Very interesting - I have a few questions - if anyone can answer them ...
1. Is that a diagram for the FGFA or just the twin seater PAK-FA?
2. Is there any plans for a bubble canopy on the PAK-FA or the FGFA? What type of coating are they planning? Or else how are they planing to keep the RCS low for the canopy?
3. What IS the planed / expected RCS for the FGFA - frontal, side and rear? Top view and bottom view?
4. Are the wingroot bays going to carry any A2A weapons? I read somewhere that the WVR missiles are supposed to be housed there - but this diagram is only showing gravity bombs.
Before we start making baseless assumptions on the radar cross section (RCS) of the latest designs some basic understanding of radar detection is necessary...
In radar detection, there are vital 'target resolutions':
- Altitude
- Speed
- Heading
- Aspect angle
In order to calculate those resolutions, a constant stream of EM energy across the distance will not help. All of those resolutions are in the time domain, as do most of everything we do: Measure in and by time indexes or (slices).
So instead of a constant stream of EM energy that give us no information in the time domain, we must create individual time slices or 'pulses'...
Each pulse has:
- Leading edge
- Trailing edge
- Duration
All three give us a characteristic call 'finite pulse length'. The corollary to 'finite pulse length' is a finite amount of energy per pulse. Therefore, a pulse has two time indexes: When the pulse begins life and when it ends. The radar system remembers this crucial information along with duration. Other pulse characteristics listed in the above illustration are for a different discussion. The above is applicable to all wavelengths, from the meters length HF/VHF/UHF bands to the ghz centimetric and millmetric bands.
For any wavelength, the shortest pulse we can create is that equal to one cycle. However, such a short pulse will deny us the ability to create other pulse characteristics such as the variable PRI and manipulate them for other purposes such as ECM and that is for another and more complex discussion.
It is then obvious that the greater the amount of time slices we have inside a duration and across a physical distance, the higher those vital target resolutions. The analogy here is high speed photography where the shutter speed creates time slices of visible wavelengths to give us those sports 'slo-mo' action. Or in nightclubs where strobe lights create 'jerky' and 'disjointed' movements. We want pulses that are greater than one cycle but not too long that it will give us too coarse information. This is a delicate balancing act for any system engineering team when designing a radar for a specific purpose.
What this mean is that each frequency band, from the meters length HF/VHF/UHF to the ghz centimetric and millimetric bands, has their advantages and disadvantages because of the characteristic 'finite pulse length' which equals to useful packets of energy. The centimetric and millimetric bands will allow us to create pulses that are much closer to each other to give high target resolutions the same way the camera's high shutter speed can give us so fine motion resolutions that we can 'slo-mo' the athlete's run.
Air defense radars will have different antennas transmitting different wavelengths because of these advantages and disadvantages:
- Long wavelengths give us long pulses that contains the highest amount of energy but the poorest target resolutions. This is useful for long distance search where coarse target information will suffice. For an airport, traffic at 200 km distance is not as important as traffic at 50 km.
- Low end centimetric and millimetric give us shorter pulses and finer grain target resolutions but because there is less amount of energy per pulse, we are restricted to use these bands at closer distances. For the military, it is for threat assessment and assignment, for the civilian airport it is for landing permission and priority.
- High end centimetric and millimetric give us even shorter pulses and higher target resolutions but the least amount of energy per pulse. Atmospheric attenuation (loss) can sap the pulse of its energy before it reach the target. This highest target resolution capability but against this disadvantage confined these bands to missile guidance, either from the ground or contained in a missile. At this point there no longer are potential threats. All targets are assigned as threats and missiles launched.
This is where the X-band proved to be useful as the best compromise for threat assignment and missile guidance =>
X band - Wikipedia, the free encyclopedia
The answer to the highlighted question is based upon some unofficial official 'laws' of RCS control measures for the military and under the fact that active cancellation is not yet available. The first and most important law is:
1- Target the threat frequency. In other words, the design should be shaped to act against the X-band.
Next are:
2- Use angled facetings technique to deny the seeking radar large expanse of surface areas as much as possible.
3- Use absorber (lossy) material whenever possible to control surface wave behaviors.
4- Enforce tolerances across surfaces.
5- Treat trailing edges to control edge diffraction signals. This includes plan forming all flight control elements.
6- Avoid corner reflectors of any degree whenever possible. If not possible, then avoid the 90 deg kind.
7- Avoid straight line cavities such as inlet tunnels whenever possible. If not possible, diffuse entrant signals prior to them entering said cavities. Inlet tunnels can create 'resonance' or 'ringing' in the EM spectrum that will exit both ends of the tunnels.
8- Shield one's own high-gain radar antenna from non-threat frequencies. In other words, use law 2 to deny other frequencies from exposing one's aircraft via direct reflection from the antenna.
9- Avoid surface discontinuities whenever possible. If not possible, see law 5.
Some examples of how these laws are applicable: During competition, the Boeing design had a single vertical stabilator, severely violating law 6 and forced the company out of competition. Law 6 is why all 'stealth' designs will have twin canted vertical stabs or in the case of the B-2 -- none at all, and law 6 is why all weapons will be carried internally. Laws 2, 5, and 9 are most evident on the F-117. Law 5 is applicable to all. Law 3 is minimally used on the F-22 and F-35.
Some critics of the F-35 called out the alleged fact that it is less 'stealthy' than the F-22. The critics missed the point completely that mission requirements can trump certain laws, in other words, mission requirements compelled the design team to focus more on some laws than others.
The result of the compromises between mission requirements and RCS control laws will give us the three generally accepted RCS shapes above. The process goes: Modeling, Estimation, and Measurement. With today's sophisticated software, the first two items can change positions but nothing is known until the measurement step, and the data from measurement will be hidden from the public.
