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Just as a watership may be anchored rather than brought into a shed, an airship may be moored rather than brought into a hangar. However, the mooring system must be such that the airship can be easily moored and unmoored, and it must be able to hold the airship even in a substantial wind.
Mooring Masts in the New Timeline
A couple of mooring masts have appeared in canon. Marlon Pridmore transports his small thermal airship Upwind to Copenhagen by wagon, arriving in November 1635. By December 1635, a mooring mast of unspecified design has been erected, and the airship is inflated and flown, then moored to the mast. ("No Ship for Tranquebar," Evans & Evans, Grantville Gazette 27).
In February-March 1636, the master carpenter of the USE's Suriname colony receives plans for a "watchtower" that is over forty feet tall, with wooden tripod supports fixed in concrete. The "watchtower" is in fact a short mooring mast, as becomes evident when the mooring cap is attached to the top of the structure after the arrival of the (uninflated) Danish nonrigid Sandterne in December 1636-January 1637. The Sandterne is field-inflated, under weighted netting, with its nose cone attached by cable to the mooring cap, and hauled into the mooring cup as it inflates.
In September 1636, the much larger USE-owned, Dutch-built rigid airship Magdeburg is moored to a temporary mast near the village of Chieming in Bavaria. This is a "low" mast, and we don't know whether it was cut from local wood, or brought there in some way (Flint, 1636: Ottoman Onslaught, Chapter 48). The mast is only intended to be used during the period that the floating hangar is being constructed. Figure a year . . . or figure longer than that—temporary military structures have a way of becoming permanent!
Late that month, the Danish rigid airship Royal Anne lands in Tranquebar (the Dutch trading post near modern Tharangambadi, India). Before mooring, it makes two preliminary passes, one to drop a crewman off to explain how to erect a mast, and the second to drop off the "long mast slung below the cargo compartment," with a "pivoting cone attachment" already in place on the mast. The mast is guyed for stability ("No Ship for Tranquebar, Part Four," Evans & Evans, Grantville Gazette 30).
Stationary Mooring Systems
Here we look at some of the design issues for a stationary mooring system.
Single Pivot vs. Complete Restraint
The first design choice is whether to moor the airship to a single pivot point or to attempt the more or less complete restraint of the airship.
The advantage of mooring the airship to a single pivot point (on a mooring mast) is that it can rotate around that point like a weathervane around its post, thus reducing the wind load on the airship and the resultant load on the mast. The advantage of complete restraint is that less real estate is needed to moor the airship; a rectangle instead of a circle.
As far as I know, complete restraint for a prolonged period of an airship in the open has never been attempted. There have been field moorings that used multiple restraints; these did allow some airship motion and, as I'll discuss later, none of them have been satisfactory.
A Goodyear study of the requirements for a nonrigid airship similar to the ZP3G (975,000 cubic feet, 324 x 73.4 ft) considered two approaches to complete restraint. One would involve attachment of the control car to the ground, either directly or through landing gear on outriggers. Bear in mind that on a nonrigid airship, the control car is attached to the envelope via an internal catenary curtain, and this curtain would have to be strengthened substantially in order to resist a full wind load.
The other approach involved "attaching external catenary curtains on each side of the envelope and providing cable attachments to anchor points in the ground." (In other words, give the airship "jowls" and tie them down!)
Either way, we are adding weight to the airship; the study calculated that the suspension system weight would increase from 2.32% of gross lift for a conventional airship, to 8.71% for one designed to resist 30 knot winds and 28.87% to resist 60 knot winds (Goodyear Table 6-6). Given that the useful lift of the ZP3G was just 43% of the gross lift, that was a prohibitive penalty.
Admittedly, a rigid airship has an internal skeleton that can be equipped with external anchor points, and if that skeleton were sufficient to resist the wind forces, then there wouldn't be a comparable weight penalty. But I have read that airships in flight would flex as a result of variations in weight and buoyancy, and airships have broken up during turns at low altitudes.
