Thank you Ozair for that very good link. Even though it is like 6 years old, it is the best paper on stealth I have read.
I'm going to add some remarks in underline to Mxaxai's comments just to extend the discussion.
I figured it might be nice to discuss current and future threats to stealth aircraft, since this might also give us a better idea what technologies '6th-gen' fighter jets or new bombers may desire.
Obviously there are different areas:Radar
- Current stealth jets are optimised around certain wavelengths. New radars that use lower or higher frequencies more effective than current radars can exploit that.
- Stealthy aircraft are not invisible, even current radar will pick them up if they get too close. Aircraft must avoid known radar locations. Stealth provides a reduced cross section not eliminated. Reductions can be 10 to 20 fold or better, a B-2 appears to be the size of a cruise missile or drone, not a 737, but still visible.
- Stealth often means reflecting radar in certain directions. Aircraft must avoid pointing these directions towards known radar locations. Yes, stealth is directional, the most reduced signature is the front, really crappy stealth from above or below with the broadside of the wing showing.
- Many current radar guided systems have the radar receiver and emitter in the same place. Dispersed, cooperating systems could exploit the directional radar reflection. This takes massive computational power and often signal conditioning to reset the 'clock'. The varying distance between sensors and the target and the travel distance from each sensor to the central processor. It appears the US has mastered this, the Chinese may have, but it takes being at the forefront of tech to do properly. Electronic jamming can really screw this up also, imagine the jammer sending out say 4 echos following the initial echo all at different delays, basically 5 echos with only one being real.
- Many current radar systems focus on few radars with large range. These are easy to avoid by stealth aircraft. Future systems may look into increased radar numbers dispersed over a large area, possibly through drones or simply much more but cheaper/smaller radars (quantity over quality, short range over long range protection).
- Stealth aircraft must not create contrails, water droplets are visible on radar and other sensors. What about icing conditions or flying through rain? All powered object create disturbances in the media it is flowing thru, yes it is the water droplets in the air that are seen but the jet wash creates lots of disturbance, detecting these disturbances is tricky to discern over ambient but it can be done.
- Stealth aircraft must remain clean. Could we see future detection systems exploit dirt on the airframe? Perhaps disperse an aerosol that coats the aircraft in something that reflects radar?
: Getting an infrared signature from the front is quite difficult, but yes any missile seeing the exhausts is a real problem.
- Engines are still hot. 6th-gen aircraft may study improved exhausts.
- High-speed flight heats up the airframe through friction. New paints or cooling systems may need to be developed, or stealth aircraft need to fly slower. Hypersonic jets are unlikely.
- Resolution and sensitivity of infrared sensors is improving. We may see new systems that can compete with short-range radar.
- Infrared lasers can be used to measure composition and velocity of the atmosphere. Could the resolution improve to the point that small disturbances like an aircraft exhaust are detectable?
The US Air Force would kill for a paint that made things invisible. But normal haze makes visibility difficult. An F-35 coming at you at 2 miles is very hard to see in sufficient time to respond. But the visual camouflage of WW2 planes is no longer done, meaning it is no longer effective at this time.
Acoustic: I'll discuss acoustic below.
- Stealth aircraft feature camouflage paint but are not invisible. Improved image processing software may help detect aircraft at short ranges, e. g. terminal guidance on missiles.
- A coarse location through low-frequency radar can guide enemy fighter aircraft into the vicinity of stealth aircraft, who could then use alternative methods (like the mk. 1 eyeball) to detect the stealth jet.
- LIDAR is already usable for short-range guidance (infrared or visible). Range may increase with future developments. Countermeasures could be active (anti-LIDAR-laser) or passive (absorbent paint).
Magnetic: Every Trident sub before its tour is demagnetized, but it is 19K tons of mostly steel, not sure if aluminum and titanium have a lot of magnetic properties. This greatly reduces the magnetic signature but scrambling each molecules profile instead of being fully aligned.
- Every aircraft creates noise. Dispersed ground sensors can form an acoustic array to locate the source. Current processing power is insufficient to resolve the location in real-time.
- Sonic booms may be easier to resolve, though. Stealth aircraft may need to remain subsonic, or develop quiet supersonic flight, if anti-aircraft systems use acoustic sensors.
- Observers on the ground may alert the target that a fighter jet recently passed them. High-altitude flight may be preferable.
- Again, might be useful as a terminal guidance system on missiles. Certainly more difficult and easier to confuse than image processing, though.
- Any aircraft disturbs earth's magnetic field by a tiny amount. Probably not by a detectable amount for the foreseeable future.
What do you think? Am I forgetting a key aspect or did I get something entirely wrong?
Totally missed electronic emissions: How does the plane emit active radar, transmit a river of data, handle sending radio. If laser up to a satellite, how does it air sufficient to provide a signal. Probably the largest weakness for an aircraft.
Acoustics: Submarines in water really can only be detected by sonar, with some ability to capture magnetic signature, cavitation and heat from the sub's propulsion. Active sonar is little used right up to the attack, torpedoes can use active sonar, passive sonar, along with heat to acquire the target.
Current submarines have a large passive sonar array at the bow with the sensors in a fluid of the same properties of the sea water within the nose behind the plastic nosecone. They also tow an array behind that is very long to get excellent 3D 360 degree detection. I have heard reports sonar operators can discern between male and female dolphins at 20 miles, quite amazing. The US sono buoy network basically tracks all ship and submarines around the world almost like flightaware does for planes.
Acoustic silencing is amazing, one cannot hear a 300 HP pump operating on the test stand at 5 feet, pumps are tested regularly for acoustic signature. Imagine making a pump that totally avoids cavitation. Same with the 15 foot diameter propellers, that are basically shrouded minutes after drydocking and takes a classified badge to enter the propeller machine shop.
With aircraft, acoustics are quite difficult as the plane is travelling toward you at .85 the speed of sound. So the sound locates the aircraft position quite late. If supersonic, the signal arrives after it flies overhead.
Reviewing the Turkish S-400 being located next to an airport with F-35's. It can download all kinds of data keyed to a known object location at close range. All those signature and signals can be recorded and analyzed at close range. Like the sonar operator in Hunt for Red October, it could detect at slow speed the cat drive signature once it knew what to look for. It could also learn what triggers the F-35 friend/foe response if it knew what to broadcast, etc. Far different than a station that sees limited F-35 activity at 10 miles. But there are probably covert listening stations near Hill AFB recording the same.