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Contrails

Contrails would be a dead giveaway for a stealth aircraft. A contrail is the condensation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines.

At typical contrail-friendly altitudes, between about 28,000 and 40,000 feet, temperatures run from about -36 to -76 degrees. If the airplane leaves a long trail, the air is not only cold but humid, allowing the ice crystals to persist. If the contrail stops, then starts up again, creating a broken line, chances are the airplane flew through a dry patch.

Attempts have been made previously to suppress the formation of contrails from the exhaust of a jet engine. One early approach was the injection of chlorosulfonic acid into the exhaust of an engine to diminish the particle size of water below the visible range. However, this substance is extremely corrosive. Chlorosulfonic Acid is a corrosive, clear liquid; melting point -80 C; boiling point 151 C; soluble in chlorinated solvents. It is hygroscopic and lachrymatory strongly. It reacts with water to form HCl. Chlorosulfonic Acid is used to produce organic sulfonic aicds for the end applications of detergent, pharmaceuticals, pesticides, and dyes. It was a starting material to produce saccharin.

Another suggested the injection of carbon black into an aircraft engine effluent. Numerous techniques are used for dispelling of fogs and clouds by dispersing various compounds therein. However, insofar as known, the various compounds or components thereof which were used for fog dispersal were never considered for use in suppressing the formation of contrails from engines operating in cold environments at high altitudes.

A hypernucleating solution for condensing water vapor in engine exhaust to particles having a size below the humanly visible range comprising in relatively sufficient proportions to effect said hypernucleation a non-corrosive surfactant in a combined carrier and nucleating agent selected from the group of water soluble monohydric, dihydric or polyhydric alcohols. A method of suppressing the formation of contrails from the exhaust of an engine including the steps of providing a solution of a non-corrosive surfactant in a combined carrier and nucleating agent selected from the group of water soluble monohydric, dihydric, trihydric or other polyhydric alcohols, forming the solution into a vapor, and injecting the solution into the exhaust of the engine. The solution may include by weight between about 0.01% to 2.5% of the non-corrosive surfactant, between about 1% and 8% water, and between about 85% and 99% ethylene glycol. Another solution may include by weight a monohydric, dihydric or polyhydric alcohol in an amount of between about 85% and 99% and the non-corrosive surfactant in an amount of between about 0.01% and 8%. Still another solution may include an inorganic nucleating or hygroscopic salt, such as ammonium iodide, ammonium fluoride, silver iodide or calcium chloride in monohydric, dihydric or polyhydric alcohols and surfactant mixtures.

The aircraft has a bay outboard of each of the main landing gear that was originally supposed to store contrail-suppression chemicals but was never used. The two bays are about 2.75 meters (9 feet) long and are as deep as the wing. The original B-2 design included tanks outboard of the main landing gear that would store a chemical to mix with the exhaust and suppress contrail formation. According to one report, "chlorofluorosulphonic acid" was to be injected into the jet exhaust to eliminate contrails. But this scheme wasn't actually used. Several other acids were tested too, but the result was that the chemicals were too corrosive.

It seems that "chlorofluorosulphonic acid" is unobtainium, as the term only occurs in a few articles about stealth. Possibly what is meant is "chloro fluoro sulfone" which goes by the IUPAC name "Sulfuryl chloride fluoride" [ClF O2 S]. This compound is employed as a solvent for highly oxidizing compounds, and reacts violently with water.

Sulfonic acid has a sulfur atom bonded to a carbon atom of a hydrocarbon and bonded also to three oxygen atoms, one of which has been attached to a hydrogen atom. Sulfonic acid is acidic due to the hydrogen atom, stronger than a carboxylic acid. Sulfonic acid is one of the most important organo sulfur compounds in organic synthesis. Aromatic sulfonic acids, such as paratoluene sulfonic acids (p-TSA), are made by sulfonating aromatic compounds with sulfonic acid, SO3, or Oleum. pTSA is comparable in strength to mineral acids such as sulphuric acid, but are especially suitable for organic reactions where an inorganic, mineral acid could cause charring,oxidation, or an unwanted chemical reaction. It is most useful as anesterification / condensation / acetylation catalyst and as an acid catalyst for resins in foundry cores. Other uses include: curing agent for Amino/phenolic/acrylic resins, descaling agent (industrial metal cleaners), electroplating bath additive, plastics, coatings, dyes, pharmaceutical intermediates,hydrotrope, coupling agent and as a wetting agent. Also applicable for use in food packaging adhesives.

In 1994 the government awarded a $63.5-million [other sources report $16 million] contract to the Northrop Corporation to fine-tune stealth bombers in a number of ways. One retrofit involved the installation of "contrail management systems." Ophir, an optical sensor manufacturer in Littleton, Colorado, saved the day. Its Pilot Alert System uses lidar (light detection and ranging) to differentiate contrails from clouds and tell the pilot to change his altitude when his aircraft is "conning."





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