Airborne Infrared and Supersonic Stealth

[Above] The mighty SR-71 Blackbird (part of the A-12 program series) detailing its operational skin temperatures when flying at speed and altitude. The aircraft depicted is the two-seat trainer.

Interestingly, an airborne SOFIA-class Infrared threat model against a supersonic stealth platform (for which the F-117 and B-2 are not) - is totally absent - from seemingly comprehensive analysis of F-22 Raptor capability.

The NASA-DLR 'SOFIA' (Stratospheric Observatory for Infrared Astronomy).

If the Stratospheric Observatory For Infrared Astronomy, (SOFIA) is flown at altitudes of over 41,000 feet [12,497 meters] - nearly the entire infrared spectrum will reach its telescope and measurement sensors: "Missions will be flown at altitudes of 39,000 to 45,000 feet, above 99 percent of the water vapor in the lower atmosphere that restricts the capabilities of ground-based observatories over most of the infrared and sub-millimeter spectral range."

When one moves the Infrared Search and Track (IRST) discussion out of military circles and into a different scientific discipline, in this case, airborne infrared astronomy - then the entire subject matter shifts with respect to what is possible.

Too many at the DoD and in the US public - simply do not understand the relationship between transmittance/wavelength-propagation and altitude/atmospheric density.  The higher the IR sensor is - the greater the distances it can see a high-altitude object. This inability to grasp basic physics could cost the lives of F-22 aircrews.

‘Fast-jets’ (stealthy or otherwise) should be understood as “energetic aerial objects.” [Above] F-22 traveling at transonic speed, note the array of physics on display. The amount of “work” needed to propel an object from transonic through to supersonic speeds - will produce heat. The visible vapor shock wave pictured is a function of air compression and therefore, heating - ahead of the wave. This relationship is called thermodynamics.

[Below] Anytime a gas (air) is compressed "squeezed" - it heats up. This is often referred to as 'the heat of compression.' Typical areas of thermodynamic heating forward the engines (not part of propulsion/combustion) are due to the aerodynamic-frictional heating “compression” of the atmosphere (which is a gas) as airspeeds increase. Keep in mind the engines themselves also use 'the heat of compression' Gas Law to operate.

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[Below] The higher an IR sensor is flown, the more - the lower - transmittance %, (Y-axis) can reach the IR sensor.

[Above/Below] Once transmittance % starts to approach 100 percent, there is effectively nothing to interfere with the travel of IR wavelengths.

Comparison (see below) of the atmospheric transmittance between the Mauna Kea observatory at 4,480m [~ 14,700 ft] which is one of the best infrared ground-based sites, and SOFIA at 13,200m [~ 43,300 ft] altitude. Note the vastly expanded red-pink areas of SOFIA over the Mauna Kea line - both on - and extending off to each side - of the chart. F-22 Raptor IR signature will have no place to hide from a high-flying infrared IR sensor.

Combat aircraft like Eurofighter Typhoon (PIRATE), Sukhoi Advanced Flanker Series (OLS), Lockheed F-35 (DAS/ EOTS), and Rafale (FSO) Infrared Search and Track (IRST) sensitivity will also be more akin to an airborne SOFIA?

Stealthy F-22 may have little defense against detection from an IRST flown at high altitude(s) if Raptor is to be flown at high speeds and altitude, due to frictional heating of her airframe and canopy, in addition to (below) her exhaust heat plumes; in surrounding air temperatures of -30F, -40F, -50F, -60F degrees below zero.

Target RCS (Radar Cross Section) [below] is determined by 1) the power transmitted in the direction of the target. 2) The amount of power that impacts the target and is reflected back in the direction of the radar. 3) The amount of reflected power that is intercepted by the radar antenna. 4) The length of time in which the radar is pointed at the target.

The issue here is an IRST (IR sensor) can be "slaved" to the radar, to follow whatever the radar is tracking, or the radar can be slaved to the IRST to track whatever the IR sensor(s) "see." So in effect: 'Time on Target' (Pay close attention to numbers “1” and "4" of the graphic above).

This would be consistent with publicly available information stating that Stealth prefers hostile “scanning” radar(s) over hostile “tracking” radar(s).