So my best guess is that the skeleton would have to be substantially strengthened for complete restraint to be feasible. The early rigid airships had empty weights of about 75% of their gross lift, so incurring this weight penalty seems a bad idea.
That said, it is common for a nonrigid airship to be restrained in the field with a weighted net while it is being inflated (Stephenson 51), as the Sandterne was in 1636: Seas of Fortune, and Khoury (152) says that restraint is feasible if wind speeds are less than five knots.
The first use of a mooring mast was in 1911, at Cavendish Dock. This was actually a floating latticework steel mast. The mast was high enough above the water to be level with the nose when the airship's gondolas were in contact with the water. The mast sat on a pontoon with a six foot draft (the water depth was eight feet) and the pontoon was anchored to a concrete pillar. The airship and the mast both carried four-inch gauge mooring cables, and these were shackled together and the airship hauled in (WWA).
With mooring masts, there are several design choices: the attachment point on the airship, the height of the mast, and the structure of the mast.
Airship Attachment Point
Usually, an airship is moored by pivotably attaching its nose to the mast. However, two other attachment points have been considered. One, called "belly" mooring (I would say "chin"!), involves attachment at a point midway between the nose and the control car The other attaches the mast to the centerpoint of the underside of the envelope.
I can document some use of belly mooring. The 1923 Vickers mast cradled the nose in a yoke (Goodyear 1-9), and later that decade some Goodyear nonrigids had a belly mooring disc (1-21).
The advantages of "belly" and "center point" mooring are two-fold; the point of attachment is lower on the hull and thus a shorter mast may be used, and the "sweep" radius of the airship as it weathervanes is shorter.
However, because these attachments lie below the central lateral plane of the airship, the wind will induce a rolling moment that is avoided by nose attachment. That would in turn mean providing a heavier landing gear and support structure. Moreover, moving the attachment point away from the bow decreases the longitudinal forces but increases the lateral forces on the airship, and the net effect is an increase in wind load (Goodyear 5-10). Even if the shorter mast would still be lighter, I think the designers will recognize that it is more important to minimize airship weight than mast weight.
For the ZP3G, Goodyear (5-14) calculated that once the attachment point was more than 90 feet from the nose, at equilibrium the airship nose would no longer point into the wind; for a 120-foot separation, the deviation was 7o. Such deviation would mean an increased equilibrium wind load.
High vs. Low Masts
The third design choice is between high and low masts. With a high mast, the issue is, how does one embark and disembark passengers, crew, cargo, fuel, and ballast? And with a low mast, we worry about the tail striking the ground if it suffers a loss of buoyancy or a downward gust of wind.
What is a low mast for a large airship might be a high mast for a small one. But since the largest airships had diameters (and heights) of about 135 feet, anything over 70 feet can certainly be labeled a high mast.
Tinker in 1922 says that the mast is "a latticed steel structure from 150 to 225 feet high, preferably not less than one-quarter the length of the airship to be moored. . . ." (696). Consider the Royal Anne; with an envelope 650 feet long and 70 feet in diameter. By Tinker's criterion, the mast would need to be at least 163 feet high.
An experimental mast was erected at Pulham in 1921; it was 120 feet high (Robinson 6). Photos show that it is essentially a narrow vertical truss, supported by a complex web of stays.
A ladder on one side provided access to the revolving top, but I don't see how it could have been used to communicate with the crew on a moored airship—the control car would be on the underside of the airship and many feet away from the mast.
One option is to provide access by way of the mast itself. That's what was done at Cardington in 1926. Its full service high (200 ft) mast was a tapered tower with a domed top. On top of the dome was the mooring mechanism. Below the dome, there was an overhanging passenger platform that encircled the tower. Once the R101 was secured to the mooring point, a drawbridge was dropped from the underside of the bow, engaging the parapet of the platform. and a movable stepped gangway was brought around to align with it. Inside the tower, there was both a spiral staircase and a central elevator (Khoury 266).
This was elegant but expensive. Worse, it wouldn't work with a nonrigid or semirigid airship—the cars would be suspended too far from the bow—or even with a rigid airship lacking the R101's matching bow structure. So I think it was a really stupid idea, and the irony is that the R101 crashed on its first regular flight, and Britain subsequently abandoned rigid airships.
A second option is a mast that is capable of lowering the airship after it has been moored. One can imagine several ways of doing this. One would be that the mast telescopes; the airship moors on to it while it is in the extended position, and then mast is then retracted until the gondola is near the ground.
At Recife, Brazil, the Zeppelin company built a stationary mast with a telescoping inner section (Grossman). It was not intended to retract with an airship attached; the telescoping feature was just to adjust to airships of different sizes (Graf Zeppelin vs. Hindenburg). The crawler mast at Lakehurst had a telescoping inner section (NLHS), probably with the same purpose.
The telescoping "railroad mast" (see below) introduced at Lakehurst in 1932 had a maximum height of 160 feet and a minimum height of 75 feet (OSU). An airship would not have been left in the open for long periods on this mast; the whole point was you moored to the mast, and then the mast moved into the hangar.
Another mechanism was built but never used. The Dearborn mast had a "vertical railway," that is, the mooring mechanism was mounted on tracks that ran down the mast and thus could be brought down, the airship hopefully moving with it (Khoury 319; Robinson 148).
A third option would be to use a truck with an extendible, turntable-mounted ladder. The first aerial ladder fire truck was built by Hayes in 1870; it had a spring-loaded wooden extension ladder with a 65-foot reach. The ladder could be raised in less than 40 seconds by turning a crank.
Finally, the airship gondola could be equipped to pump up fuel and water ballast, and hoist up people and cargo.
Even if the airship could be fully serviced at a high mast, there was another problem: a flight crew had to be on board at all times, to man the elevator and rudder controls, keep the airship properly ballasted and, in an emergency, to slip off the mast entirely (Walker 302).
Nonetheless, a high mast (160 feet, steel construction) was used 1922-1927 at Lakehurst, New Jersey for the USS Shenandoah and USS Los Angeles.
The calculation of the relationship of the maximum pitch angle to the airship length, height, and the mast height is a bit complex and dependent on the exact shape of the airship. Let's assume that the airship envelope has the shape of a cone (so the mast doesn't limit the pitch angle) and the airship height equals the envelope's maximum diameter (cone base).
If so, the mast height permitting a given pitch angle equals Z sin (alpha), where Z is the slant length, i.e., the square root of (L2 + R2), L is the envelope length, R is the envelope radius, and alpha is the sum of the half cone angle (the tangent of R/L) and the pitch angle. For the Royal Anne, we can calculate that to permit a pitch of 10 degrees, the mast height must be 142 feet, for 5 degrees, 89 feet; and for 1 degree, 46 feet. The minimum mast height, of course,is the radius (35 feet), and then no pitch at all is possible.
If we do similar calculations for Miro's stubbier hot air airships (150 L x 60 D), then to permit a pitch of 10 degrees, the mast height must be 84 feet; for 5 degrees, 72 feet, and, of course, the minimum mast height is 30 feet.
Plainly, the lower the mast, the more danger of the tail striking the ground, but even with a high mast there wasn't much of a margin for error. Consistent with my calculations, Fulton said that on a high mast, the airship can pitch 8-12 degrees before touching the ground, and the flight crew can keep it within 2-3 degrees of horizontal (Camplin 194). But note that this implies that even when moored to a high mast, the airship must be "flown."
A solution was proposed, and tested in 1927. The Los Angeles (653 feet x 90.7 feet x 104.5 feet) was moored to an experimental stub mast, only 60 feet high. It was a pole (an old wooden radio mast) braced with wire cables, and a taxi wheel was secured to the stern power car. The airship was trimmed so as to be heavy aft, to reduce the risk of kiting the stern upward. If the airship weathervaned, this wheel would trace out a 438-foot radius circle (Walker 303; NLHS). The same system was proposed for the new timeline by Evans ("Wingless Wonders," Evans, Grantville Gazette 18). Providing the tail carriage and a stub mast will be a lot cheaper and easier than building a "full service" tall mast, unless you just happen to have a tall tower that is easily convertible into a mooring mast.
