Aces High Bulletin Board
General Forums => The O' Club => Topic started by: Wolfala on July 27, 2003, 02:48:29 PM
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This month marks the 60th anniversary of the first use of aluminum chaff
On the evening of July 24th, 1943, 791 crews of Bomber Command were briefed for the "big" operation planned for that night; this was to be the first "mass attack" raid on Hamburg under the codename Operation Gomorrah. The force of 791 aircraft consisted of 347 Lancasters, 246 Halifaxes, 125 Stirlings, and 73 Wellingtons. Of some concern to those in command - indeed, among the crews as well - was the increasing aircraft loss rate that Bomber Command had been suffering as the bombing offensive against Germany had progressed to this point in 1943. This was largely due to the increasingly sophisticated application of the new German radar systems being used in their air defense: the Würzburg radar operating at the frequency of 560 MHz used for anti-aircraft and search-light control; the Giant Würzburg radar, operating on the same frequency but having a more precise tracking capability and used for ground controlled interception (GCI) by which to home fighter aircraft on to the RAF bomber stream; and the Lichenstein airborne intercept (AI) radar equipment fitted to German night fighter aircraft, which operated at a frequency slightly below that of the Würzburgs at 490 MHz. The combined use of these systems was providing a potent air-defense capability, much to the concern of the RAF in general and these crews in particular.
However, things were to be a little different on this raid. Approval had at last been given for the crews to use what had been up to then a most secretly developed countermeasure capability against these radars. This was to be the first operational use of the material codenamed "Window," which was to have a significant impact on bomber survivability - and certainly on this raid.
British Window Development
The idea of placing reflectors in the sky to produce radar echoes was almost as old as the radar developments that it sought to confuse. As early as 1937, British researchers discovered that a radar could detect a strip of wire one half of a wavelength long suspended from a balloon at ranges up to 30 km. The radar-countermeasure implications of this discovery were obvious and readily appreciated by Professor Frederick Lindemann (later to become Lord Cherwell and chief scientific advisor to Winston Churchill when he became prime minister in 1940). But in 1937, Churchill was in the political wilderness, and Lindemann was keeping him abreast of scientific developments. In 1938 he had penned a memorandum on the danger of placing too great a reliance on radar for air defense in times of war. If there were large numbers of aircraft present in a small volume of sky, it might be difficult to track them. In addition, there was little interest then in developing radar countermeasures. At that time, radar capabilities had been developed independently in Great Britain, the US, Germany, Japan, and the Soviet Union. Although such systems were in service, or being developed for service, each nation believed that they alone possessed the secret of the device and saw no point in diverting effort into the development of a countermeasure or working on the means of defeating such a countermeasure within the radar system.
Although championed since 1937 by the distinguished British wartime electronic-warfare (EW) scientist, Professor R. V. Jones (whom some credit with being the first to suggest the concept), serious work on the development of metal reflecting strips as a countermeasure to radar did not begin at the Telecommunications Research Establishment (TRE) in Britain until 1941. As Professor Jones described, "A reflector is made of a simple wire or strip of metal of length equal to half the wavelength used by the radar...a few hundred strips would reflect as much energy as a whole Lancaster bomber." Lindemann, pressed by Jones, finally agreed that flight trials could be undertaken, which were conducted by Joan Curran, the only female scientist at the TRE at that time, in late 1941/early 1942. The trials were a great success. The materials tested showed a significant effectiveness against the metric wavelength types of radars used by the enemy, and progress was very rapid.
However, as a top-secret device it had to be given a codename. According to Albert Rowe, superintendent of the TRE at the time, Dr. Robert Cockburn in charge of countermeasures development called on him one day to discuss a codename for the device. Rowe thought it essential that it should bear no relation to the device itself. Looking around his room, he said, "Why not call it something like Window?" With no other suggestions, Window became the British code-name for the material.
