Fuel consumption 20 tons + 15 tons additional = 35 tons.
Lift available for fuel and freight = 42 tons.
Fuel carried = 35 "
------------ Balance for freight = 7 "
At 45 miles per hour.
Fuel consumption 12 tons + 9 tons additional = 21 tons.
Lift available for fuel and freight = 42 "
Fuel carried = 21 "
------------ Balance for freight = 21 "
It will thus be seen that at the faster speed small commercial loads can be carried by L 70 and R 38 and not at all in the case of R 33, that is a.s.suming, of course, that the extra fuel is carried, of which 75 per cent of the total does not appear at all excessive in view of the weather continually experienced over the Atlantic.
At the cruising speed the loads naturally increase but still, in L 70, and more particularly in R 33, they are too small to be considered commercially. In R 38, however, the load that can be carried at cruising speed is sufficient to become a commercial proposition.
From this short statement it is evident that, by a comparatively small increase in volume, the lifting capacity of an airship is enormously increased, and it is in this subject that the airship possesses such undoubted advantage over the aeroplane. In the heavier-than-air machine there is no automatic improvement in efficiency resulting from greater dimensions. In the airship, however, this automatic improvement takes place in a very marked degree; for example, an airship of 10,000,000 cubic feet capacity has five times the lift of the present 2,000,000 cubic feet capacity rigid, but the length of the former is only 1.7 times greater, and therefore the weight of the structure only five times greater (1.7); that is, the weight of the structure is directly proportional to the total lift. Having seen that the total lift varies as the cube of the linear dimensions while air resistance, B.H.P.--other things being equal--vary as the square of the linear dimensions, it follows that the ratio "weight of machinery/total lift" decreases automatically.
In comparing the different methods of transport for efficiency, the resistance or thrust required is compared as a percentage of the total weight. The result obtained is known as the "co-efficient of tractive resistance." Experiments have shown that as the size of the airship increases, the co-efficient of tractive resistance decreases to a marked extent; with a proportionate increase in horse-power it is proportionally more economical for a 10,000,000 cubic feet capacity rigid to fly at 80 miles per hour than for a 2,000,000 cubic feet capacity to fly at 60 miles per hour.
As the ratio "weight structure/total lift" is in airships fairly constant, it follows that the ratio "disposable lift/total lift"
increases with the dimensions.
It is therefore obvious that increased benefits are obtained by building airships of a larger size, and that the bigger the ship the greater will be its efficiency, providing, of course, that it is kept within such limits that it can be handled on the ground and manoeuvred in the air.
The proportion of the useful lift in a large rigid, that is the lift available for fuel, crew, pa.s.sengers, and merchandise, is well over 50 per cent when compared with the gross lift. When the accompanying table is studied it will be seen that with airships of large capacity the available lift will be such that considerable weights of merchandise or pa.s.sengers can be carried.
Capacity in Gross Lift Length Diameter cubic feet in tons in feet in feet 2,000,000 60.7 643 79 3,000,000 91.1 736 90.4 4,000,000 121.4 810 99.5 5,000,000 151.8 872 107.2 6,000,000 182.2 927 113.9 7,000,000 212.5 976 119.9 8,000,000 242.8 1,021 125.5 9,000,000 273.3 1,061 130.4 10,000,000 303.6 1,100 135.1
In airships of their present capacity, in order to obtain the greatest amount of lift possible, lightness of construction has been of paramount importance. With this object in view duralumin has been used, and complicated girders built up to obtain strength without increase of weight. In a large ship with a considerable gain in lift, steel will probably be employed with a simpler form of girder work. In that way cheapness of construction will be effected together with increased rapidity of output, and in addition the strength of the whole structure should be increased.
The rigid airship of 10,000,000 cubic feet capacity will have a disposable lift of over 200 tons available for fuel, crew, pa.s.sengers, and merchandise in such proportions as are desired. The endurance of such a ship at a cruising speed of 45 miles per hour will be in the neighbourhood of three weeks, with a maximum speed of 70 to 80 miles per hour, and a "ceiling" of some 30,000 feet can be reached. This will give a range of over 20,000 miles, or very nearly a complete circuit of the globe.