Remember Stealth is effectively a 'radio spectrum airfoil' and its intent is to passively reduce the power of an opponent's "2" and "3" (see above graphic). Stealth has no capability to control "1" and "4". It could produce some type of radio/radar jamming-deception transmission(s), but this then potentially reveals its position. Not an easy problem to resolve for the Stealth fighter - that requires own/friendly radio energy transmission(s) to employ its primary weapon at range. Problems are compounded because, without own/friendly radio-transmission-targeting support, stealth aircrews could be forced to fly into the maw of modern, hostile, IRST detection envelopes before F-22 can fire its weapons.

The only American multi-sensor fighter in service at the time that could (with near certainty) detect and attack an F-22 (as Raptor was entering service with the USAF) - at militarily useful ranges - was the US-Navy Grumman F-14 (F-14D) Tomcat.

[Above] F-14D 'Super Tomcat' with her potent dual IRST-TCS sensor(s) system clearly visible under the chin.

 Photo: Lockheed Martin

[Above] F-35 EOTS Electro-optical targeting system under the nose.

The DoD would be quick to point out that the early F-14 Tomcat IRST (the AN/ALR-23) was of limited range, limited quality, and misidentified sources of IR emissions. However, a much-improved system was fielded in later USN Tomcats, the Northrop AN/AXX-1 Television Camera Set (TCS). On the F-14D Super Tomcat, the TCS was integrated with a dedicated IRST sensor, in a potent new dual-sensor package by GE Aerospace Electronic Systems. The detect/track range of the later GE Aerospace Electronic Systems Infra-Red Search and Track sensor (IRST) and TCS system fitted to F-14D, may have been in excess of 180km, 110 miles (97 nautical miles).

[Above] PIRATE-IRST on Typhoon.

Keep in mind that fighters like Flanker could employ their IR-version of the R-27ET medium-range missile; regardless of what happening (or not happening) in the radio spectrum, read: with less regard to RCS.

[Below] FSO system (IRST and IFF Television Camera) on Rafale.

[Below] Since the time of our original writing in 2008/2009 the 'Skyward-G' IRST system has been introduced on the Swedish JAS 39 Gripen series. The system can reportedly detect targets flying as slow as 300 knots - regardless of aspect - likely using some form of internal Atmospheric Propagation (target characterization) Modeling.

Photo: Saab

[Editors Note: Below - since the time of this original writing in 2008-09 images of the American F-22A have appeared with AN/AAR-56 Missile Launch Detector (MLD) that could evolve into an offensive IRST. However, this is conjecture presently. The condition of the F-22A LO (Low Observable) coating is another question, as is the future of the F-22A fleet - but both are outside this discussion.]

The Raptors AN/AAR-56 window can be seen here. 

Also, Flanker does not use radar to track an aggressively maneuvering dogfight target for its gun-firing solution. Only its IRST with laser rangefinder is needed. So it’s a fair statement to say Flanker's IRST has a robust air-to-air capability (IRST-OLS on Flanker is NOT THE SAME unit as MiG-29).

Some might argue that SOFIA looks up into space while an IRST must look out across the airspace for a target in the atmosphere. The problem is distance. SOFIA looks at celestial objects - millions or more miles away - while IRST targets need only be 50-100 miles away.

Since the time of our original writing in 2008/2009: the USAF 64th and 65th Aggressors Squadrons have begun incorporating (rushed?) the airborne IRST threat into their DACT. Whether lessons learned are disseminated up the chain of command to the DoD - remains unknown. Our concern is that stealth fighter programs will remain shielded and protected even when it is shown that stealth fighters can not deliver to justify their ultra-high unit cost(s).  

According to Tom Clancy (Fighter Wing: A Guided Tour of an Air Force Combat Wing - Clancy Tom; Penguin Group, 1995), the F-22 Raptor is an 'F-15 Eagle’ weapons platform, in a stealthy supercruising airframe. The only problem with Clancy's description is that the F-15 Eagle was simply no match for the Tomcat's combination of radar (+ IRST) and AIM-54 Phoenix missile. No match. Tomcat could also use AIM-9 and AIM-7 (AIM-120?) at longer ranges than Eagle. In fairness to Clancy, he dedicates one (1) sentence in his book, stating that long-wave infrared could be a problem in the future.

Remember that IRST integration into the weapon system may produce an aircraft that is highly resistant to the "Beaming" / "Beam-turn" / "Doppler turn" maneuver used by an opponent to break radar locks - because the target now presents increased engine heat aspect(s) to the sensor.