In the trials, Curran had tested various forms of reflectors, ranging from wires, through metal strips, to leaflets the size of a notebook page. The idea behind the latter was that if the metal foil was sandwiched between two sheets of paper, it could be disguised as a propaganda leaflet, and indeed it was found that a bundle of some 250 sheets produced a radar echo approximating to that of a Blenheim bomber. However, it became clear that if the foil was as effective as the tests had indicated, it was naive to "think that an enemy would not be clever enough to notice that there was something special about the leaflets that had been dropped." Such trial success, and realization of its obvious intention, effectively killed the disguised-propaganda-leaflet idea.
Metal strips of both copper and aluminum were also tested; both were found to be equally effective, but aluminum was favored as it was a good deal cheaper than copper. The final form of the early material was an aluminum strip between 1 and 2 cm wide and approximately 25-30 cm long - around half the wavelength of the Würzburg/Lichenstein radars. The aluminum was paper backed to give rigidity; a lamp black coating on the aluminum and the use of black backing paper reduced its visibility and avoided light reflection from searchlights. The aluminum strips, in several hundreds, were assembled into packets, each weighing about one pound or so, and by April 1942, stockpiles of the material were sufficient for it to be brought into use by Bomber Command.
But there was great concern among the British scientific and command authorities as to the effect that the use of Window against the German radar systems might have on the British radars, should the enemy use such a device in retaliation - despite the fact that German bombing raids over Britain were nothing like the intensity of the British raids over Germany. It was clear that if Window were to be employed operationally, the enemy was certain to discover its method of working, albeit there was some intelligence information that indicated that they had indeed developed their own material. Either way, as such a simple device, it was equally certain that the enemy could rapidly bring the countermeasure into use for its own purposes. Moreover, tests had certainly shown that British radars working on frequencies between 200 MHz and 600 MHz were equally as vulnerable to the Window countermeasure as the German radars might be. Thus, an initial decision to use the material in May 1942, with it loaded into the aircraft, was overruled so emphatically that the material had to be off-loaded. From then on, although a high-level ban had been placed on the operational use of Window, tests were allowed to continue with the aim of determining the optimum tactics for its use and for developing counter-countermeasures against it.
Meanwhile, throughout the remainder of 1942 and into 1943, the arguments for and against its use raged on. The logic of the Window ban was even more difficult to appreciate against the fact that radar jamming had been approved and was being used against German radars without the same concern that they might use the same techniques against the British radars. The whole business culminated in a meeting of the chiefs of staff, in late June 1943, which was chaired by the Prime Minister Churchill himself. This time the arguments fell in favor of the use of Window, not only in an attempt to stem the increasing loss rate, but in part acknowledging that, in the summer of 1943, Bomber Command was a strong and increasing force, while the Luftwaffe bomber force deployed in the west was relatively weak and ineffectual, reducing the potential effects of retaliatory use. Convinced by the scientific arguments, mindful of the effect it use would have on the morale of bomber crews if it did work as the scientists predicted, and aware of this change in the military balance, Churchill approved the use of Window with the dramatic words: "Very well. Let us open the Window!"
Operation Gomorrah
There had already been a false start to the first Hamburg raid. Bad weather had forced the cancellation on the night of July 23rd, the first date on which Window use had been finally approved. (And subsequent to the meeting of the chiefs of staff in June, there had been concern that German retaliatory use following British operational use of Window might result in an increase in civilian casualties, an issue threatened to be brought up at the next meeting of the War Cabinet. However, Churchill declared it "too technical" for the War Cabinet and personally accepted responsibility for its use.)
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Among the bomber crews that night, there was great excitement and anticipation of their improved, which the use of Window might confer, chances of returning safely from this raid. This was not without some design. Once approval had been given for the use of Window, there was some concern among the scientific community that if it was not employed correctly using the proscribed techniques, then it all might go horribly wrong, with all the effort to bring it into service being wasted. Further, there was little encouragement from the commander in chief for there to be contact between his crews and the Air Staff, in particular the "boffins." Undaunted, and with a little subterfuge, Jones managed to get approval for a couple of his men to visit most of the Bomber Command airfields in a hectic fortnight prior to the day of the first planned use and brief the crews on the use of Window and its potential effect. The simple deployment technique was that the leading bomber in a stream would throw out a packet of Window at the rate of one every minute or so while in the area of potential radar contact.
