Publication: Air Power History
Publication Date: 22-SEP-07
Delivery: Immediate Online Access
Author: Baucom, Ronald R.
Article Excerpt
Behind the engine-carrying body (fuselage or nacelle) a turbulent region or wake is formed as the airplane flies. The exhaust moisture and some of the engine heat are discharged into this wake and become diffused throughout the wake as a result of the mixing action of the turbulence. The moisture and heat do not, however, mix with the air outside the wake because there the air is "smooth.'...
It is easy to see that, if the air is so cold that it cannot hold much water as vapor, the water in the exhaust may be sufficient, when added to the moisture already in the atmosphere, to raise the humidity in the turbulent wake to or beyond the saturation value. If this condition exists, some of the water vapor will condense and a visible trail will form.
Richard V. Rhode and H. A. Pearson, Condensation Trails, NACA Wartime Report, September 1942 (1)
Introduction
As we saw in the first part of this paper, contrails were observed as early as October 1918. Yet, they remained a rare phenomelatively little interest across the 1920s and 1930s, despite developments that steadily raised the operational ceiling of military aircraft. By the time of the Spanish Civil War, state-of-the-art fighters could engage in combat in the upper regions of the troposphere where engine exhausts routinely turn into contrails. Francisco Tarazona's September 1938 report of contrails generated by dogfighting aircraft was a harbinger of things to come.
Within a year of Tarazona's report, Germany plunged Europe into a general war when she invaded Poland. In the months between the fall of Poland and the German invasion of France in May 1940, German pilots clashed in desultory combat with French and British airmen as both sides flew patrol and reconnaissance missions over Western Europe. (2) From these air operations and those that took place when Germany overran France in the spring of 1940, it was apparent that contrails were intrinsic to modern air combat and had important operational implications. These early months of the air war also spawned what may be the first published account of contrails in air combat.
The Battle for France and Saint-Exupery's Train of Frozen Stars
At the time Germany invaded France, French aviation pioneer and famed author Antoine Saint-Exupery was just short of his fortieth birthday, well past the age when men were considered fit for air combat duties. Given his age, his literary achievements, and health problems caused by earlier aircraft accidents, Saint-Exupery was not expected to volunteer for combat duty and could easily have honorably avoided it. However, he believed France was in grave danger and that all Frenchmen who could were obliged to come to her defense. (3)
True to his convictions, Saint-Exupery managed to secure an assignment flying reconnaissance planes, specifically, the Potez 63. Such an assignment was a serious challenge for a man of his age and physical condition due to the difficulties and discomforts associated with flying in the cold cockpits of high altitude aircraft. (4)
Saint-Exupery survived his combat missions against the Germans and escaped to the United States after France surrendered, settling into New York in January 1941. Here, he wrote about his wartime service and worked to build support for the war against Nazi Germany. After the United States captured North Africa, Saint-Exupery was allowed to sail aboard an American transport to Oran, Algeria. He then secured permission from French authorities to rejoin his old French reconnaissance unit, Group 2 of the 33d Reconnaissance Wing, and began flying combat missions after being retrained to fly the unit's aircraft. Shortly after he rejoined the 2/33, it was transferred to Colonel Elliott Roosevelt's 3d Photo Group, which flew the reconnaissance version of the P-38. By this time, Saint-Exupery was over forty-two years old, and regulations established thirty as the maximum age for pilots in Roosevelt's unit. Only through the intercession of a high-ranking French general with General Dwight Eisenhower's headquarters was this age requirement waived for Saint-Exupery. After being trained in the American P-38, he flew this aircraft on reconnaissance missions out of North Africa beginning in July 1943 and continuing until he was killed during a mission on July 31, 1944. (5)
In February 1942, while still living in New York, Saint-Exupery had published Flight to Arras, a memoir of his service against Nazi Germany in 1939 and 1940. Here, he described the challenges of his high altitude reconnaissance missions: the cold that could freeze the controls of his aircraft, finding and photographing enemy targets under fire, and the anxiety of knowing his plane was tailing a white streamer that pinpointed his position for enemy fighters and gunners. Regarding this last challenge, Saint-Exupery's wrote:
The German on the ground knows us by the pearly white scarf which every plane flying at high altitude trails behind like a bridal veil. The disturbance created by our meteoric flight crystallizes the watery vapor in the atmosphere. We unwind behind us a cirrus of icicles. If the atmospheric conditions are favorable to the formation of clouds, our wake will thicken bit by bit and become an evening cloud over the countryside.
The fighters are guided towards us by their radio, by the bursts on the ground, and by the ostentatious luxury of our white scarf ...
The fact is, I have absolutely no idea whether or not we are being pursued, and whether from the ground they can or cannot see us trailed by the collection of gossamer threads we sport.