Iranian Tomcats would routinely orbit on station at ~ 13,000m (~ 40,000 ft) during the Iran/Iraq war to extend their endurance.

[Above] The US-Navy F-14 Tomcat could direct six (6) ultra-long-range AIM-54 'Phoenix' rounds at six (6) separate targets simultaneously. The Tomcat weapon system quickly scans aerial targets and creates a target “track-file” for each target.  Each track (file) represents where the target(s) should be at a ‘future three-dimensional point.’ It then loads these coordinates to her AIM-54s. At weapon(s) release, Phoenix climbs to 26,000m (80,000 ft) and accelerates to Mach 4+ towards its assigned track "file" coordinate. The weapon then dives down (using kinetic energy) to kill the target. The target profile presented to the AIM-54 is - the top of the target - not head-on. The top of a target (stealthy or otherwise) provides the largest possible: surface area, physical size, and RCS aspect. F-14D (Infrared+radar) targeting in this way, with Phoenix alone - would have presented an unacceptable DACT (Dissimilar Air Combat Training) hazard for the new USAF F-22 Raptor.

Historical accounts over the Barents-Baltic Seas in the mid-late 1980s suggest that this will – indeed – be the case. The Soviet Mig-25 PD/PDS Foxbat and in particular, the MiG-31 Foxhound interceptors would routinely track American SR-71 Blackbird reconnaissance sorties on the Warsaw Pact border using only the MiG's infrared (IRST) channel. They would simply lock onto the immense thermal signature of the SR-71 flying at speed and at altitude – reportedly from ranges of over 100km (62 mi). Both Foxbat and Foxhound could then illuminate Blackbird with their main radar (cued by IRST) had the order been given to attack. 

[Above] Soviet MiG-25PD 'Foxbat-E' interceptors were a major threat to Detachment-4 SR-71 flights over the Barents/Baltic Seas. The MiG-25 Foxbat was originally designed to shoot down the high-flying Mach-3 XB-70 bomber. After 1976, the Soviets upgraded the base MiG-25P to include a new 'Sapfeer-25' radar, IRST, a BAN-75 target identification system, as well as improved R-15BD-300 engines. Note the size of the MiG's R-40 missiles in relation to the pilot. Intercepts of Det-4 Blackbirds over the Baltic were conducted by the 787th IAP flying MiG-25PD out of Finow-Eberswalde airbase (GDR).

The Foxbat could employ four massive R-40 missile rounds, (two radar and two IR homing). Foxbat R-40s were designed specifically for targets at high altitudes. The SR-71 would have, with almost certainty, been unable to cope with the R-33s from MiG-31 Foxhound - targeted in this manner.

[Above/Below] A retractible IRST on the MiG-31 is called the 'OMB’ optical multi-functional apparatus. 

OMB shown retracted.

Taking a second look at this graphic, heat management of the airframe would have been a critical function for Blackbird crews. In recent years, folklore has sprung up around the aircraft and it also incorporated stealth technology. This would have been inconsistent with Kelly Johnson’s views on stealth at the time (and in particular at the time of Oxcart). Given the extremes in metallurgy, propulsion, and thermodynamics being confronted by the design - low radar cross-section (low RCS) was unlikely a design goal consideration either for the A-12 or its derivatives; the YF-12, SR-71 (or even the D-21 drone). The Russians always knew the American spy plane was inbound several hours beforehand - due to the support asset activity that always preceded the Blackbird's arrival. It is important to remember that immense afterburner plumes are generated from Oxcarts massive J58 engines, that could be detected at great distances.

Whether a supersonic Raptor can close and maneuver into firing position unseen by a modern/future IRST system should be a source of debate and secrecy for some time. These new IRST systems are so sensitive that even weapon release of an opponent’s missile round can be detected from its rocket plume and its nose cone heating. Suffice it to say that IR sensor design cycles will certainly - outpace - stealth airframes.

[Below] F-22A 'Pitot' tube is clearly visible at the nose. The Raptor has one tube on either side. These air measurement devices are electrically heated to ~ 270 C (520 F) to keep them from icing at altitude. Indeed, they appear as two hot spots on the nose under FLIR (Forward Looking Infrared).

The IRST ‘fishbowl’ (below) on this Flanker is clearly visible just ahead of the front canopy. Note it's physical size. The sensor is fully integrated into the weapons system and can cue thermal targets for the radar, pilot, or both. During helmet-sighting: sensors, radar, and missile seeker heads - look wherever the pilot looks.

Laser-rangefinders operating in the IR wavelengths would also have extended ranges at high altitudes?

The IRST might also use its own stored 'Atmospheric Propagation Model' to effectively “make an educated guess” as to target(s) relative range, aspect, and velocity – without the radar or laser rangefinder. In effect, the sensors own performance is characterized to construct a sensitivity model against known objects at known distances and velocities. Then during wartime when IRST sees something - it compares its own “known” internal Atmospheric Propagation Model - and the weapons system then extrapolates target range and bearing.

[Above] Sukhoi IRST on both Su-35 Flanker (left) and T-50 (right). Either spells big trouble for F-22. Note the simply immense heat distortion shimmer in the background - heat produced by all-powerful aircraft - stealthy or otherwise, regardless of national origin.

John C. Mather, Senior Astrophysicist at the Goddard Space Flight Center, Mars Society, the University of Maryland 31-July, 2009; during his remarks on Doppler (red/blue) shift detection resolution-granularity in discussing astrophysics of celestial objects: “…we are able to see the velocity of a star down to one (1) meter per second.”

Advanced Flanker IRST Doppler-shift sensitivity will not require the granularity of astrophysics because an F-22 Raptor traveling at Mach 1.1 will be moving at approximately 374 meters per second. By including air combat closure rates - this figure is even higher (!)

This would seem to fall well within, the definition of militarily useful sensitivity?

So in effect, Advanced Flanker variants could affect the engagement of a supersonic radio-spectrum airfoil (an F-22) using - all available sensors - in five (5) ways:

1.)True-Positive (Doppler): IRST uses infrared Doppler-shift w/APM to determine target range.

2.) True-Positive (Laser): IRST uses infrared and verifies the range to target with laser range-finder.

3.) True-Positive (Radar): IRST uses infrared and verifies the range to target with radar.

4.) True-Positive (Cycle): IRST uses infrared and verifies the range to target by cycling through steps 1-2-3-repeat.

And finally...

5.) Conceptually one can act on a - 'False-Positive' - even if stealth is 100% effective in the radio spectrum:

a) IRST picks something up.
b) Point your radar at it.
c) No (or strange) radar return? = stealth.
d) That is not ours.
e) Select R-27T / R-77-1 / R-37M class weapon - 'Fox!’

Editor Note - Since the time of our original writing - as of March 2021 the USAF claimed a cooperative target kill w/AIM-120D of 87 nm (161 km), 100 miles; in a controlled test. Also since our original writing, the VKS (Russian Air Force) has killed a Ukrainian aircraft with R-37M at some 177 km. 

Because the R-37M published range is ~ 400 km - the noncooperative combat kill in Ukraine of 177 km is ~ 44 percent reduction in the R-37Ms published figures. To compare apples-to-apples a 44 percent reduction factor must be applied to the AIM-120D for noncooperative targets under actual combat conditions. This gives us a range of ~ 49 nm (90 km) and 56 miles respectively for the AIM-120D (for noncooperative combat employment).   

Now putting it all together we can calculate with these assumptions:

• F-22A is traveling at 65,000 ft at Mach 2.
• F-22 can fire its primary weapons 56 miles from the target.
• Su-35 moving at 400 mph. 
• 98-mile IRST detection range.
• Speed of Sound at 65,000 ft is 660 mph.
• Stacked sections of Su-35s at 45,000 and 43,000 ft, contra-rotating "lazy 8" or "racetrack"

IRST on MiG-31 (called ‘OMB’ optical multi-functional apparatus) published range is ~50 km (~ 27 nautical miles). Yet during SR-71 intercepts in the late 1980s, the OMB was able to track and target the Blackbird from below at over (over) 100km (54 nautical miles). This is double the OMB published range. Su-35S IRST OLS-35's published range is 80 km, so we assess at least ~ 160 km (~ 86 nautical miles) as a plausible detect range for a high-speed 'hot' high-flyer + IRST sensitivity improvements since events in the 1980s. 