On that night of July 24th, as the aircraft of Bomber Command entered enemy airspace dispensing the packets of Window as briefed, the effect of Window on the German radar systems met all expectations and predictions. The result on German air defenses was devastating. That night British radio monitoring stations picked up frenzied calls between German night fighters and their ground-control stations:
"It is impossible, too many hostiles."
"Wait a while, there are many more hostiles."
"I cannot control you. Try without your ground control."
"The enemy are reproducing themselves!"
The introduction of Window was a complete success. Out of an attack force of 791 aircraft, Bomber Command lost only 12 aircraft that night, or 1.6% of the force. Previous attacks against this type of heavily defended targets had resulted in a loss of some 6%, thus near 50 aircraft might have been expected to be lost had Window not been used. More personally, it can be said that some 300 aircrew were saved during that initial attack for an expenditure of some 90-million strips of metal foil.
For the remainder of the war, the RAF used Window in large quantities to safeguard its bombers during night attacks. This led to the formation of 100 Group in late 1943 to support the night bombing operations. Dedicated to the task of radar countermeasures and special operations (comms spoofing, aerial photography, etc.), 100 Group went on to comprise some 15 squadrons of specially modified Stirlings, Halifaxes, B-17 Flying Fortresses, and B-24 Liberators. With five squadrons dedicated to electronic radar jamming, they developed sophisticated Window-dropping spoofing operations that were laid on as diversionary operations to accompany most main-force operations. The objective was to confuse the enemy defenses on the direction of the actual attack and, more importantly, to hold back or divert enemy night fighters from their intended targets.
US Chaff Development
Throughout, the US had been tracking the UK's use of Window, which they called chaff (now the universally accepted term for the material). Dr. Fred Terman, head of countermeasures development at the Radio Research Laboratory (RRL) at Harvard, had been kept well informed of the British development work on Window. Terman recognized that much of the British research work had been empiric in nature, so he asked the distinguished US antenna expert, Dr L. J. Chu, to conduct a mathematical study of the electrical behavior of the dipole in their reflection of radar energy. Chu investigated the properties of radar energy scattering in the cross-section of rectangular sheets as a function of frequency and calculated the effect of varying ratios of width to length. Also, he developed formulas for the scattering cross-sections of cylinders and other geometrical shapes. Chu's findings led to the unexpected observation that applies even today: At frequencies even far off dipole resonance, the greater the length of a dipole compared to its diameter, or the narrower the dipole (length-to-diameter ratio), the greater the size of the radar echo produced by a given weight of foil. Realizing the importance of this discovery, Terman passed details of the discovery to his British counterpart, Dr. Cockburn at the TRE, where they were confirmed in tests.
As with the RAF, from the end of 1943, the consumption of chaff by the US 8th Air Force operating from Britain increased rapidly, which raised problems of production in the vast amounts now needed. While the paper backing and lamp blacking had soon been dispensed with, Window/chaff was being cut to size on the guillotines used by the paper trade. But the metal foil caused rapid blunting of the blades, and this made large-scale production difficult. The problem was solved by the RRL at Harvard. One of the RRL's scientists, Harold Elliott, came up with an ingenious solution to the problem, as co-worker Matt Lebenbaum explained:
"The British had done a lot of work on chaff, but they were having problems manufacturing it in the huge quantities required. Harold's chaff-cutting machine was the neatest thing you ever saw. It was rather like a lawnmower with 20 blades, every second one of which was ground back slightly. The blades were rotated at 800 rpm by an electric motor, and the first blade cut the foil; the second bent it along its length to give a 'V' shape for rigidity; the third cut; the fourth bent; and so on. With the blades rotating at 800 rpm, the cutter produced 8,000 chaff strips per minute. I think Harold Elliott deserves great credit for his little machine, without which we might never have produced enough chaff to protect our bombers."
