Super cruise

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F-22 in transonic flight

A supercruise is the ability of a fighter aircraft to fly continuously faster than sound without an afterburner . The term supersonic cruise was first used in the 1970s in connection with civilian passenger aircraft such as the Concorde , which can fly over long distances at supersonic speeds . A group of officers and analysts from the US Air Force took up this idea and around 1980 asked for a supercruising fighter that could fly for 20-30 minutes or 200-300 NM at Mach 1.4-1.5 without an afterburner. Since fighter planes rarely always fly in a straight line, the term supersonic cruise and maneuver is mostly used in research .

idea

The aircraft should only have one engine and be smaller than an F-5E . A thrust-to-weight ratio of over 1: 1 in dry thrust with combat load and 1.2: 1 with post-combustion was targeted. Since the aircraft should be as cheap as possible, the turbojet engine with three to four compressor stages should only have a total pressure ratio of 10: 1 to 12: 1, a turbine inlet temperature of around 1400 K and a thrust-to-weight ratio of over 10: 1 have. Complicated avionics like radar were deemed superfluous; the armament should consist of infrared guided Sidewinder missiles, and anti-radar Sparrows (Brazo). In order to achieve an acceptable range despite the high specific consumption of the low-tech engine, a fuel mass fraction of 0.40 was aimed for. A tailless delta or double delta was proposed as the wing . The idea was taken up and further developed by the military-industrial complex . The following advantages have been identified:

  • Greater endurance in supersonic flight .
  • Higher kinetic energy at the beginning of the dogfight, which u. a. increases the effective range of your own missiles.
  • The range of air-to-air and surface-to-air missiles is dependent on angle and speed. An R-77 has a range of 100 km head-on, but only 25 km in pursuit. These ranges increase or decrease as the target flies faster. By skillfully maneuvering in a faster aircraft, shots can be fired at the enemy, while you can escape the enemy weapon range by quickly turning.
  • Higher mission rate (i.e. combat missions per time) as distances can be covered faster.
  • No afterburner even with hot and high starts , which saves fuel.
  • Critical areas in which FlaRak systems with ambush tactics could lie in wait are passed faster.
  • Combat aircraft with a higher dry thrust-to-weight ratio can "sit out" fighting in turns until the enemy runs out of fuel.
  • More effective interception, or more effective evasion of enemy hunters.

Although both sides flew planes at top speeds of Mach 2+ in the Vietnam War, they were mostly at Mach 0.5-0.9, and only very rarely faster than Mach 1.1. The reason for this is that, due to aerodynamics and propulsion, these fighter aircraft are dependent on the afterburner to reach supersonic speed, which would limit the mission time to a few minutes.

implementation

aerodynamics

While almost 50% of the air resistance is due to surface friction, about 30% to the induced air resistance , about 10% to the surface roughness, about 5% to the wave resistance and about 3% to the interference resistance, this composition changes in supersonic flight. Here approx. 35% is generated by the wave resistance, 25% each by induced and frictional resistance and 5% each by interference, roughness and others. It is therefore obvious to reduce the wave resistance, which is usually done by a stronger sweep . By definition, the following applies to the arrow:

  • <35 ° subsonic aircraft
  • 35 ° to 50 ° Transonic optimized with moderate supersonic properties
  • 50 ° to 60 ° overshoot optimized with moderate transonic properties
  • > 60 ° supersonic aircraft

The second largest drag is the induced drag, which should be reduced by a higher glide ratio in the supersonic (lift to drag). The glide ratio (English Lift-to-Drag, L / D ), which for an F-16, F-18, F-4B, ​​F-111 and similar in supersonic (Mach> 1.2) at about 4–5 must therefore be increased. Since a fighter aircraft almost never flies straight ahead for longer distances, this parameter is particularly critical: Older fighter aircraft lose a lot of speed and energy when maneuvering in supersonic due to the low specific power excess and the low glide ratio, so that they quickly fall back into subsonic. That is why military super cruisers are optimized for the highest possible glide ratio in the supersonic. This in turn depends largely on the drag coefficient at zero lift in supersonic conditions c w, 0 . For this purpose, the volume of the aircraft must be distributed over the greatest possible length, according to the area rule in supersonic. Furthermore, a low wing loading and a thin profile are required.

F-106: Supersound optimized, but not very maneuverable

In practice, the concept of building a delta wing with a strong sweep and low aspect ratio on a needle-shaped, area-regulated aircraft reaches its limits. Modern combat aircraft should be highly maneuverable in supersonic and subsonic conditions, with low energy losses for high permanent turning rates. The classic Convair F-106 is therefore ruled out, although its afterburn boost could easily be achieved dry with modern engines.

In the subsonic, the profile resistance depends proportionally on the wing area, while the induced resistance is proportional to the square of the span load (W / b) ². In order to achieve higher turning rates, a large extension is required. Since the sustained turning rate in subsonic depends on thrust and air resistance, and the latter depends 75% on the induced drag when turning, this is a critical parameter. At the supersonic level, the permanent turning rate depends on the thrust and wave resistance, which is significantly influenced by the area rule and a low aspect ratio. Thus there is a conflict of objectives.

A number of measures are possible to solve the problem: An unstable design can increase the maximum lift by over 20%. The trim resistance in supersonic flight, about 10-15% of the maximum lift, can be reduced. Especially deltas with their rather poor ratio of lift to drag benefit from this, which can be improved by combining it with canards. Furthermore, the profile curvature can be manipulated in flight by means of buoyancy aids in order to give thinner supersonic profiles a more favorable polar in the subsonic. Pumping fuel in flight to reduce trim resistance is also an option. The cockpit can also be better integrated into the fuselage and the vertical stabilizer can be dispensed with.

Two F-16XL double deltas in flight

NASA started the Supersonic Cruise and Maneuver Program (SCAMP) in 1977 to develop a supercruise-optimized wing, which led to the General Dynamics F-16XL. General Dynamics and NASA examined over 150 different configurations in more than 3600 wind tunnel hours before the double delta wing was chosen. The 60 m² wing with a 70 ° / 50 ° sweep significantly reduced wing loading and wave drag, at Mach 2.2 a glide ratio of over 9 could be achieved. In subsonic the glide ratio did not change compared to the F-16. The frictional resistance increased by 22%, the internal fuel quantity by 82%. The maximum angle of attack increased from 33 ° (22 ° loaded) to 60 ° (50 ° loaded). The 9 g envelope should be doubled and go well into the supersonic range. However, since simulations revealed deficits in the flight control software, the flight envelope of the machine was limited to 7.2 g. Furthermore, the leading edge of the wing was connected to the fuselage with an S-line in order to reduce the increasing instability at high angles of attack.

Other designs such as the Experimental Aircraft Program (EAP) should start with a double delta with a 60 ° / 40 ° sweep. Ultimately, 57 ° / 45 ° was implemented in this delta canard design in order to improve the permanent turning rates in the subsonic through a higher aspect ratio. With the double deltas, the cone vortex of the inner delta reinforces the vortex that begins at the bend to the outer wing, which reduces the induced drag. Furthermore, the "missing wing piece" increases the aspect ratio due to a lower lift surface, which can improve the permanent turn rates in the subsonic, provided that the increasing wing loading does not destroy this. A simple delta with a 53 ° sweep was therefore used in the EFA.

Alternative aircraft concepts, such as the Supersonic Tactical Aircraft Configuration (STAC) developed by Grumman and NASA from 1977, should achieve a cruising speed of Mach 2 with a delta wing with a sweep of 57 °, with a glide ratio of 6. From 1983, an arrow-shaped wing was also included a Supercritical Conical Camber (SC 3 ) provided, which should be combined with the canards that are far forward. The SC 3 wing is conically curved along its length in order to generate a supercritical flow at its leading edge, which becomes subcritical in the interior. The wing is optimized for supersonic maneuvers and has a very favorable glide ratio. The trapezoidal wings later chosen for the afterburnerless but still Mach 2 capable Northrop-Dornier ND-102 , however, have very low frictional resistance. Although the wave resistance is less favorable here, a lower total resistance can still be achieved in the supersonic. The design was later transferred to the YF-23 .

drive

Despite the desire to complete the supersonic cruise flight without post-combustion, the fuel consumption in supersonic flight is high because a high thrust is required. As a result, a large amount of internal fuel must be carried in order to enable useful application radii. The following applies to the fuel mass fraction:

  • <0.29 subcruiser
  • 0.29 - 0.35 quasi-super cruiser
  • > 0.35 super cruiser

The choice of drive technology can be determined in a system comparison. The state of the art is 2010, the aim is to achieve speeds of up to Mach 4. In an afterburnerless turbojet , with a combustion chamber inlet temperature of 900 K and a turbine inlet temperature of 1900 K, a maximum pressure ratio of 46: 1 at Mach 1.2 is required to achieve the optimum consumption value of 30 g / kNs. The required pressure ratio in the compressor drops rapidly with increasing speed through the inlet, since the combustion chamber inlet temperature is limited due to the lack of cooling. A maximum speed of just under Mach 4 with 48 g / kNs is achieved. The specific thrust drops by -39%, while consumption increases by +67%. Since the pressure ratio in the compressor is above Mach 3 and below 4: 1, the “dry” turbojet as a drive above Mach 3 is inefficient.

If a turbojet is combined with afterburning and a nozzle inlet temperature of 2000 K is assumed, the following values ​​are calculated: Although the afterburner burns inefficiently, at Mach 1.2, around +38% specific thrust increase with +40% specific fuel consumption is determined. The efficiency of the afterburner increases with increasing airspeed; at about Mach 4 + 25% thrust with + 6% consumption is achieved. The cause lies in the higher turbine outlet temperature.

Ramjet engines are ruled out, as they are more or less an afterburner without a gas turbine, with inefficient combustion. At Mach 4, the specific thrust of the turbojet with post-combustion is achieved, but the specific consumption of 58 g / kNs is higher than that of the turbojet with post-combustion, which only reaches 50 g / kNs. At slower speeds the disproportion is even more drastic. Ramjets as a propulsion system for future combat aircraft are therefore a Hollywood fantasy. In the experimental area, however, there are and have been aircraft with ramjet engines that can manage well above Mach 4, but cannot take off on their own like the Boeing X-43 .

Turbofans without post-combustion with a bypass flow of 0.1 to 1 achieve lower specific fuel consumption, but the difference depends on the speed. The higher the Mach number, the lower the influence of the bypass ratio. With a bypass ratio of 1: 1, the optimal overall fan pressure ratio at Mach 1.2 is 5: 1, and at a top speed of Mach 3.6 it is almost 1: 1. Compared to the turbojet without afterburning, the specific thrust at Mach 1.2 is around −40% lower, with only −17% consumption reduction. At Mach 3.6, the consumption advantage over the turbojet is only -2.5%, but the thrust is -49% less. A turbofan engine without afterburning with a bypass ratio of 1: 1 is therefore unfavorable for supersonic flight.

Concept of the ADVENT engine

If the above-mentioned turbofan engine is combined with an afterburner, the specific thrust at Mach 1.2 is lower than that of the turbojet with afterburner; the difference is negligible at high Mach numbers. The consumption is always higher than that of the turbojet with afterburner, since the afterburner inlet temperature is lower and therefore more heat / fuel has to be supplied to the inefficient afterburner.

In theory, a turbojet (with afterburning) is the best solution. However, the engine of a supercruising fighter must also have top performance in the subsonic and not only shine in the supersonic, similar to the aerodynamics. In practice, a turbofan engine (with post-combustion) is therefore selected to reduce the specific consumption in subsonic, but with a very small bypass ratio. The long-term goal is therefore engines with a variable bypass ratio that can reduce the bypass flow to almost zero in the supersonic and maximize it in the subsonic. Examples of this are the AL-41F from the MiG-MFI or the General Electric YF120 .

Critical technologies are also inlets, which should have the highest possible total pressure recovery at the desired Mach numbers, and convergent-divergent nozzles. Military research programs such as Integrated High Performance Turbine Engine Technology (IHPTET) , which aim to produce propulsion systems for cruising speeds of Mach 3+, are also striving to increase the thrust-to-weight ratio to 20: 1 for the engine, around the thrust-to-weight ratio to improve the fighter aircraft. For this purpose, lighter materials and higher turbine inlet temperatures are being researched. Programs such as ADaptive Versatile ENgine Technology (ADVENT) should research variable bypass flow ratios and better integration of afterburner and nozzle in the airframe.

System considerations

Today's super cruisers can only cover short distances in supersonic cruising flight. If z. For example, if a specific consumption of 22.2 g / kNs is assumed for the Eurofighter and Raptor, the theoretical endurance in supersonic flight can be calculated for both systems. The F-22 has 8200 kg of fuel for 19700 kg of empty weight, which results in a fuel mass fraction of (8200) / (8200 + 19700) = 0.29. The endurance is:

8200 kg / (2 × 116 kN × 0.0222 kg / kNs × 60 s) = 26 min

For the Typhoon there are around 5000 kg of fuel per 11000 kg empty mass, which results in a fuel mass fraction of (5000) / (5000 + 11000) = 0.31. The endurance is on the combat setting:

5000 kg / (2 × 69 kN × 0.0222 kg / kNs × 60 s) = 27 min

At a cruising speed of around 1800 km / h (Mach 1.8), a maximum of around 800 km can be covered, provided the internal tanks would run dry. During Operation Allied Force , the distance from the Gioia del Colle military airfield to Belgrade was about 530 km as the crow flies. Conceptually, the following variants are possible:

  1. Subsonic flight with drop tanks and weapons to the target, dropping them on site, and supersonic flight home. The best solution for heavy external weapon loads. The penetration can be done subsonic. Through the use of weapons, interceptors are alerted, which can be escaped.
  2. Subsonic flight with drop tanks or supersonic flight with aerial refueling over the Adriatic , supersonic flight to the destination and back, aerial refueling over the Adriatic, supersonic flight home. Only with light external armament or internal loads. High usage rates; the enemy area is flown over in permanent supersonic flight, which makes the enemy air defense more difficult.
  3. Subsonic flight with drop tanks to the destination and back, the tanks are dropped when they are empty or the aircraft needs better flight performance. Mission profile like a subcruiser , the supercruiser capability is only used when required. Most missions will follow this pattern, since combat aircraft are mostly used for CAPs or as bomb trucks.
Raptor with external tanks