AIM-120D range, noncooperative (reduced by 44 percent) for actual combat conditions:

49 mn

90 km

56 miles

OLS-35 IRST published range:

43 nm

8o km

49 miles

x2 IRST detection range based on historical OMB vs. SR-71 (high, fast mover):

86 nm

160 km

98 miles

Subtract the AIM-120D range from IRST x 2 range increase (to get range delta). AIM-120 launch platform must travel these distances inside the IRST detect-sphere before it can fire:

37 nm

70 km

42 miles

Mach 2 at 65,000 ft is 1320 mph. AIM-120D speed ~ Mach 6 (Mach 4 + Mach 2 of F-22), so our hypothetical AMRAAM flies ~ 66 miles in one minute. Also hypothetical instantaneous acceleration of AIM-120 to Mach 6.

660 mph divided by 60 minutes = 11 miles per minute. 11 miles per minute x 6 (six-times Mach 1 of 660mph) = 66 miles traveled per minute at Mach 6 = 1.1 mile per second.

Divide the IRST detect range delta by mile per second AIM-120 speed; 42 / 1.1 = 38.18 seconds;  for Flanker flight to detect an AIM-120D launch/launch-platform and react.

So roughly Su-35 has a ~ 30-mile downrange engagement countermeasure window of ~ 30 seconds. 

Our hypothetical attacking AIM-120 friendly mid-course update detection by Flankers halfway to target regardless at ~ 19 seconds before weapon arrives.

i) Actual F-22 speed is more likely Mach 1.7
ii) Actual Su-35S speed likely 350 mph

In this scenario, the F-22A flies into the Su-35S IRST detect envelope for ~ 42 miles before the F-22A can release its primary weapon. 

There is just no way the F-22 maintains an element of surprise with a high/fast attack profile. IR wavelength transmittance-propagation is simply too great - as by definition - there are not enough gasses and vapor in the atmosphere to absorb the IR wavelengths. Even if we take the USAF's 100 mi test as a valid AIM-120D weapon release distance - it is still very much on the edge of higher altitude IRST detection range(s).

Whether the short-finned AIM-120 used in the F-22 has the control surface authority to get close to a 3-D thrust-vectoring fighter at higher altitudes - is unknown.

The only question in this scenario is what can (will) the Russian shoot to try and hit a high-fast mover? Possibilities could include derivatives of R-27, R-77-1, R-77M, R-77ME, R-37M? Or something we haven’t seen?

[Above] R-77 ARH (Active Radar Homing) and IR homing R-27T medium-range class weapons on this Russian Air Force Su-35S in Syria in 2016. The R-27T is on the outboard station - right side of photo.

[Above] Though not designed to hit a 2-D thrust-vectoring stealth fighter, oddly enough the Russian AA-6 Acrid R-40TD would have some capability against a high-speed, high-altitude target. Though originally designed to knock down an XB-70, the weapons large fins and large warheads are specifically designed to operate in high, thin atmosphere. The R-40 can be employed from a launch aircraft flying at high speeds.

This discussion is in a way - academic. The Russians have already identified two main areas to exploit supersonic Raptor.


They revolve around, and loop back into these two issues:


• F-22A Primary weapon.
• F-22A Thermal signature.


The infinite logic of this - is clear. If Raptor attempts to improve her kinematic situation by using high altitudes and high speeds – she will increase her thermal exposure. Any attempt to mitigate thermal propagation issues – by lower speeds or altitudes - directly impacts the power (reduces the range) of Raptors primary weapon.


There is no escape…

F-14 Tomcat w/the Northrop AN/AXX-1-TCS (IFF camera).

The F-14D 'Super Tomcat' had been slated to remain in service through at least 2008, but was retired from USN service in mid-2006 under former US Defense Secretary Donald Rumsfeld. When the Navy asked for a replacement AIM-54 capability - it was denied. Tomcat aircraft were summarily shredded and manufacturing dies were destroyed for reasons allegedly involving Iran. However, the Iranians had already been producing their own F-14 spares/electronics for their F-14A-GR fleet, since the mid-1980s with US assistance via the Iran-Contra Scandal.

A good view of the IRST-TCS on this F-14D.

The 1st Fighter Wing at Langley, Virginia, declared the F-22 Raptor operational on 12-Dec, 2007.

Your thoughts?

[Be advised that Google has deleted this original writing link dated 2009. Original publish date can be cited however by using the date stamps of the comments in the comment section]

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