The V-shaped crease in each strip was a relatively simple change that brought about a substantial improvement in chaff performance. The extra rigidity allowed the width of each strip to be reduced to 1.3mm, and that in turn meant that a heavy-bomber-sized radar echo could be obtained with only 84 grams of the material (one ninth of the weight originally required). Over 500 examples of Elliott's chaff cutter were manufactured, and the clever little machine went on to produce a large proportion of the chaff dropped by Allied aircraft over Europe. This was indeed a unique development, and this method of aluminum-foil-chaff production was to continue for the next 40 years.
To meet the US chaff requirements for WWII, a major producer of aluminum foil for the packaging industry - the Standard Rolling Mills in Brooklyn, NY, were placed under contract to produce the foil strips used for US chaff. But, as in Britain, there was intense secrecy surrounding the program, which effectively prevented the start of production for several weeks. Vince Lane, then the company's director of research, explained:
"At the time chaff was very highly classified, they wouldn't tell us what they were going to use it for. Chaff comprises half-wave dipoles, so you need to know what length to cut it. It was pretty difficult to make the stuff when the customer would not want to tell you what length he wanted, because that length was classified! In the end, we had to sit him down and say: 'Hey, buddy, where do we put the scissors?' Eventually we got him to tell us, but it took time."
With the US Army Air Forces operating from the UK also making large scale use of chaff, Lieutenant J. Dale of the 96th Bomb Group, a B-17 Flying Fortress unit based at Snetterton Heath in Norfolk, described the tactics used to counter the Würzburg flak-control radar as the aircraft ran in to bomb:
"Every plane in the lead combat wing of the division (about 80 aircraft) carries and dispenses chaff. They fly at altitudes ranging from 23,000 feet to 27,000 feet (7,000 m to 8,230 m). Each plane carries two cartons of chaff containing a total of 144 packages. The dispensing procedure starts four minutes before the target is reached and continues until three minutes after leaving the target. The material is dispensed at the rate of about one package every four seconds. This lays down a chaff lane of approximately 21 miles (34 km) in length.
"In these missions, dispensing operations are carried out by the radio operator. The window in the radio compartment of the plane has been removed, and a chute of about 4 inches by 4 inches (10 cm by 10 cm) cross-section has been installed in the side of the plane in place of the window. The chute is inclined at 45 degrees to the fuselage surface of the side of the plane and extends somewhat through the surface in such a way as to form a cowling, which produces a strong air current from the inside to the outside of the plane. When a package of chaff is inserted in the chute, it is forced out rapidly by this air blast and opens soon after it leaves the chute."
Chaff Today
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There are now only two major manufacturers of chaff in the Western world today, one of these is Armtec Defense (Coachella, CA), and the other is Chemring Countermeasures (Portsmouth, UK). Chaff for aircraft self-protection remains a key element of the electronic-countermeasures (ECM) capability of most of the world's combat aircraft - if not the only ECM capability for many. Dispenser systems and deployment techniques have become more sophisticated, and more complex tactics are employed in its application, but it continues to present itself as a real target against the most sophisticated radars, which it can defeat or reduce their ability to target track. Also, the material has changed radically in shape and form, being much shorter in dipole length to match the higher frequencies of modern threat radars and being made of aluminized-glass filaments having diameters around 25 microns - thinner than human hair. But it is its simplicity and cheapness that makes it a very cost-effective countermeasure, and chaff-dispensing systems will form an integral part of the EW fit in future combat aircraft such the US F-22 Raptor, the Eurofighter Typhoon, and most recently the F-35 Joint Strike Fighter.
Excrepts from Journal of Electronic Defense
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Unfortunately, it is becoming more difficult to test chaff.
http://www.globalsecurity.org/military/library/report/enviro/panelreport_f.pdf
http://www.globalsecurity.org/military/library/report/enviro/1997-08-01385.pdf
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"The enemy are reproducing themselves!"
lmao! sounds like the M.A.W!
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Of the 300 or so pages in those PDF's, which section(s) were the most damaging?
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The executive summaries pretty much cover it... the military considers the environmental impact to be neglible, although there are numerous special interest groups in the Fallon, NV area that would like to see the use of chaff discontinued.