days (The low-bypass is predominantly useful at low speeds for improved fuel-consumption although at high-speeds it produces some air-cooling benefits, the high-pressure ratio is predominantly useful for low-speeds and also to an extent to reduce some of the thrust-loss effects of the turbofan at altitude, however probably does produce some benefits at supersonic speeds). Since a lightweight compact engine is desirable, being able to squeeze as much pressure out of each stage is preferable, as a result, advance compressor geometry is generally a good feature, as is counter-rotating spools which eliminate the need for a guide-vane in between the two spools. Since airflow conditions will vary wildly from sitting still on the ramp at idle to racing through the stratosphere at supersonic speed, the engines need to be able to withstand these, advanced geometry helps this too as does variable guide-vanes. Because engines with high-exhaust velocities, high pressure-ratios all tend to produce high turbine temperatures, especially when you get 'em up at high mach numbers (though I suppose it depends on what speed you're going to fly at) highly efficient air-cooling schemes, and good turbine materials and fabrication are highly important. The ability to control the engines pressure ratio can be a nice touch as well, variable-guide vanes can effectively lower the AoA on the blades and drop the pressure ratio effectively at high-speeds. The J-58 and F-100 employed this particular design feature. While either the engine or the airframe can mount the nozzles, it is typically fitted to the engine -- the nozzle is highly practical in regards to producing the right exhaust-area whether idling or working at full afterburner (if applicable) so as to avoid engine surges at low RPM, and to optimize exhaust velocity at higher RPM's all the way up to full power and afterburner (if applicable). A high exhaust velocity is well-suited to high-speed so this is quite important for an airplane designed to fly at supersonic speeds for protracted periods of time as potentially small changes in the nozzle area can produce substantial changes in fuel-burn.

Note: The definition I am using for supercruise is an airplane that whether mounting afterburners or not can accelerate from subsonic speeds to a supersonic cruise speed using just dry-power without having to touch the afterburners. Airplanes that fit this description are the English-Electric Lighting, the Lockheed YF-22/F-22, the Northrop/McDonnell Douglas YF-23, the Eurofighter EF-2000 Typhoon, and possibly the Sukhoi Su-35. Airplanes like the Concorde and Tu-144D/LL's do not as they require burner to get up to a speed from which they can continue without afterburner. Planes like the B-58, XB-70, A-12/YF-12A/SR-71, and early Tu-144 models also do not qualify as they require continuous afterburner for cruise (though there is no dispute that the Concorde, Tu-144D/LL, the B-58, XB-70, A-12/YF-12A/SR-71 were all capable of sustained supersonic flight).

Supercruise aircraft, though for the nature of this post, seems to generally pertain to fighter-jets such as the F-22, are not relegated to just fighters. They can include bombers and even some supersonic-transport designs (The SST-competition by the late 1960's had already began to focus on scaled-up non-AB GE-4 designs, the HSCT program evolved into a totally supercrusing design).


KJ
 

AeroJadeXG

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Kj, I've been here 1 day, and you're rapidly becoming a favorite person of mine. Can I ask what your background is?
 

KJ_Lesnick

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AeroJadeXG,

Kj, I've been here 1 day, and you're rapidly becoming a favorite person of mine.

Thank you, I'm flattered.

Can I ask what your background is?

Oddly, I have no formal education in aerospace engineering. Education wise, I was a pre-med major, and attended med-school (I didn't graduate, but I attended).

My interest in aviation started out when I was a kid as I flew a lot as a kid either due to vacations, visiting family, or both, and I guess I wanted to know how the planes I rode on flew. At first my interest in aviation was predominantly confined to commercial aviation then began to branch out to military aviation as well. I'd say as a whole, the bulk of my knowledge of aviation comes largely from reading books (of which some are aerospace-engineering related, and others pertain to individual aircraft or aircraft engines) and asking questions online. I seem to have a photographic memory, and am generally good at remembering trivia, figures and small-details, and stuff of that nature.


KJ Lesnick
 

AeroJadeXG

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Have you gotten your hands on the Hill & Peterson "Mechanics and Thermodynamics of Propulsion" text? It's a truly terrible read, but it's also probably the most definitive text on air breathing propulsion in existence. Also, basic as it is, the Anderson text "Introduction to Flight" is probably one of the most informative books on all the basics (basic in aerospace engineering textbooks, you'll find, does not mean simple, but rather foundational) that exists. Anderson is by and large THE source on the study of aircraft, and this is probably his best book.
 

pathology_doc

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Airplanes that fit this description are the English-Electric Lightning, the Lockheed YF-22/F-22, the Northrop/McDonnell Douglas YF-23, the Eurofighter EF-2000 Typhoon, and possibly the Sukhoi Su-35.