The "tail car" also ameliorated the problem of kiting, that is, the tail being lifted. Any shift in the wind will cause the bow-moored airship to kite as well as to weathervane. Goodyear (6-7) says that kiting is the result of "ground effects," but perhaps the worst case of kiting ever was when the USS Los Angeles did a headstand on a high mast.
Even if we are happy with the airship's stability while moored, there is still the risk of the airship striking the ground while approaching the mast at low altitude. In the early days, this was ameliorated by keeping the airship "light" and using a large ground crew to hold it down until moored. Takeoff also had its dangers; in 1932, the Akron dropped its tail down, damaging the rudder (GettyImages 145881601).
Even with a stub mast, it might not be possible to simply walk out of the passenger or crew car. The car height depends on its position under the hull and how streamlined the hull is. If the car is more than a few feet above ground level, then those on board will need assistance getting on and off. To minimize weight, an airship isn't likely to carry much more than a hoist, a rope ladder, or small stepladder.
A ground base could be equipped with a rolling ladder or boarding stairs; modern airline ones have a sill height reach ranging up to about nineteen feet. However, Khoury (152) warns that an airship can move vertically or horizontally without warning, and thus such ground equipment poses a hazard to the airship. Khoury urges that boarding steps be attached to the airship without touching the ground.
The mast may be an unstayed (but anchored) pole mast, a pole mast stabilized by stays, or some sort of vertical truss like a modern radio tower. Which is chosen will depend on the loads, which in turn depend on the size of the airship and the local meteorology.
Civil engineering handbooks are likely to be available in Grantville and to provide information as to wind loads on buildings. That is fairly easily extrapolated to hangars, but less so to moored airships. The drag coefficient will not be a constant, but rather dependent on the wind speed and the yaw angle. NTL engineers may attempt to get some sense of the forces involved by placing models in wind tunnels (which themselves have to be constructed) or in the field under closely monitored conditions.
Our authors generally don't have wind tunnels, so how will they know how sturdy a structure is needed? The Goodyear report on the ZP3G (324 x 73.4 ft) is a possible starting point. They performed a dynamic analysis on a masted ZP3G and Appendix B provides airship and mast forces for it, bow-moored in a 60-knot wind, at yaw angles of 15, 30, 45, 60, 75, and 90 degrees. You can scale this to your own airship by assuming that the load is proportional to the lateral surface area and to the square of the wind speed.
Once you know the vertical, lateral, and longitudinal forces on the airship as a result of various wind speeds and yaw angles, you can attempt to calculate the forces on the mooring cable, the mooring mast, and any stays or struts used to stabilize the mast. Since there are six up-timers with a bachelor's degree in civil engineering, and another two with master's degrees, I think there's a good chance that some software for making the stress calculations for a complex structure is available. However, it may be geared to determination of static stresses, whereas computational fluid dynamics software is needed for analysis of the effect of shifting winds.
Even if our characters can do the calculations, most of the authors (including me) lack the tools and expertise and will follow the adage "vague is good." But I can provide only a simplified analysis for an unstayed pole mast. It is sufficient to show that a mast of the sort used on a large frigate is adequate to retain a Swordfish-class airship.
The wind will exert mostly a horizontal force. That horizontal force can do three things, all undesirable: cause the base of the mast to slide along the ground, cause the mast to pivot at the base and thus overturn, and cause the mast to bend, perhaps to the breaking point.
The mast is also subject to vertical loads, from its own weight, and as a result of changes in the buoyancy of the airship. There is thus the risk of the mast buckling, or (if the airship becomes positively buoyant) or being lifted off the ground. However, I am going to ignore the vertical forces.
There are basically two ways of anchoring a mast so it isn't simply toppled over (or slid away) by the wind—giving it a wide base and giving it a deep one. (They may be combined, of course.)
If the mast is not a monopole, but rather a latticework connecting three or four legs, then you may have a single foundation for the whole mast or separate ones for each leg.