Gossamer threads set me daydreaming again. An image comes into my mind which for the moment seems to me enchanting. "... As inaccessible as a woman of exceeding beauty, we follow our destiny, drawing slowly behind us our train of frozen stars." (6)
This passage in Flight to Arras may be the earliest first-hand account of combat-related contrails to be published. Although Tarazona recorded his observation of contrails in September 1938, as noted in part I of this paper, his diary was not published until the 1970s. Flight to Arras may also be the first published apprehension that contrails have major implications for air combat operations, although the significance of contrails was obvious in the Royal Air Force long before the publication of Saint-Exupery's memoir.
The Boffins Come to Grip with Contrails
Like Saint-Euxpery, Flight Lieutenant M. V. Longbottom was a pilot in an aerial reconnaissance unit, in this case, the RAF's No. 2 Camouflage Unit, a designation chosen to conceal the unit's mission. (7) Furthermore, like Saint-Euxpery and his comrades, Longbottom and the members of his unit depended upon the speed of their planes and the stealthiness provided by high altitude flight to protect them against enemy defenses. Therefore, it should come as no surprise that Longbottom was keenly interested in condensation trails.
Thus, on Christmas day 1939, over two years before the publication of Flight to Arras, Longbottom issued a SECRET report titled "Condensation Trails at High Altitudes" which begins by explaining the major implication of contrails for air warfare: a contrail aids enemy defenders by betraying the position of an aircraft that might otherwise be invisible. In Longbottom's words:
It has been found that, at high altitudes over about 8000 meters (27,000 feet), under certain conditions, aircraft in flight leave behind them a dense white trail of condensation. In its most marked form this condensation, starting from the engine exhausts, forms a dense white trail behind the aircraft, which rapidly spreads to a band many times the width of the aircraft, stretching across the sky like a long wisp of well marked cirrus cloud. From the ground, this trail appears to come to a point, sharply defined, at the exact position of the aircraft, so that although the machine itself may not be visible, every movement it makes--every turn and zig-zag--is easily visible to the naked eye of an observer on the ground, and may be very accurately plotted, enabling accurate A.A. fire to be opened. (8)
To bolster this point, Longbottom recounted the experience of a Spitfire pilot who was trailing a pronounced contrail at about 32,000 feet when he came under accurate antiaircraft fire near Trier, a town on the Moselle River near Germany's border with Luxemburg. He also noted that although antiaircraft fire had been encountered at altitudes as high as 33,000 feet, this occurred only when the target aircraft was generating a contrail. (9)
Longbottom was clearly interested in finding some means by which RAF pilots could keep their planes from producing contrails. To this end, he examined the experiences of pilots who flew missions on December 20, 21, and 22, 1939. While one pilot flying on December 22 noticed only "slight wisps of condensation, the other four, including one who also flew on December 22, reported heavy contrails. All four of the pilots reporting contrails were able to eliminate them by throttling back their engines and descending one or two thousand feet. (10)
In an effort to correlate weather conditions with the experiences of these pilots, Longbottom consulted a French meteorologist regarding conditions in No. 2 Unit's mission area on the appropriate days. This consultation suggested a correlation of contrail formation with conditions of low temperature and high humidity aloft. When these conditions prevail at the altitude where a plane is flying, "the rapidly expanding gases from the exhausts" of the plane cause "sudden condensations to form in [the plane's] wake." (11)
The information that Longbottom assembled also indicated the possible existence of layers in the atmosphere some of which would support contrail formation while others would not. The existence of such layers would account for contrail termination when a pilot reduced his altitude. It also suggested that a pilot might be able to stop contrail generation by climbing out of a layer conducive to contrail formation, provided such a climb was within the capabilities of his plane. (12)
In addition to the work carried out by Longbottom, several later contrail studies were completed under the auspices of England's Aeronautical Research Committee (ARC) that had been founded in January 1935. This was the same committee that spawned the British radar program. (13) Once radar was more fully developed and applied to the control of anti-aircraft systems, it largely nullified the importance of contrails as a means of locating high-flying aircraft and directing anti-aircraft fire. However, as we shall soon see, radar did not eliminate the significance of contrails for air operations. A measure of the continuing importance of condensation trails is the series of contrail studies sponsored by the ARC.
On February 3, 1941, the ARC's High Altitude Subcommittee issued a report that provides a glimpse of the state of knowledge of contrails in the British scientific community. "Until very recently," the report begins,
the data available on "vapour trails" was so meager that no positive conclusions could be drawn as to formation. War operations at high altitudes and high speeds have made the phenomenon more common, and data is [sic] now being accumulated in greater volume. The absence of essential information, however, makes it impossible, at present, to do more than put forward tentative ideas on the nature of the phenomenon.
As more information becomes available and knowledge of contrails increases, the subcommittee said, additional reports would be issued. (14)
At this stage, the subcommittee believed that there were three mechanisms that might account for the formation of contrails. One was the "precipitation, as ice, of water vapour previously present in the atmosphere in a...
NOTE: All illustrations and photos have been removed from this article.
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