For commercial purposes the possibilities of such a craft are enormous, and the uses to which it could be put are manifestly of great importance. Urgent mails and pa.s.sengers could be transported from England to America in under half the time at present taken by the steamship routes, and any city in the world could be reached from London in a fortnight.
In the event of war in the future, which may be waged with a nation situated at a greater distance from this country than was Germany, aircraft Of long endurance will be necessary both for scouting in conjunction with our fleets and convoy duties. The British Empire is widely scattered, and large tracts of ocean lie between the various colonies, all of which will require protection for the safe-guarding of our merchant shipping. The provision of a force of these large airships will greatly add to the security of our out-lying dominions.
We have now reached a point where it is inc.u.mbent on us to face certain difficulties which beset the airship of large dimensions, and which are always magnified by its detractors. Firstly, there is the expense of sheds in which to house it; secondly, the large number of trained personnel to a.s.sist in landing and handling it when on the ground; thirdly, the risks attendant on the weather--for the airship is still considered the general public as a fair-weather craft; and fourthly, though this is princ.i.p.ally in connection with its efficiency for military purposes, its vulnerability. We will deal with the four difficulties enumerated under these headings seriatim, and endeavour to show to what extent they may be surmounted if not entirely removed.
The solution of the first two problems may be summed up in two words: "mooring out"; on the success of this it is considered that the whole future of airships for commercial purposes rests. It will be essential that in every country which the airship visits on its voyages, one large central station is established for housing and repairs. The position of such a station is dependent on good weather conditions and the best railway facilities possible. In all respects this station will be comparable to a dry dock for surface vessels. The airship will be taken into the shed for overhaul of hull structure, renewing of gasbags or outer cover, and in short to undergo a periodical refit.
The cost of a shed capable of housing two rigid airships, even at the present time, should not greatly exceed L500,000. This sum, though considerable, is but a small item compared with the cost of constructing docks to accommodate the Atlantic liner, and when once completed the cost of maintenance is small when weighed against the amount annually expended in dredging and making good the wear and tear of a dock.
Apart from these occasional visits to a shed, the airship, in the ordinary way at the end of a voyage, will pick up its moorings as does the big steamer, and land its pa.s.sengers and cargo, at the same time replenishing its supplies of fuel, gas, provisions, etc., while minor repairs to the machinery can be carried out as she rides in the air.
A completely satisfactory solution of the mooring problem for the rigid airship has yet to reach its consummation. We saw in the previous chapter how, in the case of small non-rigids, they were sheltered in berths cut into woods or belts of trees, but for the rigid airship something more secure and less at the mercy of the elements is required.
At the present moment three systems of mooring are in an experimental stage: one, known as "the single-wire system," is now practically acknowledged to fall short of perfection; the second, "the three-wire system," and the third, "mooring to a mast," both have their champions, but it is probable that the last will be the one finally chosen, and when thoroughly tried out with its imperfections eliminated will satisfy the most exacting critics.
The single-wire system is at the same time the simplest and most obvious method which suggests itself, and means that the ship is secured by a wire cable attached to a suitable point in the ship and led to some fixed point on the ground. It has been found that an airship secured in this way requires constant attention, and that steering is always necessary to render her steady in the air.
Considerable improvement is obtained if a dragging weight is added to the wire, as it tends to check to a considerable extent lateral motion of the bow of the ship.
The three-wire system is an adaptation and an improvement on the one previously mentioned. In this case the mooring point of the ship is attached to three long wire cables, which, when raised in the air, form a pyramid to the head of which the ship is attached. These wires are led to bollards which form in plan an equilateral triangle. The lift of the ship raises these wires off the ground, and if the ship is trimmed up by the bows she will be found to resist the action of the wind. A rigid airship moored out by this method remained in the open for a considerable time and rendered the future of this experiment most hopeful. It was resolved to continue these experiments by adding a subsidiary system of wires with running blocks, the whole wiring to form a polygon revolving round a fixed centre. The disadvantages of this method appear to be rather serious. It seems that great difficulty will always be found in picking up these moorings in a high wind, and though this also applies to the method with the mast, the initial obstacles do not appear to be so great. A powerful engine driving a winch will be necessary to raise these heavy wires from the ground, although of course the lift of the airship will a.s.sist in this.
Secondly, the lowering of pa.s.sengers and cargo will not be easy as the ship will not be rigidly secured. This, however, can probably be managed when experiments have reached a further stage, and at present the system may be said to present distinct possibilities.
The third system, that of mooring to a mast, possesses several features peculiar to itself, and not embraced by the other two, which should secure it prolonged investigations. The system is by no means new and has been tried from time to time for several years, but since the question of mooring in the open has been so ventilated and is now considered of such vital importance, these experiments have been continued, and in less spasmodic fashion than in the past. In a trial with a small non-rigid airship some months ago a signal success was achieved. The ship remained attached to a mast in open country with no protection whatsoever for six weeks in two of the worst months of the year. During this period two men only were required to look after the ship, which experienced gales in which the force of the wind rose to 52 miles per hour, and not the slightest damage was sustained.
Two or three methods of attaching the airship to the mast have been proposed, but the one which appears to be most practical is to attach the extreme bow point of the ship to some form of cap, in which the nose of the ship will fit, and will revolve round the top of the mast in accordance with the direction of the wind.
For large airships, employed as pa.s.senger and commerce carriers, we can imagine the mast advanced a stage further, and transformed into a tower with a revolving head. Incorporated in this tower will be a lift for pa.s.sengers and luggage, pipes also will be led to the summit through which both gas and water can be pumped into the ship. With the airship rigidly held at the head of such a structure all the difficulties of changing crews, embarking and disembarking pa.s.sengers, shipping and discharging cargo and also refuelling, vanish at once. a.s.suming the mooring problem solved with success, and we feel correct in this a.s.sumption, the first two of our difficulties automatically disappear.
Sheds will only be necessary as repair depots and will not be extensively required, all intermediate stopping places being provided with masts and necessary arrangements for taking in gas, etc. At these intermediate stations the number of men employed will be comparatively speaking few. At the depots or repair stations the number must, of course, be considerably increased, but the provision of an enormous handling party will not be necessary. At present large numbers of men are only required to take a large airship in or out of a shed when the wind is blowing in a direction across the shed; when these conditions prevail the airship will, unless compelled by accident or other unforeseen circ.u.mstances, remain moored out in the open until the direction of the wind has changed.
Mechanical traction will also help effectually in handling airships on the ground, and the difficulty of taking them in and out of sheds has always been unduly magnified. The provision of track rails and travellers to which the guys of the ship can be attached, as is the practice in Germany, will tend to eliminate the source of trouble.
We must now consider the effect that weather will have on the big airship. In the past it has been a great handicap owing to the short hours of endurance, with the resulting probability of the ship having to land before the wind dropped and being wrecked in consequence. Bad weather will not endanger the big airship in flight, and its endurance will be such that, should it encounter bad weather, it will be able to wait for a lull to land. Meteorological forecasts have now reached a high state of efficiency, and it should be possible for ample warnings to be received of depressions to be met with during a voyage, and these will be avoided by the airship flying round them. In the northern hemisphere, depressions generally travel from west to east and invariably rotate in a counter-clockwise direction with the wind on the south side blowing from the west and on the north side blowing from the east. Going west, the airship would fly to the north of a depression to take advantage of the wind circulating round the edge, and going east the southern course would be taken.
Lastly, the vulnerability of the airship must be taken into account.
Hydrogen is, as everyone knows, most highly inflammable when mixed with air. The public still feels uncomfortable misgivings at the close proximity of an immense volume of gas to a number of running engines.
It may be said that the danger of disaster due to the gas catching fire is for peace flying to all intents and purposes negligible. At the risk of being thought hackneyed we must point out a fact which has appeared in every discussion of the kind, namely, that British airships flew during the war some 21 million miles, and there is only one case of an airship catching fire in the air. This was during a trial flight in a purely experimental ship, and the cause which was afterwards discovered has been completely eliminated.
For airships employed for military purposes this danger, due to the use of incendiary bullets, rockets and various other munitions evolved for their destruction, still exists.
Owing to its ceiling, rate of climb and speed, which we take to be from 70 to 80 miles per hour in the airship of the future, the airship may be regarded as comparatively safe against attack from the ordinary type of seaplane. The chief danger to be apprehended is attack from small scouting seaplanes, possessing great speed and the power to climb to a great height, or from aeroplanes launched from the decks of ships. If, however, the airship is fitted to carry several small scout aeroplanes of high efficiency in the manner described in the previous chapter, it will probably be able to defend itself sufficiently to enable it to climb to a great height and thus make good its escape.
