A SUBMARINE running submerged must maintain equilibrium, or approximately so, for efficient operation and control. While running submerged a submarine does not burn a quantity of fuel but uses the energy stored in its electric storage batteries for propulsion. Hence, only minor adjustments of ballast are necessary. However, when the submarine has emerged it burns fuel not only for surface propulsion but also for recharging its electric storage batteries. In order to retain diving trim, ballast (water) is admitted to its tanks to compensate for the weight of fuel burned, lubricating oil consumed, refuse thrown over the side, etc.
An airship before taking off, weighs off just as a submarine does when making a stationary dive. But once in the air the main engines of an airship are used for propulsion and fuel is consumed. To date gasoline is the fuel principally used by all airships and the only fuel used by our one rigid airship, the Los Angeles. Just as a submarine must maintain equilibrium while submerged for efficient operation and control, so must an airship while under way. As gasoline is used, if no compensation is made, an airship will become lighter and lighter. For a while, by keeping it pointed down by the head, a constant altitude (analogous to depth for a submarine) can be maintained, but at a loss of speed and an increase of fuel consumption. And finally it becomes imperative to make adjustments of the trim in order to have any control whatever.
The simplest method to adjust for the loss of weight (fuel consumed) is to release sufficient of the lifting gas to the atmosphere. Using round figures, 1 gal. of gasoline weighs 6 lbs. and 1,000 cu. ft. of helium will lift 60 lbs. or 10 gal. of gasoline. Hence, if 1,000 gal. (6,000 lbs.) of gasoline is burned it will be necessary, in order to compensate for this loss of weight, to valve 100,000 cu. ft. of helium. When once valved helium cannot be recovered. And with the scarcity and the present cost of helium such losses are prohibitive during extensive operations. Furthermore, helium is obtainable only in certain localities and must be shipped and stored at the airship bases.
Hydrogen is relatively inexpensive and can be easily produced anywhere, where there is a small supply of ferrosilicon and caustic soda. Hence it is not of particular moment whether hydrogen is valved to the atmosphere or not. This could best be explained by a hypothetical case. Consider the Los Angeles inflated with helium preparing for a cruise out 2,000 miles, fuel, and return. On the way out, fuel, 24,000 lbs. by weight, was consumed. If compensation was made by valving helium, 400,000 cu. ft. of helium was valved. It would therefore be necessary to have an equal amount of helium on hand to take into the gas cells of the ship if she were to take fuel in order to start her return flight with as much gasoline as she put out with. A volume of 400,000 cu. ft. of helium would involve 2,500 cylinder containers weighing 150 tons, or four carloads.
If the Los Angeles had been inflated with hydrogen for the same cruise, caustic soda and ferrosilicon totaling only twenty tons, less than a carload, would have been required to give her the amount of hydrogen she had valved and no compression into transporting cylinders would have been necessary.
The above has been offered to point out the imperative need of some method to provide ballast in lieu of the fuel consumed where helium is used as the lifting gas. (“Blau” gas or an equivalent will not be discussed because the safety obtained by the use of the inert helium would be offset by an inflammable fuel gas.) Hence the United States, the possessor of practically all the helium known in the world, developed “water recovery” to its present stage. Some experimenting had been done in Europe but the development of a practical “water recovery” was made in the United States.
If on some cold winter morning you will look at the exhaust pipe of your automobile immediately after starting up, you will see drops of water running from it. That water comes from the exhaust gases of your motor. If your exhaust pipe were coiled several times around your car and you began driving through the cold air a steady stream of water would run from the end of the coil—water amounting to as much as the gasoline running to your carburetor. That is “water recovery” as adapted to the motors of an airship.
If you burn 100 lbs. of gasoline you burn a mixture of 1,600 lbs., as 15 lbs. of air are required to burn 1 lb. of gasoline. The 1,500 lbs. of air will provide 350 lbs. of oxygen and, with average weather conditions, 10 lbs. of water vapor. The 100 lbs. of gasoline will provide the 15 lbs. of hydrogen. Two particles of hydrogen will combine with one particle of oxygen. The unit weight of hydrogen is 1 and that of oxygen 16; so the two particles of hydrogen weighing 2 plus the one of oxygen weighing 16 have formed a particle of water weighing 18; the water formed therefore equals 9 times the weight of the hydrogen available, or 9X15 (weight of hydrogen) = 135 lbs. Adding 10 lbs. for the water vapor originally in the mixture gives a result of 145 lbs. Therefore, after combustion there will be in your exhaust pipe 145 lbs. of H2O which can be collected if condensed. It is not necessary in an airship to continue the cooling to condense the entire amount but it is endeavored under average conditions to condense sufficient to produce water in amount equal to the weight of fuel consumed, thereby keeping the ship in continual equilibrium.
A general description of the arrangement of the water-recovery apparatus installed on the Los Angeles should make a few points clearer. The exhaust gases in two exhaust pipes from the engines’ two exhaust manifolds are led up through the top of the engine car and join into a common riser. Here is connected an atmospheric exhaust controlled by a valve in order that the condenser may be cut out. The riser, made of steel, is well finned and leads to the upper condenser header. By the time the gases reach the condenser the temperature is reduced to about 900°F. which is a safe temperature for aluminum. The gases then pass through the tubes of the condenser. There are ten rows of tubes, 45 tubes in each row. The tubes are 1 in. in diameter and 5 ft. long, the metal being .02 inches thick. The tubes at each end are pressed into headers made of cast aluminum which serve to change the direction of the exhaust approximately 180° every five feet. After passing through the lower row of tubes the gases, holding water in suspension, pass upward and through a baffled separator which gathers the water which then flows by gravity to a stowage tank within the ship while the gases pass to the atmosphere. Another drain is connected to the low point of the lower header to allow free water present to run off. This drain joins the lead from the separator. To prevent freezing of the water in cold weather, alcohol is introduced into the water lines. The flow of alcohol is controlled according to the temperature. It is also necessary to partially screen the condenser with cloth in very cold weather.
The temperature within the exhaust manifold at the engine is about 1,600°F. It has been stated that the temperature of the gases when entering the condenser is about 900°F. or a drop of 700° F. in ten feet of travel. It is interesting to note then that fifty more feet of travel are required to reduce the temperature from 800° F. to about 95 °F., at which temperature the water obtained will equal in weight the fuel consumed. The curve submitted clearly shows the drop in temperature according to the distance the exhaust travels. This curve is not exact for the apparatus on the Los Angeles but it is a typical curve. At the lower temperatures the specific heat of the gases is many times greater than at the higher temperatures, caused by the latent heat of condensation, and the difference between the gas temperature and the air temperature reduces progressively as the gas temperatures drop.
Although the water recovery as at present developed will make water, it is not entirely satisfactory. Not only is the weight considerable, but the air resistance is high. Consequently speed is reduced, fuel consumption increased, and the cruising radius reduced accordingly. It is hoped and expected that improvements will be made in the not so very distant future.