Lone Sentry: German Area Smoke-Screening (WWII Tactical and Technical Trends, No. 24, May 6, 1943)

Numerous reports indicate that the Germans have found the use of smoke screens at night to have considerable protective value against enemy bombing attack. This has been particularly true of the coastal areas, where unscreened coastlines offer a ready means of calculating distances accurately. In inland areas, where targets are not so easily located, smoke screens are generally limited to the protection of precision targets of prime importance. However, there are indications that several of the larger inland cities are protected in this way. In addition, decoy smoke screens have been effectively employed upon a number of occasions to protect nearby targets.

Relatively few instances of daylight screening have been reported to date, and on the whole the practice has not proved particularly effective. With the increasing frequency of daylight raids over the Continent, it is to be expected that the Germans will take steps to protect vital target areas with efficient smoke screens both day and night.

The Germans have used smoke extensively for screening:

(1) Battleships, docks, and naval installations;

(2) Oil refineries and storage;

(3) Important blast furnaces, factories, and buildings;

(4) Canals and harbors.

The most exact knowledge of large-scale smoke operations comes from Brest, where several large German battleships and cruisers were successfully protected from enemy bombing over a very considerable period of time. A screen was put up immediately upon the sounding of an air raid warning, and within 20 minutes the docks and town were completely enveloped in smoke. It is reported that the screen was so dense that visibility on the ground extended for only a few yards.

The generators appear to have been of the Smoke Generator 41 type, consisting of a 40- to 55-gallon steel drum fitted externally with a stopcock and a steel projection tube. They contain 20 gallons of a chlorsulfonic acid and sulfur trioxide smoke mixture, which is expelled by means of a cylinder of compressed air contained within the generator drum. Alongside the generator, a similar drum of smoke acid was provided for recharging. By this means a smoke screen could be maintained at full strength for 2 hours. The smoke is described as issuing from the generator in the form of a liquid, which immediately vaporizes. It has the color of tobacco smoke and is said to be almost odorless and harmless, but irritating to the throat.

More recent reports indicate replacement of the original equipment with an apparatus similar to the German naval chlorsulfonic acid smoke generator. This consists of two containers, one containing the chlorsulfonic acid and sulfur trioxide mixture, and the other compressed air for atomizing the smoke acid. Obviously, the newer type of equipment provides an uninterrupted generation of smoke over a considerably longer period of time. It is definitely reported that, on at least one occasion, the smoke screen was effective throughout an air raid lasting 4 hours.

The generators were scattered around Brest and its suburbs and along the docks and breakwaters, either in groups of several or at intervals of roughly 75 yards. Additional smoke was provided by about 20 small fishing craft (10 to 12 tons), each provided with a smoke generator. These boats were moored during the day at the end of a jetty, and at dusk were anchored in crescent formation in the harbor 1 or 2 kilometers from shore.

While practically all reports describe the smoke acid as composed of a mixture of chlorsulfonic acid and sulfur trioxide, one report mentions the use of a "weak mixture of titanium tetrachloride and ammonia." It is believed that the Germans have overcome the clogging difficulties formerly experienced with titanium tetrachloride when used in spray generators, and it is known that ammonia increases the density of the smoke.

Aerial photographs showing smoke screens in Norwegian fjords are evidence of the increasing effectiveness of this method of protecting primary targets. There is reason to believe, however, that development has not reached the point where desired results can be obtained irrespective of wind direction. While German warships shown in these photographs were not completely obscured by smoke, the protection afforded appears to have been considerable and would hamper raiding aircraft to a marked degree.

Reports concerning the materials and equipment employed in screening Norwegian coastal areas are less specific. According to a prisoner of war from one Norwegian port, smoke-producing liquid is stored in 40- to 55-gallon drums in the holds of fishing vessels. When poured into the water, the liquid takes the paint off the sides of the boats and causes the putty around their portholes to flake and drop off. A heavy grayish smoke develops, which clings to the water and spreads gradually upward as the concentration increases. On one occasion, a half hour elapsed before complete protection was afforded. This source reports, however, that smoke became effective over a limited dock area in a specific German port in only 5 minutes.

Another unverified report states that the main part of a smoke screen employed in a certain area in Norway was produced by 20 small fishing boats, each manned by 3 or 4 men, whose operations were supplemented by 3 land-based crews. The smoke here was more irritating to the nose and throat than the harmless, odorless smoke produced at Brest. It is said to have incapacitated men working in the vicinity, although cattle in adjoining fields apparently were not seriously affected. According to a Norwegian source, the Germans appeared reluctant to start smoke screens except when a major attack was imminent. The reason for this was not known, although it is suggested that the cost of the operations may be a factor.

It seems quite likely that smoke screen operations in German-controlled coastal areas are aided by minesweepers (R boats), which are reported to be fitted with smoke generators using chlorsulfonic acid and/or oleum. Also, German E boats doubtless contribute to these operations. In addition to carrying two of the smaller smoke apparatus (Smoke Generator '41') aft, one on each side, they are equipped with French smoke floats. The latter apparatus, weighing 40 kilograms (88 lbs.) when filled, contains 32 kilograms (70.4 lbs.) Berger-type** smoke mixture, which burns for periods variously estimated from 8 to 14 minutes.

Certain German cities are reported to be protected by extensive smoke-screen systems. RAF pilots have reported dark-gray smoke screens over Berlin, 20 to 30 miles long and 2 miles wide. The very dense smoke appeared to have come from generators 20 yards apart. (Note: 75-yard intervals appear to be the more normal spacing.)

It is reported from Kiel that a screen of gray-brown smoke covering the entire city is produced from the exhausts of automobiles racing through the streets whenever there is danger of an air raid.

Before the war, the question of pipe-line installations with jets at suitable intervals was discussed in German technical literature. There have been indications of the use of this system to screen certain factories in Germany, such as the Krupp works near Essen. Reports from Greece indicate that a pipe line for smoke screens runs the entire length (4 miles) of the Corinth canal.

*Prepared in the Office of the Chief, Chemical Warfare Service.
**Berger Mixture, named after the French chemist, consists of the following:
      Zinc (dust)          25 to 30 percent
      Carbon tetrachloride 45 to 55 percent
      Zinc oxide           10 to 20 percent
      Kieselguhr             5 to 10 percent