The airship, moreover, will be more or less immune from such dangers if the non-inflamable gas, known as "C" gas, becomes sufficiently cheap to be used for inflating airships. In the past the expense of this gas has rendered its use absolutely prohibitive, but it is believed that it can be produced in the United States for such a figure as will make it compare favourably with hydrogen.
The navigation of an airship during these long voyages proposed will present no difficulty whatever. The airship, as opposed to the aeroplane, is reasonably steady in the air and the ordinary naval instruments can be used. In addition, "directional" wireless telegraphy will prove of immense a.s.sistance. The method at present in use is to call up simultaneously two land stations which, knowing their own distance apart, and reading the direction of the call, plot a triangle on a chart which fixes the position of the airship. This position is then transmitted by wireless to the airship. In the future the airship itself will carry its own directional apparatus, with which it will be able to judge the direction of a call received from a single land station and plot its own position on a chart.
We have so far confined our attention to the utilization of airships for transport of pa.s.sengers, mails and goods, but there appear to be other fields of activity which can be exploited in times of peace. The photographic work carried out by aeroplanes during the war on the western front and in Syria and Mesopotamia has shown the value of aerial photography for map making and preliminary surveys of virgin country. Photography of broken country and vast tracks of forest can be much more easily undertaken from an airship than an aeroplane, on account of its power to hover for prolonged periods over any given area and its greater powers of endurance. For exploring the unmapped regions of the Amazon or the upper reaches of the Chinese rivers the airship offers unbounded facilities. Another scope suggested by the above is searching for pearl-oyster beds, sunken treasure, and a.s.sisting in salvage operations. Owing to the clearness of the water in tropical regions, objects can be located at a great depth when viewed from the air, and it is imagined that an airship will be of great a.s.sistance in searching for likely places. Sponges and coral are also obtained by diving, and here the airship"s co-operation will be of value. Small ships such as the S.S. Zero would be ideal craft for these and similar operations.
The mine patrol, as maintained by airships during the war, encourages the opinion that a systematic search for icebergs in the northern Atlantic might be carried out by airships during certain months of the year. As is well known, icebergs are a source of great danger to shipping in these waters during the late spring and summer; if the situation becomes bad the main shipping routes are altered and a southerly course is taken which adds considerably to the length of the voyage. The proposal put forward is that during these months as continuous a patrol as possible should be carried out over these waters.
The airship employed could be based in Newfoundland and the method of working would be very similar to anti-submarine patrol. Fixes could be obtained from D.F. stations and warnings issued by wireless telegraphy.
Ice is chiefly found within five hundred miles of the coast of Newfoundland, so that this work would come within the scope of the N.S.
airship. The knowledge that reliable information concerning the presence of ice will always be to hand would prove of inestimable value to the captains of Atlantic liners, and would also relieve the shipping companies and the public of great anxiety.
There are possibly many other uses to which airships can be put such as the policing of wide stretches of desert country as in Arabia and the Soudan. The merits of all of these will doubtless be considered in due course and there for the present we must leave them.
Finally, a few words must be written regarding the means to be adopted in introducing the airship into the realms of commerce. As we said at the beginning of the chapter it is not likely that the formation of a company to exploit airships only will at the present moment appeal to business men. Airships are very costly and are still in their infancy, which means that the premiums demanded for their insurance must of necessity be enormous. One suggestion is to place a reasonable scheme before the great shipping companies in case they will care to find the necessary capital and form subsidiary companies.
Another suggestion is that the Government should make arrangements to subsidize commercial airships. The subsidy might take the form of insuring them. If the burden of insurance is taken off their shoulders, it is considered feasible to promote companies which will give an adequate return for capital invested. The Government could also give a financial guarantee if mails are carried, in the same manner as is done by shipping companies.
In return for this the Government could at the outbreak of hostilities commandeer all or any of the airships for war purposes and so save the number to be kept in commission.
By this means the Government will have a large number of highly-trained and efficient personnel to call upon when the emergency arises, in the same way as the fleet can call upon the R.N.R. This system appears to be the best in every respect, and it cannot be denied that in the long run it would be the most economical for the country.