This post was updated on 1 May 2019.
For more than two decades, there has been significant interest in the use of modern lighter-than-air craft and hybrid airships in a variety of military, commercial and other roles, including:
- Heavy cargo carriers serving remote and/or unimproved sites
- Persistent optionally-manned surveillance platforms
- Maritime surveillance / search and rescue
- Disaster relief, particularly in areas not easily accessible by other means
- Unmanned aerial vehicle (UAV) / unmanned air system (UAS) carrier
- Commercial flying cruise liner
- Airship yacht
In spite of the significant interest, actual military, commercial and other customers have been slow coming to the marketplace with firm orders, the airship manufacturers have been slow in developing and delivering advanced airships that meet their customer’s needs, and funding was prematurely curtailed for several ambitious projects. This uncertain business climate seems likely to change by the early 2020s, when several different heavy-lift airships are expected to be certified by airworthiness authorities and ready for mass production and sale to interested customers.
One of the very significant factors driving interest in modern airships is that they offer the potential to link isolated regions with the rest of the world while doing so in a way that should have lower environmental impacts than other transportation alternatives for those regions. This target market for airships exists in more than two-thirds of the world’s land area where more than half the world’s population live without direct access to paved roads and reliable ground transportation.
This matter is described well in a 21 February 2016 article by Jeanne Marie Laskas, “Helium Dreams – A new generation of airships is born,” which is posted on The New Yorker website at the following link:
In this article, Boris Pasternak, CEO of airship manufacturer Worldwide Aeros Corp., commented:
“The biggest challenge in using lighter-than-air technology to lift hundreds of tons of cargo is not with the lifting itself—the larger the envelope of gas, the more you can lift—but with what occurs after you let the stuff go. ‘When I drop the cargo, what happens to the airship?’ Pasternak said. ‘It’s flying to the moon.’ An airship must take on ballast to compensate for the lost weight of the unloaded cargo, or a ground crew must hold it down with ropes.”
Among the many current designers and manufacturers of large airships, this matter of “load exchange” (i.e., maintaining the airship’s net buoyancy within certain limits while loading and unloading cargo and passengers) is handled in several different ways depending on the type of airship involved. Some load exchange solutions require ground infrastructure for external ballast handling, while others require no such infrastructure. The solution chosen for accomplishing a load exchange strongly influences how an airship can be operationally employed and where it can deliver its payload.
- Types of modern airships
The term “aerostat” broadly includes all lighter than air vehicles that gain lift through the use of a buoyant gas. Aerostats include unpowered balloons (tethered or free-flying) and powered airships.
There are three main types of powered airships: conventional, hybrid, and variable buoyancy / fixed volume. The basic characteristics of each airship type are described below.
Basic characteristics of conventional airships
Conventional airships are lighter-than-air (LTA) vehicles that operate at or near neutral buoyancy. Airships of this type include non-rigid blimps, rigid zeppelins, and semi-rigid airships. The lifting gas (helium) generate 100% of the lift at low speed, thereby permitting vertical takeoff and landing (VTOL) operations and hovering. Various types of propulsors may be used for cruise flight propulsion and for low-speed maneuvering and station keeping.
- Non-rigid airships (blimps): These airships have a flexible envelope that defines the shape of the airship, contains the lifting gas cells and ballonets, and supports the load of a gondola, engines and payload.
- Rigid airships (zeppelins):These airships have a lightweight, rigid airframe that defines their exterior shape. The rigid airframe supports the gondola, engines and payload. Lifting gas cells and ballonets are located within the rigid airframe.
- Semi-rigid airships: These airships have a rigid internal structural framework that supports loads. A flexible envelope is installed over the structural framework and contains the lifting gas cells and ballonets.
After being loaded and ballasted before flight, conventional airships have little control over the in-flight buoyancy of the airship. Control can be exercised over ballast, lifting gas and the ballonets as described below.
- Ballast: Conventional airships require adjustable ballast (i.e., typically water or sand) that can be added or removed as needed to establish a desired net buoyancy before flight. Load exchanges require a corresponding ballast adjustment. If an airship is heavy and the desired buoyancy can’t be restored with the ballonets, ballast can be dumped in flight to increase buoyancy.
- Lifting gas: Normally, there is no significant loss of lifting gas during flight. If an airship is light and the desired buoyancy cannot be restored with the ballonets, it is possible to vent some lifting gas to the atmosphere to decrease static lift.
- Ballonets:In conventional airships, the gas envelope is divided into a sealed main helium gas volume and separate gas volumes called “ballonets” that contain ambient air at atmospheric pressure. The ballonets are used to compensate for change in the volume of lifting gas and to make small changes in buoyancy by expanding or contracting the air volume to change the gross weight or the fore-and-aft trim of the airship.
On the ground, the ballonets may be inflated with air to make the airship negatively buoyant to simplify ground handling. To takeoff, the ballonets would be vented to the atmosphere, reducing the mass of air carried by the airship, allowing the helium gas volume to expand, and increasing buoyant lift.
As the airship gains altitude, external air pressure decreases, allowing the helium gas volume to expand within the gas envelope, into space previously occupied by the air in the ballonets, which vent a portion of their air content overboard. The airship reaches its maximum altitude, known as its “pressure height,” when the helium gas volume has expanded to fill the gas envelope and the ballonets are empty. At this point, the airship’s mass is at a minimum and the helium lifting gas can expand no further.
