Lighter-than-air craft have been used with great success by the US Navy in the 20th Century. Consideration of the unique capabilities of these aircraft, particularly rigid airships, suggests that they would be well suited to some present-day civilian and possibly military missions. This article discusses why a lighter-than-air ship may still be a practical and low-cost airborne platform with modern applications.
The earliest successful attempts to fly were made with balloons, at first tethered and free flight, later powered flight. The airship has had a history of successful operation both military and civilian for many years. Unfortunately, in 196l the US Navy closed out its Lighter-than-Air program. The Hindenburg disaster seems to have effectively curtailed design, construction and use of these vehicles, even though in that instance flammable hydrogen was used rather than inert helium, and even though the suspicion of sabotage has remained strong.
History of US Navy Light-than-air craft
Lighter-than-air craft possess unique operational characteristics which is reflected in a unique combination of mission capabilities. Historically, the most significant of these capabilities were long flight endurance and high load capacity. Since early 1900s, US Navy applications have ranged from the use of large, aircraft-carrying rigid airships as fleet scouts to the use of the non-rigid blimps as pickets and convoy escorts.
During the 1920'3 and 1930's the US Navy gained a great deal of experience with rigid airships of all sizes. The operational and logistic problems associated with these craft are well known, and eventually practical remedies were worked out. Further, some of the most substantial problem areas of the past may be essentially eliminated by modern materials and technology.
For example, the (over-emphasized) hazard of flight in rough weather would be reduced by the employment of aeronautical instrumentation, control systems, radar, and fly-by-wire pilot assistance. Also, the structure of the rigid airship would be improved (in strength, lightness and skin smoothness) by using new materials, fabrication techniques, and procedures for structural design and analysis. Still another area where great strides have been made over the last significant airships flew is that of power sources. Not only are aviation engines far lighter, more powerful and more efficient than early inefficient piston engines.
Classification of Airships
Airships may be taken to comprise the class of self-powered dirigibles. The term dirigible in itself properly applies to any steerable aerostat, powered or not. Airships of the past have been of three basic designs: rigid, semi-rigid and non-rigid. The first features a mechanically strong hull of the desired form, while the other two rely on (slight) internal overpressure to maintain the proper hull form. The semi-rigid type is distinguished from the non-rigid by having a keel, a strong member which carries the operational forces, while the non-rigid distribute the forces throughout the fabric of the airship itself.
The US Navy, amongst other sources, have developed a list of applications that may be satisfied by a lighter-than-air craft. Some of these applications have been previously used and some are possibilities. These applications may be categorised as either research applications or general applications. This list was first discussed in 1972 in the US Navy Research Laboratory and developed upon since then.
General uses for Lighter-than-air Ships
- Passenger and tourist flights
- Mobile wifi and Internet broadband services in remote areas
- Airborne visual inspection and survey
- Airborne stable elecstronic sensor platform for oceanography, meteorology, visual mapping and IR mapping
- Detailed mapping services
- Temporary emitter and broadcasting services
- Transport of speciality cargo packages
- Airborne navigation and weather radar platform
- Civilian emergency airborne command post
- Quick response transport vehicle for civilian rescue and disaster relief
Military uses for lighter-than-air craft
- Military fleet support, such as mine sweeping, anti-submarine platform
- Development of radio-location and navigation equipment training
- Anti-Submarine Warfare (ASW) operations
- Advanced electronic surveillance
- Disarmament inspection vehicle
- Drone support
Crazy proposals for lighter-than-air ships
- Development of nuclear propulsion for rigid lighter-than-air ships
A large rigid airship would appear to be the ideal vehicle for nuclear propulsion.
In 1972, the US Navy prepared an internal review paper that readily considered nuclear power units for power and propulsion. The paper stated that a "large rigid airship would appear to be the ideal vehicle for nuclear propulsion. The combination of nuclear propulsion and a thoroughly modernised airship airframe would form a vehicle whose performance eclipses that of any hitherto known. It is reasonable to expect significant payload capacities and virtually unlimited flight endurance." I am sure that there are even more significant concerns about pollution and nuclear fallout.
Operational Parameters for Rigid Airships
Some operational characteristics of rigid airships are functions of their size, while others of general to lighter-than-air craft. Amongst the latter are:
- Vertical take-off and landing
- Ability to hover
- Altitude can be controlled without power
- Speed is slow, probably about 100 knots (115mph) as a practical limit
- Altitide is relatively low, with a general ceiling of 7500ft (2200m) although this is not an absolute limit
- Angular and altitude stability in flight are very good
- Noise and vibration levels in flight are low
- The greatest risk of damage lies in collision with ground-based objects
- The volume available for crew space is large
- The volume available as cargo space for a given payload is large
- Loss of gas by leakage is negligible. Gas valved intentionally for manoeuvring may amount to about 1% per mission.
Airships of all types have the characteristic that the bigger they are, the better; their virtues are proportional to volume, while their faults are proportional to surface area.
Design and Construction of Airships
All airships have had hull forms approximating an ellipsoid of revolution. During World War I, rigid airships (Zeppelins) were built as cylinders with elliptical ends since this was quicker and cheaper. The peacetime practice was to taper the central section somewhat from fore to aft. The non-rigid and semi-rigid types have largely had hull forms more nearly ellipsoidal (save for some very early specimens).
The fineness ratio (length/diameter) strongly affects the dynamic stability of the airship. However, values of 4 to 8 give satisfactory results and the practice has been to build rigid airships with ratios of 6 and non-rigid with about 4.5 to 5. The slenderness of the rigid airships was a matter of manufacturing convenience more than a reasoned choice.
The structure of the rigid airship was a frame of ring girders and stringers (of wood or metal) covered with doped fabric. The interior of the hull was occupied by gas cells and passenger/cargo space. Gas cells were constructed of goldbeater's skin, and bulkheads between them usually were a group of taut wires. The semi-rigid and non-rigid types had envelopes of rubberized fabric and the envelope was filled by the gas cells, save for small communication passages. One non-rigid airship, the US Navy AMC-2, had an envelope of very thin aluminum.
The rigid type has the advantages that its hull is not deformed appreciably by external pressure, so is capable of higher speeds than the other types, and that space is available within the hull.
Problems of Rigid Airships
The majority of airship problem areas were of the sort which are part of developing any new device, and gradually vanished as experience was gained. Some were solved by the introduction of new materials and techniques throughout industry generally. Yet others are intrinsic in the nature of the airship and to these some accommodation must be made; it is to these areas that attention will be directed.
The area which has always presented airships with their greatest peril is that of ground-handling. Being large and lighter than air, airships are blown around easily by the wind and can be difficult for a group of men on the end of a line to control. Many of the early airships (e.g., until the end of World War I) were damaged by being blown into buildings, etc., while being held or moved by the ground crews.
Wind per se is no threat to an airship. Since it normally flies at 100 mph or more, it is obviously capable of tolerating winds of such speeds. However, it is the nature of an airship to head into the wind, and if it is tethered in such a way that the nose does so while the tail is prevented, from moving, the resulting bending moment can do damage. This situation can be avoided by tethering by the nose only, using a short mooring mast to keep vertical, motions small. This method was used successfully with fairly large vessels. When the very large ships Akron, Macon, Graf Zeppelin and Hiadenburg appeared, the stern beam was added. This was a heavy carriage running on circular railroad tracks. The tail of the airship was tied to this, the nose to a mooring post in the center of the circle and the ship oriented to point into the prevailing wind.
The stem beam also restrained the buoyancy when the ships were unloaded. At their home bases these large ships were kept in hangars, and the mooring post and stem beam ran on tracks so that the moored ship could be moved directly from the mooring circle into its hangar. With the development of this procedure ground handling of even such large vessels ceased to be a problem, but it should be remembered that there is an intrinsic propensity to trouble in this regard ready to develop if proper procedures are not followed.
Although it was not identified as such, the next most troublesome problem area was that of structural analysis. This was not an intrinsic problem, of course, but the inability to calculate airship structural responses made it impossible to predict its behavior accurately, let alone optimize its design. This can now be easily overcome with modern finite element design techniques and the abundance of computer processing power.
Modern Improvements to Airship Design
The rigid airship presents a structure in which gigantic improvements could be made today. First, with today's large, fast computers and modem knowledge of structural dynamics it is possible to analyse the airship's structure. The basic procedure was to lay out a moment diagram and determine requisite member strengths by simple beam theory. In time, the development of relaxation techniques allowed fairly accurate analysis of some of the structural components (e.g. ring girders), but the structure as a whole retained elements of mystery. Proven design details were changed with reluctance and to as small an extent as possible. The evolution of rigid airship structures was accordingly slow. Most post World War I airships were basically of Zeppelin-type due to the fact that Zeppelin Company had built far more than anyone else. It war a proven, dependable design, and while it was recognized as inefficient, it was not possible to improve on it at that time by any method other than trial and error. The present ability to analyse an airship structure as a complete frame can be relied on to produce more efficient designs.
Modem materials would also have great impact. The early airships used wood, duralumin, and steel wire for strength members, rubberized cotton cloth and cow's intestines for fabrics. Materials available today include plastics and metals with much better strength/density ratios, and plastic films are far superior in every respect to the fabrics. Use of Mylar film for gas cells, for example, should increase gas retention times from a few months to several thousand years. Even present-day engineered woods and age-hardenable aluminium alloys are greatly superior to those available at the dawn of aviation. In addition, the modem technology of composite materials would permit the properties of the individual structural elements to be tailored to the requirements established by structural analysis.
Other improvements available today over early designs are superior powerplants. There may be opportunities for purely electric capability supplemented with solar power arrays. Autopilot and system automation can make flying almost as easy as driving a car and in line with advanced AI capability could be fully autonomous. This would make airships ideal for intercity transport and logistics, thereby clearing congested highways.
Ground Facility requirements
Ground facility requirements. For small rigids (up to about 3 x 106 cu ft), ground facilities need be no more than a cleared circular area about 1500 ft across with a mooring post in the center. For short terra use, the same arrangement (with a larger cleared area) will also suffice for larger airships (e.g., 'Hindenberg' at Lakehurst), but as a permanent base it is desirable to have a mooring circle with stern beam, a hangar, and some helium storage capability. Satisfactory ground facility requirements and operating procedures required for rigids of up to 107 cu ft volume are well known and can be provided, while those required for larger vehicles are known only from sizable, but straightforward extrapolation.
About this article
This article is significantly based on research conducted in the 1970s and released into the public domain by the US Navy. Although there are many modern firms building and operating lighter-than-air craft, these vehicles have not become a significant model of transport. The practical benefits continue to be available today and the author felt that airships should be discussed today as a potential platform for current innovative ideas and applications.