Definition and history
Beth Hagenauer from Dryden has defined supercruise as an ability to fly supersonically without using afterburner; USAF Flight Test Center at Edward Air Force Base defines supersonic speeds as being above Mach 1, without regard for transonic region (which is different for all aircraft – F-16s transonic region is from Mach 0,9 to Mach 1,1, and Gripen’s is even narrower). According to this definition, list of supercruisers is quite long. English Electric Lightning prototype exceeded Mach 1 on dry thrust on August 11, 1954, and could achieve Mach 1,22 without reheat. Term “supercruise” was actually first applied to the Lightning. Mirage IIIO with Avon engine was able to reach Mach 1,3 in 1962. F-104 Starflighter was capable of maintaining Mach 1,1 in level flight in military power, and in fact could maintain it for 15 minutes. A clean F-16 (that is, no external stores except two missiles at wingtip stations) can cruise at Mach 1,1. With 6 missiles, Gripen C can cruise at Mach 1,1, Gripen E at Mach 1,3, Rafale C at Mach 1,4, Typhoon at Mach 1,5. F-22 with 8 missiles can cruise at Mach 1,7. Tornado F3 is also capable of supercruise. As it can be seen, supercruise is nothing new or special.
But just the ability to fly at supersonic speed without afterburner is not enough if it does not give an operational advantage. Two to one advantage in cruise (or maximum) speed is of no use if it only lasts for half a minute. Therefore a fighter aircraft has to be able to achieve a supersonic flight for a useful period of time with standard air-to-air load, regardless of wether it is done with or without the afterburner (even though many take form over the function and state that supersonic flight without reheat is automatically supercruise). Necessary supersonic cruise performance has been defined here as 20+ minutes of supersonic patrol after a 200+ mile (322+ km) subsonic leg to the enemy territory. This should also include combat.
Efficient supersonic flight at high altitude (above 50.000 feet) requires engines to be in a low afterburner. Fuel fraction required for effective supercruise is over 40 per cent.
Supercruise capability is important in several aspects of air to air combat. First is the fact that fighters cruise at their maximum cruise speed when in the combat zone. Higher cruise speed allows pilot to surprise the enemy by approaching him from the rear, zone of poorest detection, and to avoid getting surprised by a slower-cruising opponent. It also allows the fighter to choose a time and place of engagement.
In the beyond visual range combat, supercruise capability increases range of the missile shot, and reduces the effective range of adversary’s missiles. If pilot decides to pursue a merge or a visual-range attack pass, its excess kinetic energy again allows it to dictate terms of the engagement. It can also offset a possible situational awareness disadvantage – knowing where the enemy is is of little use if you can’t engage him. In fact, it could be easily possible that supercruise, not radar stealth, is the main advantage of the F-22 over the US legacy fighters – chase fighters were not able to keep up with the F-22, and modest 20 degree heading changes caused teen fighters to abort intercepts, having exhausted fuel supply.
Comparing some of supercruising fighters
F-22s maximum supercruise speed is greater than Mach 1,4, most likely between Mach 1,5 and 1,7. While F-22 isn’t any more stealthy than the F-117, its higher cruise speed compresses the enemy’s OODA loop.
At 30.000 feet, range for the F-22 is 0,02 nautic miles per lb of fuel at Mach 1,5 (1.636,5 kph; 589 kts), and 0,066 nautic miles per lb at Mach 0,9. 20 minutes of supercruise equals 196 nautic miles covered for expenditure of 9.817 lbs of fuel, and two minutes of afterburning combat uses up 1.377 lbs of fuel, leaving 6.806 lbs of fuel. This allows the F-22 a combat radius of 224 nautic miles or 415 kilometers. Realistically, radius will be less than that as this calculation does not account for takeoff, landing, or climb, leading to borderline useful supercruise performance.
Rafale’s maximum supercruise speed is Mach 1,4 with 6 missiles.
Fuel consumption is 2.250 l or 1.800 kg per hour at Mach 0,9 (987 kph at 30.000 feet). At full dry thrust, fuel consumption is 8.000 kg per hour, and 26.250 kg/h at maximum afterburner. Thus, 2 minutes of combat uses up 875 kg of fuel, while 20 minutes of supercruise use up 2.667 kg of fuel, leaving 1.178 kg of fuel. This allows for a flight time of 39 minutes at cruise thrust, allowing a combat radius of 320 km. Again, the calculation does not account for takeoff, landing or climb, meaning that Rafale will have to refuel in the air after the takeoff or carry a centerline fuel tank to have useful supercruise performance.
F-35 cannot supercruise according to either definition used. Only way for it to achieve supersonic flight at dry thrust is to achieve maximum speed with afterburner (Mach 1,6) and then switch to dry thrust. After that, it takes 150 miles for the F-35 to decelerate to subsonic speed. But to cruise means to sustain the speed. Following is the quote that started the “F-35 can supercruise” hype:
“What we can do in our airplane is get above the Mach with afterburner, and once you get it going … you can definitely pull the throttle back quite a bit and still maintain supersonic, so technically you’re pretty much at very, very min[imum] afterburner while you’re cruising,” Griffiths said. “So it really does have very good acceleration capabilities up in the air.”
As it can be seen, it can only maintain supersonic flight at low afterburner settings, which would be around 30.000 lbf or 13.608 kgf. At SFC of 2 kg/kgf h (real value is far higher as the cited value is for the most efficient – maximum – afterburner setting), it will be consuming 27.216 kg per hour, or 9.072 kg in 20 minutes. Two minutes of combat will use up 1.300 kg of fuel. Since F-35C has fuel capacity of 8.860 kg, and both other variants have less than that, it means that unlike the Rafale and the F-22, F-35 cannot achieve even the barest minimum requirement to be defined as a supercruising aircraft. Its cruise speed at maximum dry thrust is Mach 0,95. F-/15 is actually in similar situation – a clean F-15 at Mach 1,3 would maintain supersonic speed for several minutes in dry thrust.
This quote gives further proof that the F-35 cannot supercruise:
“While a fine bomb-hauler and (one hopes) a good multi-service airframe, the F-35 is a mediocre performer. The problem with the F-35 … is speed. It doesn’t have the capability to supercruise. Speed lets us get inside the decision cycle of the bad guy.”
1st Fighter Wing commander Brigadier General Burton Field
The F-18 transonic region ends at M 1,21, F-16 M 1,15, Rafale M 1,08, Typhoon M 1,05. Ergo, Gripen C flying at M 1,1 will be either just outside or very close to leaving the transonic region.
F-35 is supposed to supercruise for 150 miles at Mach 1,2. At 40.000 ft, turbojet would have its fuel consumption reduced to 1/3. This reduction will be far less for a turbofan, especially one with comparatively high bypass ratio such as F135. Still, assuming such reduction, F-35 would consume 3.696 kg/h. F-22 has a requirement for 20 minute supercruise, during which it consumes 55% of its internal fuel. For 150 miles at Mach 1,2, F-35 would need 11,4 minutes, for fuel consumption of 700 kg. Even assuming 1.400 kg of fuel consumed for supercruise, that is still only 17% of F-35’s internal fuel. In other words, F-35 might be able to achieve supersonic speed without afterburner, but that cannot be compared to supercruise capability of F-22, Typhoon, Rafale or even Gripen, because it can do it only with such a limited fuel load that it is useless in practice.