To descend, a fan is used to inflate the ballonets with outside air, adding mass and slightly compressing the helium into a smaller volume. This action decreases buoyant lift. As the airship continues to descend into the denser atmosphere, the helium gas volume continues to compress and the ballonets become proportionately larger. Ballonet inflation is controlled to manage buoyancy as the airship approaches the ground for a landing.
In flight, inflating only the fore or aft ballonet, and allowing the opposite ballonet to deflate, will make the bow or stern of the airship heavier and change the pitch of the airship. These operating principles are shown in the following diagrams of a blimp with two ballonets, which are shown in blue.
Blimp with ballonets (blue). Source: zeppelinfan.de
Basic characteristics of hybrid airships
These are heavier-than-air (HTA) vehicles that are “semi-buoyant.” This means that the lifting gas provides only a fraction of the needed lift (typically 60 – 80%) and the balance of the lift needed for flight is generated by other means, such as vectored thrust engines and aerodynamic lift from the fuselage and wings during forward flight.
Sources of lift for a semi-rigid, inflated hybrid airship. Source: DoD 2012
General characteristics of hybrid airships include the following:
- This type of airship requires some airspeed to generate aerodynamic lift.Therefore, it typically makes a short takeoff and landing (STOL).
- Some hybrid airships may be capable of limited VTOL operations (i.e., when lightly loaded, or when equipped with powerful vectored thrust engines).
- Like conventional airships, the gas envelope in hybrid airship is divided into helium gas volumes and separate volumes containing ambient air.
- Hybrid airships are heavier-than-air and are easier to control on the ground than conventional airships.
There are two types of hybrid airships: semi-rigid and rigid.
- Semi-rigid hybrid airships: These airships have a structural keel or spine to carry loads, and a large, lifting-body shaped inflated fuselage containing the lifting gas cells and ballonets. Operation of the ballonets to adjust net buoyancy and pitch angle is similar to their use on conventional airships. These wide hybrid airships may have separate ballonets on each side of the inflated envelope to adjust the roll angle. While these airships are heavier-than-air, they still generally require adjustable ballast to handle a load exchange involving a heavy load.
- Rigid hybrid airships: These airships have a more substantial structure that defines the shape of the exterior aeroshell. In some respects, these are semi-buoyant aircraft, with less buoyancy than the semi-rigid hybrid airships. In exchange for the reduced buoyancy, handling on the ground is more like a conventional fixed-wing aircraft and load exchanges do not require external ballast.
Basic characteristics of variably buoyancy / fixed volume airships
These are rigid airships that can become LTA or HTA, as the circumstances require. These airships become heavier by compressing the helium lifting gas or ambient air:
- Compressing the helium lifting gas into smaller volume tanks aboard the airship reduces the total lift generated by helium.
- Compressing ambient air into pressurized tanks aboard the airship adds weight to the airship and thus decreases the net lift.
These airships become lighter by venting gas from the pressurized tanks:
- Compressed helium lifting gas is vented back into the helium lift cells, increasing their volume and increasing lift.
- Compressed air is vented to the atmosphere, reducing the weight of the airship and thus increasing net lift.
This buoyancy control process is accomplished without taking on external ballast or venting the lifting gas to the atmosphere.
General characteristics of variable buoyancy airships include the following:
- Variable lift airships are capable of VTOL operations and hovering with a full load.
- The buoyancy control system enables in-flight load exchanges from a hovering airship without the need for external ballast.
- On the ground, variable lift airships can make themselves heavier-than-air to facilitate load exchanges without external infrastructure or ballast.
- The scale of large cargo airships
Some of the advanced airship concepts being developed, especially for future heavy cargo carriers, will result in extremely large air vehicles on a scale we haven’t seen since the heyday of the giant zeppelins in the 1930s. Consider the following semi-rigid hybrid airships shown to scale with contemporary fixed-wing cargo aircraft.
Size comparison for hybrid airships required for various lift applications. Source: DoD 2012
- Specific airships
Details on the airships listed in the following tables are provided in individual sections.
Megalifter, CargoLifter CL160, Project Walrus and SkyCat are included because they are of historical interest as early, though unsuccessful, attempts to develop very large cargo airships. Concepts and technologies developed on these airship projects have promoted the development of other modern airships.
Among the airships in the above list, the following have actually flown:
- Zeppelin NT 07
- Skybus 80K
- Aeroscraft Dragon Dream
- Hybrid Air Vehicles HAV-304
- Hybrid Air Vehicles Airlander 10 prototype
- Lockheed Martin P-791
As of May 2019, the Zeppelin NT 07 is the only advanced airship in this list that is flying regularly in commercial service. The others in the list are under development or remain as concepts only.
By the early 2020s, we likely will see several advanced airships on this list completing their development cycle and airworthiness certification. The leading candidates seem to be:
- Aeroscraft ML866 / Aeroscraft Gen 2
- Airlander 10
- Lockheed Martin LMH-1
Here are the links to the individual airships descriptions:
Variable buoyancy, fixed volume airships:
Hybrid (semi-buoyant) airships: