Blip/scan: a different kind of "stealth"

[Archibald]

I kept reading that A-12 & Fish-Kingfish were designed for stealth, and did not understood how this could be, being Mach 3 rather than HAVE BLUE / F-117 subsonic.

I've found the answer to that puzzle at least. It wasn't RCS reduction (at least not to F-117 level) but rather a fascinating stuff called "Blip scan".

Google Search

www.google.com

More detail later, I have to go to work. But it explains at least the why of the A-12 & Kingfish and how could they pretend to be stealth while flying at Mach 3 with a different shape than HAVE BLUE.

Kingfish

www.globalsecurity.org

Also it explains somewhat the D-21 drone. That one seemingly tried to get both blip/scan and RAINBOW reduced RCS (since it lacked the U-2 cockpit and intakes).

 

[Archibald]

The Wikipedia entry seems pretty good.

Blip-to-scan ratio - Wikipedia

en.wikipedia.org

And this is the reason why they pushed so hard on speed and ceiling after RAINBOW failed to turn the U-2 into, well, a reduced RCS aircraft. The intakes and cockpit did not wanted to cooperate.

Most salient points

Consider a typical early-era jet aircraft flying at 1000 km/h. With every complete rotation of the antenna, which takes 10 seconds, the aircraft will move 1000 km/h = 278 m/s * 10 = 2780 m, a little less than 3 km. On a display with a 300 km radius this represents a movement of only 0.5% across the display's face (600 km diameter), producing a tiny line segment between the two dots that is easy for an operator to interpret as an aircraft.

But if the target speed is increased, its movement becomes more pronounced on the scope, making it less recognizable and more difficult to track. At Mach 3 (3500 km/h at 25,000 m) the same ten seconds of movement represent over 1.5% of the display's face. At this point, the slowly moving dot turns into a series of dim individual spots, which can more easily be mistaken for clutter. Additionally, since the spots are separated on the display, the returns no longer "add up," potentially reducing the returns to the same level as background noise, making it invisible.

An operator seeing a line of small dots across their screen might eventually recognize the return as an aircraft. To frustrate even this, aircraft were designed to fly as high as possible. Assuming a high-speed aircraft flying at 90,000 feet, or 27 km, this means the aircraft will be above the maximum angle of the radar when it approaches within about 100 kilometres (62 mi) of the station. Assuming it is first detected at 250 kilometres (160 mi), that means it is only visible over a range of 150 kilometres (93 mi). At Mach 3, this means it would be visible, even in theory, for about 3 minutes. This leaves very little time to arrange an interception.

A replacement for the U-2 had been under consideration even before its operational missions began. Originally these studies focused entirely on the reduction of the radar cross section (RCS), but after Franklin Rodgers introduced the idea of spoofing the blip/scan in 1957, the plans were changed to research high-speed, high-altitude designs instead. Lockheed calculated that in order to be effective against known Soviet radars, an aircraft would have to travel between Mach 2 and Mach 3 at 90,000 ft and have an RCS of about 10 square meters. This led to a number of proposals which were down-selected to the Lockheed A-12 and Convair Kingfish.

Wait, so this mean Rainbow failure plus blip/scan red herring delayed (somewhat) stealth by two decades ? Or maybe not, since 1950's computers would be unable to compute RCS of even "Hopeless diamond", HAVE BLUE and F-117.

It was during the development of these aircraft that problems with blip/scan avoidance became clear. It was discovered that the high-temperature exhaust of these aircraft engines reflected radar energy at certain wavelengths, and persisted in the atmosphere for some time. It would be possible for the Soviets to modify their radars to use these frequencies, and thereby track the targets indirectly but reliably.

It was also realized that since blip/scan avoidance relied more heavily on a problem in Soviet displays rather than in the principles of radar, changing these displays could render the technique moot. A system that recorded the radar returns in a computer and then drew the targets on the display as an icon whose brightness was independent of the physical return (a system in which returns did not have to "add up" in order to appear on the display) eliminated the potential for operator confusion. This was particularly worrying, because the USAF was itself in the process of introducing precisely this sort of display as part of their SAGE project.

Finally, the introduction of the first effective anti-aircraft missiles dramatically changed the game. Radars for plotting an air intercept were generally made as long-range as possible in order to give the operators ample time to guide intercept aircraft onto the target as it moved across the display. This led to low blip/scan ratios and inaccurate prediction of aircraft trajectories. This had been compounded by the difficulty of quickly scrambling intercept aircraft.