Of interest are the design and first-flight dates of the first of these aircraft relative to all of the others. Though I freely admit the Lightning's limitations!
 

doolyii

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I've read few article about F-111 / Gripen / F-16 (with IPE or latest GE engine) going super-cruising too..If it's all about super-cruising only, high core thrust (low bypass, or near turbo-jet cycle) with lowest possible supersonic drag airframe would be the way...but like you say, including stealth, maneuver, payload requirements will make problem all too complex.

As for engine, I remember the airflow speed at fan face (and compressor) and turbine exhaust temperature (TET) were two major limiting factors. Supersonic fan/compressor design (can take faster air-stream flow without separation), Hotter (bigger) core for high thrust (advanced material, advanced cooling), and higher turbine temperature.

I cannot stop being amazed by F-22...with apparently higher wet area (for large fuel volume and internal weapon) and more control surface (and close-coupled like F-18...should have more drag than delta or V tail type) still managed to have this kind of super-cruising capability..I wonder whether it's coming mostly from F-119 or low drag airframe..

Does anybody have latest take off weight (typical air-to-air, air-to-ground) / fuel fraction / mission profile info available ?
 

Sundog

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Supercruise simplified;

1) Low drag, especially wave drag, natch.
2) High, installed, dry thrust. The F-22's F-119s achieve this by having a very high mass flow and higher operating temps than previous engines. See the IHPTET program, which has been replaced by VAATE for reference.

Of course, in the F-22's case, it isn't just about supercruise, it's about supercruise, combined with a use-able payload and low observability tech.. There are a few military aircraft that have the ability to supercruise, but without a relevant payload it doesn't mean anything.
 

pathology_doc

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AeroJadeXG said:
Also, basic as it is, the Anderson text "Introduction to Flight" is probably one of the most informative books on all the basics (basic in aerospace engineering textbooks, you'll find, does not mean simple, but rather foundational) that exists. Anderson is by and large THE source on the study of aircraft, and this is probably his best book.

Having now bought and read it - yes, yes, and yes again.
 

BDF

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Abraham Gubler said:
The F-22A burns about half a tank of gas for the 200 NM of super cruising and the other half for takeoff, climb, cruise out 310 NM, cruise back 310 NM and land. The ATF was conceived to have a lot more fuel on board so it could do a lot more super cruising. That 200 NM of 'supercruising' lasts only 14 minutes.
One wonders how the 49th conducted a training sortie by flying from HO to the UTTR at M 1.5+ then recovering at Hill if the F-22 burns as you say "half it's tank of gas" for 200nm. HO to UTTR is approximately 580nm straight line. Even allowing for climb and acceleration that's still on the order of 450-500nm. For your numbers to work the jet would use up all it's useable fuel and have none for start, taxi climb, descent and the require recovery reserve. This doesn't account for the fact that those numbers come up roughly 100nm short. The correct ratio of super to sub fuel burn is roughly 2:1. Even Stevenson states the F-22's specific range is roughly 0.04nm/lb of fuel at M 1.5 @ 40K vs. roughly 0.08nm/lb at M 0.9. and his numbers appear to be on the low side. The USAF stated in a 2006 AvWeek article the jet can supercruise for 41 minutes at "around Mach 1.5" which suggests a specific range of roughly 0.485 nm/lb of fuel.
 

Ogami musashi

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As i posted in the F-22 thread, a recent report by USAF on F-22 stated that the plane burns up to 4 times less fuel in supersonic than 4th generation fighter; since the MAX to MIL thrust fuel consumption is roughly 10:1 on those planes this means the F-22 would consume about 2,5 times more at mach 1.5 than in sub cruise mode; So it goes with stevenson low figures and actually if you do some maths you end up with the 310+100 radius which has an important parameter: it is without loiter time; So for sure the plane can supercruise far longer than that.


The max radius seems to be somewhere near 450 miles (without sub cruise).


This is still far from the original plan (200sub+400sup combat radius) but not that bad.
 

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