Stealth through the years
Many people believe that stealth is a recent development, but stealth has been around almost as long as detection systems to detect aircraft have been
around. As far back as WWII detection countermeasures were being used by both sides. By the late portion of the war, aircraft on both sides had been
developed that had reduced radar cross sections.
Acoustic Mirrors and Countermeasures
Prior to WWII, acoustic radar systems were in use. These systems are designed to detect the sound of incoming aircraft by using parabolic
"mirrors". Multiple large reflectors were placed along areas that aircraft may approach from to listen for them approaching. The systems weren't
very effective, and as aircraft got faster, their effectiveness decreased, as the time from detection to the aircraft being overhead decreased.
To counter this, the German aircraft would fly towards England with their engines unsynchronized. Normally a multiengine propellor aircraft flies with
both engines synchronized to run as smoothly as possible. This also causes the sound to be multiplied, which can be detected farther away. By flying
with the engines unsynchronized it changed the sound signature of the aircraft, so the detection range was decreased by the mirrors.
Radar and the Begining of Stealth
In 1935, tests showed that a radio signal could hit an aircraft, and be detected by a receiver. The first radar system detected an aircraft at 17
miles in June of 1935. By the outbreak of WWII radar systems were becoming more common, and by 1939 the British had a system that could be mounted on
an aircraft, allowing for night interceptions, without having to make visual contact first.
One of the interesting early stealth attempts wasn't even designed as a stealth design. The de Havilland Mosquito was originally designed as an
unarmed fast bomber, built almost entirely out of wood. It eventually became a multipurpose aircraft for everything from photo reconnaissance, to
target towing. It had a low radar signature, because of the wooden structure, along with rubber dampening structures in the aircraft, all of which
didn't leave much to reflect radar signals.
Later in the war, both the US and Germans had flown flying wing aircraft, which have an inherently low radar cross section, due to their shape. The
Germans eventually developed and flew the Horton Ho.229 flying wing. Lockheed Martin built a radar test model of the aircraft, and tested the RCS on
their test range. The tests were shocking, and showed that if they had gotten it into production in any kind of numbers, they could have caused a lot
of damage to the RAF. The detection time was reduced significantly, giving much less time to react and launch interceptors.
Flying wings have been found to be stealthy due to the fact that there isn't a large fuselage for the radar to hit and return from. The drawback to
a flying wing design, however, is that it is also inherently unstable and hard to control. Especially until recent years when computers were finally
able to compensate for the unstable airframe.
The Post War Years
During WWII, the Army Air Force started looking for a new bomber. Requirements were for a 10,000 pound payload, 10,000 miles, with a 275 mph cruise
speed, at 45,000 feet. This would allow bombing of Europe if Britain fell. In 1946, the first XB-35 flew. Thanks to politics, and other factors the
program was eventually cancelled and the aircraft scrapped in 1953.
The Cold War
After the war, the US began overflights of the USSR, using the Lockheed U-2. The aircraft was non-stealthy, and many attempts were made to make it
stealthy. In the 1950s, Lockheed attempted to reduce the RCS by putting printed circuits on the skin, to suppress S-Band radar. The next attempt was
to put wires near the leading and trailing edge of the wings and tail, that were 180 degrees out of phase of 70 MHz radar, so they would, in theory,
cancel the return. The result of both was to reduce the cruising altitude and speed, with a negligible affect on RCS, so the ideas were dropped.
When Lockheed flew the SR-71, they attempted several radar reduction systems on the airframe. Lead in the leading edges, a long flat shape, and other
features. As a result it had a reduced radar return compared to other aircraft. The aircraft never overflew the USSR, but it was the first aircraft
designed from the begining with stealth characteristics in mind.
The Petr Ufimtsev Years
Petr Ufimtsev was a Soviet engineer in the 1960s. He began working on a formula that would describe the reflection of lasers from two and three
dimensional objects. He was told by his superiors that his work was useless and was given permission to publish it internationally. After
publishing, an engineer at Lockheed realized that he had created a theory that would allow them to predict how radar would react to hitting an
aircraft, allowing them to create shapes and test how the EM energy would react to hitting them.
Lockheed used this formula to map out the radar cross section of the F-117 design. Later Northrop was able to use it to predict the B-2 RCS when they
were developing it. With the F-117, the model used, known as Echo-1, was only able to calculate in 2 dimensions, which led to a faceted shape,
designed to significantly reduce the RCS. The SR-71 had an RCS of 0.01 m2, or roughly that of a bird. By the time of the F-117, they had that down
to 0.003 m2, and it continued to drop on later platforms. Both the F-22 and B-2 have a public RCS of 0.0001 m2, using a more conventional shape, and
Stealth continues to advance, even as countermeasures do. The Air Force B-21 was designed to have a reduced cross section against UHF and VHF radar,
compared to the B-2, both of which are becoming much more prevalent in many parts of the world. Experiments have been done with active RAM, which
will work to counter the radar as it hits the aircraft, as well as with optical stealth, which will reduce the visual signature of the aircraft,
making it harder to see and visually intercept. No public RCS data has yet been released on the B-21, but with modern materials and shapes, it should
be significantly lower than even the B-2 and F-22.
This thread isn't meant to cover every single aspect of stealth, but to give a broad generalization of stealth through the years. I know that some
things have been left out, but at the same time, it gives a good picture as to just how long ago stealth work actually began, even inadvertantly.