THE VICKERS AEROPLANE WORKS AT WEYBRIDGE, ENGLAND
COMFORT CAN BE ENJOYED IN AIR TRAVEL TO-DAY
The rigid airship is as yet only at the beginning of its development, particularly as regards size and carrying capacity. The airship of three million, five hundred thousand cubic feet capacity, for immediate use on the fast passenger services, carrying a load of passengers of fifteen tons for a distance of forty-eight hundred miles, might be built immediately, and could be housed in sheds at present available. As the lift and speed efficiency of a rigid airship increases rapidly in proportion to the vessel's size, it will be advantageous to use the largest airships that can be economically operated. A rigid dirigible able to carry fifty tons of passengers and freight for ten thousand miles at a speed of eighty miles an hour is quite feasible; and the design and construction of such an airship could be undertaken immediately if it were justified by the demand for air transport.
The ships of three million, five hundred thousand cubic feet capacity, which can be housed and flown for commercial purposes as soon as the required terminals and navigational facilities are ready, will approximate to those described as being suitable for a transatlantic service. If standardized for adaptation to all conditions and world routes, they should be capable of a non-stop flight of about eighty hours, at an average speed of sixty miles an hour.
To prevent wastage and reduce the running costs, several economical devices for dealing with height equilibrium are needed. On long flights the greatest problems are maintenance of the airship at a constant height, and avoidance of the loss of gas consequent on expansion when the ship rises as it loses weight by the consumption of fuel. Owing to the great variation in temperature between day and night, the ship becomes heavy at night owing to the lower temperature, and light during the day, as a result of the higher temperature. A discharge of ballast at nightfall, and of gas in the morning, is needed to keep it in equilibrium. To obviate discharge of gas, and the necessity of starting with a large weight of ballast, it is proposed to run a proportion of the engines on hydrogen fuel, so that the hydrogen can be consumed at such a rate that the loss of lift equals the loss of weight of fuel consumed by the other engines, thus economically using hydrogen which otherwise would be lost through the discharge of the gas valves.
I make the supposition that hydrogen, and not helium, will be the sustaining gas. For commercial aviation it has many advantages, for helium is dearer and rarer, and has about twenty per cent. less lift. Contrary to general belief, a flight in an airship filled with hydrogen, subject to proper precautions, has no greater fire risk than living near a gas factory. Helium is a necessity only for airships used in war, as, unlike hydrogen, it is not ignited by incendiary bullets from hostile aircraft. The United States has almost a monopoly of the world's quantitative supply of helium, which fact should be a tremendous asset in wartime.
The ballast difficulty can be met by apparatus to condense the water of combustion from the exhaust gases of the engines. Experiments have shown that it is practicable to recover water of slightly greater weight than the gasoline fuel consumed, thus avoiding any variation in lift due to gasoline consumption. Further, water ballast could be picked up periodically from the sea by descending and taking in water through a pump suspended from a flexible hose, or direct into tanks in the gondolas through sea-valves.
Still further reduction of running costs may be effected by fuel economy. This would be difficult with internal combustion engines of the type in use at present, for greater thermal efficiency (the ratio between the amount of heat contained in the fuel consumed and the amount of useful work delivered by the engine) necessitates heavier machinery. The reduction in gasoline consumption is thus offset by a decrease in the disposable lift. It is probable that a saving on large dirigibles might result from substituting for the internal combustion method of generating power engines that burn cheap oils. Although such engines are much heavier, and although the crude oils weigh a good deal more than gasoline, the difference would be more than covered on long flights, for gasoline is nearly four times dearer than crude oil. Moreover, the weight of oil actually consumed would be about twenty per cent. less than that of the gasoline burned by internal combustion engines over the same distance.
The solution may be in the employment of steam. For the rather low standards of horsepower on which dirigibles are driven, heavy steam engines of the ordinary type, although much more reliable, would be less economical than internal combustion engines, owing to the latter's better thermal efficiency. Engineers are attempting to evolve a light type of steam turbine that will overcome this drawback.