As can be seen, the problem is the insufficient fuel mass fraction of the machines. With (2) in particular, continuous refueling becomes a problem when the opponent goes on the offensive . A hypothetical variant of covering the entire distance to the destination with a light load in supersonic conditions fails due to the lack of fuel for the return flight. Supersonic flight with external tanks may be possible, but it is extremely inefficient: In subsonic, only about half of the content is available to increase the range, the remaining fuel is used to overcome the aircraft's higher aerodynamic drag. There is no rule of thumb for supersonic , but the disparity is likely to be even more unfavorable. With a “correct” fuel mass fraction of 0.35 or higher, Typhoon or Raptor would have to carry at least 6000 kg or 10700 kg of fuel internally. The Eurofighter will still have the option of having conformal fuel tanks with around 2000 kg of fuel on the fuselage. Assuming that 72 kN dry thrust per engine can compensate for the additional resistance of the CFTs, ​​the endurance is:

(2000 kg + 5000 kg) / (2 × 72 kN × 0.0222 kg / kNs × 60 s) = 36 min

Without external loads, a maximum flight distance of around 1100 km would result at a cruising speed of around 1800 km / h (Mach 1.8). If external loads reduce the speed to 1400 km / h (Mach 1.4), the flight distance drops to 840 km. It is interesting in this context that neither the drafts of the US Air Force nor those of Grumman and NASA, or the ND-102 developed explicitly on Supercruise, had weapon bays. The ATF drafts also had none before the stealth requirements were tightened. No reasons are given. It is conceivable, however, that this was not done because the majority of the weapons used in the air war are 500-pound bombs or AGM-88 HARMs . The latter alone fired over 2000 pieces in the Second Gulf War . In order to carry an acceptable amount of these weapons internally, a very large weapon bay would be required, which is impractical for agile combat aircraft. The loss of speed due to air-to-air weapons is also low: an F-4E with 4 × AIM-7 can reach Mach 2 instead of Mach 2.2 without weapons. An F-15A / C with 4 × AIM-9, 4 × AIM-7 and an object on the lower fuselage achieved Mach 1.8 instead of Mach 2.5. The increase in air resistance compared to an unarmed aircraft can be greatly reduced by using weapon mounts that conform to the fuselage or fixed start rails. The return flight to the base, half of the flight route, is practically unarmed anyway.

Individual evidence

  1. a b c d e f POGO: Comparing the Effectiveness of Air-to-Air Fighters: F-86 to F-18 , April 1982 (PDF; 5.9 MB)
  2. a b c d e f g h i j k l Ray Whitford: DESIGN FOR AIR COMBAT , Jane's Publishing Inc, ISBN 0-7106-0426-2 online ( Memento from January 4, 2012 in the Internet Archive )
  3. Mason / Virginia Tech: 10. Supersonic Aerodynamics (PDF; 12.5 MB), accessed September 9, 2013
  4. ^ B. Probert: Aspects of Wing Design for Transonic and Supersonic Combat Aircraft , British Aerospace, 1998 ( Memento of May 17, 2011 in the Internet Archive )
  5. a b c W.H. Mason: Some Supersonic Aerodynamics , Virginia Tech (PDF; 71.1 MB), accessed September 9, 2013
  6. NASA: Flight Test Results for the F-16XL With a Digital Flight Control System , March 2004 (PDF; 619 kB)
  7. NASA: Review of Cranked-Arrow Wing Aerodynamics Project: Its International Aeronautical Community Role , 2007 (PDF; 2.2 MB)
  8. a b NASA: Control Definition Study for Advanced Vehicles , November 1983 (PDF; 11.7 MB)
  9. NASA: A Wing Concept for Supersonic Maneuvering , November 1983 ( Memento from April 29, 2014 in the Internet Archive ) (PDF; 4.3 MB)
  10. I. Kroo: UNCONVENTIONAL CONFIGURATIONS FOR EFFICIENT SUPERSONIC FLIGHT , Stanford University, 2005 (PDF; 1.2 MB)
  11. POGO: The F-22 Program: Fact Versus Fiction , 2005
  12. a b c d e f g NATO RTO / Joachim Kurzke: The Mission Defines the Cycle: Turbojet, Turbofan and Variable Cycle Engines for High Speed ​​Propulsion ( Memento from February 17, 2013 in the Internet Archive )
  13. Flightglobal: US propulsion looks beyond ATF , May 27, 1989 (PDF; 2.1 MB)
  14. F-4E sustained g turn capabilities
  15. Doghouse-Polt of F-4C / D / E and F-15A / C  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.worldaffairsboard.com