Missiles solved both of these problems. Missiles stations guided their missiles with their own radar systems, which had maximum ranges only slightly longer than the missile's own flight range, about 40 km in the case of the SA-2 Guideline; therefore they had much higher PRFs, and as a result the blip/scan problems were greatly reduced. Defenders would still have the problem of finding the target in time to prepare for a missile counterattack, but this was by no means as difficult or as time consuming as scrambling manned aircraft and relying on the radar operator to guide them onto the target before the aircraft left radar range.

By the time the A-12 was operational in the early 1960s the blip/scan avoidance technique was no longer considered useful. The A-12 never flew over the Soviet Union (although it came close to doing so) and was limited to missions against other countries, like Vietnam. Even here the performance of the aircraft proved questionable, and A-12s were attacked by SA-2 missiles on several occasions, receiving minor damage in one case.

 

[jsport]

would a unmanned bomber flying m3 @ 90k 'then still make sense?

 

[Archibald]

What, the Valkyrie ? that one had an IR and RCS signature the size of Mount Rushmore... it wasn't optimized for stealth or blip/scan by any mean.

 

[Archibald]

Good summary of how the U-2, despite valiant efforts, failed at stealth.

The birth of stealth: Lockheed and the CIA

Paul Crickmore explains early US efforts to hide aircraft from radar following the shooting down of Gary Powers’ U-2.

www.key.aero

Note that the effort was not totally vain: they managed to significantly reduce the U-2 RCS. But the inlets, cockpit and exhaust ruined the effort (and there comes the F-117 with a peculiar cockpit, weird intakes and bizarre exhausts... makes a lot of sense).

Two U-2s dedicated to Rainbow were based at Watertown, Nevada, (a purpose-built base for the clandestine operation, later known variously as The Ranch and Area 51). Modified flight-test results revealed that ‘Trapeze’ reduced 70 MHz returns by 20 decibels (dB) and S-band returns by about 10 dB, so overall, the aircraft’s radar cross section (RCS), was reduced to half its earlier detection range. But the “draggy” modifications reduced the U-2’s range by 20% and its altitude by 5,000ft (1,524m).

In an operational evaluation, two U-2 penetration flights of the Soviet Union were made from Turkey on July 21 and 30, 1957. Analysis of the aircraft’s System 5 (a multiband radar recorder), revealed Soviet radars had detected the U-2’s presence when flying directly towards and directly away from the radar head. Further tests showed the source of these returns had emanated from the aircraft’s inlets, cockpit and exhaust – none of which could be treated by what had thus far been developed. Clearly a far more radical approach was required.

 

[red admiral]

From Rainbow to Gusto: Stealth and the Design of the Lockheed Blackbird (Library of Flight) https://amzn.eu/d/3ZaJabd

There's plenty more in here

 

[Fluff]

would a unmanned bomber flying m3 @ 90k 'then still make sense?

Sadly not, I think the technique or 'hack' is only applicable to a human radar operator, having to 'remember' what he was tracking. Doppler and computers, and lots of memory have rendered this obsolete.

Having said that, B21 is the closest we are going to get, for now........maybe.......

 

[quellish]

I kept reading that A-12 & Fish-Kingfish were designed for stealth, and did not understood how this could be, being Mach 3 rather than HAVE BLUE / F-117 subsonic.

I've found the answer to that puzzle at least. It wasn't RCS reduction (at least not to F-117 level) but rather a fascinating stuff called "Blip scan".

KINGFISH and the A-12 were designed for stealth. Their goals were aggressive for the capabilities of the time. The A-12 was designed to have an RCS of around 1m2. One of the reasons that these aircraft were designed for high altitude and high speed was that previous efforts to design a subsonic aircraft with low RCS and high altitude (GUSTO, etc.) could not be made to work.

In the 1970s there were more serious efforts to reduce RCS as a primary design goal. The Navy, Air Force, and Army all had programs looking at reduced RCS aircraft. Several RPV programs demonstrated promising results. Ken Perko at DARPA had several contractors study what RCS values would be "game changers" against projected air defense threats. The output of those efforts set the goals for the XST program.

If the XST design had been produced even a few years earlier it would have gone nowhere. There would not even have been a way to measure it accurately.

 

[Dilandu]

Basically the idea was to exploit the limited capabilities of 1950s cathode ray indicators, and became obsolete with the display improvements.